Determinants of ganglion-receptor cell interaction during development of the inner ear. A heterochronic ganglia study

Morphological and morphometric study on human Scarpa ganglion development

CONCLUSION

In Scarpa neurons the cell and nuclear area increases and nuclear/cytoplasm ratio decreases with fetal age (p < 0.0001). There are statistically significant differences in cell area between all fetal groups, except for the interval 45-74 mm crown-rump-length (CRL). Displacement of a neuron within the internal auditory meatus (IAM) occurs from 9 weeks in the fetus until the neonate.

METHODS

A light microscopic histomorphometric study of the Scarpa ganglion in human fetuses from spontaneous abortions measuring 45, 74, 90, 134, 145 and 270 mm CRL and a from a 1-day-old neonate (360 mm) was carried out. Cell and nuclear area, ganglion area and distances from the Scarpa ganglion neurons to the endocranial porus of the IAM were measured.

RESULTS

In the 45, 74, 90 and 134 mm CRL human fetuses the cartilaginous labyrinthine capsule appears divided by the facial nerve and the Scarpa ganglion into two compartments: rostral and dorsal. Ovoidal Scarpa ganglion in the 45 mm CRL lies within the IAM near its endocranial porus (15 µm). In the otic capsule of the 145 mm CRL fetus an endochondral ossification appears in the IAM base, where Scarpa ganglion neurons are displayed in two groups: superior and inferior divided by a vascular-connective septum. This anatomy remains from this specimen until the neonate specimen.

Neural network and "ganglion" formations in vitro: a video microscopy and scanning electron microscopy study on adult cultured spiral ganglion cells

HYPOTHESIS

To analyze if adult-dissociated spiral ganglion cells may be propagated in vitro for later use in transplantation models to form integrated neural networks.

BACKGROUND

Hearing loss is often associated with primary or secondary spiral ganglion cell degeneration. New strategies for cell repair and tissue engineering warrants further elucidation of the regenerative capacity of the auditory nerve.

METHODS

We used in vitro/in video microscopy in combination with immunocytochemistry and field emission scanning electron microscopy to analyze neural development and network formation from dissociated adult guinea pig spiral ganglion cells. Cells were cultured in serum-free medium and in the presence of brain-derived neurotrophic factor, neurotrophin 3, and glia cell line-derived neurotrophic factor for up to 8 weeks.

RESULTS

Time-lapse video microscopy and scanning electron microscopy exposed the propagation of auditory neurons and the role of neural growth cones in axon locomotion, fasciculation, and nuclear migration, often ensuing in cell congregation (ganglion-like formations) during network formation. Axons were sometimes ensheathed by adjoining S-100/glia fibrillary acidic protein-expressing cells. A few expanding neurons were nestin positive and sometimes incorporated the markers of proliferating cells Ki67 and 5'-bromo-2-deoxyuridine. Neurons expressed the markers and transcription factors for neural development neurogenin 1, neurogenic differentiation factor 1, Brn3a, and GATA binding protein 3, as well as the neural markers beta-III tubulin, NeuN, and neurofilament 160 during this process.

CONCLUSION

This method of culturing and expanding spiral ganglion neurons in vitro may be useful in further studies of cell transplantation models aiming to restore the injured inner ear.

Structure and locomotion of adult in vitro regenerated spiral ganglion growth cones – A study using video microscopy and SEM

Neuronal development and neurite regeneration depends on the locomotion and navigation of nerve growth cones (GCs). There are few detailed descriptions of the GC function and structure in the adult auditory system. In this study, GCs of adult dissociated and cultured spiral ganglion (SG) neurons were analyzed in vitro utilizing combined high resolution scanning electron microscopy (SEM) and time lapse video microscopy (TLVM). Axon kinesis was assessed on planar substratum with growth factors BDNF, NT-3 and GDNF. At the nano-scale level, lamellipodial abdomen of the expanding GC was found to be decorated with short surface specializations, which at TLVM were considered to be related to their crawling capacity. Filopodia were devoid of these surface structures, supporting its generally described sensory role. Microspikes appearing on lamellipodia and axons, showed circular adhesions, which at TLVM were found to provide anchorage of the navigating and turning axon. Neurons and GCs expressed the DCC-receptor for the guidance molecule netrin-1. Asymmetric ligand-based stimulation initiated turning responses suggest that this attractant cue influences steering of GC in adult regenerating auditory neurites. Hopefully, these findings may be used for ensuing tentative navigation of spiral ganglion neurons to induce regenerative processes in the human ear.

Relationship between the development of outer hair cell electromotility and efferent innervation: a study in cultured organ of corti of neonatal gerbils

Outer hair cell (OHC) electromotility, which powers the cochlear amplifier, develops at a later stage of hearing ontogeny. There has been speculation whether efferents play a necessary role in directing or achieving OHC maturation in mammals. In this study, we examine whether the development of OHC motility depends on the establishment of efferent innervation of the cells' synaptic pole by measuring electromotility of OHCs grown in cultures, deprived of efferent innervation. Tissue cultures of the organ of Corti were prepared from the cochleas of newborn gerbils. Solitary OHCs were obtained from 4- to 15-d-old cultures by enzymatic digestion and mechanical trituration. Length changes evoked by transcellular electrical stimulation were detected and measured with a photodiode sensor. Results show that OHCs develop electromotility between 6 and 13 d in culture without the presence of efferent innervation. The timetable for the onset of OHC electromotility is comparable with that in vivo. This demonstrates that the ontogeny of OHC electromotility is an intrinsic process that does not require the influence of efferent innervation.

Complementary roles of BDNF and NT-3 in vestibular and auditory development

The physiological role of BDNF and NT-3 in the development of the vestibular and auditory systems was investigated in mice that carry a deleted BDNF and/or NT-3 gene. BDNF was the major survival factor for vestibular ganglion neurons, and NT-3, for spiral ganglion neurons. Lack of BDNF and NT-3 did not affect ingrowth of nerve fibers into the vestibular epithelium, but BDNF mutants failed to maintain afferent and efferent innervation. In the cochlea, BDNF mutants lost type 2 spiral neurons, causing an absence of outer hair cell innervation. NT-3 mutants showed a paucity of afferents and lost 87% of spiral neurons, presumably corresponding to type 1 neurons, which innervate inner hair cells. Double mutants had an additive loss, lacking all vestibular and spiral neurons. These results show that BDNF and NT-3 are crucial for inner ear development and, although largely coexpressed, have distinct and nonoverlapping roles in the vestibular and auditory systems.

Morphologic and morphometric study of human spiral ganglion development

A light microscopic study of the spiral ganglion was done in human embryos and fetuses measuring 45, 50, 60, 74, 90, 134, 270 mm crown-rump length (crl), and in a one-day-old neonate. Morphometric evaluations of i) cell and nuclear area, ii) nuclear area/cell area ratio, iii) ganglion area, iv) cell concentration/surface unit, and v) distance between the first neuron and the receptor were made, and the results statistically evaluated. In earlier stages of development, spiral ganglion primordia appeared as a cluster of neuroblasts and some schwannoblasts immersed in the mesenchymal tissue, close to the ductus cochlearis. A honeycomb pattern in the spiral ganglion neurons was observed in the basal turn of a 74 mm crl fetus. In later stages, the basal turn of a 90 mm crl fetus showed a spatial organization. Peripheral and central fibers of the acoustic nerve appeared stratified in early periods of development (45 mm crl embryo). From this stage on, both phenomena progress apicalwards until the neonatal period. A significant decrease in the nuclear area/cell area ratio was observed from the 134 mm crl fetus (17 weeks) to the neonatal stage in all turns. This led to a significant increase in cellular area from the 270 mm crl fetus (32 weeks) to the neonate, with no significant variation in nuclear area. The distance from the primordium of the organ of Corti to the spiral ganglion in the interval between 45 and 74 mm crl showed a significant increase in all turns.(ABSTRACT TRUNCATED AT 250 WORDS)

Tropic effects of otic epithelium on cochleo-vestibular ganglion fiber growth in vitro

Sensory nerve fibers of the cochleo-vestibular ganglion (CVG) innervate the otic epithelium in the early chick embryo by directed growth. To see if the target tissue could exert a tropic influence, we co-cultured CVGs from chick embryos (Hamburger-Hamilton stages 16-30) in a 3D collagen matrix with their normal target epithelium or with other epithelial tissues taken from the same or different stages of development. The pattern of neurite outgrowth and the viability of the CVG after five days in vitro were assessed histologically with a silver method. On the basis of the patterns of neurite outgrowth directed toward the epithelium, the cultures were classified as having slightly, mostly, exclusively, or no directed outgrowth. Of 49 cultures containing otic epithelium, 33 had mostly or exclusively directed growth patterns. This effect did not depend on any particular stage difference between co-cultures or on their viability in vitro. Cultures of non-sensory otic epithelium (endolymphatic duct) also presented directed growth patterns. Co-cultures with ectoderm from forelimb or visceral arch had little, if any, directed growth. The directed growth could not be explained simply as a result of guidance by non-neuronal cells or of the viability of the explants. The results are consistent with the hypothesis that the otic epithelium provides a tropic factor that attracts growing CVG fibers.

Temporal pattern of innervation in the developing mouse inner ear: an immunocytochemical study of a 66-kD subunit of mammalian neurofilaments

We have examined the expression of a 66-kD neurofilament protein (NF-66) in the developing inner ear. Mouse embryos, fetuses, and neonates were fixed in Methacarn, embedded in paraffin, and sectioned. A polyclonal antiserum raised specifically to NF-66 and unreactive to NF-L, -M, -H, and peripherin was used for immunocytochemical staining. NF-66 immunostaining was first detectable in the rhombencephalon at embryonic day (E) 9.5. Immunoreactivity was first detected in the statoacoustic ganglion (SAG) early on E10.5. By late E10.5, the first SAG axons were detectable within the intraepithelial spaces of the otocyst. At E12, NF-66 positivity was detectable in neurites that projected into areas of presumptive vestibular sensory epithelium. Neurites projecting into the presumptive acoustic sensory epithelium were negative. However, at E13, the projections from both the vestibular and the acoustic ganglion (i.e, cochlear duct) were both NF-66 positive. In the cell bodies, NF-66 expression appeared earlier in the vestibular than in the auditory neurons. By E16, neuronal somas in both ganglia were NF-66 positive.