Investigation of oogenesis in hydra. Communication I. Ultrastructure of interstitial cells at early stages of their transformation into oocytes.

We studied ultrastructure of the interstitial cells (i-cells) at successive stages of their transformation into oocytes. The i-cells that form the epidermal thickening in the genital zone of the animal start to proliferate in hydras approaching sexual reproduction. Undifferentiated i-cells are characterized by a high density of diffusely disposed ribosomes and almost total absence of membranous structures. Femal sex cells starting to undergo differentiation are characterized by the appearance of aggregations of finely filamentous material in the cytoplasm. This material is of nuclear origin and is often surrounded by mitochondria. Dimensions of the i-cells increase rapidly, and single elongated cisterns of the granular endoplasmic reticulum appear in them. These cisterns run parallel to the cell surface and give rise to minute smooth-surfaced vesicles, whose membranes fuse wtih the cytoplasmic membrane in such a way that their contents are emptied into the intercellular space. Different sectors of the cell are marked by the appearance of grouped cisterns of the granular endoplasmic reticulum. Such cisterns give rise to smooth-surfaced vesicles, which form numerous Golgi complexes. The number of mitochondria increases simultaneously; aggregations of lipid drops with glycogen granules among them start to appear; and escape of nuclear material into the cytplasm intensifies. Such cells at the stage of cytoplasmic growth and accumulation of organelles and yolk inclusions. In connection with this, it is suggested that the term "oogonium" (which is used to designate such cells in the literature) be retained only for proliferating i-cells containing finely filamentous nuclear material in the cytoplasm in the genital zone of the hydra.