Expression of KIT in the ovary, and the role of somatic precursor cells.

KIT is a type III receptor protein tyrosine kinase, and KITL its cognate ligand. KIT can mediate its effects via several intracellular signalling pathways, or by formation of a cell-cell anchor with its ligand. Through these mechanisms, KIT controls fundamental cellular processes, including migration, proliferation, differentiation and survival. These cellular processes are modulated by soluble KIT, a cleavage product of KIT, generated at the cell membrane. A cell-retained KIT cleavage fragment also arises from this cleavage event. This cleavage fragment must be distinguished from truncated KIT (trKIT), which originates through cryptic promoter usage. The expression of trKIT is highly restricted to postmeiotic germ cells in the testis. In contrast, KIT, together with its cleavage products, is present in somatic cells and germ cells in the gonads of both sexes. A functional KITL/KIT system is mandatory for normal population of the gonads by germ cells. Signalling via the KITL/KIT system promotes the growth, maturation, and survival of germ cells within the gonads, and prevents meiotic entry and progression. In addition to its importance in germ cell biology, the KITL/KIT system is crucial for gonadal stromal differentiation. During foetal life, KIT is expressed by testicular stromal precursor cells, which develop into Leydig cells. In the ovary, stromal cell KIT expression accompanies theca layer development around advanced follicles. After ovulation, KIT-immunopositive cells translocate from the theca layer to the luteal ganulosa where they contribute to a delicate cellular network that extends between the fully luteinised large luteal cells. In the outer regions of the developing corpus luteum, a highly conspicuous subpopulation of KIT/CD14-double-immunopositive cells can be observed. KIT/CD14-double-immunopositive cells are also seen in the haematopoietic-like colonies of long-term granulosa cultures established from late antral follicles. These cultures demonstrate expression of pluripotency marker genes such as octamer binding transcription factor-3/4 and sex determining region Y-box 2. The KIT/CD14-double-immunopositive cells can be purified and enriched by KIT-immunopositive magnetic cell sorting. Subsequent exposure of the KIT-expressing cells to the hanging drop culture method, combined with haematopoietic differentiation medium, provides the signals necessary for their differentiation into endothelial and steroidogenic cells. This suggests that monocyte-derived multipotent cells are involved in ovarian tissue remodelling. In summary, multicelluar KITL/KIT signalling organizes the stroma in the ovary and testis; monocyte-derived multipotent cells may be involved.