Postnatal development of spiral ganglion cells in the rat

Abstract

The postnatal development of the spiral ganglion in the albino rat was studied using light and electron microscopy. The morphological characteristics distinguishing type 1 from type 2 spiral ganglion cells were defined, and the critical period for distinguishing the two types of neurons was identified. At birth, the spiral ganglion consists of a homogeneous population of small, densely packed, spherical cells that have large cytoplasmic-to-nuclear ratios. During the first postnatal week, the cells mature slowly. At this period the myelin sheath around the cell bodies generally consists of only a few layers of loose myelin. Long glial fingers extend around the cell processes and soma, particularly the filopodial extensions of the somatic membrane. Type 2 spiral ganglion cells can be distinguished at postnatal day 8. Viewed with the phase-contrast microscope these cells are smaller and have more darkly staining nuclei and more lightly staining cytoplasm than the type 1 cells. The most characteristic ultrastructural features of the type 2 neurons are the densely packed neurofilaments in the cytoplasm and lack of compact myelin around the cell soma. By day 14, spiral ganglion cells are morphologically mature, although the myelin sheath continues to thicken. The results are discussed in relation to the electrophysiological development of the auditory system and the morphological maturation of the organ of Corti.

Developmental regulation of neuron‐specific P2X3 receptor expression in the rat cochlea

“…P2X 3 R immunoreactivity was observed in SGN at E16, but labeling in the neurites extending to the IHC in the greater epithelial ridge was only detected at E18, after synaptic contact between IHC and afferent nerve fibers has been established (Echteler, 1992;Pujol, 1998). OHC innervation by SGNII does not occur until P2 (Schwartz et al, 1983;Romand and Romand, 1990;Simmons et al, 1991;Echteler, 1992). At this time, P2X 3 R expression develops in the afferent neurites innervating the OHC, mimicking the IHC innervation pattern established earlier.…”

Developmental expression of the actin depolymerizing factor ADF in the mouse inner ear and spiral ganglia

“…The spiral ganglion is composed of two populations of neurons: 1) Type I SGNs innervated by IHCs, comprising the majority (90%-95%) of SGNs; 2) Type II SGNs forming a small population (5%-10%) and receiving input from OHCs (Raphael and Altschuler, 2003). Type II SGNs are located at the periphery of the spiral ganglion (Schwartz et al, 1983) and are distin- guishable from Type I SGNs by their smaller cell bodies (cf. fig.…”

“…ADF weak SGNs were randomly distributed throughout the ganglion. A peripheral location and a relative amount of 5%-10% are hallmarks for Type II SGNs (Schwartz et al, 1983;Raphael and Altschuler, 2003). We therefore asked whether ADF strong neurons indeed represent Type II SGNs and investigated the number and spatial distribution of both neuron populations in more detail.…”

Spatiotemporal definition of neurite outgrowth, refinement and retraction in the developing mouse cochlea

“…We hypothesize that the smaller-diameter type II nerve fibers reseal before the dextran crystals can be taken up by these fibers. Additional potential mechanisms that underlie TMRD-specific labeling of type I fibers include a lower dextran diffusion efficiency in type II versus type I fibers due to a high density cytoskeletal meshwork in type II fibers (Schwartz et al, 1983;Dau and Wenthold, 1989). In addition, a higher dextran transport efficiency in the anterograde direction (Fritzsch, 1993;Kobbert et al, 2000) may have enabled TMRD labeling of efferent nerve fibers in the later stage of cochlear neural development.…”