Electron microscopy of the developing hearing organ

Cytological changes related to maturation of the organ of Corti and opening of Corti's tunnel

Maturation of the organ of Corti in the gerbil was analyzed between 2 and 16 days after birth (DAB) by electron microscopy and immunostaining for beta-tubulin. At 2 DAB, the organ of Corti consisted of stratified epithelium bearing immature sensory hair cells (HCs) and supporting cells. Maturation of OHCs and Deiters cells progressed in a medial-to-lateral direction and cytoskeletal development in inner pillar cells preceded that in outer pillar cells at the single location studied along the frequency-place map. Pillar cell differentiation progressed through a unique stage characterized by the appearance and stratification of structural features apparently concerned with opening of Corti's tunnel and subsequently showed other structural changes related to maturity toward the adult form. Development of the microtubule cytoskeleton occurred first in the cell's apex and proceeded basally. Ruffling of a middle region of the cell surface by microvilli appeared to promote separation between inner and outer pillar cells and initiate tunnel opening at 4 DAB. Proliferation of distended cisternae of granular reticulum evidenced proteinaceous secretion by these cells between 4 and 8 DAB. Subsequent tunnel expansion at about 14 DAB coincided with appearance in outer pillar cells of tubulocisternal endoplasmic reticulum and associated Golgi complexes that are thought to mediate fluid and ion secretion. Sixteen days postnatally after disappearance of granular and tubulocisternal reticula and Golgi complexes and at the time of clearing of tunnel fluid, lysosomes interpreted as mediating catabolism of endocytosed protein congregated beneath the apical and apicolateral plasmalemmae of inner pillar cells. As with pillar cells, development of the microtubule system in Deiters cells proceeded from the cell's apex to base. Following differentiation of their microtubule system by 8 DAB, Deiters cells showed expansion of Golgi cisternae between 10 and 15 DAB and development of tubulocisternal endoplasmic reticulum at 15 DAB. Hair cells possessed abundant, distinctively large mitochondria from 4 to 10 DAB. The subsurface cisternae matured earlier in medial as opposed to lateral outer hair cells. Vesicles budding from underlying cisternae appeared associated with development of subsurface cisternae and at 16 DAB were still observed in third row but not in more mature first row HCs.

Differentiation of the rat stria vascularis

The differentiation of the rat stria vascularis (SV) was investigated by light- and electron microscopy and by immunocytochemistry. Loss of the basal lamina at the epithelial-mesenchymal interface of SVs as indicated by immunoreactions of laminin and fibronectin induces the formation of vascular feet by basal infoldings of the marginal cells (MCs), and the development of the strial capillaries (SCs) by mesenchymal cells in a manner of vasculogenesis is progressing at the same time. The production of fibronectin in the rough endoplasmic reticulum of mesenchymal cells and the involvement of this glycoprotein in a mechanical linkage between the vasoformative mesenchymal cells and endothelial ones of the SCs are indicated by immunocytochemistry. The plasma membrane of the marginal cells (MCs) begins to show immunoreactions of Na+.K+ ATPase at postnatal day 5 and is conjugated to each other by tight junctions at postnatal day 14. The apical tubules of the differentiating MCs do not seem to be involved in the endocytotic activity but are involved in the plasma membrane supply for the rapid differentiation.

Strial dysfunction in a melanocyte deficient mutant rat (Ws/Ws rat)

A homozygous mutant rat at the white spotting (Ws) locus showing a deficiency of melanocytes has recently been found (4, 5). The function and morphology of the inner ear of the Ws/Ws rat were examined by auditory brainstem response (ABR), endocochlear DC potential (EP), and electron microscopy. The mean ABR threshold of the Ws/Ws rat was significantly higher than that of the control +/+ rat. Most Ws/Ws rats showed no or very little EP. In electron microscopy, the stria vascularis of the Ws/Ws rat proved to be very thin and flat with poor interdigitation of marginal cells, and absence of intermediate cells. The organ of Corti appeared to be intact in both the +/+ rat and the Ws/Ws rat. These electrophysiological and morphological findings suggest that the Ws/Ws rat suffered from severe hearing loss caused by strial dysfunction.

Development of inner ear vessels. A scanning electron microscopic study

The development of the inner ear vessels of guinea pigs were studied by SEM. Employing 32- to 55-gestational-day-old guinea pig embryos, corrosion casts of inner ear blood vessels were prepared using Mercox resin and then observed by scanning electron microscopy. Between the 35th and 40th gestational days, the vascular structure of the vestibule was sufficiently developed, but the capillary network in the wall of the cochlea remained immature. Thus, in the guinea pig, the development of the vestibular structure precedes the development of the cochlear vessels. Moreover, at both sites, the density of the embryonic blood vessels was high, and the vessels were believed to transport a large blood flow. From about the 40th day of gestation, the coiled vessels and the capillary network in the lateral wall of the cochlea underwent rapid development, and the vascular structure in both the vestibule and cochlea was nearly complete by the time of birth.

Strial dysfunction in mice with cochleo-saccular abnormalities

Most viable dominant spotting (Wv/Wv) mutant mice, which show cochleo-saccular degeneration, were found to have an endocochlear potential (EP) around zero together with a structurally abnormal stria vascularis. Inner hair cells were well preserved, but outer hair cells in the basal half of the cochlea were degenerating, possibly as a result of primary strial dysfunction. Thresholds for the detection of a compound action potential were raised to around 100 dB SPL in the mutants with no EP, and there was little if any cochlear microphonic at the round window. Of the 20 Wv/Wv mice studied, five partially escaped the effects of the mutation and had measurable positive potentials (15-86 mV) in scala media in the basal turn; responses in these animals were intermediate between control responses and those of mutants with no EP. These findings confirm that the pathological processes in this mutant, with cochleo-saccular abnormalities, are fundamentally different from the pathological processes in animals with neuroepithelial abnormalities reported previously [see Steel and Bock (1983) Arch. Otolaryngol. 109, 22-29, for references].

The vascular anatomy of the gerbil cochlea

The vascular pattern of the cochlea was studied in 12 adult gerbils (Meriones unguiculatus) using the Berlin blue (Prussian blue) contrast injection technique. The capillary areas of the gerbil cochlea are similar to those of other mammals studied. As in some other mammals, the vessel of the basilar membrane was inconsistently identified in the basal turn. It was usually replaced by a larger uninjected channel, supporting the suggestion that the vessel of the basilar membrane may be more functionally important in fetal life than in adulthood. The vascular pattern of the external cochlear wall is well maintained from base to apex, although a simplification of vasculature can be observed apically.

Postnatal maturation of cochlear sensory hairs in the mouse

The surface morphology of inner hair cells in the cochlea of the mouse is fully developed at birth. At this time a regular pattern of elongated microvilli covers the surfaces of outer hair cells at all levels of the cochlea. The sensory hair on outer hair cells undergo a morphological maturation during the first days postnatally. A mature surface morphology is reached 5-7 days after birth. There are two gradients in hair cell maturation in the cochlea: (1) basal to apical, and (2) from inner hair cells to the 3rd row of outer hair cells. Polarization of sensory hairs on an outer hair cell, i.e. polar differentiation of the sensory hairs in the radial direction, occurs as a stepwise increase in length of those facing the stria vascularis.

Early development and maturation of the spiral ganglion

The maturation of the spiral ganglion and its peripheral pathways to the cochlea has been morphologically analysed from birth to the 14th day postnatally, i.e. to the time from the onset and during the maturation of cochlear potentials (mouse). The spiral ganglion cells and the neurons between the ganglion and habenula perforata must attain myelination before action potentials can be elicited. Earlier observations that a certain maturation of the organ of Corti is a prerequisite for the development of electrophysiological potentials is probably only coincidental in time with the structural development of the ganglion and the neurons.

Development of the epithelium of the middle ear. Electron microscopic study of fine structure, including junctional complexes and basal lamina

A detailed light and electron microscopic study of the development of the middle ear lining epithelium of the rat was undertaken in order to determine whether a portion of the middle ear lining is normally derived from a condensation of mesenchymal connective tissue. We learned that pharyngeal epithelium outpouches early in development and persists during the formation and growth of the ossicles and inner ear structures in a relatively constant anatomic relationship. We found no evidence of a contribution of mesodermal tissue to the lining epithelium itself. Certain features of the junctional complexes between adjacent cells were used as a means of identifying pharyngeal and middle ear epithelial cells. It is concluded that at no time is connective tissue found within the middle ear space (as defined by its epithelial limits); pneumatization occurs by absorption of mesenchymal tissue that occupied the space between the outer part of the epithelium and bone structures.

Organ of Corti in the human fetus: scanning and transmission electronmicroscope studies

The organ of Corti in the five-month human fetus was studied by transmission and scanning electronmicroscopy. Differentiation of the surface organization of the organ of Corti into a single row of inner and three to four rows of outer hair cells was complete at this stage except at the apical end. The morphological aspects of the hair bundles changed with maturation of the sensory cells; the inner hair cells preceded the outer hair cells in cytodifferentiation at a given location.

Pigmentation of the stria vascularis. The contribution of neural crest melanocytes

Because all stages of melanogenesis (premelanosome, melanosome and melanin granules) were found in intermediate cells of the rat stria vascularis, they can be classified as melanocytes. The marginal and basal cells of these rat specimens contained no pigment. Early in development, the melanocytes or future intermediate cells are located beneath the strial basal lamina. They penetrate this lamina to insert themselves between marginal cells. Both the melanocytes and marginal cells are believed to participate actively in the tissue rearrangements which ultimately bring the two cell types into intimate structural relationship. The ingrowing melanocytes and the mature, pigmented, intermediate cells which they form are frequently associated with blood vessels. Pretreatment with Dopa to enhance the melanin content of early melanocytes did not enable us to identify their route of migration into the stria.

Receptor maturation and synaptogenesis in the golden hamster cochlea

Maturation of hamster cochlea was studied using light and electron microscopy. Critical stages of receptor and neural structure development have been determined. At birth the hamster cochlea shows a pronounced immaturity, but innervation can already be found. 2 or 3 days later, characteristic afferent synapses can be recognized beginning at the inner hair cell level. Similarly, efferent endings first appear on the inner side at the end of the first week. The onset of auditory function must be related to structures depicted at around 10 days, and cochlear maturation is achieved at about 25 days. The sequence of synaptic development in the cochlea is discussed regarding the general morphogenesis of synapses within the nervous system. Some determinations of the timing of peripheral myelination are given. This process begins almost a week before the presumable date of the onset of function.

Organ culture of the mammalian and avian embryo otocyst

The use of a chemically defined medium supplemented with serum has proved most suitable for the growth of the isolated embryonic otocyst in vitro. Complete differentiation of the cochlear duct and of the vestibular apparatus ensued and is described. The system is reliable for organ cultures of both the avian and mammalian otocyst and for the study of various lesions affecting the inner ear and middle ear.

The vascular pattern of the chinchilla cochlea

The vasculature of the chinchilla cochlea was demonstrated with injected Prussian blue contrast. By and large, the vascular pattern is similar to other mammals. The vasculature appears to be richly developed, but separate types of vessels appear to be of small caliber. Particular findings for the chinchilla cochlea were the veins of the scala tympani, formed by a merging of collecting venules, running parallel to the spiral modiolar vein. Furthermore, no vessel of the basilar membrane under the tunnel of Corti was found, and which, when present, is of such great presumed importance for the oxygen supply to the organ of Corti. The scala vestibuli wall appears to be supplied completely arterially, in contrast with that of other mammals. The stria vascularis is broad and well developed, both at the apex and at the basal end.

The ciliary-cone sensory cell of anemones and cerianthids

An ultrastructural study of the tentacles of Stomphia and of Ceriantheopsis has revealed that the so-called 'ciliary-cone sensory cell' consists of a cluster of five to seven apparent receptors rather than just one cell as reported previously. At the center of a cluster is a single cell, whose dendrite bears one cilium surrounded by about ten large stereocilia. Surrounding this cell are a number of peripheral cells whose dendrites bear large numbers of small stereocilia and, in Ceriantheopsis, one cilium. The sensory apparatuses of all cells in a cluster unite to form a single unit projecting above the tissue surface: the ciliary cone. Their possible physiological role is discussed in relation to new behavioural observations.

Melanocytes of the vestibular labyrinth and their relationship to the microvasculature

The melanocytes of the vestibular labyrinth as seen in colored guinea pigs show a characteristic pattern of distribution in the wall of the utricle and in the ampullae, but they are not present in the wall of the saccule. They are found mainly in well-vascularized regions of apparent secretory or metabolic importance, including the “dark cell” areas. Their dendrite-like processes are often in intimate contact with the capillaries, although no pinocytotic vesicles or other indications of transfer of substances between the melanocytes and the capillary endothelium are seen under the electron microscope. In the human ear, the apparent density of the melanocytes varies with skin color. They are numerous in the cochlea, especially in the bony wall of the modiolus and on the osseous spiral lamina, and they occur also in Reissner's membrane and in the stria vascularis. In the vestibular system they are found in the wall of the saccule as well as in the utricle, the crus commune, and the ampullae, but not in the semicircular canals. They tend to be diffusely scattered, rather than to form the well-defined, intensely pigmented areas that are characteristic of the guinea pig. Close contacts with capillaries are seen mainly in the tympanic portion of the spiral ligament. The significance of melanin and the melanocytes in the labyrinth is unknown, but both the anatomical relation of certain melanocytes to capillaries, and the biochemical relation of melanin to the catecholamines support the hypothesis that they may have a vasomotor function. Other evidence, including the not infrequent association of sensorineural deafness with hereditary disorders of pigmentation also suggests that the melanocytes may play a role of some biological consequence in the inner ear.