An electron microscope study on the oocyte, test cells and follicular envelope of the tunicate, Molgula manhattensis.

Both inner follicle cells and test cells in the tunicate ovary appear to engage in active synthesis and secretion during growth of the oocyte. Both cell types contain prominent Golgi systems, numerous vesicles and cisternae of endoplasmic reticulum, and typical mitochondria. Two types of special inclusions develop in the inner follicle cells. The first is a homogeneous inclusion of low density which persists until the end of oogenesis. The second is a heterogeneous inclusion containing dense spheres in a less dense matrix. The Golgi material may synthesize the dense bodies but the matrix, at least in part, is probably contributed by endoplasmic reticulum. The heterogeneous inclusions degenerate and large vacuoles appear in the inner follicle cell late in oogenesis. Alignment of vesicles along the plasma membrane of follicle cells and formation of caveolae from the surface of oocytes suggest that pinocytosis may contribute to the transport of nutrients from follicle cells to oocyte. A distinctive feature in test cells is the differentiation of the nucleus into inner-dense and outer-light zones. The outer zone contains dense spots which give it a porous appearance. Positive reaction with the periodic acid-Schiff technique and metachromasia with toluidine blue suggest that the outer nuclear zone of the test cells may contain polysaccharide. The test cells appear to degenerate late in the development of the oocyte. The outer layer of follicle cells remains thin throughout oogenesis and does not appear active in synthesis and secretion. Changes occurring in the cytoplasm of the oocyte during oogenesis are also discussed.