Ultrastructural changes in mouse yolk sac associated with the initiation of vitelline circulation.

Abstract

AbstractThe endodermal cells of the visceral yolk sac have been studied from the seventh to eleventh day of gestation. The absorptive capacity of the cells is established by the seventh day as indicated by the presence of microvilli, coated invaginations and vacuoles, the apical canalicular system and abundance of absorptive droplets and vacuoles in these cells. Changes in cellular structure during the next three to four days include the development of the cisternal system of the rough endoplasmic reticulum, an increase in mitochondrial number and their localization near absorptive droplets and the formation of multiple Golgi complexes. Intercellular spaces form between the endodermal cells by eight to eight and one‐half days and coincide with the disappearance of the basal lamina separating the endoderm and mesoderm (angioblastic cords); these changes correlate with the formation of the vitelline vessels. As the vitelline circulation becomes functional, the visceral basal lamina is re‐established, the intercellular clefts decrease in prominence and the absorptive storage droplets and vacuoles decrease in size and number.The apical junctional complex of the endodermal cells forms a continuous barrier for the direct passage of material from the yolk sac cavity to the developing vitelline vessels as established by the use of ruthenium red. The absorption and intracellular storage of macromolecules in the visceral endodermal cells was traced through the intermicrovillous apical coated invaginations, coated vesicles, apical canaliculi and storage vacuoles using ferritin.Immunofluorescent studies indicate the presence of immunoglobulin (anti‐mouse gamma globulin) in the cytoplasm of the visceral endodermal cells as early as 9 to 11 days. Several blast‐like cells which also were observed in the vitelline vessels at 11 days exhibited positive fluorescence for immunoglobulin.