The development of the endolymphatic duct and sac. A light microscopical study

CHD7 variants associated with hearing loss and enlargement of the vestibular aqueduct

Enlargement of the endolymphatic sac, duct, and vestibular aqueduct (EVA) is the most common inner ear malformation identified in patients with sensorineural hearing loss. EVA is associated with pathogenic variants in SLC26A4. However, in European-Caucasian populations, about 50% of patients with EVA carry no pathogenic alleles of SLC26A4. We tested for the presence of variants in CHD7, a gene known to be associated with CHARGE syndrome, Kallmann syndrome, and hypogonadotropic hypogonadism, in a cohort of 34 families with EVA subjects without pathogenic alleles of SLC26A4. In two families, NM_017780.4: c.3553A > G [p.(Met1185Val)] and c.5390G > C [p.(Gly1797Ala)] were detected as monoallelic CHD7 variants in patients with EVA. At least one subject from each family had additional signs or potential signs of CHARGE syndrome but did not meet diagnostic criteria for CHARGE. In silico modeling of these two missense substitutions predicted detrimental effects upon CHD7 protein structure. Consistent with a role of CHD7 in this tissue, Chd7 transcript and protein were detected in all epithelial cells of the endolymphatic duct and sac of the developing mouse inner ear. These results suggest that some CHD7 variants can cause nonsyndromic hearing loss and EVA. CHD7 should be included in DNA sequence analyses to detect pathogenic variants in EVA patients. Chd7 expression and mutant phenotype data in mice suggest that CHD7 contributes to the formation or function of the endolymphatic sac and duct.

Programmed cell senescence during mammalian embryonic development

Cellular senescence disables proliferation in damaged cells, and it is relevant for cancer and aging. Here, we show that senescence occurs during mammalian embryonic development at multiple locations, including the mesonephros and the endolymphatic sac of the inner ear, which we have analyzed in detail. Mechanistically, senescence in both structures is strictly dependent on p21, but independent of DNA damage, p53, or other cell-cycle inhibitors, and it is regulated by the TGF-β/SMAD and PI3K/FOXO pathways. Developmentally programmed senescence is followed by macrophage infiltration, clearance of senescent cells, and tissue remodeling. Loss of senescence due to the absence of p21 is partially compensated by apoptosis but still results in detectable developmental abnormalities. Importantly, the mesonephros and endolymphatic sac of human embryos also show evidence of senescence. We conclude that the role of developmentally programmed senescence is to promote tissue remodeling and propose that this is the evolutionary origin of damage-induced senescence.

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.

Morphology of the developing human endolymphatic sac

The adult human endolymphatic sac (ES) has been described as a complex network of interconnected tubules. Embryologic examination describes the human ES as a single-lumen, pouch-like structure. Transition from saccular shape to tubules during the entire fetal period has not been previously reported. Tubular ES structure is thought to be unique to humans. Animal investigations describe similar saccular appearance, but without tubules in mature sacs. The authors examined 45 human fetal temporal bones to trace ES development and reviewed six types of animal sacs. Results in humans reveal tubular structure as early as 26 weeks' gestation. Maturation variably occurred in the fetal period and postnatally. For the first time, the tubular system is noted in the animal, the rhesus monkey. These findings suggest that the tubular system may represent more advanced specialized function.

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.

Three-dimensional ultrastructure of the endolymphatic sac

The subcellular structures of the epithelial cells of the guinea pig endolymphatic sac were studied. By using a newly developed scanning electron microscopy technique, the intracellular organelles could be studied three-dimensionally and the topographic relationships analyzed. The light epithelial cell has an extensive network of endoplasmic reticulum which is characteristically arranged in a baso-apical direction. The connections between the inner surface of the plasmalemma and the endoplasmic reticulum were observed, as were connections between the Golgi complex and the endoplasmic reticulum. Our findings support the hypothesis that the endoplasmic reticulum might form transcellular channels through which the cell can transport water and ions from the lumen of the endolymphatic sac out into the subepithelial tissue. The dark epithelial cells seen in particular contained the smooth type of endoplasmic reticulum. Lysosomes were also observed in the dark cells, indicating that these cells probably have more of a secretory function.