Control of Pregnancy

Ovulation

During the follicular phase of the menstrual cycle, one or more follicles are prepared for ovulation. FSH and estrogens are the most important hormones for this developmental process. Complete follicular maturation cannot occur in the absence of LH. Rupture of a mature follicle follows the midcycle peak of LH and FSH by about 24 hours. In humans, usually one mature ovum is released per cycle. During the luteal phase of the menstrual cycle and under the influence of LH, the ovarian granulosa cells of the corpus luteum become vacuolated and accumulate a yellow pigment called lutein.

Implantation

The lining of the uterus, that is, the endometrium, is critical for implantation of the fertilized ovum. The en-dometrium consists of a layer of epithelial cells overlying a layer of vascularized stromal cells. Under the influence of estrogen and progesterone, the endome-trium undergoes cyclical changes that prepare it for the implantation of a fertilized ovum. The follicular phase of the menstrual cycle also may be called the proliferative phase when referring to changes that occur in the uterus. Estrogens induce endometrial cell division and growth.

During the luteal phase, when the uterus is exposed to high concentrations of progesterone and moderate estradiol levels, the mitotic activity in the endometrial cells is suppressed. The action of progesterone on the endometrium converts it from a proliferative state to a secretory state. The epithelial cell structure assumes a more glandular appearance. Vascularization of the stroma increases, and some stromal cells begin to look like the decidual cells of early pregnancy. Estrogens and progesterone are key hormones in the maintenance of pregnancy. Estriol is produced in high concentrations by the placenta in pregnant women.

Cervical mucus is secreted by the endocervical glands and is regulated by estrogens and progestins. Under the influence of high levels of estrogen or progesterone, the physicochemical composition of cervical mucus may reduce sperm motility and provide a barrier to fertilization.

When implantation of the ovum does not occur, estrogen and progesterone levels fall and menstrual bleeding ensues. The endometrial lining, but for a single layer of epithelial cells, is shed.

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