Transmission of symbiotic algae through sexual reproduction in hydra: movement of algae into the oocyte.

The histological pathway by which intracellular symbiotic Chlorella move into the developing oocytes of hydra was investigated at the ultrastructural level. Algae move from within the digestive cells of the endoderm to within the oocytes of the ectoderm in a three-step process. First, the algae are released by digestive cells into the mesolamella (basement membrane). Second, the algae move as individual cells into the adjacent intercellular spaces of the ectoderm. Third, they are taken up by the oocyte by phagocytosis. This transfer occurs only in the central regions of the ovary, and only after oocytes have reached an advanced stage. Normally the mesolamella is separated from the ectodermal interstitial spaces by a layer of epitheliomuscular cell muscle processes. This layer degenerates in the region where algae will move into the ectoderm. This study shows that algae move as individual cells and not intracellularly within processes of the epithelial cells.