Distinct transcriptomic profile of satellite cells contributes to preservation of neuromuscular junctions in extraocular muscles of ALS mice

Amyotrophic lateral sclerosis (ALS) is a fatal neuromuscular disorder characterized by progressive weakness of almost all skeletal muscles, whereas extraocular muscles (EOMs) are comparatively spared. While hindlimb and diaphragm muscles of end-stage SOD1G93A (G93A) mice (a familial ALS mouse model) exhibit severe denervation and depletion of Pax7+satellite cells (SCs), we found that the pool of SCs and the integrity of neuromuscular junctions (NMJs) are maintained in EOMs. In cell sorting profiles, SCs derived from hindlimb and diaphragm muscles of G93A mice exhibit denervation-related activation, whereas SCs from EOMs of G93A mice display spontaneous (non-denervation-related) activation, similar to SCs from wild-type mice. Specifically, cultured EOM SCs contain more abundant transcripts of axon guidance molecules, including Cxcl12, along with more sustainable renewability than the diaphragm and hindlimb counterparts under differentiation pressure. In neuromuscular co-culture assays, AAV-delivery of Cxcl12 to G93A-hindlimb SC-derived myotubes enhances motor neuron axon extension and innervation, recapitulating the innervation capacity of EOM SC-derived myotubes. G93A mice fed with sodium butyrate (NaBu) supplementation exhibited less NMJ loss in hindlimb and diaphragm muscles. Additionally, SCs derived from G93A hindlimb and diaphragm muscles displayed elevated expression of Cxcl12 and improved renewability following NaBu treatment in vitro. Thus, the NaBu-induced transcriptomic changes resembling the patterns of EOM SCs may contribute to the beneficial effects observed in G93A mice. More broadly, the distinct transcriptomic profile of EOM SCs may offer novel therapeutic targets to slow progressive neuromuscular functional decay in ALS and provide possible 'response biomarkers' in pre-clinical and clinical studies.

Developmental, Physiological and Phylogenetic Perspectives on the Expression and Regulation of Myosin Heavy Chains in Craniofacial Muscles

This review deals with the developmental origins of extraocular, jaw and laryngeal muscles, the expression, regulation and functional significance of sarcomeric myosin heavy chains (MyHCs) that they express and changes in MyHC expression during phylogeny. Myogenic progenitors from the mesoderm in the prechordal plate and branchial arches specify craniofacial muscle allotypes with different repertoires for MyHC expression. To cope with very complex eye movements, extraocular muscles (EOMs) express 11 MyHCs, ranging from the superfast extraocular MyHC to the slowest, non-muscle MyHC IIB (nmMyH IIB). They have distinct global and orbital layers, singly- and multiply-innervated fibres, longitudinal MyHC variations, and palisade endings that mediate axon reflexes. Jaw-closing muscles express the high-force masticatory MyHC and cardiac or limb MyHCs depending on the appropriateness for the acquisition and mastication of food. Laryngeal muscles express extraocular and limb muscle MyHCs but shift toward expressing slower MyHCs in large animals. During postnatal development, MyHC expression of craniofacial muscles is subject to neural and hormonal modulation. The primary and secondary myotubes of developing EOMs are postulated to induce, via different retrogradely transported neurotrophins, the rich diversity of neural impulse patterns that regulate the specific MyHCs that they express. Thyroid hormone shifts MyHC 2A toward 2B in jaw muscles, laryngeal muscles and possibly extraocular muscles. This review highlights the fact that the pattern of myosin expression in mammalian craniofacial muscles is principally influenced by the complex interplay of cell lineages, neural impulse patterns, thyroid and other hormones, functional demands and body mass. In these respects, craniofacial muscles are similar to limb muscles, but they differ radically in the types of cell lineage and the nature of their functional demands.

fhl2b mediates extraocular muscle protection in zebrafish models of muscular dystrophies and its ectopic expression ameliorates affected body muscles

In muscular dystrophies, muscle fibers loose integrity and die, causing significant suffering and premature death. Strikingly, the extraocular muscles (EOMs) are spared, functioning well despite the disease progression. Although EOMs have been shown to differ from body musculature, the mechanisms underlying this inherent resistance to muscle dystrophies remain unknown. Here, we demonstrate important differences in gene expression as a response to muscle dystrophies between the EOMs and trunk muscles in zebrafish via transcriptomic profiling. We show that the LIM-protein Fhl2 is increased in response to the knockout of desmin, plectin and obscurin, cytoskeletal proteins whose knockout causes different muscle dystrophies, and contributes to disease protection of the EOMs. Moreover, we show that ectopic expression of fhl2b can partially rescue the muscle phenotype in the zebrafish Duchenne muscular dystrophy model sapje, significantly improving their survival. Therefore, Fhl2 is a protective agent and a candidate target gene for therapy of muscular dystrophies.

Extraocular muscle stem cells exhibit distinct cellular properties associated with non-muscle molecular signatures

Skeletal muscle stem cells (MuSCs) are recognised as functionally heterogeneous. Cranial MuSCs are reported to have greater proliferative and regenerative capacity when compared with those in the limb. A comprehensive understanding of the mechanisms underlying this functional heterogeneity is lacking. Here, we have used clonal analysis, live imaging and single cell transcriptomic analysis to identify crucial features that distinguish extraocular muscle (EOM) from limb muscle stem cell populations. A MyogeninntdTom reporter showed that the increased proliferation capacity of EOM MuSCs correlates with deferred differentiation and lower expression of the myogenic commitment gene Myod. Unexpectedly, EOM MuSCs activated in vitro expressed a large array of extracellular matrix components typical of mesenchymal non-muscle cells. Computational analysis underscored a distinct co-regulatory module, which is absent in limb MuSCs, as driver of these features. The EOM transcription factor network, with Foxc1 as key player, appears to be hardwired to EOM identity as it persists during growth, disease and in vitro after several passages. Our findings shed light on how high-performing MuSCs regulate myogenic commitment by remodelling their local environment and adopting properties not generally associated with myogenic cells.

Obscurin Maintains Myofiber Identity in Extraocular Muscles

Purpose

The cytoskeleton of the extraocular muscles (EOMs) is significantly different from that of other muscles. We aimed to investigate the role of obscurin, a fundamental cytoskeletal protein, in the EOMs.

Methods

The distribution of obscurin in human and zebrafish EOMs was compared using immunohistochemistry. The two obscurin genes in zebrafish, obscna and obscnb, were knocked out using CRISPR/Cas9, and the EOMs were investigated using immunohistochemistry, qPCR, and in situ hybridization. The optokinetic reflex (OKR) in five-day-old larvae and adult obscna-/-;obscnb-/- and sibling control zebrafish was analyzed. Swimming distance was recorded at the same age.

Results

The obscurin distribution pattern was similar in human and zebrafish EOMs. The proportion of slow and fast myofibers was reduced in obscna-/-;obscnb-/- zebrafish EOMs but not in trunk muscle, whereas the number of myofibers containing cardiac myosin myh7 was significantly increased in EOMs of obscurin double mutants. Loss of obscurin resulted in less OKRs in zebrafish larvae but not in adult zebrafish.

Conclusions

Obscurin expression is conserved in normal human and zebrafish EOMs. Loss of obscurin induces a myofiber type shift in the EOMs, with upregulation of cardiac myosin heavy chain, myh7, showing an adaptation strategy in EOMs. Our model will facilitate further studies in conditions related to obscurin.

MYH13, a superfast myosin expressed in extraocular, laryngeal and syringeal muscles

MYH13 is a unique type of sarcomeric myosin heavy chain (MYH) first detected in mammalian extraocular (EO) muscles and later also in vocal muscles, including laryngeal muscles of some mammals and syringeal muscles of songbirds. All these muscles are specialized in generating very fast contractions while producing relatively low force, a design appropriate for muscles acting against a much lower load than most skeletal muscles inserting into the skeleton. The definition of the physiological properties of muscle fibres containing MYH13 has been complicated by the mixed fibre type composition of EO muscles and the coexistence of different MYH types within the same fibre. A major advance in this area came from studies on isolated recombinant myosin motors and the demonstration that the affinity of actin-bound human MYH13 for ADP is much weaker than those of fast-type MYH1 (type 2X) and MYH2 (type 2A). This property is consistent with a very fast detachment of myosin from actin, a major determinant of shortening velocity. The MYH13 gene arose early during vertebrate evolution but was characterized only in mammals and birds and appears to have been lost in some teleost fish. The MYH13 gene is located at the 3' end of the mammalian fast/developmental gene cluster and in a similar position to the orthologous cluster in syntenic regions of the songbird genome. MYH13 gene regulation is controlled by a super-enhancer in the mammalian locus and deletion of the neighbouring fast MYH1 and MYH4 genes leads to abnormal MYH13 expression in mouse leg muscles.

Effects of myostatin gene knockout on porcine extraocular muscles

Myostatin (MSTN), a negative regulator of skeletal muscle mass, is not well known in extraocular muscles (EOMs). EOMs are specialized skeletal muscles. Hence, in this study, the effect of MSTN on the superior rectus (SR) and superior oblique (SO) of 2-month-old MSTN knockout (MSTN-/-) and wild-type (WT) pigs of the same genotype was investigated. SR (P < 0.01) and SO (P < 0.001) fiber cross-sectional areas of MSTN-/- pigs were significantly larger than those of WT pigs. Compared with WT pigs, MSTN-/- SO displayed a decrease in type I fibers (WT: 27.24%, MSTN-/-: 10.32%, P < 0.001). Type IIb fibers were higher in MSTN-/- pigs than in WT pigs (WT: 30.38%, MSTN-/-: 62.24%, P < 0.001). The trend in SR was the same as that in SO, although the trend in SO was greater than that in SR. The expression of myogenic differentiation factor (MyoD) and myogenic (MyoG) showed a significant increase in MSTN-/- SO (about 2.5-fold and 2-fold, respectively at the gene expression level, about 1.5-fold at the protein level) compared with WT pigs. MSTN plays an important role in the development of EOMs and regulates the muscle fiber type by modulating the gene expression of MyoD and MyoG in pigs.

Identification and characterization of differentially expressed circular RNAs in extraocular muscle of oculomotor nerve palsy

BACKGROUND

Oculomotor nerve palsy (ONP) is a neuroparalytic disorder resulting in dysfunction of innervating extraocular muscles (EOMs), of which the pathological characteristics remain underexplored.

METHODS

In this study, medial rectus muscle tissue samples from four ONP patients and four constant exotropia (CXT) patients were collected for RNA sequencing. Differentially expressed circular RNAs (circRNAs) were identified and included in functional enrichment analysis, followed by interaction analysis with microRNAs and mRNAs as well as RNA binding proteins. Furthermore, RT-qPCR was used to validate the expression level of the differentially expressed circRNAs.

RESULTS

A total of 84 differentially expressed circRNAs were identified from 10,504 predicted circRNAs. Functional enrichment analysis indicated that the differentially expressed circRNAs significantly correlated with skeletal muscle contraction. In addition, interaction analyses showed that up-regulated circRNA_03628 was significantly interacted with RNA binding protein AGO2 and EIF4A3 as well as microRNA hsa-miR-188-5p and hsa-miR-4529-5p. The up-regulation of circRNA_03628 was validated by RT-qPCR, followed by further elaboration of the expression, location and clinical significance of circRNA_03628 in EOMs of ONP.

CONCLUSIONS

Our study may shed light on the role of differentially expressed circRNAs, especially circRNA_03628, in the pathological changes of EOMs in ONP.

Development of Nystagmus With the Absence of MYOD Expression in the Extraocular Muscles

Purpose

Myoblast determination protein 1 (MYOD) is a critical myogenic regulatory factor in muscle development, differentiation, myofiber repair, and regeneration. As the extraocular muscles significantly remodel their myofibers throughout life compared with limb skeletal muscles, we hypothesized that the absence of MYOD would result in their abnormal structure and function. To assess structural and functional changes in the extraocular muscles in MyoD-/- mice, fiber size and number and optokinetic nystagmus reflex (OKN) responses were examined.

Methods

OKN was measured in MyoD-/- mice and littermate wild-type controls at 3, 6, and 12 months. The extraocular muscles were examined histologically for changes in mean myofiber cross-sectional area, total myofiber number, and nuclei immunostained for PAX7 and PITX2, markers of myogenic precursor cells.

Results

The MyoD-/- mice developed nystagmus, with both jerk and pendular waveforms, in the absence and in the presence of moving visual stimulation. At 12 months, there were significant losses in mean myofiber cross-sectional area and in total number of orbital layer fibers in all rectus muscles, as well as in global layer fibers in the superior and inferior rectus muscles. Haploinsufficient mice showed abnormal OKN responses. PITX2-positive cell entry into myofibers of the MyoD-/- mice was significantly reduced.

Conclusions

This study is the first demonstration of the development of nystagmus in the constitutive absence of expression of the muscle-specific transcription factor MYOD. We hypothesize that myofiber loss over time may alter anterograde and/or retrograde communication between the motor nerves and extraocular muscles that are critical for maintaining normalcy of extraocular muscle function.

Oculomotor nerve guidance and terminal branching requires interactions with differentiating extraocular muscles

Muscle function is dependent on innervation by the correct motor nerves. Motor nerves are composed of motor axons which extend through peripheral tissues as a compact bundle, then diverge to create terminal nerve branches to specific muscle targets. As motor nerves approach their targets, they undergo a transition where the fasciculated nerve halts further growth then after a pause, the nerve later initiates branching to muscles. This transition point is potentially an intermediate target or guidepost to present specific cellular and molecular signals for navigation. Here we describe the navigation of the oculomotor nerve and its association with developing muscles in mouse embryos. We found that the oculomotor nerve initially grew to the eye three days prior to the appearance of any extraocular muscles. The oculomotor axons spread to form a plexus within a mass of cells, which included precursors of extraocular muscles and other orbital tissues and expressed the transcription factor Pitx2. The nerve growth paused in the plexus for more than two days, persisting during primary extraocular myogenesis, with a subsequent phase in which the nerve branched out to specific muscles. To test the functional significance of the nerve contact with Pitx2+ cells in the plexus, we used two strategies to genetically ablate Pitx2+ cells or muscle precursors early in nerve development. The first strategy used Myf5-Cre-mediated expression of diphtheria toxin A to ablate muscle precursors, leading to loss of extraocular muscles. The oculomotor axons navigated to the eye to form the main nerve, but subsequently largely failed to initiate terminal branches. The second strategy studied Pitx2 homozygous mutants, which have early apoptosis of Pitx2-expressing precursor cells, including precursors for extraocular muscles and other orbital tissues. Oculomotor nerve fibers also grew to the eye, but failed to stop to form the plexus, instead grew long ectopic projections. These results show that neither Pitx2 function nor Myf5-expressing cells are required for oculomotor nerve navigation to the eye. However, Pitx2 function is required for oculomotor axons to pause growth in the plexus, while Myf5-expressing cells are required for terminal branch initiation.

Muscle Progenitors Derived from Extraocular Muscles Express Higher Levels of Neurotrophins and their Receptors than other Cranial and Limb Muscles

Extraocular muscles (EOMs) show resistance to muscle dystrophies and sarcopenia. It has been recently demonstrated that they are endowed with different types of myogenic cells, all of which present an outstanding regenerative potential. Neurotrophins are important modulators of myogenic regeneration and act promoting myoblast proliferation, enhancing myogenic fusion rates and protecting myotubes from inflammatory stimuli. Here, we adapted the pre-plate cell isolation technique to obtain myogenic progenitors from the rat EOMs, and quantified their in vitro expression of neurotrophins and their receptors by RT–qPCR and immunohistochemistry, respectively. The results were compared with the expression on progenitors isolated from buccinator, tongue and limb muscles. Our quantitative analysis of brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF) and neurotrophin-3 (NT-3) transcripts showed, for the first time, that EOMs-derived cells express more of these factors and that they expressed TrkA, but not TrkB and TrkC receptors. On the contrary, the immunofluorescence analysis demonstrated high expression of p75^NTR on all myogenic progenitors, with the EOMs-derived cells showing higher expression. Taken together, these results suggest that the intrinsic trophic differences between EOMs-derived myogenic progenitors and their counterparts from other muscles could explain why those cells show higher proliferative and fusion rates, as well as better regenerative properties.

Distinct segregation of the pathogenic m.5667G>A mitochondrial tRNAAsn mutation in extraocular and skeletal muscle in chronic progressive external ophthalmoplegia

Chronic progressive external ophthalmoplegia (CPEO) is a frequent clinical manifestation of disorders caused by pathogenic mitochondrial DNA mutations. However, for diagnostic purposes skeletal muscle tissue is used, since extraocular muscle tissue is usually not available for work-up. In the present study we aimed to identify causative factors that are responsible for extraocular muscle to be primarily affected in CPEO. We performed comparative histochemical and molecular genetic analyses of extraocular muscle and skeletal muscle single fibers in a case of isolated CPEO caused by the heteroplasmic m.5667G>A mutation in the mitochondrial tRNA^Asn gene (MT-TN). Histochemical analyses revealed higher proportion of cytochrome c oxidase deficient fibers in extraocular muscle (41%) compared to skeletal muscle (10%). However, genetic analyses of single fibers revealed no significant difference either in the mutation loads between extraocular muscle and skeletal muscle cytochrome c oxidase deficient single fibers (extraocular muscle 86% ± 4.6%; skeletal muscle 87.8 %± 5.7%, p = 0.246) nor in the mutation threshold (extraocular muscle 74% ± 3%; skeletal muscle 74% ± 4%). We hypothesize that higher proportion of cytochrome c oxidase deficient fibers in extraocular muscle compared to skeletal muscle might be due to facilitated segregation of the m.5667G>A mutation into extraocular muscle, which may explain the preferential ocular manifestation and clinically isolated CPEO.

Changing muscle function with sustained glial derived neurotrophic factor treatment of rabbit extraocular muscle

Recent microarray and RNAseq experiments provided evidence that glial derived neurotrophic factor (GDNF) levels were decreased in extraocular muscles from human strabismic subjects compared to age-matched controls. We assessed the effect of sustained GDNF treatment of the superior rectus muscles of rabbits on their physiological and morphological characteristics, and these were compared to naïve control muscles. Superior rectus muscles of rabbits were implanted with a sustained release pellet of GDNF to deliver 2μg/day, with the contralateral side receiving a placebo pellet. After one month, the muscles were assessed using in vitro physiological methods. The muscles were examined histologically for alteration in fiber size, myosin expression patterns, neuromuscular junction size, and stem cell numbers and compared to age-matched naïve control muscles. GDNF resulted in decreased force generation, which was also seen on the untreated contralateral superior rectus muscles. Muscle relaxation times were increased in the GDNF treated muscles. Myofiber mean cross-sectional areas were increased after the GDNF treatment, but there was a compensatory increase in expression of developmental, neonatal, and slow tonic myosin heavy chain isoforms. In addition, in the GDNF treated muscles there was a large increase in Pitx2-positive myogenic precursor cells. One month of GDNF resulted in significant extraocular muscle adaptation. These changes are interesting relative to the decreased levels of GDNF in the muscles from subjects with strabismus and preliminary data in infant non-human primates where sustained GDNF treatment produced a strabismus. These data support the view that GDNF has the potential for improving eye alignment in subjects with strabismus.

Pattern of Anesthetic Diffusion during Peribulbar Infiltration: A Ct Evaluation

PURPOSE

The aim of this study was to evaluate the changes undergone by orbital structures during peribulbar anesthesia.

METHODS

CT-thin slices were acquired and 8-9 ml of anesthetic, with a small part of low-density non-ionic water-soluble contrast medium were injected under CT guide once to follow extraconal spreading and diffusion to intraconal structures and above all, the surgically important structures such as recti muscles, orbicular and optic nerve sheath. Conjunctival sack involvement was assessed and conjunctival chemosis was rated. Induced ptosis was evaluated, verifying change before and after eyepressure.

RESULTS

The results help in optimizing anesthetic technique, above all concerning the site, materials and quantity, and suggest the amount of anesthesia could be reduced.

CONCLUSIONS

The peribulbar anesthetic technique proved effective and safe for patients.

Effects of retinoic acid signaling on extraocular muscle myogenic precursor cells in vitro

One major difference between limb and extraocular muscles (EOM) is the presence of an enriched population of Pitx2-positive myogenic precursor cells in EOM compared to limb muscle. We hypothesize that retinoic acid regulates Pitx2 expression in EOM myogenic precursor cells and that its effects would differ in leg muscle. The two muscle groups expressed differential retinoic acid receptor (RAR) and retinoid X receptor (RXR) levels. RXR co-localized with the Pitx2-positive cells but not with those expressing Pax7. EOM-derived and LEG-derived EECD34 cells were treated with vehicle, retinoic acid, the RXR agonist bexarotene, the RAR inverse agonist BMS493, or the RXR antagonist UVI 3003. In vitro, fewer EOM-derived EECD34 cells expressed desmin and fused, while more LEG-derived cells expressed desmin and fused when treated with retinoic acid compared to vehicle. Both EOM and LEG-derived EECD34 cells exposed to retinoic acid showed a higher percentage of cells expressing Pitx2 compared to vehicle, supporting the hypothesis that retinoic acid plays a role in maintaining Pitx2 expression. We hypothesize that retinoic acid signaling aids in the maintenance of large numbers of undifferentiated myogenic precursor cells in the EOM, which would be required to maintain EOM normalcy throughout a lifetime of myonuclear turnover.

Phenolic Elderberry Extracts, Anthocyanins, Procyanidins, and Metabolites Influence Glucose and Fatty Acid Uptake in Human Skeletal Muscle Cells

Uptake of glucose and fatty acids in skeletal muscle is of interest for type 2 diabetes treatment. The aim was to study glucose and fatty acid uptake in skeletal muscle cells, antioxidant effects, and inhibition of carbohydrate-hydrolyzing enzymes by elderberries. Enhanced glucose and oleic acid uptake in human skeletal muscle cells were observed after treatment with phenolic elderberry extracts, anthocyanins, procyanidins, and their metabolites. The 96% EtOH and the acidified MeOH extracts were highly active. Of the isolated substances, cyanidin-3-glucoside and cyanidin-3-sambubioside showed highest stimulation of uptake. Phloroglucinol aldehyde was most active among the metabolites. Isolated anthocyanins and procyanidins are strong radical scavengers and are good inhibitors of 15-lipoxygenase and moderate inhibitors of xanthine oxidase. As α-amylase and α-glucosidase inhibitors, they are considerably better than the positive control acarbose. The antidiabetic property of elderberry phenolics increases the nutritional value of this plant and indicates potential as functional food against diabetes.

Distinct inflammatory gene expression in extraocular muscle and fat from patients with Graves' orbitopathy

OBJECTIVE

This study sought to compare patients with thyroid eye disease (TED) and normal controls with respect to the expression of the NR3C1, CHUK, IKBKB, FOS, NFKB and HSD11B1 genes in orbital fat (OF) and extraocular muscle (EOM).

DESIGN AND METHODS

A prospective study design was used to evaluate 34 TED patients and 38 healthy controls. OF was harvested from 33 TED patients and 27 controls. EOM biopsies were obtained from 32 TED patients and 18 controls. Samples were examined by real-time PCR and evaluated using appropriate statistical analyses with a significance cut-off of P < 0.05.

RESULTS

NR3C1 mRNA levels were higher in TED EOM (median 213 (96-376)) than those in control EOM (78 (34-138)) (P < 0.001), and NFKB expression was elevated in TED muscle (223 (31-520)) relative to that in control muscle (8 (6-31)) (P < 0.001). HSD11B1 expression was higher in TED EOM (0.78 (0.47-2.01)) than that in control EOM (0.22 (0.09-0.51)) (P < 0.001). Levels of CHUK, IKBKB, and FOS were higher in TED EOM (115 (20-223), 111 (54-299) and 0.11 (0.03-0.19), respectively) than those in control EOM (5.8 (2-13), 21 (5-52) and 0.05 (0.001-0.03) respectively) (P < 0.001).

CONCLUSION

Tissues involved in GO exhibited different mRNA levels of NR3C1, CHUK, IKBKB, FOS, NFKB and HSD11B1. Gene expression in OF was similar for TED patients and controls. CHUK, IKBKB, FOS, NFKB, and HSD11B1 mRNA levels were higher in TED EOM than those in control EOM. NFKB was disproportionally elevated compared with NR3C1; this finding was indicative of a local proinflammatory profile.

Sparing of the extraocular muscles in mdx mice with absent or reduced utrophin expression: A life span analysis

Sparing of the extraocular muscles in muscular dystrophy is controversial. To address the potential role of utrophin in this sparing, mdx:utrophin(+/-) and mdx:utrophin(-/-) mice were examined for changes in myofiber size, central nucleation, and Pax7-positive and MyoD-positive cell density at intervals over their life span. Known to be spared in the mdx mouse, and contrary to previous reports, the extraocular muscles from both the mdx:utrophin(+/-) and mdx:utrophin(-/-) mice were also morphologically spared. In the mdx:utrophin(+/)(-) mice, which have a normal life span compared to the mdx:utrophin(-/-) mice, the myofibers were larger at 3 and 12 months than the wild type age-matched eye muscles. While there was a significant increase in central nucleation in the extraocular muscles from all mdx:utrophin(+/)(-) mice, the levels were still very low compared to age-matched limb skeletal muscles. Pax7- and MyoD-positive myogenic precursor cell populations were retained and were similar to age-matched wild type controls. These results support the hypothesis that utrophin is not involved in extraocular muscle sparing in these genotypes. In addition, it appears that these muscles retain the myogenic precursors that would allow them to maintain their regenerative capacity and normal morphology over a lifetime even in these more severe models of muscular dystrophy.

Correlation between infectivity and disease associated prion protein in the nervous system and selected edible tissues of naturally affected scrapie sheep

The transmissible spongiform encephalopathies (TSEs) or prion diseases are a group of fatal neurodegenerative disorders characterised by the accumulation of a pathological form of a host protein known as prion protein (PrP). The validation of abnormal PrP detection techniques is fundamental to allow the use of high-throughput laboratory based tests, avoiding the limitations of bioassays. We used scrapie, a prototype TSE, to examine the relationship between infectivity and laboratory based diagnostic tools. The data may help to optimise strategies to prevent exposure of humans to small ruminant TSE material via the food chain. Abnormal PrP distribution/accumulation was assessed by immunohistochemistry (IHC), Western blot (WB) and ELISA in samples from four animals. In addition, infectivity was detected using a sensitive bank vole bioassay with selected samples from two of the four sheep and protein misfolding cyclic amplification using bank vole brain as substrate (vPMCA) was also carried out in selected samples from one animal. Lymph nodes, oculomotor muscles, sciatic nerve and kidney were positive by IHC, WB and ELISA, although at levels 100-1000 fold lower than the brain, and contained detectable infectivity by bioassay. Tissues not infectious by bioassay were also negative by all laboratory tests including PMCA. Although discrepancies were observed in tissues with very low levels of abnormal PrP, there was an overall good correlation between IHC, WB, ELISA and bioassay results. Most importantly, there was a good correlation between the detection of abnormal PrP in tissues using laboratory tests and the levels of infectivity even when the titre was low. These findings provide useful information for risk modellers and represent a first step toward the validation of laboratory tests used to quantify prion infectivity, which would greatly aid TSE risk assessment policies.

Analysis of neurotrophic factors in limb and extraocular muscles of mouse model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is currently an incurable fatal motor neuron syndrome characterized by progressive weakness, muscle wasting and death ensuing 3–5 years after diagnosis. Neurotrophic factors (NTFs) are known to be important in both nervous system development and maintenance. However, the attempt to translate the potential of NTFs into the therapeutic options remains limited despite substantial number of approaches, which have been tested clinically. Using quantitative RT-PCR (qRT-PCR) technique, the present study investigated mRNA expression of four different NTFs: brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), neurotrophin-4/5 (NT-4) and glial cell line-derived neurotrophic factor (GDNF) in limb muscles and extraocular muscles (EOMs) from SOD1^G93A transgenic mice at early and terminal stages of ALS. General morphological examination revealed that muscle fibres were well preserved in both limb muscles and EOMs in early stage ALS mice. However, in terminal ALS mice, most muscle fibres were either atrophied or hypertrophied in limb muscles but unaffected in EOMs. qRT-PCR analysis showed that in early stage ALS mice, NT-4 was significantly down-regulated in limb muscles whereas NT-3 and GDNF were markedly up-regulated in EOMs. In terminal ALS mice, only GDNF was significantly up-regulated in limb muscles. We concluded that the early down-regulation of NT-4 in limb muscles is closely associated with muscle dystrophy and dysfunction at late stage, whereas the early up-regulations of GDNF and NT-3 in EOMs are closely associated with the relatively well-preserved muscle morphology at late stage. Collectively, the data suggested that comparing NTFs expression between limb muscles and EOMs from different stages of ALS animal models is a useful method in revealing the patho-physiology and progression of ALS, and eventually rescuing motor neuron in ALS patients.

Dystrophic changes in extraocular muscles after gamma irradiation in mdx:utrophin(+/-) mice

Extraocular muscles (EOM) have a strikingly different disease profile than limb skeletal muscles. It has long been known that they are spared in Duchenne (DMD) and other forms of muscular dystrophy. Despite many studies, the cause for this sparing is not understood. We have proposed that differences in myogenic precursor cell properties in EOM maintain normal morphology over the lifetime of individuals with DMD due to either greater proliferative potential or greater resistance to injury. This hypothesis was tested by exposing wild type and mdx:utrophin(+/-) (het) mouse EOM and limb skeletal muscles to 18 Gy gamma irradiation, a dose known to inhibit satellite cell proliferation in limb muscles. As expected, over time het limb skeletal muscles displayed reduced central nucleation mirrored by a reduction in Pax7-positive cells, demonstrating a significant loss in regenerative potential. In contrast, in the first month post-irradiation in the het EOM, myofiber cross-sectional areas first decreased, then increased, but ultimately returned to normal compared to non-irradiated het EOM. Central nucleation significantly increased in the first post-irradiation month, resembling the dystrophic limb phenotype. This correlated with decreased EECD34 stem cells and a concomitant increase and subsequent return to normalcy of both Pax7 and Pitx2-positive cell density. By two months, normal het EOM morphology returned. It appears that irradiation disrupts the normal method of EOM remodeling, which react paradoxically to produce increased numbers of myogenic precursor cells. This suggests that the EOM contain myogenic precursor cell types resistant to 18 Gy gamma irradiation, allowing return to normal morphology 2 months post-irradiation. This supports our hypothesis that ongoing proliferation of specialized regenerative populations in the het EOM actively maintains normal EOM morphology in DMD. Ongoing studies are working to define the differences in the myogenic precursor cells in EOM as well as the cellular milieu in which they reside.

Cutting edge: retrobulbar inflammation, adipogenesis, and acute orbital congestion in a preclinical female mouse model of Graves' orbitopathy induced by thyrotropin receptor plasmid-in vivo electroporation

Graves' orbitopathy (GO) is a complication in Graves' disease (GD) but mechanistic insights into pathogenesis remain unresolved, hampered by lack of animal model. The TSH receptor (TSHR) and perhaps IGF-1 receptor (IGF-1R) are considered relevant antigens. We show that genetic immunization of human TSHR (hTSHR) A-subunit plasmid leads to extensive remodeling of orbital tissue, recapitulating GO. Female BALB/c mice immunized with hTSHR A-subunit or control plasmids by in vivo muscle electroporation were evaluated for orbital remodeling by histopathology and magnetic resonance imaging (MRI). Antibodies to TSHR and IGF-1R were present in animals challenged with hTSHR A-subunit plasmid, with predominantly TSH blocking antibodies and were profoundly hypothyroid. Orbital pathology was characterized by interstitial inflammation of extraocular muscles with CD3+ T cells, F4/80+ macrophages, and mast cells, accompanied by glycosaminoglycan deposition with resultant separation of individual muscle fibers. Some animals showed heterogeneity in orbital pathology with 1) large infiltrate surrounding the optic nerve or 2) extensive adipogenesis with expansion of retrobulbar adipose tissue. A striking finding that underpins the new model were the in vivo MRI scans of mouse orbital region that provided clear and quantifiable evidence of orbital muscle hypertrophy with protrusion (proptosis) of the eye. Additionally, eyelid manifestations of chemosis, including dilated and congested orbital blood vessels, were visually apparent. Immunization with control plasmids failed to show any orbital pathology. Overall, these findings support TSHR as the pathogenic antigen in GO. Development of a new preclinical model will facilitate molecular investigations on GO and evaluation of new therapeutic interventions.

Ghrelin in ocular pathophysiology: from the anterior to the posterior segment

Ghrelin is a 28 amino acid acylated peptide produced in several organs that binds the growth hormone secretagogues receptor type 1a (GHSR-1a). It acts over a wide range of systems, e.g. the endocrine, cardiovascular, musculoskeletal and immune systems and the eye. The aim of this work is to review the physiologic and pathologic implications of the ghrelin-GHSR-1a in the eye. A systematic revision of studies published between 2000 and 2013 in English, Spanish or Portuguese in MEDLINE, EMBASE and Scopus was performed. Search words used included: ghrelin, GHSR-1a, ocular production, iris muscular kinetics, ciliary body, glaucoma, retinopathy and uvea. The production of ghrelin by the ocular tissue has been detected both in the anterior and posterior segments, as well as the presence of GHSR-1a. This peptide promotes the relaxation of the iris sphincter and dilator muscles, being this effect independent from GHSR-1a and dependent on prostaglandins release in the first case and dependent on GHSR-1a in the second. Regarding ocular pathology, ghrelin levels in the aqueous humor appear to be decreased in individuals with glaucoma. Moreover, ghrelin has been shown to decrease the intraocular pressure in animal models of ocular hypertension through GHSR-1a. In the posterior segment, the ghrelin-GHSR-1a system interferes with the development of oxygen-induced retinopathy, being protective in the vaso-obliterative phase and deleterious in the vaso-proliferative stage of the disease. Thus, the ghrelin-GHSR-1a system presents as a possible local regulatory mechanism in the eye, with pathophysiological implications, constituting a target for future clinical and therapeutic research and interventions.

Postulating a role for connective tissue elements in inferior oblique muscle overaction (an American Ophthalmological Society thesis)

PURPOSE

To compare the localization and density of collagens I, IV, VI, and elastin, the major protein components of connective tissue, in the inferior oblique muscle of patients with overelevation in adduction and in controls and to characterize changes that develop following surgery. Biomechanical studies suggest that the connective tissue matrix plays a critical role in extraocular muscle function, determining tensile strength and force transmission during contraction.

METHODS

Prospective laboratory-based case-control study of inferior oblique muscle specimens from 31 subjects: 16 with primary inferior oblique overaction, 6 with craniofacial dysostosis, and 9 normal controls. Collagen I, IV, VI, and elastin were localized and quantified using immunohistochemical staining. Densities were compared using analysis of variance and post hoc comparisons.

RESULTS

In primary inferior oblique overaction, all connective tissue components in unoperated specimens were elevated compared to controls (P<.0001). Previously operated muscles showed normal levels of collagens IV and VI (P>.27) but increased collagen I. In unoperated craniofacial dysostosis specimens, only elastin was elevated (P=.03), whereas density of collagens IV and VI was lower in previously operated vs unoperated specimens (P=.015).

CONCLUSIONS

Elevated collagen and elastin levels in the cohort with primary inferior oblique overaction are consistent with the clinical finding of muscle stiffness. Contrarily, normal connective tissue densities in craniofacial dysostosis support the hypothesis that overelevation in this group reflects anomalous muscle vectors rather than tissue changes. Surgical intervention was associated with changes in the connective tissue matrix in both cohorts. These results have ramifications for treating patients with overelevation in adduction.

Distinct changes in synaptic protein composition at neuromuscular junctions of extraocular muscles versus limb muscles of ALS donors

The pathophysiology of amyotrophic lateral sclerosis (ALS) is very complex and still rather elusive but in recent years evidence of early involvement of the neuromuscular junctions (NMJs) has accumulated. We have recently reported that the human extraocular muscles (EOMs) are far less affected than limb muscles at the end-stage of ALS from the same donor. The present study aimed to compare the differences in synaptic protein composition at NMJ and in nerve fibers between EOM and limb muscles from ALS donors and controls. Neurofilament light subunit and synaptophysin decreased significantly at NMJs and in nerve fibers in limb muscles with ALS whereas they were maintained in ALS EOMs. S100B was significantly decreased at NMJs and in nerve fibers in both EOMs and limb muscles of ALS donors, but other markers confirmed the presence of terminal Schwann cells in these NMJs. p75 neurotrophin receptor was present in nerve fibers but absent at NMJs in ALS limb muscles. The EOMs were able to maintain the integrity of their NMJs to a very large extent until the end-stage of ALS, in contrast to the limb muscles. Changes in Ca²⁺ homeostasis, reflected by altered S100B distribution, might be involved in the breakdown of nerve-muscle contact at NMJs in ALS.

[Study on the changes of fibronectin in the resected medial rectus of patients with concomitant exotropia]

OBJECTIVE

To study the changes of fibronectin in the resected medial rectus muscles in patients with concomitant exotropia and to have a better understanding about the pathomorphological changes of extraocular muscles in the genesis and development of concomitant exotropia.

METHODS

Thirty-five concomitant exotropic patients were chosen as exotropic group and 20 normals were selected as control group. In the exotropic group, the age range 2 to 46 years old (mean, 20.9 ys), deviation range -45(Δ) to -90(Δ) (mean, -67.5(Δ)), 19 males, 16 females, and 8 cases have the family history. Exotropic patients were divided into intermittent exotropic group 16 cases and constant exotropic group 19 cases. In the control group, the age range 15 - 28 years old (mean, 18.8 years), 12 males and 8 females. All the resected tissues of the medial rectus muscles were obtained from patients with concomitant exotropia during resection surgery and cornea donators. Fibronectin was measured its mean average optical by Sp method of immunohistochemistry using image analyzer. The amounts of fibronectin in the resected muscle of patients were compared with normals, as well as correlated clinical data of patients such as different types of exotropia, age, degree of deviation, gender and positive family history.

RESULTS

The amounts of fibronectin in the resected medial rectus muscle of patients with constant exotropia (0.4729 ± 0.0872) were significantly lower than those of normals (0.5914 ± 0.0547) and intermittent exotropic group (0.5821 ± 0.0792) respectively (q = 7.0314, 6.1177, P < 0.01), while patients with intermittent exotropia did not show significantly lower amounts of fibronectin than those with normals (q = 0.5271, P > 0.05). Neither gender nor positive family was correlated with the amounts of fibronectin (t = 1.1317, 1.6327, P > 0.05) and the amounts of fibronectin had no correlation with age and degree of deviation.

CONCLUSION

The amounts of fibronectin were correlated with the course of development of concomitant exotropia. Some of these changes may play a role in the pathogenesis of exotropia from intermittence to constancy, which indicates a further clue to the nature of this disease.

Effects of sequential injections of hepatocyte growth factor and insulin-like growth factor-I on adult rabbit extraocular muscle

PURPOSE

To determine whether hepatocyte growth factor (HGF) and insulin-like growth factor-I (IGF-I) have synergistic effects in promoting extraocular muscle fiber growth and force generation.

METHODS

A superior rectus muscle of adult rabbits was treated with either a single injection of HGF or sequential injections of HGF followed 1 week later by IGF-I. One week after HGF alone and 1 week after the IGF-I injection, the superior rectus muscles from treated and control orbits were examined for alterations in force generation as well as changes in myofiber size.

RESULTS

Injection of HGF alone did not result in changes to muscle force, specific tension, or myofiber cross-sectional area; however, it did result in a significant increase in numbers of satellite cells. Sequential injection of HGF and IGF-I resulted in significantly increased force, specific tension, and myofiber cross-sectional areas as well as increased numbers of satellite cells.

CONCLUSIONS

Preinjection with HGF augments the treatment effect of IGF-I. This synergistic effect is likely a result of HGF-induced activation of satellite cells and should allow a reduction in IGF-I dosing required to produce a given increase in extraocular muscle force generation.

Genomic profiling reveals Pitx2 controls expression of mature extraocular muscle contraction-related genes

PURPOSE

To assess the influence of the Pitx2 transcription factor on the global gene expression profile of extraocular muscle (EOM) of mice.

METHODS

Mice with a conditional knockout of Pitx2, designated Pitx2(Δflox/Δflox) and their control littermates Pitx2(flox/flox), were used. RNA was isolated from EOM obtained at 3, 6, and 12 weeks of age and processed for microarray-based profiling. Pairwise comparisons were performed between mice of the same age and differentially expressed gene lists were generated. Select genes from the profile were validated using real-time quantitative polymerase chain reaction and protein immunoblot. Ultrastructural analysis was performed to evaluate EOM sarcomeric structure.

RESULTS

The number of differentially expressed genes was relatively small. Eleven upregulated and 23 downregulated transcripts were identified common to all three age groups in the Pitx2-deficient extraocular muscle compared with littermate controls. These fell into a range of categories including muscle-specific structural genes, transcription factors, and ion channels. The differentially expressed genes were primarily related to muscle contraction. We verified by protein and ultrastructural analysis that myomesin 2 was expressed in the Pitx2-deficient mice, and this was associated with development of M lines evident in their orbital region.

CONCLUSIONS

The global transcript expression analysis uncovered that Pitx2 primarily regulates a relatively select number of genes associated with muscle contraction. Pitx2 loss led to the development of M line structures, a feature more typical of other skeletal muscle.

Pitx2 regulates myosin heavy chain isoform expression and multi-innervation in extraocular muscle

Extraocular muscle is fundamentally distinct from other skeletal muscle and demonstrates specific anatomical divisions, unique innervation, diverse myosin isoform expression patterns, a distinct genomic profile and differential involvement in neuromuscular disorders. The paired-like homeodomain transcription factor 2 (Pitx2) is known to regulate the formation of extraocular muscle development and in this report we show that its expression in adulthood also defines certain extraocular muscle traits. We found that expression of slow-MyHC and slow-tonic MyHC, along with contractile regulatory proteins troponin I and troponin T, is reduced during the first 3 weeks after birth in mice with conditional knockout of Pitx2, designated Pitx2(Δflox/Δflox). En grappe endplates, which are normally only found on slow-MyHC expressing fibres, were not identified in the Pitx2(Δflox/Δflox) extraocular muscle, suggesting that altered innervation was responsible for the loss in slow-MyHC expression. Extraocular muscle (EOM)-specific MyHC expressing fibres were dramatically reduced at P14 and rarely detected at 3 months in the Pitx2(Δflox/Δflox) mice. 2A-MyHC fibres, which are excluded from mid-belly region in wild-type mice, dominated the orbital layer with no apparent longitudinal variation in the Pitx2(Δflox/Δflox) mice. Pure 2X-MyHC fibres, only present at distal ends in the wild-type mice, populated the outer global layer in the mid-belly region of the Pitx2(Δflox/Δflox) mice. Pitx2 influences slow-MyHC, slow-tonic MyHC and EOM-MyHC expression in extraocular muscle and its absence leads to increased expression of 2X-MyHC and 2A-MyHC. Precise definition of the regulation of MyHC isoforms in extraocular muscle may allow their rational manipulation, in order to alter muscle contractility for therapeutic purposes.

Expression of insulin-like growth factor 1 isoforms in the rabbit oculomotor system

OBJECTIVE

The insulin-like growth factor-1 (IGF-1) gene encodes two isoforms, IGF-1Ea and IGF-1Eb. Both isoforms can regulate skeletal muscle growth and strength. It has been suggested that IGF-Eb may be more potent in promoting skeletal muscle hypertrophy. Precise contractile force regulation is particularly important in the oculomotor system. However, expression of these isoforms in mammalian extraocular muscles (EOMs) is unknown. Here, we examined their expression in rabbit EOMs and the innervating nerve, two potential sources for myogenic growth factors, and compared isoform expression between EOMs and limb skeletal muscles.

DESIGN

Expression of IGF-1 isoforms was quantified by real-time RT-PCR in adult rabbit EOMs, trochlear and ophthalmic nerves, and compared with expression in rabbit limb skeletal muscles. The presence of mature IGF-1 peptide in the muscles was further examined by Western blot.

RESULTS

Both IGF-1Ea and IGF-1Eb were expressed in the EOM and the trochlear nerve. Both isoforms were expressed at significantly higher levels (9-fold) in EOM than in limb skeletal muscle. Transcripts of IGF-1 isoforms, of IGF-1 receptor and of IGF binding proteins showed a gradient distribution along the EOM from proximal to distal. The mature IGF-1 protein showed the same gradient distribution in the EOM.

CONCLUSIONS

Expression of relatively abundant amounts of both IGF-1 splicing isoforms in EOMs, and at a significantly higher level than in limb skeletal muscle, underscores the potential relevance of these myogenic growth factors in EOM plasticity and force regulation.

Development of extraocular muscles requires early signals from periocular neural crest and the developing eye

OBJECTIVES

To identify and explain morphologic changes of the extraocular muscles (EOMs) in anophthalmic patients.

METHODS

Retrospective medical record review of patients with congenital anophthalmia, using magnetic resonance imaging and intraoperative findings to characterize EOM morphology. We then used molecular biology techniques in zebrafish and chick embryos to determine the relationships among the developing eye, periocular neural crest, and EOMs.

RESULTS

In 3 human patients with bilateral congenital anophthalmia and preoperative orbital imaging, we observed a spectrum of EOM morphologies ranging from indiscernible muscle tissue to well-formed, organized EOMs. Timing of eye loss in zebrafish and chick embryos correlated with the morphology of EOM organization in the orbit (eye socket). In congenitally eyeless Rx3 zebrafish mutants, or following genetic ablation of the cranial neural crest cells, EOMs failed to organize, which was independent of other craniofacial muscle development.

CONCLUSIONS

Orbital development is dependent on interactions between the eye, neural crest, and developing EOMs. Timing of the ocular insult in relation to neural crest migration and EOM development is a key determinant of aberrant EOM organization. Additional research will be required to study patients with unilateral and syndromic anophthalmia and assess for possible differences in clinical outcomes of patients with varied EOM morphology.

CLINICAL RELEVANCE

The presence and organization of EOMs in anophthalmic eye sockets may serve as a markers for the timing of genetic or teratogenic insults, improving genetic counseling, and assisting with surgical reconstruction and family counseling efforts.

Do palisade endings in extraocular muscles arise from neurons in the motor nuclei?

PURPOSE

The purpose of this study was to localize the cell bodies of palisade endings that are associated with the myotendinous junctions of the extraocular muscles.

METHODS

Rhesus monkeys received tract-tracer injections (tetramethylrhodamine dextran [TMR-DA] or choleratoxin subunit B [CTB]) into the oculomotor and trochlear nuclei, which contain the motoneurons of extraocular muscles. All extraocular muscles were processed for the combined immunocytochemical detection of the tracer and SNAP-25 or synaptophysin for the visualization of the complete muscle innervation.

RESULTS

In all muscles--except the lateral rectus--en plaque and en grappe motor endings, but also palisade endings, were anterogradely labeled. In addition a few tracer-labeled tendon organs were found. One group of tracer-negative nerve fibers was identified as thin tyrosine hydroxylase-positive sympathetic fibers, and a second less numerous group of tracer-negative fibers may originate from the trigeminal ganglia. No cellular or terminal tracer labeling was present within the mesencephalic trigeminal nucleus or the trigeminal ganglia.

CONCLUSIONS

These results confirm those of earlier studies and furthermore suggest that the somata of palisade endings are located close to the extraocular motor nuclei--in this case, probably within the C and S groups around the periphery of the oculomotor nucleus. The multiple en grappe endings have also been shown to arise from these cells groups, but it is not possible to distinguish different populations in these experiments.

Histochemical analysis of muscle fiber types of rat superior rectus extraocular muscle

Extraocular muscles (EOMs) represent a distinctive class among mammalian skeletal muscles in exhibiting unique anatomical and physiological properties. To gain insight into the basis for the unique structural/functional diversity of EOM fiber types and to explain their high fatigue resistance, rat superior rectus muscle (SRM) was studied using histochemical techniques. Muscle fibers were typed with regard to their oxidative and glycolytic profiles generated from succinic dehydrogenase (SDH) and phosphorylase activities in combination with their morphologic characteristics. Superior rectus muscle is organized into two layers, a central global layer (GL) of mainly large diameter fibers and an outer C-shaped orbital layer (OL) of principally small diameter fibers. Five muscle fiber types were recognized within the SRM: I, II, III, IV and V. In the global layer, four muscle fiber types were identified: type I (18.25±0.96μm; 32%) showed intermediate SDH (coarse type) and high phosphorylase activity. Type II fibers (14.45±0.82μm; 22%) exhibited high SDH (fine type) and intermediate phosphorylase activity. Low SDH (granular type) and high phosphorylase activity were demonstrated by type III fibers (22.65±1.73μm; 36%). Type IV fibers (26.24±1.32μm; 10%) were recognized by their low oxidative and glycolytic reactions. In the orbital region, only three muscle fiber types were recognized; fiber types I and II were found to compose approximately two-thirds of the layer. The third orbital fiber type (type V, 10.05±0.99μm) exhibited low SDH and low phosphorylase profiles. In this paper, the functional significance of the histochemical characteristics of the EOM fiber types is discussed.

Effects of recession versus tenotomy surgery without recession in adult rabbit extraocular muscle

PURPOSE

Surgical recession of an extraocular muscle (EOM) posterior to its original insertion is a common form of strabismus surgery, weakening the rotational force exerted by the muscle on the globe and improving eye alignment. The purpose of this study was to assess myosin heavy chain (MyHC) isoform expression and satellite cell activity as defined by Pax7 expression in recessed EOMs of adult rabbits compared with that in muscles tenotomized but not recessed and with that in normal control muscles.

METHODS

The scleral insertion of the superior rectus muscle was detached and sutured either 7 mm posterior to its original insertion site (recession surgery) or at the same site (tenotomy). One day before euthanization, the rabbits received bromodeoxyuridine (BrdU) injections. After 7 and 14 days, selected EOMs from both orbits were examined for changes in fast, slow, neonatal, and developmental MyHC isoform expression, Pax7 expression, and BrdU incorporation.

RESULTS

Recession and tenotomy surgery resulted in similar changes in the surgical EOMs. These included a decreased proportion of fast MyHC myofibers, an increased proportion of slow MyHC myofibers, and increased BrdU-positive satellite cells. Similar changes were seen in the non-operated contralateral superior rectus muscles. The ipsilateral inferior rectus showed reciprocal changes to the surgical superior rectus muscles.

CONCLUSIONS

The EOMs are extremely adaptive to changes induced by recession and tenotomy surgery, responding with modulations in fiber remodeling and myosin expression. These adaptive responses could be manipulated to improve surgical success rates.

Nonmuscle myosin IIB, a sarcomeric component in the extraocular muscles

Extraocular muscles (EOMs) are categorized as skeletal muscles; however, emerging evidence indicates that their gene expression profile, metabolic characteristics and functional properties are significantly different from the prototypical members of this muscle class. Gene expression profiling of developing and adult EOM suggest that many myofilament and cytoskeletal proteins have unique expression patterns in EOMs, including the maintained expression of embryonic and fetal isoforms of myosin heavy chains (MyHC), the presence of a unique EOM specific MyHC and mixtures of both cardiac and skeletal muscle isoforms of thick and thin filament accessory proteins. We demonstrate that nonmuscle myosin IIB (nmMyH IIB) is a sarcomeric component in approximately 20% of the global layer fibers in adult rat EOMs. Comparisons of the myofibrillar distribution of nmMyHC IIB with sarcomeric MyHCs indicate that nmMyH IIB co-exists with slow MyHC isoforms. In longitudinal sections of adult rat EOM, nmMyHC IIB appears to be restricted to the A-bands. Although nmMyHC IIB has been previously identified as a component of skeletal and cardiac sarcomeres at the level of the Z-line, the novel distribution of this protein within the A band in EOMs is further evidence of both the EOMs complexity and unconventional phenotype.

Histochemical and immunohistochemical profile of human and rat ocular medial rectus muscles

PURPOSE

To compare the organization of human and rat ocular medial recti muscles (MR).

METHODS

The cryosections of human and rat MR were processed for myofibrillar ATPase (mATPase), succinate dehydrogenase and glycerol-3-phosphate dehydrogenase. To reveal myosin heavy chain (MyHC) isoforms, specific monoclonal antibodies against MyHC-1/beta- slow, alpha-cardiac (-alpha), -2a, -2x, -2b, -extraocular (eom), -embryonic (-emb) and -neonatal (-neo) were applied. The MyHC gene expression was studied by in situ hybridization in human muscle.

RESULTS

The muscle fibers were arranged in two distinct layers in both species. In the orbital layer most fibers were highly oxidative and expressed fast MyHC isoforms, whereas slow and oxidative fibers expressed MyHC-1 and -alpha, some of them also MyHC-2a, -2x, -eom, very rarely -emb, and -neo. In the global layer, slow fibers with very low oxidative and glycolytic activity and three types of fast fibers, glycolytic, oxidative and oxidative-glycolytic, could be distinguished. The slow medium-sized fibers with mATPase activity stable at pH 4.4 expressed mostly MyHC-1 and -alpha in rat, while in humans they co-expressed MyHC-1 with -2b, -2x, -eom, and -neo. In both species, the fast fibers showed variable mATPase activity after preincubation at pH 9.4, and co-expressed various combinations of MyHC-2b, -2x, -2a and -eom but not -emb and -neo. MyHC-2b expressing fibers were larger and glycolytic, while MyHC-2a expressing fibers were smaller and highly oxidative in both species. To our knowledge, the present study is the first that demonstrated the expression of MyHC-2b in any of human skeletal muscles. Though the expression of MyHC genes did not correlate with the immunohistochemical profile of fibers in human MR, the expression of MyHC-2b gene was undoubtedly confirmed.

CONCLUSIONS

Rat MR represent a good model that can be applied to study human MR in experiment or disease, however certain differences are to be expected due to specific oculomotor demands in humans.

An altered phenotype in a conditional knockout of Pitx2 in extraocular muscle

PURPOSE

To determine the temporal and spatial expression of Pitx2, a bicoid-like homeobox transcription factor, during postnatal development of mouse extraocular muscle and to evaluate its role in the growth and phenotypic maintenance of postnatal extraocular muscle.

METHODS

Mouse extraocular muscles of different ages were examined for the expression of Pitx2 by RT-PCR, q-PCR, and immunostaining. A conditional mutant mouse strain, in which Pitx2 function is inactivated at postnatal day (P)0, was generated with a Cre-loxP strategy. Histology, immunostaining, real-time PCR, in vitro muscle contractility, and in vivo ocular motility were used to study the effect of Pitx2 depletion on extraocular muscle.

RESULTS

All three Pitx2 isoforms were expressed by extraocular muscle and at higher levels than in other striated muscles. Immunostaining demonstrated the presence of Pitx2 mainly in extraocular muscle myonuclei. However, no obvious expression patterns were observed in terms of anatomic region (orbital versus global layer), innervation zone, or muscle fiber types. The mutant extraocular muscle had no obvious pathology but had altered muscle fiber sizes. Expression levels of myosin isoforms Myh1, Myh6, Myh7, and Myh13 were reduced, whereas Myh2, Myh3, Myh4, and Myh8 were not affected by postnatal loss of Pitx2. In vitro, Pitx2 loss made the extraocular muscles stronger, faster, and more fatigable. Eye movement recordings found saccades to have a lower peak velocity.

CONCLUSIONS

Pitx2 is important in maintaining the mature extraocular muscle phenotype and regulating the expression of critical contractile proteins. Modulation of Pitx2 expression can influence extraocular muscle function with long-term therapeutic implications.

Palisade endings are present in canine extraocular muscles and have a cholinergic phenotype

Classical proprioceptors, like Golgi tendon organs and muscle spindles are absent in the extraocular muscles (EOMs) of most mammals. Instead, a nerve end organ was detected in the EOMs of each species including sheep, cat, rabbit, rat, monkey, and human examined so far: the palisade ending. Until now no clear evidence appeared that palisade endings are also present in canine EOMs. Here, we analyzed dog EOMs by confocal laser scanning microscopy, 3D reconstruction, and transmission electron microscopy. In EOM wholemount preparations stained with antibodies against neurofilament and synaptophysin we could demonstrate typical palisade endings. Nerve fibers coming from the muscle extend into the tendon. There, the nerve fibers turn 180 degrees and return to branch into preterminal axons which establish nerve terminals around a single muscle fiber tip. Fine structural analysis revealed that each palisade ending in dog EOMs establish nerve terminals on the tendon. In some palisade endings we found nerve terminals contacting the muscle fiber as well. Such neuromuscular contacts have a basal lamina in the synaptic cleft. By using an antibody against choline acetyltransferase (ChAT) we proved that canine palisade endings are ChAT-immunoreactive. This study shows that palisade endings are present in canine EOMs. In line with prior findings in cat and monkey, palisade endings in dog have a cholinergic phenotype.

Lower respiratory capacity in extraocular muscle mitochondria: evidence for intrinsic differences in mitochondrial composition and function

PURPOSE

The constant activity of the extraocular muscles is supported by abundant mitochondria. These organelles may enhance energy production by increasing the content of respiratory complexes. The authors tested the hypothesis that extraocular muscle mitochondria respire faster than do mitochondria from limb muscles because of the higher content of respiratory complexes.

METHODS

Inner mitochondrial membrane density was determined by stereological analysis of triceps surae (a limb muscle) and extraocular muscles of adult male Sprague-Dawley rats. The authors measured respiration rates of isolated mitochondria using a Clark-type electrode. The activity of respiratory complexes I, II, and IV was determined by spectrophotometry. The content of respiratory complexes was estimated by Western blot.

RESULTS

States 3, 4, and 5 respiration rates in extraocular muscle mitochondria were 40% to 60% lower than in limb muscle mitochondria. Extraocular muscle inner mitochondrial membrane density was similar to that of other skeletal muscles. Activity of complexes I and IV was lower in extraocular muscle mitochondria (approximately 50% the activity in triceps), but their content was approximately 15% to 30% higher. There was no difference in complex II content or activity or complex III content. Finally, complex V was less abundant in extraocular muscle mitochondria.

CONCLUSIONS

The results demonstrate that extraocular muscle mitochondria respire at slower rates than mitochondria from limb muscles, despite similar mitochondrial ultrastructure. Instead, differences were found in the activity (I, IV) and content (I, IV, V) of electron transport chain complexes. The discrepancy between activity and content of some complexes is suggestive of alternative subunit isoform expression in the extraocular muscles compared with limb muscles.

[Observation on the morphology differences of ocular rectus pulley between preschool children and adults]

OBJECTIVE

To study the morphology differences of ocular rectus muscles pulley between preschool children and adults.

METHODS

It was an experimental research. Ten fresh cadaver orbits (five children and five adults) were fixed in situ and serially sectioned at 4 microm thickness. Every section was stained with Masson's trichrome and picrosirius red methods. After being stained, extraocular muscle pulley in typical sections was examined through gross anatomy and light microscopy. Two-factor analysis of variance for quantitative data was used in statistical treatment.

RESULTS

Near the equator of the globe, the children and adults' extraocular muscle pulleys were all at the points where the rectus penetrate posterior Tenon's fascia, encircling the rectus orbit side by fiber ring structure. There were pulley bands among rectus pulleys. The rectus pulley length and the origin distance from the corneoscleral limbus of the children were significantly less than those from the adult group (P < 0.05). The main composition of the children rectus pulley was similar to that of the adults, but the smooth muscle and collagen type III fibers contents of the medial rectus pulley in children were significantly greater than those in the adults group (P < 0.05). Collagen type I fibers in the children were significantly less than that of the adults group (P < 0.05).

CONCLUSION

There are morphological differences in rectus pulley between preschool children and adults, which provides an anatomic basis for the better understanding of the pathogenesis of strabismus.

Human CHN1 mutations hyperactivate alpha2-chimaerin and cause Duane's retraction syndrome

Duane's retraction syndrome (DRS) is a complex congenital eye movement disorder caused by aberrant innervation of the extraocular muscles by axons of brainstem motor neurons. Studying families with a variant form of the disorder (DURS2-DRS), we have identified causative heterozygous missense mutations in CHN1, a gene on chromosome 2q31 that encodes alpha2-chimaerin, a Rac guanosine triphosphatase-activating protein (RacGAP) signaling protein previously implicated in the pathfinding of corticospinal axons in mice. We found that these are gain-of-function mutations that increase alpha2-chimaerin RacGAP activity in vitro. Several of the mutations appeared to enhance alpha2-chimaerin translocation to the cell membrane or enhance its ability to self-associate. Expression of mutant alpha2-chimaerin constructs in chick embryos resulted in failure of oculomotor axons to innervate their target extraocular muscles. We conclude that alpha2-chimaerin has a critical developmental function in ocular motor axon pathfinding.

Measurement of contractile force of skeletal and extraocular muscles: effects of blood supply, muscle size and in situ or in vitro preparation

Contractile forces can be measured in situ and in vitro. To maintain metabolic viability with sufficient diffusion of oxygen, established guidelines for in vitro skeletal muscle preparations recommend use of relatively thin muscles (< or =1.25 mm thick). Nevertheless, forces of thin extraocular muscles vary substantially between studies. Here, we examined parameters that affect force measurements of in situ and in vitro preparations, including blood supply, nerve stimulation, direct muscle stimulation, muscle size, oxygenated or non-oxygenated buffer solutions and the time after interruption of vascular circulation. We found that the absolute forces of extraocular muscle are substantially lower when examined in vitro. In vitro preparation of 0.58 mm thick extraocular muscle from 3-week-old birds underestimated contractile function, but not of thinner (0.33 mm) muscle from 2-day-old birds. Our study shows that the effective criteria for functional viability, tested in vitro, differ between extraocular and other skeletal muscle. We conclude that contractile force of extraocular muscles will be underestimated by between 10 and 80%, when measurements are made after cessation of blood supply (at 5-40 min). The mechanisms responsible for the declining values for force measurements are discussed, and we make specific recommendations for obtaining valid measurements of contractile force.

[The changes of extracellular matrix molecules in the resected medial rectus muscle of patients with concomitant exotropia]

OBJECTIVE

In order to have a better understanding about the role of the extracellular matrix molecules in the intermittent and constant exotropia, we measured the amounts of fibronectin and proteoglycan in the resected medial rectus muscles of patients with concomitant exotropia.

METHODS

Thirty-one exotropic patients (including intermittent exotropia 17 cases and constant exotropia 14 cases) and 21 normal were chosen. In exotropic group, there were 7 cases with positive family history. Tissues of the medial rectus muscles were obtained from patients with concomitant exotropia during resection surgery and the specimen of medial rectus muscle from 21 normal persons was as control. All the tissues were weighted and then pulverized to obtain supernatant for enzyme linked immunosorbent assay (ELISA). The total amounts of fibronectin and proteoglycan were measured. The correlation of fibronectin and proteoglycan with age, gender, positive family history in different groups of exotropia was analyzed.

RESULTS

The amount of fibronectin in the resected medial rectus muscle of patients with concomitant exotropia was significantly lower than that of normal individual (P < 0.01), and the difference of proteoglycan amount between patients and the normal control was not significant (P > 0.05). Patients with intermittent exotropia showed significantly higher concentration of fibronectin than those with constant exotropia (P < 0.01), and the amount of proteoglycan had no significant difference between two groups (P > 0.05). The amounts of proteoglycan decreased significantly with the advance of age (r = -0.8712, P < 0.01), while the amounts of fibronectin had no correction with age (r = -0.1718, P > 0.05). Neither gender nor positive family was correlated with the amounts of proteoglycan and fibronectin (P > 0.05).

CONCLUSIONS

The change of fibronectin may be related to the concomitant exotropia and the development from intermittent to constant exotropia. An attention should be paid to the roles of fibronectin in the development of strabismus.

Myostatin shows a specific expression pattern in pig skeletal and extraocular muscles during pre- and post-natal growth

Myogenesis is driven by an extraordinary array of cellular signals that follow a common expression pattern among different animal phyla. Myostatin (mstn) is a secreted growth factor that plays a pivotal role in skeletal muscle mass regulation. The aim of the present study was to investigate mstn expression in a large mammal (the pig) in order to ascertain whether distinct expression changes of this factor might be linked to the fiber-type composition of the muscle examined and/or to specific developmental stages. To assess the expression pattern of mstn in relation to myogenic proliferative (Pax7 and MyoD) and differentiative (myogenin) markers, we evaluated muscles with different myosin heavy-chain compositions sampled during pre- and post-natal development and on myogenic cells isolated from the same muscles. Skeletal muscles showed higher levels of mRNA for mstn and all other genes examined during fetal development than after birth. The wide distribution of mstn was also confirmed by immunohistochemistry experiments supporting evidence for cytoplasmic staining in early fetal periods as well as the localization in type 1 fibers at the end of the gestation period. Extraocular muscles, in contrast, did not exhibit decreasing mRNA levels for mstn or other genes even in adult samples and expressed higher levels of both mstn mRNA and protein compared with skeletal muscles. Experiments carried out on myogenic cells showed that mstn mRNA levels decreased when myoblasts entered the differentiation program and that cells isolated at early post-natal stages maintained a high level of Pax7 expression. Our results showed that mstn had a specific expression pattern whose variations depended on the muscle type examined, thus supporting the hypothesis that at birth, porcine myogenic cells continue to be influenced by hyperplastic/proliferative mechanisms.

LYVE-1-positive macrophages are present in normal murine eyes

PURPOSE

A functioning lymphatic system is necessary not only to permit the organism to mount a rapid and effective immune response but, even more so, to maintain tissue fluid homeostasis. However, no clear evidence of lymphatic vessels draining intraocular and orbital tissues--retina, choroid, sclera, and extraocular muscles--exists.

METHODS

Ocular tissue flatmounts from normal or enhanced green fluorescence protein (EGFP) chimeric mice were immunostained for lymphatic endothelium hyaluronan receptor (LYVE-1, a routinely used lymphatic endothelial marker), podoplanin, Flt4/VEGFR3, Sca-1, CD11b, or F4/80 and were observed by confocal microscopy. Single-cell suspensions from ocular tissues were also prepared and were analyzed by flow cytometry.

RESULTS

Lymphatic vessels were detected in the posterior regions of the extraocular muscles and the connective tissues of the extraocular muscle cones in the normal mouse. No typical lymphatic vessels were found within the eye. A large population of LYVE-1(+) nonendothelial cells, distributed as single cells, was detected in all ocular tissues except the central cornea. These cells also express another lymphatic endothelial cell marker, Flt4/VEGFR3, but not podoplanin, and they have hyaluronan-binding ability. Bone marrow chimerism studies indicated that the LYVE-1(+) cell populations are bone marrow derived and have a slow turnover in ocular tissues (3-6 months). Phenotype analysis revealed that nonendothelial LYVE-1(+)cells in the sclera, choroid, and iris included CD11b(+)F4/80(+) macrophages, CD11b(+)F4/80(-) macrophages, and CD11b(-)F4/80(-) bone marrow-derived cells. All LYVE-1(+) cells in the retina were CD11b(+)F4/80(+) macrophages. Cells in the limbus and the iris root also express Sca-1, suggesting that they are hematopoietic lymphatic vessel progenitor cells.

CONCLUSIONS

These observations suggest that a lymphatic system exists for the transport of immune cells and fluids from the posterior segment of the eye, that ocular tissues are rich in bone marrow-derived LYVE-1(+) macrophages under normal physiological conditions, and that a subpopulation of these cells may represent resident precursor cells necessary for the de novo formation of ocular/orbital lymphatic vessels in pathologic conditions.

Identification and characterization of layer-specific differences in extraocular muscle m-bands

PURPOSE

To examine and characterize the expression of M-bands (or M-lines) in the orbital layer (OL) and global layer (GL) of adult rat extraocular muscles (EOMs).

METHODS

Semiquantitative polymerase chain reaction (PCR), quantitative (q)PCR, immunohistochemistry, and confocal microscopy were used to analyze expression of the major gene and protein constituents of M-bands in freshly dissected and cryosectioned rectus extraocular muscles (EOMs) and tibialis anterior (TA) muscles. Electron microscopy (EM) was performed on perfusion-fixed EOMs and TA muscles in a layer-specific manner, to determine, characterize, and quantify laminar-specific differences in M-band expression.

RESULTS

These studies demonstrate EOM layer-specific differences in the expression of M-bands and their major constituents, myomesin1 (Myom1) and myomesin2 (Myom2 or M-protein) at the structural, mRNA, and protein levels by using EM, semiquantitative PCR, qPCR, immunohistochemistry, and confocal microscopy. Differences in thick filament lattice order were quantified by using EM-based inter-thick-filament distance and variance measurements and were found to be TA > GL > OL.

CONCLUSIONS

The expression pattern of M-bands and their constituents in EOMs provides mechanistic insight for their allotypic and layer-specific viscoelastic properties. Modeling the differential expression of M-bands between EOMs and TA predicts increased elasticity but reduced force and eccentric contraction (ECC)-mediated damage in EOMs and suggests a potential mechanism for the clinical sparing of EOMs noted in Duchenne's muscular dystrophy (DMD).

Histochemical and immunohistochemical study on muscle fibers in human extraocular muscle spindles

Human extraocular muscles are unique in several ways including their endowment with proprioceptive organs. Aim of this study was to establish a classification of intrafusal muscle fibers of human extraocular muscles based on their histochemical and immunohistochemical properties and to determine their relationship to extrafusal extraocular muscle fiber types in this respect. Using light microscopy, intrafusal muscle fibers were followed on consecutive cross-sections and classified according to the localization of their myonuclei and to their enzyme- and myosin-immunohistochemical characteristics. Sixteen muscle spindles in human extraocular muscles counted as 'true' spindles revealed 27% nuclear chain fibers [40.1 microm+/-10.4; perimeter+/-SD] and 73% anomalous fibers [44.1 microm+/-12]. Seven 'false' muscle spindles showed only anomalous fibers [43.8 microm+/-11.1] and entirely lacked nuclear chain fibers. Six fiber types were distinguished according to their histochemical and myosin heavy chain immunohistochemical properties. Fiber type 1 [46.3 microm+/-13.3] was made up of fast-twitch myosin heavy chain isoform. Fiber type 2 [39.5 microm+/-10] additionally expressed a developmental myosin heavy chain isoform. Fiber type 3 [42.8 microm+/-10.4] consisted of pure slow-twitch positive muscle fibers. Slow-twitch MHC and fast-twitch myosin heavy chain isoform were found in fiber type 4 [43.3 microm+/-9]. Fiber types 5 and 6 showed different myosin heavy chain patterns than fiber types 1-4. The vast majority of nuclear chain fibers displayed fiber type 2 features, but 12% of nuclear chain fibers were found to be of fiber type 1. Among anomalous fibers in true spindles the frequency of fiber type 1 was much higher than in false spindles. On the other hand, fiber type 4 was found more often in false than in true spindles. With regard to their histochemical and immunohistochemical properties intrafusal muscle fibers in human extraocular muscles differ both from intrafusal muscle fibers in other skeletal muscles and from extrafusal muscle fibers in extraocular eye muscles. These conspicuous differences to skeletal muscle spindles relate to their morphology and myosin heavy chain characteristics. In particular, the occurrence of anomalous fibers might reflect dynamic neuronal processes and might be necessary for modulating and adapting processes in advancing age, as well as maintaining proprioceptive input during the whole life.

[Variation of nestin expression in SD rat eyes in different stages of postnatal development]

OBJECTIVE

To observe the position and quantity of nestin expression in SD rat eyes in different stages of postnatal development.

METHODS

Immunocytochemical method was used to identify nestin expression in the eyes of SD rats of 1 to 30 days old.

RESULTS

Nestin expression was detected in the retina and extraocular muscles of SD rats. The expression varied with the time of postnatal development, distributing in the entire retina layers in earlier stages and confined in the nerve fiber layer in later stages. The quantities of nestin expression in the extraocular muscles decreased gradually with growth.

CONCLUSION

Nestin expression in the retinas and extraocular muscles of SD rats decreases during the postnatal development.

Anatomical and pharmacological relationship between acetylcholine and nitric oxide in the prepositus hypoglossi nucleus of the cat: Functional implications for eye-movement control

The prepositus hypoglossi (PH) nucleus has been proposed as a pivotal structure for horizontal eye-position generation in the oculomotor system. Recent studies have revealed that acetylcholine (ACh) in the PH nucleus could mediate the persistent activity necessary for this process, although the origin of this ACh remains unknown. It is also known that nitric oxide (NO) in the PH nucleus plays an important role in the control of velocity balance, being involved in a negative feedback control of tonic signals arriving at the PH nucleus. As it could be expected that neurons taking part in eye-position generation must control their tonic background inputs, the existence of a relationship between nitrergic and cholinergic neurons is hypothesized. In the present study we analyzed the distribution, size, and morphology of choline acetyltransferase-positive neurons, and their relationship with neuronal nitric oxide synthase in the PH nucleus of the cat. As presumed, some 96% of cholinergic neurons were also nitrergic in the PH nucleus, suggesting that NO is regulating the level of ACh released by cholinergic PH neurons. Furthermore, we studied the alterations induced by muscarinic-receptor agonists and antagonists on spontaneous and vestibularly induced eye movements in the alert cat and compared them with those induced in previous studies by modification of NO levels in the same animal preparation. The results suggest that ACh is necessary for the generation of saccadic and vestibular eye-position signals, whereas the NO is stabilizing the eye-position generator by controlling background activity reaching cholinergic neurons in the PH nucleus.

Structural Abnormalities in the Levator Palpebrae Superioris Muscle in Patients With Congenital Blepharoptosis

BACKGROUND AND OBJECTIVE

To evaluate structural and ultrastructural abnormalities of the levator palpebrae superioris (LPS) complex in patients with congenital blepharoptosis.

PATIENTS AND METHODS

Samples of the LPS complex were obtained from patients operated on for congenital blepharoptosis between 2000 and 2001 and studied under light microscopy (15 cases) and electron microscopy (9 cases).

RESULTS

Findings of light microscopy evaluation of the LPS complex correlated closely with the clinical grading of congenital blepharoptosis-hypoplasia, decreased number and varying diameter of muscle fibers, and fibrous tissue hyperplasia in the endomysium and perimysium. The Müller's muscle preserved a normal appearance. Mild blepharoptosis revealed proliferation of collagen fibers on electron microscopy. Moderate blepharoptosis showed abnormal distribution of myofibrils and distortion of the tubular system and mitochondria in addition to the changes observed in mild blepharoptosis. Severe blepharoptosis showed mitochondria loss, cytoplasm thinning, and homogenous fiber areas in addition to the changes observed in mild and moderate blepharoptosis.

CONCLUSIONS

The clinical degree of severity of congenital blepharoptosis correlates positively with the degree of histopathologic changes in the levator palpebrae superioris muscle.