[Light and electron microscopic studies of the greater epithelial ridge and its relationship to the developing tectorial membrane in the cochlear duct of the guinea pig (author's transl)]

Early identification of inner pillar cells during rat cochlear development

Although the structure of the auditory organ in mature mammals, the organ of Corti, is clearly established, its development is far from being elucidated. Here, we examine its spatio-temporal development in rats from embryonic day 16 (E16) to E19 by using cytochemical and immunocytochemical methods at the light- and electron-microscope levels. We demonstrate that the organ of Corti develops from a non-proliferating cell zone that is located in the junctional region between two edges of the dorsal epithelium of the cochlear duct. We also reveal that the first cells to develop in this zone are the inner pillar cells, a particular type of non-sensory supporting cell, which arise in the base of the cochlear duct at the boundary between the two ridges at E16. Cell differentiation in this prosensory region continues according to a base-to-apex gradient; the inner hair cells appear in the greater epithelial ridge at E17 and the outer hair cells in the lesser epithelial ridge at E18. At E19, the various cell types of the organ of Corti are in place. Finally, we show that unlike the development of all the supporting cell types of the organ of Corti, the development of inner pillar cells within the prosensory domain seems not to involve Notch1 activation. These results highlight the central role that the inner pillar cells probably play in the development of the organ of Corti.

Modification of tonotopic representation in the auditory system during development

During the early development of the bird and the mammalian peripheral auditory system, a restricted range of low--mid frequencies is recorded in immature animals. These early recordings are correlated to the base or mid-basal region of the cochlea which codes high frequencies in the adult. In order to reconcile the functional observations with anatomical ones, two main hypotheses have been put forward: one called the development of the place principle derived from observations of acoustic trauma in chick cochlea and a second derived from auditory nerve fiber recordings in kittens. Whatever the theories, the tonotopic shift during development is a well-established phenomenon in both birds and mammals that could be explained by a synthetic theory including active and passive cochlear processes. The tonotopic shift observed in the central auditory system mimics quite closely the frequency representation of the peripheral auditory system. The same trend is observed in all auditory nuclei including the cortex, except that the frequency representation is more complex because it shows tonotopic maps that can be twisted in three dimensions. From current observations, there is a simultaneous onset of tonotopic maps across auditory nuclei up to the cortex. A hypothesis is presented related to the frequency changes observed in the cochlea that affect the central auditory pathway, along with possible consequences on auditory behavior.

Incorporation of D3H glucosamine to the adult and developing cochlear tectorial membrane of normal and hypothyroid rats

The uptake of D-3H-glucosamine by the developing cochlea of normal and hypothyroid rats was examined using light microscopic radioautography. During postnatal development, normal and hypothyroid rat cochleas exhibited a layer of radiolabelling in the tectorial membrane (TM). This layer first appeared in the TM region which covers the spiral limbus and the Kölliker's organ (KO), then progressively reached the apical part of the TM covering the organ of Corti. Radiolabelling was significantly greater in hypothyroid than in normal cochleas. These findings suggests that the enormous size reached by the TM in the congenital hypothyroidism could be related to an increase of epithelial secretion, at least for carbohydrates. It also suggests that TM, in normal and hypothyroid cochleas, could be formed during development by the addition of successive layers. Older layers could be displaced upwards by the new ones. Cochleas of normal young adult rats, treated with D-3H-glucosamine, showed a very scarce and diffuse radiolabelling. Cochleas of hypothyroid young adult rats exhibited a thickened and distorted TM, which incorporated a significant amount of carbohydrates. These results suggest that TM secretion is highly reduced in young adult normal animals, while in young adult hypothyroid ones it is still active. During cochlear maturation, thyroxine seems to be necessary, not only for the synthesis of normal glycoproteins (as suggested by previous reports), but also for the control of glycoprotein secretion.

Incorporation of D-[3H]-glucosamine and L-[3H]-fucose into the developing rat cochlea

The uptake of two tritiated carbohydrates, D-[3H]-glucosamine and L-[3H]-fucose, to the developing rat cochlea was examined using light and electron microscopic radioautography. Both carbohydrates, administered to in vitro developing rat cochleas, shared a similar ultrastructural labeling pattern on the microvilli and apical cell region and on the tectorial membrane (TM) fibrils. On embryonic day 18, the radiolabeling appeared on the apical surface of the undifferentiated epithelium that will develop into both spiral limbus and Kölliker's organ (KO), while on postnatal day (PD) 1, it was only located on the apical surface of the KO. When D-[3H]-glucosamine was administered in vivo to newborn rats, the radiolabeling was observed in the TM covering the KO at PD 3. Lastly, D-[3H]-glucosamine administered in vivo to PD 7 rats, appeared at PD 9 in the TM region lying just above the organ of Corti. The present findings support the previously suggested leading role of the spiral limbus and KO in the secretion of the TM during cochlear development. The secretion of carbohydrates, and probably of other matrix components, starts on the spiral limbus and KO region and progressively extends to the organ of Corti.

Tonotopic evolution during development

The evolution of cochlear tonotopy can be observed by the study of the modification of tuning curves as obtained from auditory nerve fibers in the kitten. The differential development of the two components of the tuning curves, i.e. the tail, which is restricted to lower frequencies that appear at first during ontogenesis, and the tip which is restricted to higher frequencies that appear later, can explain the ontogenetic variations of tonotopy observed in the past. An hypothesis is presented where the tail of the tuning curve is related to the first functioning of the basal inner hair cells during development, whereas the delayed appearance of the tip is related to the basal outer hair cells. The base-apex gradient of maturation of receptors, along with their lateral gradient of development can explain the tonotopic shift observed during the cochlear ontogenesis.

Factors affecting the onset of inner ear function

The developing inner ear receptors have a very significant influence on the onset of stato-acoustic function and on its evolution. The factors which prevent the stato-acoustic system from functioning are called 'the limiting factors'. At present, it is possible to postulate that these factors are restricted to the inner ear cells and related structures. At least four places are particularly relevant for the onset of function: (1) connections of the apical part of hair cell with the tectorial membrane; (2) the internal structure of hair cell; (3) connections between the base of the hair cell and nerve fibers; (4) the ganglion cell with its processes. Special emphasis is devoted to the apical part of the inner hair cell and its connections to the tectorial membrane which are considered as very important for the onset of the cochlear function. For the labyrinth, it is technically difficult to determine precisely the onset of function because of its early prenatal onset. Nevertheless, it is postulated that the limiting factors for the onset of function are also related to certain components of hair cells.

Lectins demonstrate the presence of carbohydrates in the tectorial membrane of mammalian cochlea

Histological sections of rat and guinea pig cochleas were exposed to lectins to identify the carbohydrates present in the tectorial membranes. N-Acetylglucosamine, galactose, mannose and fucose were found to be present in both rats and guinea pigs, but N-acetylgalactosamine was not detected. In addition, two control experiments were performed. In the first, each lectin was preincubated with its specific inhibitory sugar. In the second, the unfixed tectorial membranes were exposed to lectins. Radioautographic studies confirmed the presence of glucosamine and fucose in the tectorial membrane of 1-day-old rats.

[Light and electron microscopy studies of the development of Reissner's membrane in the cochlear duct of guinea pig fetuses]

Differentiation of the Reissner's membrane (membrana vestibularis) is described in precisely dated guinea pig fetuses. In newborn and adult animals, the Reissner's membrane consists of two layers of flat cells, an epithelial one and a mesenchymal one, separated by a basement membrane. The epithelial part develops from the initially simple columnar epithelium of the upper wall of the cochlear duct. The mesenchymal part arises from the embryonic connective tissue, which in early developmental stages occupies the space of the later scala vestibuli. The studies show that during fetal life, a mixture of the endolymph of the cochlear duct with the perilymph of the scala vestibuli is prevented by zonulae occludentes between the epithelial cells of Reissner's membrane. On the other hand, even at an early stage an exchange of material between the two fluids is to a small extent possible by means of cytopempsis through the cytoplasm of the epithelial cells.

The blood vessels of the limbus spiralis

The capillaries of the spiral limbus in the cochlea of the laboratory rat were investigated under light and electron microscopy. A stereological examination of the capillary volumes of specimens injected with horseradish peroxidase was performed. The volume of capillaries amounted to 2.87%, and, as such, is very high. The ultrastructure of the capillaries showed continuous endothelial cells and basal laminae, discontinuous pericytes and varying relations towards the surrounding tissue. Morphological criteria, such as microvilli and plasmalemmal vesicles, suggest a significant transendothelial transport. The causes for the abundance of vessels in the spiral limbus are discussed: the transport of oxygen to the perilymph and the need of substances for the formation of the tectorial membrane seem to be important factors. The transport processes are obviously controlled by the endothelial cells as well as by the spiral limbus cells.

The limbus spiralis and its relationship to the developing tectorial membrane in the cochlear duct of the Guinea pig fetus

The development of the interdental cells of the limbus spiralis and of the inner spiral sulcus cells as well as the formation of the mesenchymal teeth of Huschke are described during fetal life up to the day of birth in the guinea pig. Additionally, the changes of the developing tectorial membrane are studied. The ultrastructural observations allow the conclusion that during fetal development at least a considerable part of the material of the tectorial membrane is secreted by the interdental cells of the limbus spiralis.