The endometrium, the inner lining of the uterus, is perhaps the single most dynamic tissue in the human body, rapidly growing, functionally differentiating, and then degenerating during each menstrual cycle (Figure 1). Growth and regression of the arteries, veins, and capillaries of the endometrial vasculature are an intrinsic part of this cycle. This occurs throughout the decades-long reproductive life span in response to a precise pattern of circulating steroid hormones produced by the ovaries. This cycling is punctuated normally only by pregnancy, during which the endometrial vasculature undergoes further extensive modification for delivery of maternal blood to the placenta. In humans and other primates, the endometrium is composed of two main layers, the functionalis and the basalis (Figure 2). The functionalis, the innermost portion surrounding the lumen, is composed of a richly vascularized stroma covered on its luminal surface by a simple epithelial cell layer. The functionalis undergoes dramatic changes in thickness (fourfold or more) during the menstrual cycle, first rapidly regenerating itself (during the first half or prolifera-tive phase of the cycle), then functionally differentiating in preparation for implantation of a developing embryo (called the secretory phase of the cycle, in recognition of the full development and function of the epithelial cell-lined endometrial "glands"). If pregnancy does not occur, the functionalis is shed during menstruation. The basalis, the thinner, basal layer that remains after menstruation, is the foundation and tissue reservoir from which the functionalis, including its vasculature, is regenerated during the subsequent cycle.
To support the regrowth of the endometrium, and pregnancy if it occurs, a rich new vasculature regenerates within it during each proliferative phase. This contrasts to the situation in almost all other tissues of the body where blood vessels are highly stable once formed. Thus, the uterus is one of the few sites in the body in which angiogenesis, the development of new blood vessels, is a regular occurrence. In addition to the standard role of supplying oxygen and nutrients and removing wastes for surrounding cells, the endometrial microvessels also play a special role, via increases in their permeability, in the rapid regrowth of the endometrium, secretion by endometrial glands, and the process of implantation. This last role, and the factors that mediate it, will be emphasized in this review.
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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.