Extracellular matrix of the developing ovarian follicle.

Ovaries can be considered tissues in which endocrine organs--follicles and corpora lutea--continually grow and regress. Follicles have both epithelial and stromal layers in which cell migration or movement, cell division, specialization and differentiation, and death occur. A fluid-filled antrum develops and at ovulation the epithelial cells undergo an epithelial to mesenchymal transition into luteal cells. Although growth factors and hormones are very important in some of these processes, the extracellular matrix participates in all of them. Importantly, the matrix is diverse in composition and cells rarely behave without reference to the composition and structure of the matrix. When follicles commence growing, the follicular basal lamina changes in its composition from containing all six alpha chains of collagen type IV to only alpha 1 and alpha 2. Perlecan and nidogen 1 subsequently become components of the follicular basal lamina, and there is an increase in the amount of laminin chains alpha 1, beta 2 and gamma 1, at least in cows. Late in follicular development and on atresia some follicles contain laminin alpha 2. On atresia the follicular basal lamina is not degraded as occurs at ovulation, but can be breached by cells from the thecal layer if granulosa cells no longer align it. Other matrix components are present and also change during follicular development. Versican was identified in all the follicular layers and has been found to play a key role together with inter-alpha-trypsin inhibitor, tumour necrosis factor alpha-stimulated gene 6 (TSG-6) and hyaluronan in cumulus oocyte expansion and fertility. Recent studies are directed at investigating the regulation of the matrix and its function in the ovary.