Corpus luteum (CL) function: local control mechanisms

LH and PGF(2alpha) are the principal luteotrophic and luteolytic hormones in domestic animals, however, it is becoming increasingly apparent that intra-ovarian factors can modulate luteal function. For example, the insulin-like growth factors (IGF-I and -II) can regulate ovarian function, and have direct effects on ovarian cells. An important role for the IGFs in regulating ovarian function is suggested by the multiple effects of IGFs on both follicular and luteal steroidogenesis. Expression of mRNA encoding IGF-I, IGF-II and the type 1 IGF receptor has also been detected in the ruminant CL and is suggestive of autocrine/paracrine roles for both IGF-I and -II in the regulation of luteal function. The actions of the IGFs are further modulated by their association with specific binding proteins (IGFBPs), which regulate the transport of IGFs and their presentation to specific receptors. IGFBPs have been detected in the CL of domestic animals, and inhibitory effects on IGF-I-stimulated progesterone production have been demonstrated. The rapid cyclical changes in luteal growth and regression are associated with rapid changes in vasculature. The principle angiogenic factors include the fibroblast growth factors (FGFs), vascular endothelial growth factor (VEGF) and the angiopoietins (Ang). Other locally produced factors include cytokines such as TNF-alpha and IL-1beta. One such factor is monocyte chemoattractant protein (MCP-1), which increases after exogenous PGF(2alpha). An influx of macrophages takes place in the CL around luteolysis, possibly in response to MCP-1 release, but these changes are not observed in cattle when luteolysis is inhibited. In conclusion locally produced factors are important in the control of luteal function, although their roles have yet to fully elucidated.