Abstract
The ovaries are located within the broad ligament, close to the lateral wall of pelvic cavity, and extend onto the peritoneal cavity.
1 The Ovaries
The ovaries are located within the broad ligament, close to the lateral wall of the pelvic cavity, and extend onto the peritoneal cavity.
Each ovary is divided into an outer cortex and inner medulla.
The dense cellular stroma of the cortex houses the ovarian follicles, which are the functional units, performing both gametogenic and endocrine function.
The stages in the development of the follicle are shown in Figure 13.1.
During fetal development, the ovaries contain over 7 million primordial follicles, which reduces to a million at birth and around 300 000–400 000 at puberty.
They undergo part of the first meiotic division at birth and survive in prophase arrest until adulthood. Just before ovulation, the first meiotic division is completed. One of the daughter cells, the secondary oocyte, receives most of the cytoplasm, and immediately begins the second meiotic division, but this division stops at metaphase and is completed only when a sperm penetrates the oocyte and forms a haploid ovum.
Figure 13.1 Stages in the development of a follicle.
2 The Menstrual Cycle
The reproductive system of women shows regular cyclic changes, which teleologically may be regarded as periodic preparations for fertilisation and pregnancy. See Figure 13.2.
In humans and other primates, the cycle is a menstrual cycle, and its most conspicuous feature is the periodic vaginal bleeding that occurs with the shedding of the uterine mucosa (menstruation).
The length of the cycle is roughly 28 days, divided into the following phases:
◦ days 1–5 (menstrual phase)
◦ days 6–14 (follicular/preovulatory phase)
◦ days 14–28 day (luteal/postovulatory phase)
3 Ovarian Cycle
3.1 Follicular Phase
At the start of each cycle, several follicles enlarge, and a cavity forms around the ovum (antrum formation), filled with follicular fluid.
Usually one of the follicles (dominant follicle) grows rapidly from day six, which is related to the ability of the follicle to secrete the estrogen needed for maturation, while the others regress, forming atretic follicles.
When women are injected with highly purified human pituitary gonadotropin preparations, many follicles develop simultaneously.
Ovarian (graafian) follicle: is lined by theca interna cells, which produce estrogen as they secrete androgens that are aromatised to estrogen by the granulosa cells.
3.2 Ovulation
On the fourteenth day of the cycle, the distended follicle ruptures and the ovum is extruded into the abdominal cavity, picked up by the fimbriated ends of the uterine tubes, and transported to the uterus.
The follicle that ruptures at the time of ovulation promptly fills with blood, forming a corpus haemorrhagicum. Minor bleeding from the follicle into the abdominal cavity may cause peritoneal irritation and transient lower abdominal pain (mittelschmerz).
3.3 Luteal Phase
The granulosa and theca cells of the follicle lining rapidly proliferate, and the clotted blood is replaced with yellowish, lipid-rich luteal cells, forming the corpus luteum, which secretes estrogen and progesterone.
An adequate blood supply is crucial for the growth of the corpus luteum, in which the vascular endothelial growth factor plays an important role.
If pregnancy occurs, the corpus luteum persists; whereas if it does not, the corpus luteum begins to degenerate around the twenty-fourth day of the cycle) and is eventually replaced by scar tissue, forming a corpus albicans.
4 Uterine Cycle
The uterine cycle is divided into the menstrual phase followed by the proliferative phase (fifth to fourteenth day) when a new endometrium regrows under the influence of estrogens from the developing follicle.
The endometrium increases rapidly in thickness and simultaneously the uterine glands are drawn out so that they lengthen.
Also called the preovulatory or follicular phase of the cycle.
Secretory or luteal phase: represents preparation of the uterus for implantation of the fertilised ovum.
The length of this phase is remarkably constant at about 14 days.
After ovulation, the endometrium becomes very highly vascularised and slightly oedematous, and the glands become coiled and tortuous, and begin secreting a clear fluid.
The endometrium is supplied by two types of arteries. The superficial two-thirds of the endometrium that is shed during menstruation (the stratum functionale) is supplied by long, coiled spiral arteries, whereas the deep layer that is not shed (the stratum basale) is supplied by short, straight basilar arteries.
When fertilisation fails to occur during the secretory phase, the endometrium is shed and a new cycle starts.
Regression of the corpus luteum leads to withdrawal of hormonal support for the endometrium, which reduces in thickness, causing the coiling of the spiral arteries, leading to spasm and degeneration of walls. The vasospasm is produced by prostaglandin F2 (PGF2) released by the secretory endometrium.
5 Normal Menstruation
Seventy-five percent of menstrual blood is arterial in origin.
It contains tissue debris, prostaglandins and relatively large amounts of fibrinolysin from endometrial tissue.
The usual duration is three to five days.
The average amount of blood lost is 30 ml, which can be affected by various factors, including the endometrial thickness, medication and clotting disorders.
6 Anovulatory Cycles
In such cycles, ovulation fails to occur during the menstrual cycle.
It is common for the first 12 to 18 months after menarche and again before the onset of the menopause.
7 Cyclical Changes in the Uterine Cervix
The cervical mucosa does not undergo cyclical desquamation, but there are regular changes in the cervical mucus.
estrogen makes the mucus thinner and more alkaline. These changes promote the survival and transport of sperms.
The mucus is thinnest at the time of ovulation, and its elasticity, or spinnbarkeit, increases so that, by midcycle, a drop can be stretched into a long, thin thread, which may be 8 to 12 cm or more in length. In addition, it dries in an arborising, fern-like pattern when a thin layer is spread on a slide.
Progesterone makes it thick, tenacious and cellular; hence, after ovulation and during pregnancy, it becomes thick and fails to form the fern pattern.
8 Cyclical Changes in the Breasts
Cyclical changes also take place in the breasts during the menstrual cycle.
estrogens cause proliferation of mammary ducts, whereas progesterone causes growth of lobules and alveoli.
The breast swelling, tenderness, and pain experienced by many women during the 10 days preceding menstruation are probably due to distention of the ducts, hyperaemia and interstitial oedema, which regress during menstruation.
9 Indicators of Ovulation
It is important to know when ovulation occurs to plan management for fertility or contraception.
A convenient indicator of the time of ovulation is the rise in the basal body temperature, which starts one to two days after ovulation and can be recorded using a digital thermometer in the morning before getting out of bed. The cause of the temperature change is probably the increase in progesterone secretion, since progesterone is thermogenic.
A surge in luteinising hormone (LH) secretion triggers ovulation, and ovulation normally occurs about nine hours after the peak of the LH surge at midcycle.
Ultrasound examination is an accurate, noninvasive method.
10 Ovarian Hormones
10.1 Biosynthesis
The naturally occurring estrogens are 17-estradiol, (E2) (most potent), estrone (E1) (less potent and predominant in menopause), and Estriol (E3) (least potent and predominant in pregnancy), which are C18 steroids and secreted primarily by the granulosa cells of the ovarian follicles, corpus luteum and placenta.
Their biosynthesis depends on the enzyme aromatase (CYP19), which converts testosterone to estradiol and androstenedione to estrone.
Theca interna and mature granulosa cells have many LH receptors, and LH acts via cAMP to increase conversion of cholesterol to androstenedione.
The theca interna cells supply androstenedione to the granulosa cells, which make estradiol when provided with androgens, which is secreted into the follicular fluid.
Granulosa cells have many follicle stimulating hormone (FSH) receptors, and FSH facilitates their secretion of estradiol by acting via cAMP to increase their aromatase activity.
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