Stem cells are defined by their ability to self-renew and differentiate into different cell types. Adult stem cells have a more restricted lineage than embryonic stem cells (ESCs) but are able to proliferate for protracted periods of time and their differentiated progeny are essential for the ongoing maintenance and repair of tissue over an animal’s lifespan. This includes sustaining the circulating blood cell populations and allowing turnover of intestinal luminal cells. Germline stem cells (GSCs) are a unique type of unipotent adult stem cell as they differentiate into the gametes that are vital for the continuation of a species.
The concept of a GSC was first proposed in 1901 during studies on spermatogenesis (Regaud, 1901a, Regaud, 1901b). The theory was that the post-pubertal testis must contain a population of cells that could both self-renew and differentiate into mature sperm. Decades of subsequent study have validated this hypothesis and male GSCs, known as spermatogonial stem cells (SSCs), have been recognised to participate in spermatogenesis in all species in which the process has been studied (Brinster, 2007). Research into the equivalent cell type in females has produced more conflicting data, with female Germ line Stem Cells (fGSCs) having a confirmed role in post-natal neo-oogenesis in some species).
Equally as important as the GSCs themselves is their surrounding microenvironment, known as the germ cell niche. These specialised somatic areas within the gonad are responsible for controlling the self-renewal and differentiation of GSCs, thus maintaining the population (Spradling et al., 2011). Furthermore, it is thought that it is the presence of germ cell niches that differentiates those species that undergo post-natal neo-oogenesis from those that cannot (Spradling et al., 2011). The exact mechanisms by which the niche “holds” GSCs in an undifferentiated state have not been elucidated, but bi-directional paracrine and juxtacrine signaling between the somatic cells and GSCs likely plays a critical role. In mammals, communication between daughter germ cells is also possible due to the cytoplasmic bridges formed between the cells during incomplete mitotic cytokinesis of the founder cell. These bridges are large, measuring 0.5 - 3µm, and can allow both molecules and organelles as large as mitochondria to transfer between cells (Greenbaum et al., 2011). Such “cytoplasmic sharing” is hypothesised to regulate the synchronisation of mitosis and control entry into meiosis (Greenbaum et al., 2011). The germ cell niche has been investigated in detail in both Drosophila and the nematode, Caenorhabditis elegans (C. elegans). 
