Uterine (menstrual) cycle - phases

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• 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.