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From Citizendium, the Citizens' Compendium
The human ovary is the female gonad, the paired organ that contains the germ cell population of oocytes, and provides for their cyclic maturation though her childbearing years. Ovaries are an important part of the reproductive system for this reason, and because-irrespective of whether an individual woman bears children, normally produce the hormones that are intrinsic for the development of the physical characteristics of womanhood. During the childbearing years, from menarche to menopause, the ovaries produce ovum, the mature egg cell that, if fertilized by a sperm cell, can develop into a baby. The ovaries also are a major source of the hormones estrogen and progesterone, that are produced in varying levels throughout the ovulatory cycle.
The development of the ovaries in the embryo
In humans, like all mammals, germ cells proliferate outside of the embryo proper and then migrate into the gonad. Here, in the embryonic ovary, these stem cells stop dividing, enter meiosis, and become organized into primordial follicles. These primordial follicles are a histologic arrangement in which each germ cell is surrounded by another tye of cell, called pregranulosa cells. "A female will have the highest number of germ cells, approximately 6 million, in her ovaries at 20 weeks gestational age. Henceforth, atresia depletes the follicular pool at a brisk pace with only 1–2 million oocytes remaining at the time of birth."((Ashim Kumar, MD, Shahin Ghadir, MD, Niloofar Eskandari, MD, & Alan H. DeCherney, MD:Chapter 55 Infertility in Alan H. DeCherney and Lauren Nathan:Current Diagnosis & Treatment Obstetrics & Gynecology, 10th edition, Copyright © 2007 by The McGraw-Hill Companies, Inc) So, newborn girls "are endowed with a vast, but finite, nonrenewable, and postmitotic reserve of dormant follicles that will later be recruited to grow, differentiate, and acquire the ability to synthesize estradiol."(Wise PM. Krajnak KM. Kashon ML. Menopause: the aging of multiple pacemakers. Science. 273(5271):67-70, 1996 UI: 8658198 )
The ovaries in infancy and childhood
At birth, each ovary contains approximately 2 million follicles - most of them in the outer layer of the organ, which is called the cortex of the ovary. Each ovary is suspended with its respective fallopian tube in a flexble sheet of connective tissue that contains the arterial and venous vasculature that supply blood circulation to the organ.
The ovaries in adolescence
The ovaries undergo a major change with the coming of puberty. The initial period of the menstrual cycle, which ushers in the female milestone called menarche, is usually anovulatory - that is, no ovum are produced.
"Premature ovarian failure, which is defined as cessation of menstruation (hypergonadotrophic amenorrhea) before the age of 40 years, occurs in up to 0.9% of women in the general population and has multiple causes."(Marhhom E. Cohen I. Fertility preservation options for women with malignancies. Obstetrical & Gynecological Survey. 62(1):58-72, 2007 Jan. UI: 17176489).
Ovarian failure: Menopause and after
At some point in the lifespan of every normal woman who ives long enough, the ovaries are said to "fail". This medical term has a harsh tone, but simply refers to the loss of the ovulatory cycle. The classic health science view of this pre-programmed change is that with normal living, the primordial follicle population continues to become smaller, and once the number of oocytes falls below a threshold population, then the menstrual cycle ceases and menopause occurs. More recently, there has been an alternative theory that the primary change is not in the ovary, but in the central nervous system.
As menopause approaches, the remaining functional life of the ovaries is referred to as "ovarian reserve". "The concept of ovarian reserve represents the remaining follicular pool of the ovaries." (ref for quote: (Ashim Kumar, MD, Shahin Ghadir, MD, Niloofar Eskandari, MD, & Alan H. DeCherney, MD:Chapter 55Infertility in Alan H. DeCherney and Lauren Nathan:Current Diagnosis & Treatment Obstetrics & Gynecology, 10th edition, Copyright © 2007 by The McGraw-Hill Companies, Inc). When ovarian reserve is high, then regular ovulation is likely. For women who have not conceived a child despite a year's attempt to do so, estimating ovarian reserve is part of an evaluation for infertility.
As the ovaries begin to decline in function, serum levels of FSH rise in the early part of the ovulatory cycle, because a higher level is needed to recruit follicles. On day 3 of the menstrual cycle, serum FSH levels are usually less than 10 mIU per ml. FSH levels that are more than 15 mIU per mL on day 3 suggest decreased ovarian reserve. Despite this, in clinical studies in which these "subfertile" women were compared to women with low values of FSH, both groups had successfu pregnancies without a significant difference in the percentage of each group who became pregnant. 9reference: Current Opinion in Obstetrics & Gynecology. 16(3):231-7, 2004 Jun. UI: 15129052). Apparently, the ovaries "turn off" in a sudden way, and up until that point provide adequate function for childbearing.
Having the last menstrual period before age 40 is considered pre-mature, and the average woman enters menopause at 50 -55 years of age. When menstrual periods continue past the age of , physicians recommend an evaluation to be sure that an endocrine abnormality is not present. Since the average lifespan of people is increasing, this means that women alive today have more years (on average) of post-menopausal life than did the women of centuries past. One of the major differences in a woman's body before and after menopause is in the level of estrogen, but there are additional hormonal changes. "By the time a woman is 65 years old, the ovary is virtually devoid of follicles [10,11] and is no longer the primary site of estradiol or progesterone synthesis". (reference:Wise PM. Krajnak KM. Kashon ML. Menopause: the aging of multiple pacemakers. Science. 273(5271):67-70, 1996 Jul 5. UI: 8658198). After menopause, the level of Leutinizing hormone (LH) rises.