Chapter 10. Reproductive Cycles: The Menstrual (uterine) Cycle - Review of Medical Embryology Book

Introduction: the menstrual cycle is caused by changes in the uterine endometrium. Menstruation is a discharging of blood secretion and tissue debris from the uterus that recurs in nonpregnant, breed-age, primate females at about monthly intervals and represents a readjustment of the uterus to the nonpregnant state

ANOVULATORY CYCLE: the ovary fails to produce a mature follicle, endometrial changes are minimal, and the proliferative endometrium develops as usual

There is no ovulation, and there is no corpus luteum formation
The endometrium does not progress to a secretory phase but continues to be of proliferative type until the beginning of menstruation
This state also can be produced by giving steroid hormones which act on the hypothalamus and hypophysis to inhibit secretion of hypothalamic releasing factors and pituitary gonadotropins needed for ovulation (like taking birth control pills)
Birth control pills: estrogen inhibits ovulation; progesterone induces the secretory phase of the menstrual cycle

OVARIAN HORMONES cause the cyclic changes in the endometrium of the uterus. There are 3 cyclic phases, each passing into the next in a continuous process

The proliferative or follicular phase occurs between days 6 to 14 and coincides with the growth of the ovarian follicles

Controlled by estrogens secreted by the theca interna (around follicle)
Two-to threefold increase in endometrial thickness (repair and proliferation)
Continuous surface epithelium covers the endometrium; the glands increase in length and number, and spiral arteries elongate but do not reach the surface

The secretory or luteal phase occurs between days 16 to 28 and coincides with the formation and growth of the corpus luteum (CL)

Progesterone secreted by the CL stimulates the glandular epithelium to produce a glycogen-rich material. Endometrium thickens due to fluid in stroma
The glands become wide, tortuous, and saccular
Spiral arteries grow into the superficial compact layer and become coiled
If an oocyte, released at ovulation, is fertilized, the blastocyst begins to implant in the endometrium at about day 20 of this phase
If fertilization does not occur, the secretory endometrium goes into an ischemic or premenstrual phase during the last 2 days of the menstrual cycle

Ischemic phase: last part of the secretory phase with ischemia due to blood deficiency, endometrium pales, and spiral arteries constrict intermittently due to decreased hormone secretion by the degenerating corpus luteum
Hormonal decrease results in stoppage of glandular secretion, loss of interstitial fluid, and a shrinking of the endometrium
As the ischemic period nears its end, the spiral arteries constrict longer, blood seeps through the arterial ruptured walls into the surrounding stroma, and pooled blood breaks through the endometrial surface, resulting in bleeding into the uterine lumen and the beginning of a new menstrual phase

ENDOMETRIAL DETACHMENT AND ARTERIAL BLEEDING INTO THE UTERINE CAVITY leads to the loss of about 35 ml of blood and, over the 3-to-6-day period, the entire compact and most of the spongy uterine layers are lost in the menstrual flow. The remaining spongy and basal layers undergo regeneration during the next proliferative phase
MENSTRUAL CYCLES normally continue until the end of the reproductive life, approximately to the ages of 47 to 52 years
IF PREGNANCY OCCURS, menstruation ceases, and the endometrium passes into a pregnancy phase. After pregnancy, ovarian and menstrual cycles resume after a variable period of time, about 6 to 10 weeks later, if there is no breast feeding