from Part II - Cell Death in Tissues and Organs
Published online by Cambridge University Press: 07 September 2011
Introduction
The ovaries are major endocrine organs in females that, in mammals, serve two principal functions: (1) to produce a female germ cell (oocyte) that is capable of fertilization and successful embryonic development, yielding a viable, healthy offspring; and (2) to secrete a number of hormones that drive development of primary and secondary sex characteristics in the female and, during adulthood, prepare the uterus for establishment and maintenance of pregnancy. These functions are carried out by structures termed follicles, which are often referred to as the functional units of the ovaries. Each follicle is composed of an oocyte that is surrounded by one or more layers of somatic granulosa cells and, at more advanced stages of follicle development, theca cells. The granulosa and theca cells are responsible for much of the ovarian hormone production and support the maturation and growth of the enclosed oocyte. There are several different types of follicles present in the ovaries, which, depending on the size of the oocyte as well as the number of granulosa and theca cell layers, are classified as primordial (resting oocyte surrounded by a single layer of quiescent granulosa cells), primary (the first stage of immature follicles activated to initiate growth, characterized by oocyte enlargement and granulosa cell mitotic activity), secondary or preantral (larger maturing follicles with several layers of mitotically active granulosa cells, as well as some theca cells), and antral (mature follicles that have a fluid-filled cavity called an antrum and the ability to be ovulated in response to the pituitary gonadotropin, luteinizing hormone). The growth and maturation of a primordial follicle to the antral stage capable of ovulation can take weeks to months, depending on species, during which time the majority of follicles actually fail to mature and are eliminated by a degenerative process involving apoptosis that is referred to as atresia.
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