Ultrastructural features of the trophonema and oogenesis in the starlet sea anemone, Nematostella vectensis (Edwardsiidae)

The starlet sea anemone, Nematostella vectensis S tephenson 1935, is a burrowing, estuarine species that has become a model organism for fundamental studies of cnidarian and metazoan development. During early oogenesis, oocytes appear in the basal region of the gastrodermis in the reproductive mesenteries and gradually bulge into the adjacent connective tissue space (mesoglea) where the majority of oocyte growth and vitellogenesis occurs. However, oocytes do not physically contact the cellular and amorphous matrix of the mesogleal compartment due to a thin, intervening basal lamina. Oocytes retain limited contact with the basal gastrodermal epithelium via groups of ultrastructurally modified gastrodermal cells called trophocytes. Trophocytes are monociliated accessory cells of somatic origin that collectively form a structure called the trophonema, a unique accessory cell/oocyte association not observed outside the Cnidaria. The trophonema consists of 50-60 trophocytes that maintain contact with