Functionally the endometrium can be divided into two zones: 1) the functionalis, which is built up and sloughed monthly during menstruation, and 2) the basalis, the epithelium and glandular elements that remain and supply the replicative cells to generate the next month’s functionalis.
The Menstrual Cycle
The average menstrual cycle is 28 days, however only a very small percentage of cycles are exactly 28 days, most cycles range from 25-36 days. Menstrual cycles usually start between the ages of 12 and 15 and continue to about the age ot 45-50 when menopause occurs.
Females have four major hormones involved in the menstrual cycle: follicle-stimulating hormone (FSH), luteinizing hormone (LH), estrogen (estradiol) and progesterone. FSH and LH are protein hormones produced by cells of the anterior pituitary within the brain, in response to small peptide hormones from the hypothalamus (hypothalamic releasing factors). These pituitary hormones travel in the blood to the ovary where they stimulate the development of one or more eggs, each within a follicle. A follicle consists of an ovum surrounded by cells responsible for the growth and nurturing of the ovum. As the cycle progresses, one follicle becomes dominant and all others regress.
The menstrual cycle can be divided into three phases: the follicular (proliferative) phase, the ovulatory phase, and the luteal (secretory) phase. The follicular phase begins with the first day of menses (menstrual flow) and continues to approximately day 13 or 14 when ovulation takes place. During the follicular phase, FSH and LH are slowly rising in preparation for the LH surge (very high level of LH) at the time of ovulation. FSH is stimulating the growth of follicles in the ovary. Estrogen and progesterone are relatively low throughout this time but slowly begin to rise toward the end of this phase. Estrogen, and progesterone to a lesser degree, are steroid hormones produced by cells of the developing ovarian follicle. Estrogen causes the endometrium to increase in thickness and vascularization (i.e.blood supply). At the end of the proliferative (follicular) phase, the endometrium is 2-3 mm thick and the glands are straight tubules with narrow lumens.
LH surges and peaks during the ovulatory phase (around day 14) and estrogen peaks at the same time. These peaks trigger ovulation. The ovum lives about 72 hours after ovulation, but it is fertilizable for only about 36 hours. Just before ovulation, progesterone levels begin to rise rapidly. Changes in cervical mucous accompany ovulation. The amount of mucous increases and it becomes clear and thin. This facilitates conception by aiding the passage of sperm through the cervical canal. Sperm can live for up to 72 hours in the female reproductive system. Therefore, the fertile period during a 28-day cycle is only about 4-5 days.
After ovulation (at the midpoint of the cycle), under the influence of LH, the ovarian follicular cells shift to the production of progesterone becoming a yellowish structure called the corpus luteum (luteal phase). The corpus luteum remains intact for the remainder of the cycle. Progesterone causes the endometrial lining to become secretory and nutritive in anticipation of implantation of a fertilized egg. The uterine glands become very coiled and the endometrial lining reaches maximum thickness of about 5 mm during the luteal (secretory phase). Progesterone also inhibits the contractions of smooth muscle cells of the myometrium. The breast swelling, tenderness and pain experience by some women is most likely due to the effects of progesterone on breast tissue.
In the luteal phase progesterone levels are very high–progesterone is important during this phase because if the egg is fertilized, and implanted in the uterus, progesterone keeps the uterus intact so that the pregnancy is maintained. The continued health of the corpus luteum (progesterone secretion) is assured by the production of human chorionic gonadotropin (hCG) by the implanted embryo, until the placenta develops and can take over. The detection of hCG in urine is the basis of laboratory and home pregnancy tests.
If fertilization and implantation have occurred, than the corpus luteum will be stimulated by hCG to continue its production of estrogen and progesterone to maintain the pregnancy. This is important because the corpus luteum dies 14-22 days after ovulation if fertilization and implantation do not occur. At the end of the secretory (luteal) phase, blood levels of estrogen and progesterone drop rapidly. The coiled arteries serving the endometrial lining contract, causing ischemia leading to tissue death in the functionalis. The blood vessels above the vasoconstriction rupture and bleeding begins resulting in the monthly menstrual flow that normally lasts about 5 days.