Uterine Cycle

Uterine Cycle

The uterine cycle refers to changes within the endometriumThe inner lining of the uterus that thickens during the menstrual cycle to support a potential pregn the innermost lining of the uterusThe muscular organ where a fertilized egg implants and develops.. The endometrium is subdivided into two layers. The functional zone lines the lumenThe inside space of a hollow organ or structure. of the uterine cavity. The basal zone lies beneath it between the myometriumThe thick muscular layer of the uterus and the functional zone. The functional zone is the layer of the endometrium that thickens and sheds each month. The entire cycle averages about 28 days. The cycle is divided into 3 phases the menstrual proliferative and secretory phases. These three phases can be aligned with the ovarian cycleThe cycle of follicle development, ovulation, and corpus luteum formation. via ovulationThe release of a mature oocyte from the ovary..  Remember that the uterine cycle discusses only the events in the uterus. The ovarian cycle talks only about the events in the ovary.  Yes, the events of the ovarian cycle dictate the events of the uterine cycle.

Menstrual Phase

The menstrual phaseThe phase of the cycle when the functional layer of the endometrium is shed. occurs in days one through 5 of the uterine cycle. During this phase, blood flow to the endometrium is reduced. This reduction starves the tissues in the functional zone of their oxygen and other nutrients. As a result these tissues deteriorate. When they break away from the uterine lining it causes blood loss.  The sluffing off of these tissues and associated blood is called menstruation lasts one to seven days. During menstruation the functional zone is reduced but the basal zone remains intact.

Estrogen and progesteroneA hormone that supports pregnancy and regulates the menstrual cycle. are dropping at this point in the ovarian cycle.  The drop in progesterone triggers spasms in those coiled arteriesBlood vessels that carry oxygenated blood away from the heart (except pulmonary arteries, which carr. These spasms cause menstrual cramps, which can sometimes be so debilitating in the first few days of the menstrual phase.  As these two hormones drop, so does body temperature. 

Proliferative Phase

The proliferative phase The phase when the endometrium rebuilds after menstruation. Is day 6 through 13. Estrogens induce all the changes throughout this phase. During this phase of the uterine cycle, estrogen is rising as folliclesStructures in the ovaries that contain developing oocytes. are growing.  As mitosis creates new cellsThe basic structural and functional units of life., estrogen inserts receptorsProteins located on the surface or inside cells that bind specific molecules (e.g., neurotransmitter for progesterone into the endometrial cells.  The end of the proliferative phase corresponds with ovulation, after which, progesterone will rule.

The major event is to reestablish the functional zone loss during menstruation via mitosis.  The word “proliferation” is often used to refer to cells that are doing lots of mitosis.  Blood vessels grow back into the functional zone, creating the coiled arteries. Endometrial glands appear which produce a mucus containing the starch glycogenA storage form of glucose found in animals.. 

Secretory Phase

The secretory phaseThe phase of the menstrual cycle when the endometrium is prepared for implantation. is days fifteen through 28 of the uterine cycle. This last and longest phase covers about 14 days.  Progesterone is very high in this phase and spends its time inserting receptors for estrogen into endometrial tissue.

Think of this phase form the uterus’s point of view.  The uterus knows an ovumA mature egg cell has been ovulated. It is hopeful there has been a copulatory event to bring spermatozoa to the uterine tubesAlso called fallopian tubes, they transport the oocyte to the uterus..  The uterus wants to prepare for the implantation of the zygote.  More coiled arteries spring up to deliver more blood to the endometrium.  The endometrial glands get bigger and produce more mucus. The glycogen in the mucus on the endometrial glands will serve to nourish the developing embryo if implanted. In general, the endometrial layer thickens with blood and mucus, getting ready for a hopeful implantation.

If no implantation occurs, the corpus luteum in the ovary dies. Estrogen and progesterone levels fall. GnRH is released from inhibition to start the cycle over again.