FIGURE 23.15 • Schematic diagram illustrating arterial blood supply to the endometrium of the uterus. The two layers of the endometrium, the stratum basale and stratum functionale, are supplied by branches of the uterine artery. The spiral arteries located at the interface between these two layers degenerate and regenerate during the menstrual cycle under the influence of estrogens and progesterone. (Based on Weiss L, ed. Cell and Tissue Biology: A Textbook of Histology, 6th ed. Baltimore: Urban & Schwarzenberg, 1988.)  The stratum functionale or functional layer is the thick part of the endometrium, which is sloughed off at men struation.  The stratum basale or basal layer is retained during menstruation and serves as the source for the regeneration of the stratum functionale.
The stratum functionale is the layer that proliferates and degenerates during the menstrual cycle.
During the phases of the menstrual cycle, the endometrium varies from 1 mm to 6 mm in thickness. It is lined by a simple columnar epithelium with a mixture of secretory and ciliated cells. The surface epithelium invaginates into the underlying lamina propria, the endometrial stroma, forming the uterine glands. These simple tubular glands, containing fewer ciliated cells, occasionally branch in the deeper aspect of the endometrium. The endometrial stroma, which resembles mesenchyme, is highly cellular and contains abundant inter-cellular ground substance. As in the uterine tube, no submucosa separates the endometrium from the myometrium.
The vasculature of the endometrium also proliferates and degenerates during each menstrual cycle.
The endometrium contains a unique system of blood vessels (see Fig. 23.15). The uterine artery gives off 6 to 10 arcuate arteries that anastomose in the myometrium. Branches from these arteries, the radial arteries, enter the basal layer of the endometrium where they give off small straight arteries that supply this region of the endometrium. The main branch of the radial artery continues upward and becomes highly coiled; it is therefore called the spiral artery. Spiral arteries give off numerous arterioles that often anastomose as they supply a rich capillary bed. The capillary bed includes thin-walled dilated segments called lacunae. Lacunae may also occur in the venous system that drains the endometrium. The straight arteries and the proximal part of the spiral arteries do not change during the menstrual cycle. The distal portion of the spiral arteries, under the influence of estrogens and progesterone, undergoes degeneration and regeneration with each menstrual cycle.
Cyclic Changes During the Menstrual Cycle
Cyclic changes of the endometrium during the menstrual cycle are represented by the proliferative, secretory, and menstrual phases.
The menstrual cycle is a continuum of developmental stages in the functional layer of the endometrium. It is ultimately controlled by gonadotropins secreted by the pars distalis of the pituitary gland that regulate the steroid secretions of the ovary. The cycle normally repeats every 28 days, during which the endometrium passes through a sequence of morphologic and functional changes. It is convenient to describe the cycle as having three successive phases:  The proliferative phase occurs concurrently with follicular maturation and is influenced by ovarian estrogen secretion.  The secretory phase coincides with the functional activ ity of the corpus luteum and is primarily influenced by progesterone secretion.
 The menstrual phase commences as hormone produc tion by the ovary declines with the degeneration of the cor pus luteum (see Folder 23.3).
The phases are part of a continuous process; there is no abrupt change from one to the next.
The proliferative phase of the menstrual cycle is regulated by estrogens.