The Ovarian Follicle: Reproductive and Endocrine Organ
The female body is endowed with the uterus and ovaries as reproductive organs. But having babies is not their only function. Initial breast bud and pubic hair formation occurs because of hormones produced by the ovaries. Later, a girl will have her first period.
A girl begins to menstruate because her ovaries are producing estrogen and progesterone. The 28-day menstrual cycle may not begin with the first period. It may take 2-3 years for a girl to be ‘established’ with monthly periods, as she may have her periods only once or twice in the first year, then more often as time progresses.
Women are born with two ovaries, one on each side of the uterus (See Figure 1).
Figure 1. The female pelvis. The uterus is behind the urinary bladder. 1 = Fallopian tube; 2 = urinary bladder; 3 = pubic symphesis; 4 = vagina; 5 = clitoris; 6 = urethral opening; 7 = vagina; 8 = ovary; 9 = fascia; 10 = uterus; 11 = posterior cervix; 12 = cervix; 13 = colon; 14 = rectum.
It is important to understand that the human ovary serves two functions: reproduction and endocrine. Both of these functions are tightly coupled, as the release of hormones makes the uterus ready for fertilization of an oocyte that comes from the ovarian follicles (See Figure 2).
Figure 2. Reproduction: The Ovarian Follicle and the Cycles of Menstruation. With the monthly cycle, the ovarian follicle prepares an oocyte for maturation and release to the Fallopian tube, with the possibility of fertilization and reproduction.
There are three types of cells in the human ovary: the oocyte or mature egg, the granulosa cells, and the external thecal layers. The follicle houses the oocyte that is maturing to the time of release. The granulosa cells are in the follicle, and they surround the oocyte.
Hormone production dictates what happens to the follicle. When testosterone increases, the number of granulosa cells decrease. When gonadotropins (i.e., protein hormones produced by the anterior pituitary gland) increase, the granulosa cells increase in number, not size. Pituitary gonadotropins include: follicle-stimulating hormone (FSH) and lutenizing hormone (LH). FSH tells the granulosa cells to make LH receptors on the cell surface so that when LH is produced and binds to the receptors, the end of the cycle proliferation occurs. This makes the period stop (see Figure 3).
Another human gonadotropin is produced by the placenta, and this is known as human chorionic gonadotropin (hCG). The hCG is the hormone test for pregnancy that is commonly used on pregnancy strips. If hCG is present, placenta is making it. As the placenta increases in size during the early stages of pregnancy, the hCG also increases in number. During pregnancy, the placenta also produces estrogen.
During the nonpregnancy state, the human female ovaries produce estrogen, progesterone, and testosterone. Granulosa cells in the ovarian follicles and the surrounding corpora lutea make estrogen. Other organ cells participate in estrogen production, but to a lessor extent: the fat or adipose, liver, breasts, and the adrenal gland. Postmenopausal estrogen production can still occurs from these extra-ovarian sources, but a woman's individual blood levels must be measured to know what phase her ovaries are in. In the nonpregnant female, the highest levels of estrogen occur just prior to ovulation, near the end of the Follicular Phase (see Figure 3).
Figure 4. The metabolism of cholesterol. A variety of biochemical reactions exist whereby cholesterol is metabolized to progesterone, then on to dehydroepiandrosterone, testosterone, dihydrotestosterone, or estradiol. Cholesterol is not all bad, and our bodies must produce cholesterol not just in order to procreate, but to develop neurologically. Cholesterol is important to the structure of cells, as well as being a precursor of oxysterols, bile acids, and steroid hormones.
Cholesterol is the "Mother Molecule" of androgen and estrogen steroids (See Figure 4). Actually, you may be surprised to learn that the cholesterol molecule is a major part of the human brain, and there is no organ in the human body that contains more cholesterol than the human brain (Orth, 2012). In fact, about 20% of the body’s cholesterol is contained in the brain. The brain does not have the same metabolic pathway as other organs, and the brain is responsible for what is called de novo synthesis of cholesterol. This means that the brain makes it freshly. It was Couerbe who, in 1836, described the cholesterol molecule as being “un element principal”, meaning ‘a key element’ in the central nervous system (Couerbe, 1834).
It is important to note that the ovary is uniquely tied into the hormones that they produce. The ovaries are an organ, and they synthesize and coordinate the lifecycle of a girl and a woman. In old age, the same ovaries dictate how menopause is approached.
If a woman undergoes a hysterectomy and the surgeon also removes the ovaries, this is 'surgical menopause'. A woman undergoing a hysterectomy gets a 'crash course' in menopause if the ovaries are removed, and she should be offered a discussion of whether or not she should be placed on hormone replacement therapy (HRT). Backing up for a moment, wait just one moment. Actually, we must first question whether the ovaries should be removed at all. Stay tuned for the next article, which will address this issue.
Couerbe JP. Du cerveau, considere sous le point du vue chimique et physiologique. Annales De Chimie Ed De Physique. 1834;56:160-193.
Orth M., and Bellosta S. Cholesterol: its regulation and role in central nervous system disorders. Cholesterol, 2012;2012:292598, doi:10.1155/2012/292598. Epub 2012 Oct 17. http://www.ncbi.nlm.nih.gov/pubmed/23119149
Additional Articles by Dr. Margaret Aranda