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Oogonial stem cells (OSCs), also known as egg precursor cells or female germline cells, are diploid germline cells with stem cell characteristics - the ability to self-renew and differentiate into other cell types of their tissue of origin.
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OSCs are known to exist in invertebrate and some lower vertebrate species and have been well studied in C. elegans, Drosophila melanogaster and Xenopus laevis. These OSCs allow for the production of new female reproductive cells (oocytes) through the process of oogenesis, during the organism's reproductive life-cycle.
The possible existence of mammalian oogonial stem cells has been a controversial topic.
C. Elegans
Caenorhabditis elegans (C. elegans) are invertebrates that can have either hermaphroditic or male reproductive capabilities. In males only spermatogenesis occurs, whilst hermaphrodites have the ability to produce spermatozoa until adulthood, after which oogenesis takes over.
All oogonial stem cells in C. elegans are derived from one Distal Tip Cell (DTC) which acts as niche to ensure that germline proliferation continues. As the DTC undergoes mitosis, the cells move proximally along the organism, passing from the "mitotic/proliferative region" into the meiotic cycle. During the meiotic cycle the cells complete meiotic prophase before passing into the zone of oogenesis/spermatogenesis depending on the sex and age of the organism (Fig 1).
D. Melanogaster
Drosophila melanogaster (D. melanogaster), which is commonly known as the fruit fly, is a deoecious organism of invertebrates. In female D. melanogaster, there are 2 ovaries and each of that is made of 16 ovarioles (Fig 2). The movement of the development from oogonial stem cells to mature oocyte is presented in a linear pattern which is similar to that of C. elegans. In D. melanogaster, the 14-stage development of oocyte starts from the anterior of the ovariole to the posterior. The mature oocytes then temporarily stored in the uterus after passing through the oviduct to wait for the egg deposition.
Vertebrates
Bar findings in two species of loris and three species of bat, mammalian OSCs have proved elusive and controversial.
Mice have been produced from putative OSCs in two studies, but questions still persist over whether in vitro culture techniques simply impart the regenesis capability or whether this is in fact something truly innate. There are also questions over cell-sorting techniques used to isolate the OSCs. Hence, some researchers prefer the less conclusive term 'female germline stem cells' over 'OSCs'.