The ultrastructure of the layers enveloping yolk-forming oocytes from Triturus viridescens

The peripheral membranous and extracellular layers of oocytes at the onset of yolk formation were studied by electron microscopy. It was shown that three cellular layers are present at this time. The outer or surface epithelium contains typical squamous cells. The middle or theca is the connective tissue layer which contains fibroblasts, blood vessels, and collagen fibers. The inner or follicular epithelium proper consists of compactly arrayed follicle cells that have distinct cell boundaries. Two extracellular layers were observed, a coarse granular homogeneous layer and a dense zona radiata. Macrovilli (0.2 μ in diameter), extensions from the follicle cells, project through the extracellular layers into the peripheral cytoplasm while more numerous microvilli (0.1 μ in diameter) project up to the dense matrix of the zona radiata. The plasmolemma separating the peripheral cytoplasm from the follicle cells is completely irregular; it forms microvilli. The relations of the enveloping layers as seen with both light and electron microscopes are discussed.