Developmental aspects of the rat endolymphatic sac and functional implications

The purpose of this study was to determine specific characteristics of endolymphatic sac (ES) cells of the developing rat that are considered to be involved in endolymph homeostasis. Because intermediate filament proteins (IFPs) are regarded as markers of cell differentiation and basal lamina proteins (BLPs) are essential in cell<=>matrix interactions, we determined the presence of IFPs [cytokeratins (CKs) and vimentin] and BLPs [collagen IV, heparan sulphate proteoglycan (HSPG) and laminin] at different developmental stages before and after birth. In addition, we studied the expression of two enzymes of oxidative metabolism: cytochrome oxidase and succinate dehydrogenase. The presence of CKs 8, 18 and 19 in all epithelial cells of the ES during the embryonic stage is characteristic of simple (glandular) epithelial cells. Interestingly, a distinct population of these cells shows additional expression of CK 7, which is a feature of secretory cells. These CK 7-positive cells also contain a high concentration of oxidative enzymes and are rich in mitochondria, indicating that they are light cells. It is suggested that light cells possess specific energy-requiring transport capabilities. Loss of CK 19 expression in the distal part and in a large region of the intermediate part of the ES implies that these cells do not differentiate any further and acquire the capacity to proliferate. Furthermore, prominent co-expression of vimentin with the CKs in the distal part of the ES may confer viscoelastic properties on this epithelium. This may facilitate expansion and thus enable cushioning of pressure fluctuations. Finally, the early prominent occurrence of HSPG in the basal lamina of the ES enables transport of ions. In this light our recent observations of early functioning NaK-ATPases in certain ES cells are interesting.