Maturation of junctional complexes during embryonic and early postnatal development of inner ear secretory epithelia

The development and maturation of tight junctions in secretory epithelia of the inner ear, i.e., the stria vascularis in the cochlea and the dark cell epithelium around the vestibular organs, have been analyzed with the freeze-fracture technique. Inner ears were followed from the otocyst stage to the mature stage (mouse; gestational age 20 to 21 days). Before epithelial cells differentiate into specific tissues, tight junctions have a loose network of sealing elements with zero to four sealing strands. During and after cytodifferentiation, the numbers of strands increase both in the marginal cells of the stria vascularis and in the dark cells around the vestibular organs. Large intra- and intercellular variations in the numbers of sealing strands of the tight junction occur prior to birth in both types of secretory epithelia. In the dark cells the mature structure of the tight junction with five to eight sealing elements is reached at birth. Maturation of marginal cell junctions occurs during the first few days postnatally. Thus, the tight junctions are morphologically mature before the development of the high potassium concentration in the endolymph.