Intracellular localization of basement membrane precursors in the endodermal cells of the rat parietal yolk sac. I. Ultrastructure and phosphatase activity of endodermal cells.

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Abstract
The parietal layer of the rat yolk sac includes a 5 microliter thick sheet known as Reichert's membrane that exhibits properties of basement membranes. Its inner side is lined by a single layer of loosely distributed cells referred to as endodermal cells. Both Reichert's membrane and endodermal cells were examined at 13-14 days' gestation with emphasis on the ultrastructure of the Golgi apparatus, the identification of its component parts by specific phosphatase activities, and its possible role in the cells' secretory process. Reichert's membrane is composed of a series of stacked layers similar to basal laminae and composed of a network of fibrils with a diameter of 2-8 nm along which dots are located at irregular intervals. The endodermal cells contain the usual organelles, including interconnected rough endoplasmic reticulum (rER) cisternae and a prominent Golgi apparatus. With the help of phosphatase reactions, the stacks of Golgi saccules were divided into a) "phosphatase-free" saccules, the first ones on the cis or forming side, b) one or two "intermediate" saccules in the middle of the stacks, containing nicotinamide adenine dinucleotide phosphatase activity, c) one or two "last" saccules rich in thiamine pyrophosphatase activity on the trans or mature side, and d) continuing beyond the trans side, the GERL element displaying acid phosphatase activity. The latter is associated with profiles equally rich in acid phosphatase and tentatively considered to be prosecretory granules. Finally, the ectoplasm adjacent to Reichert's membrane displays large, acid phosphatase-containing structures tentatively considered to be secretory granules. Thus, the extensive rER network, the well-compartmentalized Golgi apparatus, and the presence of structures which may be prosecretory and secretory granules indicate that the endodermal cells are well-equipped for the secretion of the components of Reichert's membrane.