Ovarian morphology and oogenesis in Aurelia aurita (Scyphozoa: Semaeostomae): ultrastructural evidence of heterosynthetic yolk formation in a primitive metazoan

In Aurelia aurita, the ovaries arise as horseshoe-shaped evaginations of the gastrodermis in the floor of four interradial gastric pouches. Germ-cell islands arise within endodermally-derived gastrodermal cells. Oocytes grow and gradually bulge into the mesoglea while maintaining physical contact throughout vitellogenesis with specialized cells called trophocytes. Ultrastructural changes suggest that these cells transport yolk precursors from the coelenteron to the oocytes in a manner similar to that reported for the trophonema cells of anthozoan ovaries. Vitellogenesis involves both the autosynthetic activity of the oocyte organelles (Golgi complex and rough endoplasmic reticulum) and the heterosynthetic incorporation of precursors through endocytotic processes involving both coated pits and vesicles and smooth-surfaced tubules. Ultrastructural data suggest that different types or classes of yolk precursors enter the oocyte through the trophocytes and via the surrounding mesoglea. To our knowledge, this is the most primitive animal in which this type of yolk synthesis has been described. The trophocyte-oocyte relationship in oocytes of A. aurita is reminiscent of the trophonema-oocyte relationship in anthozoans and supports the belief that the Anthozoa and Scyphozoa share a close phylogenetic relationship.