At the end of the menstrual phase, the endometrium consists of a thin band of connective tissue, about 1 mm thick, containing the basal portions of the uterine glands and the lower portions of the spiral arteries (see Fig. 23.15). This layer is the stratum basale; the layer that was sloughed off was the stratum functionale. Under the infuence of estrogens, the proliferative phase is initiated. Stromal, endothelial, and epithelial cells in the stratum basale proliferate rapidly, and the following changes can be seen:  Epithelial cells in the basal portion of the glands reconstitute the glands and migrate to cover the denuded endometrial surface.  Stromal cells proliferate and secrete collagen and ground substance.  Spiral arteries lengthen as the endometrium is reestablished; these arteries are only slightly coiled and do not extend into the upper third of the endometrium.
The proliferative phase continues until 1 day after ovulation, which occurs at about day 14 of a 28-day cycle. At the end of this phase, the endometrium has reached a thickness of about 3 mm. The glands have narrow lumina and are relatively straight but have a slightly wavy appearance (Fig. 23.16a). Accumulations of glycogen are present in the basal portions of the epithelial cells. In routine histologic preparations, extraction of the glycogen gives an empty appearance to the basal cytoplasm.
The secretory phase of the menstrual cycle is regulated by progesterone.
Under the infuence of progesterone, dramatic changes occur in the stratum functionale, beginning a day or two after ovulation. The endometrium becomes edematous and may eventually reach a thickness of 5 mm to 6 mm. The glands enlarge and become corkscrew shaped, and their lumina become sacculated as they fill with secretory products (Fig. 23.16b). The mucoid fluid produced by the gland epithelium is rich in nutrients, particularly glycogen, required to support development if implantation occurs. Mitoses are now rare. The growth seen at this stage results from hypertrophy of the epithelial cells, an increase in vascularity, and edema of the endometrium. The spiral arteries, however, lengthen and become more coiled. They extend nearly to the surface of the endometrium (Plate 97, page 882).
The sequential influence of estrogens and progesterone on the stromal cells enables their transformation into decidual cells. The stimulus for transformation is the implantation of the blastocyst. Large, pale cells rich in glycogen result from this transformation. Although the precise function of these cells is not known, it is clear that they provide a favorable environment for the nourishment of the embryo and that they create a
FIGURE 23.16 • Photomicrographs of the uterine lining in proliferative, secretory, and menstrual phases of the menstrual cycle.