Ultrastructural and electrophysiological maturation of the chick tegmentum vasculosum.

We have examined the ultrastructural and electrophysiological events associated with the embryonic development of the tegmentum vasculosum, the ion-transporting epithelium in the chick cochlear duct. The cytodifferentiation of the light and dark cells in the epithelium from embryonic day 6 through post-hatching day 7 was studied with transmission electron microscopy. The predominant ultrastructural change in the developing tegmentum vasculosum was the elaboration of the complex basolateral infoldings on the dark cells from embryonic day 11 through post-hatching day 7. The relationship between the development of the endocochlear transepithelial electrical potential difference (PD) and the cytodifferentiation of the tegmentum vasculosum was examined with in vitro studies. Microelectrode impalements of the scala media showed that the positive endocochlear PD was first detectable on embryonic day 20 but did not reach a mature value of +16 mV until post-hatching day 7. Maturation of the endocochlear PD paralleled the time during which the dark cells in the tegmentum vasculosum displayed the most extensive increase in basolateral infoldings. This correlation suggests that the development of the endocochlear PD may result from an increase in the number of Na+-K+ pump sites located on the basolateral infoldings of the dark cells.