Uterine (menstrual) cycle - phases
• The uterus goes through a cyclical developmental pattern to be ready for implantation and support of an embryo. • The uterine, or menstrual, cycle is under the control of ovarian horrnones. • The uterine cycle also has three phases: • Menstrual phase • Proliferative phase • Secretory phase Menstrual phase • The beginning of the menstrual cycle is considered to be the first day of menstrual flow. • Changes in the endometrium are triggered by changes in levels of ovarian hormones. • Declining progesterone levels lead to the reduction of blood supply to the endometrium. • This results in the shedding of the stratum functionalis layer of the uterine wall. Proliferative phase • Estrogens secreted from the developing follicles in the ovaries stimulate the regeneration of the stratum functionalis. • This cell proliferation repairs and begins to thicken the endometrium. Secretory phase • Following ovulation, the stratum functionalis reaches maximum preparedness for implantation. • Under the influence of rising levels of ovarian hormones: • The endometrium continues to thicken. • Endometrial glands secrete glycogen. • If fertilized, implantation of an embryo occurs. • The new embryo is nutritionally maintained by the endometrium. Secretory phase • If fertilization does not occur: • The corpus luteum disintegrates. • Levels of progesterone and estrogen drop. • The stratum functionalis of the uterine wall sloughs Off during menstruation.
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