Extracellular matrix of the corpus luteum.

The potential importance of the extracellular matrix to luteal formation and development, additional development in response to pregnancy hormones in some species, and luteal function and regression is possibly under-appreciated. Collagens I and III and fibronectin change dynamically during the formation of the corpus luteum and probably reflect the necessity for directional migration of cells in the establishment of a vascularized corpus luteum. Extracellular proteins may also be essential for the maintenance of luteal cell phenotype. Laminins, collagens type IV, and nidogen-1 have been localized to varying degrees of completeness in different species. Each capillary has a subendothelial basal lamina that changes in composition during luteal formation. These subendothelial basal laminas are often adjacent to luteal cells. The high vascularity of corpora lutea may have led to the assumption that luteal cells are surrounded by basal laminas. However, in rat, bovine, and human corpora lutea, there is no evidence of basal laminas surrounding luteal cells. Instead there are fibers or aggregates of basal lamina material rich in laminins interspersed throughout the luteal tissue. Versican appears to be localized to the capsule in human corpora lutea but is widely dispersed in the bovine corpus luteum, similar to the distribution of thecal derived cells, and is not associated with capillaries. Hyaluronan is also present in the luteal parenchyma. Clearly more studies of corpora lutea are required for a fuller understanding of the roles of extracellular matrix in luteal function.