| Abstract: | Specimens of murine yolk sac prepared for light and electron microscopy at 8, 12, and 18 days of gestational age revealed a changing morphology of the endodermal cells in the visceral layer as gestation progressed. Basophilia increased between 8 and 12 days and decreased slightly between 12 and 18 days. These changes corresponded to the development of a highly organized rough-surfaced endoplasmic reticulum by 12 days and its partial replacement by smooth-surfaced elements by 18 days. Staining of frozen sections with tetrazolium dyes revealed a substrate dependent localization of the staining reaction within the cells and suggested an underlying compartmentalization of metabolic activities. Two types of apical granules were observed on the basis of both cytochemistry and electron microscopy. These granules were most numerous at 12 days of gestational age. The observations suggest that although synthesis and absorption doubtless occur simultaneously throughout gestation, the emphasis is on synthesis during the first half of the period and absorption during the second half. It is suggested that the change in fine structure may be due to the rupture of Reichert's membrane and the subsequent establishment of direct contact between the endodermal cells and the uterine wall. |
