Follicle development during the normal menstrual cycle

An understanding of the factors which determine initiation of follicle growth, recruitment and dominant follicle selection may increase our understanding of the underlying process of ovarian aging. In this article, these aspects of the normal menstrual cycle are reviewed. The morphological and endocrinological development in the early follicle is described from the primordial follicle stage. The degree of follicle-stimulating hormone (FSH) dependency is discussed, as is the relationship of estradiol (E-2) production to follicle diameter. The principles governing mono-follicular selection are outlined, and the FSH 'threshold' and 'window' concepts are highlighted. Maximum FSH levels in the early follicular phase are shown to be variable between individuals. The relevance of this and the means by which individual sensitivity to FSH may be altered at the ovary in the context of ovarian aging are discussed. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.