Microautophagy in nutritive phagocytes of sea urchins.

Two types of cells were observed in germinative epithelium of male and female sea urchins: germ cells and somatic accessory cells; the latter referred to as nutritive phagocytes. At the onset of gametogenesis, nutritive phagocytes accumulate nutrients and greatly increase in their size. As gametogenesis progresses, the accumulated nutrients are transferred from nutritive phagocytes into developing gametes, and size of the nutritive phagocytes decreases. An electron microscopic study of nutritive phagocytes in sea urchins, Strongylocentrotus intermedius, at different stages of annual reproductive cycle showed for the first time that both macro- and microautophagy take place in nutritive phagocytes. Both processes occur simultaneously and regulate size and composition of nutritive phagocytes in male and female sea urchins. Nutritive phagocytes consume redundant cytoplasm via macroautophagy. Microautophagy is probably involved in consumption of redundant membranes that appear within nutritive phagocytes due to destruction of nutrient-storing globules, macroautophagy, and phagocytosis of germ cells or their remnants.