A contribution to the structural pattern differences between the apical and basal spiral ganglions in mammals

The purpose of this paper was to establish a mammalian comparative study between the neuronal spaces of the upper versus the basal cochlear turns. We studied with light microscopy i) Webster albino rat embryos at various gestational days (GD), and a postnatal (PN) series; ii) kittens 3 weeks old; iii) pigmented guinea pig embryos and also PN specimens; iv) diverse human fetuses. As from the 16th GD, the apical spiral ganglion primordium of the rat appears as a spherical cluster of cells immersed in the mesenchymal modular tissue whereas the basal spiral primordium displays a pyramidal shape with its apex towards the auditory receptor. Both ganglions keep this shape until the rat becomes adult. This structural pattern also appears in the cat, guinea pig and in human material. This morphological model enables a radial stratification of the peripheral and central auditory nerve projections. A thin uni- or bicellular connective layer covers the tympanic scala side of the apical spiral ganglion as from 8 hours after birth in the rat. No gap is detected in this layer which is: mesenchymal in the embryonic and fetal stages (rat, guinea pig, human), connective in PN rat and kitten, and osseous in PN guinea pig and in the human neonate.