This manuscript reviews the present knowledge related to folliculogenesis in sheep.Folliculogenesis starts with formation of primordial follicles before birth, present as a pool containing a certain number of follicles. By the attainment of puberty, a group of follicles from that pool starts to grow, a process known as primordial follicle activation or recruitment. The number of growing primordial follicles directly affects the number of available oocytes for fertilization. Many studies, especially in rodents, have been performed to understand the mechanisms that control the primordial follicle growth. Therefore, many autocrine, paracrine, intacrin factors from oocyte, granulasa and surrounding stroma have been identified. These regulating factors depend on species, age, physiological condition of the gonad and the environment. Also, interactions between these regulating factors have been observed and some discrepancies among results were found. The factors regulating primordial follicle activation probably act in concert with gonadotropins and regulate follicle growth and atresia before the antrum formation. Although many studies have been performed to understand the mechanisms controlling preantral follicle growth, no precise key points have been identified. The majority of the factors that affect primordial follicle transition have not yet been studied in sheep, while many studies have been performed to understand the controlling mechanisms of follicular growth through the antral the stages. Therefore new experimental studies are needed to understand the controlling mechanisms of preantral follicle growth. Manuscript profile

Studies have shown that estradiol-17β (E2) regulates gonadotropin-releasing hormone (GnRH) and GnRH receptor expression in hypothalamus and pituitary. Several studies have shown that GnRH and its receptor are also expressed in peripheral reproductive organs and little is known about their regulations. In this study, GnRH and GnRH receptor structures, functions, their peripheral expressions and regulations by E2 were reviewed. Several in vivo and in vitro conducted studies indicate that E2 decreases the expression of GnRH mRNA and regulates GnRH receptor expression in a time dependent manner. Nevertheless, the exact mechanism has not been clearly explained yet. Manuscript profile