Angiogenesis and vascular function in the ovary.

Ovarian function is dependent on the establishment and continual remodelling of a complex vascular system. This enables the follicle and/or corpus luteum (CL) to receive the required supply of nutrients, oxygen and hormonal support as well as facilitating the release of steroids. Moreover, the inhibition of angiogenesis results in the attenuation of follicular growth, disruption of ovulation and drastic effects on the development and function of the CL. It appears that the production and action of vascular endothelial growth factor A (VEGFA) is necessary at all these stages of development. However, the expression of fibroblast growth factor 2 (FGF2) in the cow is more dynamic than that of VEGFA with a dramatic upregulation during the follicular-luteal transition. This upregulation is then likely to initiate intense angiogenesis in the presence of high VEGFA levels. Recently, we have developed a novel ovarian physiological angiogenesis culture system in which highly organised and intricate endothelial cell networks are formed. This system will enable us to elucidate the complex inter-play between FGF2 and VEGFA as well as other angiogenic factors in the regulation of luteal angiogenesis. Furthermore, recent evidence indicates that pericytes might play an active role in driving angiogenesis and highlights the importance of pericyte-endothelial interactions in this process. Finally, the targeted promotion of angiogenesis may lead to the development of novel strategies to alleviate luteal inadequacy and infertility.

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.

Insulin-like growth factor (IGF) system in the oocyte and somatic cells of bovine preantral follicles.

Many studies have highlighted the role of the insulin-like growth factor (IGF) system in the control of antral follicular growth. However, much less is known about the involvement of the IGF system in the regulation of preantral follicular development. In an attempt to address this lack of knowledge, the present study describes the spatial and temporal patterns of expression of mRNA encoding components of the IGF system in bovine follicles during preantral stages of development. mRNA was detected by in situ hybridization using frozen sections (14 microm) of bovine ovarian tissue. Serial sections were probed with 35S-labelled bovine riboprobes. Type 1 IGF receptor mRNA was detected in granulosa cells and in the oocyte of preantral follicles; however, in this study, as in previous studies, it was not possible to detect mRNA encoding either IGF-I or -II. IGF binding protein (IGFBP)-2 mRNA was present in granulosa cells and oocytes of preantral follicles, and immunoreactive IGFBP-2 was detected around granulosa cells during this early stage of development. Occasionally, preantral follicles were identified in which there was no expression of IGFBP-2 in granulosa cells or the oocyte. IGFBP-3 mRNA was detected in the oocyte of preantral follicles and in the surrounding stromal tissue. mRNAs encoding IGFBP-2 and -3, and type 1 IGF receptor were first detected in type 2 follicles. In conclusion, although the IGF ligands are not expressed in preantral follicles, mRNAs encoding the type 1 IGF receptor, and IGFBP-2 and -3 were present and showed unique spatial patterns of expression within preantral follicles.

Effect of dietary energy and protein on bovine follicular dynamics and embryo production in vitro: associations with the ovarian insulin-like growth factor system.

Heifers were assigned either low or high (HE) levels of energy intake and low or high concentrations of dietary crude protein. The effect of these diets on the plasma concentrations of insulin, insulin-like growth factor (IGF)-I, and urea on follicular growth and early embryo development is described. We propose that the observed dietary-induced changes in the ovarian IGF system increase bioavailability of intrafollicular IGF, thus increasing the sensitivity of follicles to FSH. These changes, in combination with increased peripheral concentrations of insulin and IGF-I in heifers offered the HE diet, contribute to the observed increase in growth rate of the dominant follicle. In contrast to follicular growth, increased nutrient supply decreased oocyte quality, due in part to increased plasma urea concentrations. Clearly a number of mechanisms are involved in mediating the effects of dietary energy and protein on ovarian function, and the formulation of diets designed to optimize cattle fertility must consider the divergent effects of nutrient supply on follicular growth and oocyte quality.

Expression of mRNA encoding insulin-like growth factors I and II and the type 1 IGF receptor in the bovine corpus luteum at defined stages of the oestrous cycle.

Previous studies have implicated insulin-like growth factors I and II (IGF-I and -II), in the regulation of ovarian function. The present study investigated the localization of mRNA encoding IGF-I and -II and the type 1 IGF receptor using in situ hybridization to determine further the roles of the IGFs within the bovine corpus luteum at precise stages of the oestrous cycle. Luteal expression of mRNA encoding IGF-I and -II and the type 1 IGF receptor was detected throughout the oestrous cycle. The expression of IGF-I mRNAvaried significantly during the oestrous cycle. IGF-I mRNA concentrations were significantly higher on day 15 than on day 10, and IGF-I mRNA in the regressing corpus luteum at 48 h after administration of exogenous prostaglandin was significantly greater than in the early or mid-luteal phase (days 5 and 10). In contrast, there was no significant effect of day of the oestrous cycle on expression of mRNA for IGF-II and the type 1 IGF receptor in the corpus luteum. Expression of IGF-II mRNA was localized to a subset of steroidogenic luteal cells and was also associated with cells of the luteal vasculature. mRNA encoding the type 1 IGF receptor was widely expressed in a pattern indicative of expression in large and small luteal cells. These data demonstrate that the bovine corpus luteum is a site of IGF production and reception throughout the luteal phase. Furthermore, this study highlights the potential of IGF-II in addition to IGF-I in the autocrine and paracrine regulation of luteal function.

Expression of mRNA encoding IGF-I, IGF-II and type 1 IGF receptor in bovine ovarian follicles.

IGFs regulate gonadotrophin-stimulated proliferation and differentiation of granulosa and theca cells in vitro. However, the detailed pattern of mRNA expression of IGFs in bovine follicles remains controversial. The objectives of this study were therefore to describe the temporal and spatial pattern of expression of mRNA encoding IGF-I, IGF-II and the type 1 IGF receptor in bovine follicles in vivo. The expression of mRNA encoding IGF-II was detected in theca tissue from around the time of antrum formation up to and during the development of dominance. No IGF-II mRNA expression was detected in granulosa cells. In the majority of follicles we were unable to detect mRNA encoding IGF-I in either granulosa or theca tissue from follicles at any stage of development. Occasionally low amounts of mRNA encoding IGF-I were detected in the theca externa and connective tissue surrounding some follicles. Type 1 IGF receptor mRNA was detected in both granulosa and theca cells of preantral and antral follicles. Expression was greater in granulosa tissue compared with theca tissue. We also measured IGF-I and -II mRNA in total RNA isolated from cultured granulosa and theca cells using reverse transcriptase PCR. In contrast to the in vivo results, IGF-II mRNA was detected in both granulosa and theca tissue. IGF-I mRNA was detected in theca tissue and in very low amounts in granulosa cells. Using a specific IGF-I RIA we were unable to detect IGF-I immunoreactivity in granulosa conditioned cell culture media. Using immunohistochemistry we detected IGF-I immunoreactivity in some blood vessels within the ovarian stroma. We conclude from these results that IGF-II is the principal intrafollicular IGF ligand regulating the growth of bovine antral follicles. In preantral follicles the expression of mRNA encoding type 1 IGF receptor but absence of endogenous IGF-I or -II mRNA expression, highlights a probable endocrine mechanism for the IGF regulation of preantral follicle growth.