A fine structural and cytochemical study of the rhesus monkey yolk sac: endoderm and mesothelium

This study has examined the fine structure and some cytochemical characteristics of the endodermal and mesothelial cells of the rhesus monkey yolk sac between 25 and 66 days of gestation. The endodermal cells were characterized by abundant granular endoplasmic reticulum, some agranular endoplasmic reticulum, a well-developed Golgi apparatus, and numerous large mitochondria. During the earlier part of the period studied, endodermal cells had a few acid phosphatase and arylsulfatase-positive lysosomes and moderate numbers of catalase-positive microperoxisomes. During the later stages of development, large granules (believed to be lysosomes) with a heterogeneous content were numerous in the cytoplasm. Mesothelial cells showed fewer development changes. Throughout this period they were usually flattened cells with long microvilli, small mitochondria, and limited amounts of granular endoplasmic reticulum. The mesothelial cells had acid phosphatase reaction product in the Golgi region and occasional large vesicles, but were negative for arylsulfatase and catalase. One specimen was incubated at 37 degrees C in the presence of horseradish peroxidase in order to examine endocytosis. Both the mesothelial cells and endodermal cells internalized the peroxidase into a variety of cytoplasmic vesicles. Based on their cytology, the endodermal cells may function in the synthesis of serum proteins during this period, as has been suggested in other species. They may also be involved in lipid metabolism. The mesothelial cells appeared less synthetically active, but evidence suggested that they may be involved in collagen and extracellular matrix production. The endocytic activity displayed by both cell types may indicate a role in fluid and metabolite transfer across the epithelia. The cytology of both cell types was very similar to that described for human yolk sacs, suggesting that the rhesus monkey may be a useful species in which to study the maturation of yolk sac function.