Apoptosis of germ cells during human prenatal oogenesis

During human oogenesis two contrasting processes can be observed: germ cell proliferation and differentiation, and contemporaneous germ cell death. It is well known that apoptosis is a type of physiological cell death that occurs in proliferating and differentiating tissues. The aim of this study is to demonstrate, through ultrastructural and in-situ 3' end labelling observations in intact sections of human fetal ovaries, that germ cell loss during fetal life is due to a phenomenon of apoptosis. We evaluated the presence of programmed cell death in female germ cells in fetal ovaries at 18-20 weeks of postconceptional age. The alterations that occur during apoptosis were detected by the electron microscope and include cytoplasmic condensation, organelle relocalization and compaction, chromatin condensation, and finally, production of membrane-enclosed particles containing intracellular material, known as apoptotic bodies, that are phagocytosed. The fragmentation of DNA, characteristic of apoptotic cells, was detected by the use of the in-situ 3' end labelling procedure on histological sections of ovaries where only some nuclei of germ cells were positively stained. The parallel use of these two methods on human ovaries of 18-20 weeks postconceptional age has allowed us to show that the numerical decrease of human female germ cells during the fetal period is due to an apoptotic phenomenon.