The pre- and postnatal maturation of the epithelium in the endolymphatic sac. An electron microscopic survey

Dynamic expression of the retinoic acid-synthesizing enzyme retinol dehydrogenase 10 (rdh10) in the developing mouse brain and sensory organs

Organs develop through many tissue interactions during embryogenesis, involving numerous signaling cascades and gene products. One of these signaling molecules is retinoic acid (RA), an active vitamin A derivative, which in mammalian embryos is synthesized from maternal retinol by two oxidative reactions involving alcohol/retinol dehydrogenases (ADH/RDHs) and retinaldehyde dehydrogenases (RALDHs), respectively. The activity of RALDHs is known to be crucial for RA synthesis; however, recently a retinol dehydrogenase (RDH10) has been shown to represent a new limiting factor in this synthesis. We investigated the spatiotemporal distribution of Rdh10 gene transcripts by in situ hybridization and quantitative polymerase chain reaction (PCR) during development of the brain and sensory organs. Although Rdh10 relative mRNA levels decline throughout brain development, we show a strong and lasting expression in the meninges and choroid plexuses. Rdh10 expression is also specifically seen in the striatum, a known site of retinoid signaling. In the eye, regional expression is observed both in the prospective pigmented epithelium and neural retina. In the inner ear Rdh10 expression is specific to the endolymphatic system and later the stria vascularis, both organs being involved in endolymph homeostasis. Furthermore, in the peripheral olfactory system and the vibrissae follicles, expression is present from early stages in regions where sensory receptors appear and mesenchymal/epithelial interactions take place. The distribution of Rdh10 transcripts during brain and sensory organ development is consistent with a role of this enzyme in generating region-specific pools of retinaldehyde that will be used by the various RALDHs to refine the patterns of RA synthesis.

Localization and functional studies of pendrin in the mouse inner ear provide insight about the etiology of deafness in pendred syndrome

Immunolocalization studies of mouse cochlea and vestibular end-organ were performed to study the expression pattern of pendrin, the protein encoded by the Pendred syndrome gene (PDS), in the inner ear. The protein was restricted to the areas composed of specialized epithelial cells thought to play a key role in regulating the composition and resorption of endolymph. In the cochlea, pendrin was abundant in the apical membrane of cells in the spiral prominence and outer sulcus cells (along with their root processes). In the vestibular end-organ, pendrin was found in the transitional cells of the cristae ampullaris, utriculi, and sacculi as well as in the apical membrane of cells in the endolymphatic sac. Pds-knockout (Pds-/-) mice were found to lack pendrin immunoreactivity in all of these locations. Histological studies revealed that the stria vascularis in Pds-/- mice was only two-thirds the thickness seen in wild-type mice, with the strial marginal cells showing irregular shapes and sizes. Functional studies were also performed to examine the role of pendrin in endolymph homeostasis. Using double-barreled electrodes placed in both the cochlea and the utricle, the endocochlear potential and endolymph potassium concentration were measured in wild-type and Pds-/- mice. Consistent with the altered strial morphology, the endocochlear potential in Pds-/- mice was near zero and did not change during anoxia. On the other hand, the endolymphatic potassium concentration in Pds-/- mice was near normal in the cochlea and utricle. Together, these results suggest that pendrin serves a key role in the functioning of the basal and/or intermediate cells of the stria vascularis to maintain the endocochlear potential, but not in the potassium secretory function of the marginal cells.

Differences in endolymphatic sac mitochondria-rich cells indicate specific functions

OBJECTIVE/HYPOTHESIS

The purpose of the study was to examine the specific involvement of endolymphatic sac mitochondria-rich cells in endolymph homeostasis.

STUDY DESIGN

Transmission electron microscopy and immunohistochemistry were performed on the endolymphatic sac of young adult rats, and two important developmental stages were also investigated.

METHODS

Ultrastructural characteristics of endolymphatic sac mitochondria-rich cells were studied more concisely and compared with renal mitochondria-rich cells (i.e., the intercalated cells). In addition, expression of cytokeratins 7 and 19 was determined.

RESULTS

Until birth, only one type of mitochondria-rich cell is observed in the rat endolymphatic sac. In young adult animals, distinct differences in mitochondria-rich cell ultrastructure in the endolymphatic sac enables classification into subtypes or configurations. Comparison of endolymphatic sac mitochondria-rich cells with renal intercalated cells reveals striking similarities and provides additional information on their specific function in endolymph homeostasis. Furthermore, differences in cytokeratin expression are determined in endolymphatic sac mitochondria-rich cells.

CONCLUSIONS

Differences in morphology of endolymphatic sac mitochondria-rich cells develop after birth and may reflect a distinct functional or physiological state of the cell. In analogy to renal intercalated cells, the distribution patterns of H+-adenosine triphosphatase and Cl-/HCO3- exchanger may differ between subtypes. We propose that subtype A mitochondria-rich cells, from which protruding A mitochondria-rich cells are the activated state, are involved in proton secretion (apical H+-adenosine triphosphatase) and thus are potential candidates for hearing loss accompanying renal tubular acidosis. Subtype B mitochondria-rich cells are the most likely candidates to be affected in Pendred syndrome because of the assumed function of pendrin as apical Cl-/HCO3- exchanger.

The endolymphatic sac, a potential endocrine gland?

A previous investigation indicated that the chief cells of the endolymphatic sac produce an endogenous inhibitor of sodium re-absorption in the kidneys, which has tentatively been named "saccin". In this study, the ultrastructure of the endolymphatic sac and in particular the chief cells is described, demonstrating that this organ fulfils the morphological criteria of a potential endocrine gland. Accordingly, the chief cells are shown to exhibit all the organelles and characteristics of cells that simultaneously synthesize, secrete, absorb and digest proteins.

Development of the endolymphatic sac and duct in the Japanese red-bellied newt, Cynops pyrrhogaster

The development and maturation of the endolymphatic sac (ES) and duct (ED) were studied in the newt Cynops pyrrhogaster. The ES first appears as an oval capsule at the dorsal-medial tip of the otic vesicle at stage 39, about 11 days after oviposition. The ES consists of polymorphous epithelial cells with a minimum of cytoplasm. The intercellular space (IS) between the epithelial cells is narrow and has a smooth surface. At stage 44, the size of the ES increases as many vacuoles in the IS become filled. At stage 46, 18 days after oviposition, the ES elongates markedly and a slit-like lumen is found in the ES. The epithelium contains a few cell organelles which are scattered in the cytoplasm. The vacuoles in the IS are fused, which expands the IS. Two days later (stage 48), floccular material (endolymph) is present in the expanded lumen. The IS dilates and has a wide and irregular appearance. At stage 50, approximately 26 days after oviposition, the ES extends and expands significantly and crystals (otoconia) can now be seen in the widened lumen of the ES. The cytoplasm of the cuboidal epithelial cells contains an abundance of vesicles surrounded by ribosomes and Golgi complexes. Intercellular digitations are formed in the expanded IS. At stage 54, the ES forms a large bellow-like pouch. Numerous otoconia accumulate in the lumen. Free floating cells and cell debris can be seen in the lumen at this stage. The epithelial cells contain numerous cytoplasmic organelles which are evenly distributed in the cytoplasm. Granules are found in the apical and lateral cytoplasm. The IS is loose and displays a labyrinthine appearance. The primitive ED first appears as a connection between the ES and the saccule but no lumen is present inside at stage 39. At stage 46, a narrow lumen is formed in the ED, which corresponds to the formation of the ES lumen. At stage 50, as the ED extends, floccular material is seen in the lumen. At stage 54, the ED bears numerous microvilli on its luminal surface. Otoconia and endolymph are present in the ED. Tight junctions between the epithelial cells are formed at stage 46. A fully developed intercellular junctional complex is produced at stage 54. Based on the development of the ES and ED, the maturation of function of the ES and ED are discussed.

The endolymphatic duct and sac of the rat: a histological, ultrastructural, and immunocytochemical investigation

A study of the ultrastructure, vascularization, and innervation of the endolymphatic duct and sac of the rat has been performed by means of light- and electron-microscopic and immunocytochemical methods. Two different types of epithelial cells have been identified: the ribosome-rich cell and the mitochondria-rich cell. These two cell types make up the epithelium of the complete endolymphatic duct and sac, although differences in their quantitative distribution exist. The morphology of the ribosome-rich cells varies between the different parts of the endolymphatic duct and sac; the morphology of the mitochondria-rich cells remains constant. According to the epithelial composition, vascularization, and structural organization of the lamina propria, both duct and sac are subdivided into three different parts. A graphic reconstruction of the vascular network supplying the endolymphatic duct and sac shows that the vascular pattern varies among the different parts. In addition, the capillaries of the duct are of the continuous types, whereas those supplying the sac are of the fenestrated type. Nerve fibers do not occur within the epithelium of the endolymphatic duct and sac. A few nerve fibers regularly occur in the subepithelial compartment close to the blood vessels; these fibers have been demonstrated in whole-mount preparations by the application of the neuronal marker protein gene product 9.5. Single beaded fibers immunoreactive to substance P and calcitonin-gene related peptide are observed within the same compartment. Dopamine-beta-hydroxylase-immunoreactive axons are restricted to the walls of arterioles. Morphological differences between the different portions of the endolymphatic duct and sac are discussed with regard to possible roles in fluid absorption and immunocompetence.

Murine endolymphatic sac development in tissue culture: an in vitro model for sac function

Numerous studies have attempted to elucidate the function of the mammalian endolymphatic sac (ELS). All of these studies have been performed on in vivo specimens and are thus influenced by humoral and tissue factors extraneous to the sac. In contrast, an in vitro model would provide an opportunity to study ELS cells in a carefully controlled environment. This report presents our experience with tissue culturing the murine endolymphatic sac removed from 16 and 18 gestational day fetuses. Light (LM) and transmission electron microscopical (TEM) evaluations of the developing endolymphatic sac were performed over periods of one, four, and seven days in tissue culture. In order to confirm growth and maturation, three-dimensional reconstructions from serial sections of the cultured ELS were made and compared with published accounts of in vivo murine ELS development for equivalent periods of time. Both whole and dissected otocysts were grown in tissue culture and compared with one another. Two different tissue culture medias were investigated, each with and without the addition of collagenase, used to soften the dense fibrous capsule of the otocyst and thus facilitate dissection and histological preparation. The impact of collagenase and the tissue culture medias on endolymphatic sac growth were studied. Results demonstrated that murine ELS cells were able to differentiate and mature in tissue culture, as confirmed by LM, TEM, and three-dimensional reconstructions. After an initial delay, in vitro maturation of cells in tissue culture paralleled normal in vivo growth and in some specimens appeared to show accelerated maturation. This in vitro model should prove useful in efforts to define ELS function and in providing a technique for tissue culturing human ELS from normal and diseased ears.

Scanning electron microscopy and immunoglobulins of the endolymphatic sac in normal human subjects and sensorineural deafness. With special reference to Menière's disease

The pathophysiology of the endolymphatic sac (ES) in Meniere's disease was studied by scanning electron microscopy and staining of immunoglobulins in the intradural portion of the ES. The peroxidase-antiperoxidase method by means of paraffin sections was used for staining of immunoglobulins. First, subjects without hearing impairment and malformation of the temporal bone, ranging from a 7-month-old fetus to an 80-year-old adult, were investigated. All subjects, including fetuses, showed well-arranged epithelial cells by scanning electron microscopy. The epithelial cells in the proximal portion of the intradural ES were oval, showing a tendency of transitional change to be flat as they drew near the distal portion of the ES. The epithelial cells consisted mostly of light cells, but sporadic dark cells were seen. Regarding the immunoglobulins. IgG was slightly positive in the epithelial and subepithelial layers. All 15 patients with Meniere's disease showed various types of degeneration of the epithelial cells though to varying degrees. However, these findings were also seen in cases of cochlear deafness. On the other hand, the ES of acoustic tumors, with retrocochlear or neural deafness revealed a normal finding, as found in healthy subjects. Inner ear deafness experimentally produced in animals by Kanamycin sulfate (KM) injection showed degeneration of the epithelial cells of the ES similar to that found in human cochlear deafness. IgG of the ES in Meniere's disease showed moderately evident deposits compared to normal subjects. However, this was also found not only in inner ear deafness other than Meniere's disease, but also in animal deafness experimentally produced by KM injection. It is very interesting to note that moderate endolymphatic hydrops was found in animals one year after Preyer's reflex had disappeared. It is postulated that endolymphatic hydrops develop because of impairment of endolymphatic fluid resorption at the rugose portion and stenosis of the lumen in the same portion, due to degeneration of the epithelial cells. From the above results, it is argued that degenerated epithelial cells and immunoglobulins of the ES in Meniere's disease may arise from the sequelae of cochlear deafness. It is also hypothesized that endolymphatic hydrops--at least in the terminal stage of Meniere's disease--may be consistent with the same pathophysiological conditions as in animal experiments.