Theca externa

Abstract: 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.

Abstract: To investigate the involvement of extracellular matrix (ECM) in folliculogenesis in the sheep, parallel changes in ECM components and key steroidogenic enzymes were studied by quantitative immunohistochemistry and immunoblotting during follicular growth and atresia. Growth of ovarian follicles from 1 to 5 mm in diameter was characterized by a progressive increase in P450 cholesterol sidechain cleavage levels in both thecal (p < 0.001) and granulosa cells (p < 0.001), an increase in P450 aromatase levels in granulosa cells of follicles larger than 3.5 mm (p < 0.001), and an increase in levels of P450 17 alpha-hydroxylase C17,20 lyase (P450(17 alpha)) in the theca interna. In addition, during follicular growth, a change in localization of cells expressing P450(17 alpha) within the theca interna was observed, positive cells being sparse within the theca interna of small follicles and specifically located close to the basal laminae in large follicles. In parallel, follicular growth was associated with an increase in levels of type I collagen in granulosa cell layers (p < 0.01) and an increase in levels of fibronectin (p < 0.05), particularly the specific ED-A alternatively spliced variant of fibronectin, in the theca externa. Follicular atresia was characterized by a loss of P450 aromatase in granulosa cells (p < 0.001) and a decrease in levels of P450(17 alpha) in the theca interna (p < 0.05). Simultaneously, levels of fibronectin (p < 0.05), particularly the ED-A variant of fibronectin, decreased in the theca externa of atretic follicles. Within the wall of granulosa cells, levels of fibronectin (p < 0.05), laminin, type IV collagen, and heparan sulfate proteoglycans strongly increased during follicular atresia. Overall, these results show that follicular growth and atresia were associated with distinct changes in levels of ECM components, suggesting that ECM components may play a role in the regulation of proliferation, differentiation, and apoptosis of follicular cells.

Abstract: This study was undertaken to assess the role of follicular cells in the maintenance of meiotic arrest (germinal vesicle [GV] stage). Bovine cumulus-oocyte complexes (COC) were obtained by puncture of ovaries collected at a slaughterhouse. Different monolayers of follicular cells-granulosa cells, theca interna, theca externa, or both types of theca cells together-were cultured in 24-well plates with 1 ml of TCM-199 supplemented with 10% fetal calf serum. Theca cells were obtained by digesting theca layers with collagenase. The medium was renewed 48 h before coculturing selected COC with confluent follicular cell monolayers. Oocytes were maintained in GV stage when cocultured for 12 h directly on monolayers of theca cells. The percentage of oocytes in GV stage was not significantly different between treatments using different types of theca cells (51-66%). Whether COC were cocultured in contact or not in contact with theca cells, the percentage of GV stage was similar (61%). The reversibility of this inhibition was high (85%). However, granulosa cells did not exert meiotic arrest (10%). When oocytes were denuded of their cumulus cells and cocultured with theca cells, only 3% were maintained in GV stage; 59% were maintained in GV stage when COC were not removed before culture. This data provided evidence of the essential role of cumulus cells in maintaining GV stage. In conclusion, we have demonstrated that theca cells, in vitro, maintained bovine oocytes in meiotic arrest. The inhibitory factor(s) produced by theca cells is soluble in the medium and acts through the cumulus cells.

Abstract: The cells of the theca externa of large, antral ovarian follicles in the rat were studied by electron microscopy. Many resembled fibroblasts, while others possessed cytoplasmic filaments and dense bodies characteristic of smooth muscle cells. Filament-containing cells of intermediate structure appeared to represent transitional forms between the two previous types. Small numbers of smooth muscle-like cells were found also around small antral follicles, and many corpora lutea possessed locally well-defined coats of smooth muscle. These observations indicate that cells with a probable contractile function are present in the theca externa of ovarian follicles in the rat. These cells may play a role in the process of ovulation.