Evolution and development of extraocular motor neurons, nerves and muscles in vertebrates

Isolated oculomotor nerve palsy due to mesencephalic infarction diagnosed by ZOOM DWI

BACKGROUND

Oculomotor nerve palsy is a common neurological presentation in daily practice.

CASE PRESENTATION

A 55-year-old man presented with a 3-h history of diplopia and drooping of his bilateral especially left eyelids. Examination revealed an isolated oculomotor nerve palsy consisting of left medial rectus, inferior oblique, superior rectus, inferior rectus with intact pupillary reflexes and bilateral especially left superior palpebral levator. Conventional diffusion weighted imaging (DWI) of the brain showed a suspicious restriction in the left midbrain periaqueductal region. If the clinical symptomatology indicates a lesion in the midbrain, of which a high signal intensity was encountered from neurologically healthy older adults, the limited spatial resolution of conventional axial DWI is an enormous disadvantage. Zonally magnified oblique multislice (ZOOM) DWI correlated with apparent diffusion coefficient map providing higher accuracy for accurate diagnosis can identify signal alterations of mesencephalic interpeduncle area.

CONCLUSIONS

This is a rare presentation of isolated oculomotor nerve palsy due to pure mesencephalic infarction especially verified by ZOOM DWI.

Numerical Aberrations of the Extraocular Muscles and the Levator Palpebrae Superioris: An Anatomical and Clinical Insight

PURPOSE

To comprehensively review the literature about numerical aberrations of the orbital muscles of ocular motility (here referred to as extraocular muscles [EOMs]) and the levator palpebrae superioris (LPS).

METHODS

The authors summarize the embryologic bases and the possible etiopathogenetic causes of numerical aberrations of the EOMs and the LPS and organize these lesions into several broad categories. The clinical and radiologic diagnostic challenges are discussed.

RESULTS

Numerical aberrations of the EOMs include: 1) the complete absence of EOMs, 2) duplication of an entire EOM, 3) the presence of muscle bands that connect 2 EOMs, and 4) minor morphological variations such as bifid muscles (partial splitting of the muscle). Some cases may defy categorization into any of the above or may resemble atavistic remnants of the retractor bulbi muscle. Broadly speaking, numerical aberrations of the LPS generally fall into the same categories although the LPS has several peculiar numerical anomalies of its own.

CONCLUSIONS

Although numerical EOM and LPS variations are relatively rare and of little clinical significance, raising awareness about their presence is a fundamental clinical keystone not just for the strabismus surgeon but for the orbital surgeon as well. During orbital surgery, this may spare the surgeon from pursuing an orbital witch hunt for these benign innocuous accessory orbital structures that were accidentally discovered by the radiologist and misinterpreted as sinister etiologies. For the strabismus surgeon, the failure to identify them may result in an unfavorable surgical outcome if these structures are missed because of a lack of awareness despite being responsible for generating complex strabismus patterns or having a restrictive potential of their own.

The role of FGF15/FGF19 in the development of the central nervous system, eyes and inner ears in vertebrates

Fibroblast growth factor 19 (FGF19), and its rodent ortholog FGF15, is a member of a FGF subfamily directly involved in metabolism, acting in an endocrine way. During embryonic development, FGF15/FGF19 also functions as a paracrine or autocrine factor, regulating key events in a large number of organs. In this sense, the Fgf15/Fgf19 genes control the correct development of the brain, eye, inner ear, heart, pharyngeal pouches, tail bud and limbs, among other organs, as well as muscle growth in adulthood. These growth factors show relevant differences according to molecular structures, signalling pathway and function. Moreover, their expression patterns are highly dynamic at different stages of development, in particular in the central nervous system. The difficulty in understanding the action of these genes increases when comparing their expression patterns and regulatory mechanisms between different groups of vertebrates. The present review will address the expression patterns and functions of the Fgf15/Fgf19 genes at different stages of vertebrate embryonic development, with special attention to the regulation of the early specification, cell differentiation, and morphogenesis of the central nervous system and some sensory organs such as eye and inner ear. The most relevant anatomical aspects related to the structures analysed have also been considered in detail to provide an understandable context for the molecular and cellular studies shown.

VEGF, but Not BDNF, Prevents the Downregulation of KCC2 Induced by Axotomy in Extraocular Motoneurons

The potassium-chloride cotransporter KCC2 is the main extruder of Cl- in neurons. It plays a fundamental role in the activity of the inhibitory neurotransmitters (GABA and glycine) since low levels of KCC2 promote intracellular Cl- accumulation, leading to the depolarizing activity of GABA and glycine. The downregulation of this cotransporter occurs in neurological disorders characterized by hyperexcitability, such as epilepsy, neuropathic pain, and spasticity. KCC2 is also downregulated after axotomy. If muscle reinnervation is allowed, the KCC2 levels recover in motoneurons. Therefore, we argued that target-derived neurotrophic factors might be involved in the regulation of KCC2 expression. For this purpose, we performed the axotomy of extraocular motoneurons via the monocular enucleation of adult rats, and a pellet containing either VEGF or BDNF was chronically implanted in the orbit. Double confocal immunofluorescence of choline acetyl-transferase (ChAT) and KCC2 was carried out in the brainstem sections. Axotomy led to a KCC2 decrease in the neuropil and somata of extraocular motoneurons, peaking at 15 days post-lesion, with the exception of the abducens motoneuron somata. VEGF administration prevented the axotomy-induced KCC2 downregulation. By contrast, BDNF either maintained or reduced the KCC2 levels following axotomy, suggesting that BDNF is involved in the axotomy-induced KCC2 downregulation in extraocular motoneurons. The finding that VEGF prevents KCC2 decrease opens up new possibilities for the treatment of neurological disorders coursing with neuronal hyperactivity due to KCC2 downregulation.

Specification and survival of post-metamorphic branchiomeric neurons in a non-vertebrate chordate

Tunicates are the sister group to the vertebrates, yet most species have a life cycle split between swimming larva and sedentary adult phases. During metamorphosis, larval neurons are replaced by adult-specific ones. The regulatory mechanisms underlying this replacement remain largely unknown. Using tissue-specific CRISPR/Cas9-mediated mutagenesis in the tunicate Ciona, we show that orthologs of conserved hindbrain and branchiomeric neuron regulatory factors Pax2/5/8 and Phox2 are required to specify the 'neck', a cellular compartment set aside in the larva to give rise to cranial motor neuron-like neurons post-metamorphosis. Using bulk and single-cell RNA-sequencing analyses, we characterize the transcriptome of the neck downstream of Pax2/5/8. We present evidence that neck-derived adult ciliomotor neurons begin to differentiate in the larva and persist through metamorphosis, contrary to the assumption that the adult nervous system is formed after settlement and the death of larval neurons during metamorphosis. Finally, we show that FGF signaling during the larval phase alters the patterning of the neck and its derivatives. Suppression of FGF converts neck cells into larval neurons that fail to survive metamorphosis, whereas prolonged FGF signaling promotes an adult neural stem cell-like fate.

Gene networks and the evolution of olfactory organs, eyes, hair cells and motoneurons: a view encompassing lancelets, tunicates and vertebrates

Key developmental pathways and gene networks underlie the formation of sensory cell types and structures involved in chemosensation, vision and mechanosensation, and of the efferents these sensory inputs can activate. We describe similarities and differences in these pathways and gene networks in selected species of the three main chordate groups, lancelets, tunicates, and vertebrates, leading to divergent development of olfactory receptors, eyes, hair cells and motoneurons. The lack of appropriately posited expression of certain transcription factors in lancelets and tunicates prevents them from developing vertebrate-like olfactory receptors and eyes, although they generate alternative structures for chemosensation and vision. Lancelets and tunicates lack mechanosensory cells associated with the sensation of acoustic stimuli, but have gravisensitive organs and ciliated epidermal sensory cells that may (and in some cases clearly do) provide mechanosensation and thus the capacity to respond to movement relative to surrounding water. Although functionally analogous to the vertebrate vestibular apparatus and lateral line, homology is questionable due to differences in the expression of the key transcription factors Neurog and Atoh1/7, on which development of vertebrate hair cells depends. The vertebrate hair cell-bearing inner ear and lateral line thus likely represent major evolutionary advances specific to vertebrates. Motoneurons develop in vertebrates under the control of the ventral signaling molecule hedgehog/sonic hedgehog (Hh,Shh), against an opposing inhibitory effect mediated by dorsal signaling molecules. Many elements of Shh-signaling and downstream genes involved in specifying and differentiating motoneurons are also exhibited by lancelets and tunicates, but the repertoire of MNs in vertebrates is broader, indicating greater diversity in motoneuron differentiation programs.