Oogenesis in Hydra carnea: A new model based on light and electron microscopic analyses of oocyte and nurse cell differentiation

Oogenesis in Hydra carnea starts with an accumulation of a great number of I-cells in the interstitial spaces of the ectoderm of the body column. One centrally located I-cell becomes the future oocyte, the others differentiate into nurse cells. Presumptive oocyte and nurse cells are not easily distinguishable at that time. The earliest stage of an oocyte we could identify on ultrastructural criteria was in prophase of its first meiotic division. Only at this stage autosynthesis of nutritive substances predominates, the following rapid increase of the oocyte volume relies on the successive adoption of cytoplasmic fragments from nurse cells. Extending fingerlike processes between the epitheliomuscular cells, the oocyte then starts to phagocytose apoptotic nurse cells. Nurse cell differentiation is indicated by the appearance of lipid vesicles in I-cells. As differentiation proceeds glycogen, rEr and Golgi complexes appear and the cells increase due to a continuous production and accumulation of lipid, glycogen and yolk-like electron dense material. Then the loss of cytoplasmic fragments and degenerative changes typical of apoptosis, a morphologically defined form of cell death, converts the nurse cells into apoptotic bodies. The bulk of nurse cells becomes phagocytosed by the oocyte at late stages of their transformation into apoptotic bodies. At the end of oogenesis which in Hydra carnea takes about 4 days, the egg consists for the largest part of apoptotic nurse cells which persist in the developing embryo until hatching.