Menstruation is triggered by a fall in progesterone production by the ovary at the end of an infertile cycle. Constriction of blood vessels, specifically the spiral arteries, was long believed to cause the ischemic degeneration and shedding of the functionalis layer. A review of the literature, however, offers little direct support for this theory. In recent years, more emphasis has been placed on the role of proteases in the degradation of the endometrium. This theory proposes that the loss of blood vessel integrity and decreased blood flow is secondary to a large increase in protease production, triggered by the fall in progesterone, resulting in extensive proteolytic degradation of the extracellular matrix and basement membranes throughout the endometrium, including those in the walls of blood vessels . In part, this theory is favored over the vasoconstriction one because of a lack of evidence that ischemia, or hypoxia, occurs prior to the major breakdown in the endometrium. However, it was recently reported that there is a large increase in VEGF expression in the superficial endometrium during the early stages of menstruation in monkeys . Since this occurs in tissue that will be shed, and did not seem to be due to steroid stimulation, the most likely explanation for it is tissue ischemia and hypoxia. If so, to what degree that ischemia is due to vasoconstriction or to the general loss of structural integrity of vessels remains to be determined. The increase in VEGF at this point in time could be functionally significant. VEGF induces protease production by endothelial cells, which might further contribute to a breakdown in vessel integrity. Furthermore, the increase in VEGF was matched by an increase in stromal cell expression of VEGF receptors. Thus it is possible that VEGF may trigger a similar increase in protease production by stromal cells.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.