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The inability to regulate ovarian follicular development is a major obstacle to improving reproductive efficiency in cattle whether the objective be estrous cycle regulation, multiple ovulations and births, superovulation for embryo transfer, a shorter postpartum anestrous period or younger age at puberty. Similarly, normal growth and selection of ovulatory follicles are required for sexual behavior, for maturation and release of viable oocytes and for preparation of the uterine environment for gamete transport, fertilization and embryonic development. Increasing reproductive efficiency is critical to increasing livestock production as reproductive losses in cattle range from 20 to 35%. Research outlined in this report is focused on providing insight into how circulating hormones (Le., the endocrine system) and factors produced within the ovary (Le., paracrine and autocrine systems) are involved in regulating follicular development and ovulation rate.
It is well documented that hormones secreted from the pituitary (Le., LH and FSH) stimulate follicle development. However, it now appears that factors produced within the ovaries may also be important for regulating follicular development. Recently, the following peptides and proteins have been shown to exert either stimulatory or inhibitory effects on ovarian follicular growth, maturation and steroidogenesis: insulin-like growth factors (IGF-I and -II), transforming growth factors (TGF) alpha and beta, epidermal growth factor, platelet-derived growth factor, fibroblast growth factor, plasminogen activator, interleukin-I, activin, and inhibin. Although this list contains several factors, it is by no means complete. In fact, many new factors are identified each year that appear to have roles in regulating follicular development. Obviously, the mechanisms involved in controlling follicular development are probably very complex and depend on the actions and interactions of both circulating hormones and substances produced in the ovary. Research is currently being conducted to determine the role that IGF-I, TGF-alpha, TGF-beta, inhibin, and interleukin-I have in regulating follicle development and ovulation rate.