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

The cellular development of the endolymphatic sac was studied in the CBA/CBA mouse, starting from day 10 of gestation following the different stages of maturation up to an adult age of one month. The first immature cylindrical cells lining the future sac in several cell layers are seen at day 12 of gestation. At day 18 of gestation, a true sac appears and a floccular precipitate is frequently found in its lumen together with signs of increased activity in the still immature epithelial cells. Approximately one day before birth the first signs of the future light and dark cells can be distinguished. At day 4 post partum the cells are more differentiated with some showing signs of secretory activity indicating that these cells start to function at this stage. Eight days after birth differentiation into distinguishable almost mature light and dark cells is seen. Two days later these epithelial cells have obtained a fully mature appearance. At 14 days after birth widened lateral intercellular spaces separating the epithelial cells can be visualized and a few free floating cells are found in the sac lumen. The sac epithelium is thus considered to have completed its maturation process at this stage.