Scanning electron microscopy of the developing chick tegmentum vasculosum.

The developing tegmentum vasculosum in the chick cochlea was analyzed by means of scanning electron microscopy. Chick embryos representative of Hamilton and Hamburger stages 35, 37, 41 and 45 were examined in order to study the morphogenesis of the "lamellae" (Knowlton, 1967), or folds, of the tegmentum vasculosum and the differentiation of the light and dark cell luminal surfaces. At stage 35 low folds are distinguishable in the proximal segment of the cochlear duct only. In subsequent stages a complex series of lamellae develop throughout the entire tegmentum vasculosum in conjunction with a developing capillary network. Dark cell luminal surfaces are clearly distinguishable from the adjacent light cell surfaces by stage 37. Stages 41 and 45 show a progressive increase in dark cell surface area as well as a considerable increase in the number and size (height and diameter) of surface microvilli. The avian tegmentum vasculosum is thought to be the functional equivalent of the stria vascularis in mammals. Analysis of the morphogenesis of the tegmentum vasculosum is vital to a thorough understanding of its functional relationships with the maturing endolymphatic fluid ionic composition and endocochlear potential.