By convention, the first day of vaginal bleeding is called day 1 of the menstrual cycle. After menses, the decidua basalis is composed of primordial glands and dense scant stroma in its location adjacent to the myometrium. The proliferative phase is characterized by progressive mitotic growth of the decidua functionalis in preparation for implantation of the embryo in response to rising circulating levels of estrogen (61). At the beginning of the proliferative phase, the endometrium is relatively thin (1–2 mm). The predominant change seen during this time is evolution of the initially straight, narrow, and short endometrial glands into longer, tortuous structures (62). Histologically, these proliferating glands have multiple mitotic cells, and their organization changes from a low columnar pattern in the early proliferative period to a pseudostratified pattern before ovulation. Throughout this time, the stroma is a dense compact layer, and vascular structures are infrequently seen.
In the typical 28-day cycle, ovulation occurs on cycle day 14. Within 48 to 72 hours following ovulation, the onset of progesterone secretion produces a shift in histologic appearance of the endometrium to the secretory phase, so named for the clear presence of eosinophilic protein-rich secretory products in the glandular lumen. In contrast to the proliferative phase, the secretory phase of the menstrual cycle is characterized by the cellular effects of progesterone in addition to estrogen. In general, progesterone’s effects are antagonistic to those of estrogen, and there is a progressive decrease in the endometrial cell’s estrogen receptor concentration. As a result, during the latter half of the cycle, estrogen-induced DNA synthesis and cellular mitosis are antagonized (61).
During the secretory phase, the endometrial glands form characteristic periodic acid–Schiff positive–staining, glycogen-containing vacuoles. These vacuoles initially appear subnuclearly and then progress toward the glandular lumen (57) (Fig. 7.10). The nuclei can be seen in the midportion of the cells and ultimately undergo apocrine secretion into the glandular lumen, often by cycle day 19 or 20. At postovulatory day 6 or 7, secretory activity of the glands is generally maximal, and the endometrium is optimally prepared for implantation of the blastocyst.
The stroma of the secretory phase remains unchanged histologically until approximately the seventh postovulatory day, when there is a progressive increase in edema. Coincident with maximal stromal edema in the late secretory phase, the spiral arteries become clearly visible and then progressively lengthen and coil during the remainder of the secretory phase. By around day 24, an eosinophilic-staining pattern, known as cuffing, is visible in the perivascular stroma. Eosinophilia then progresses to form islands in the stroma followed by areas of conﬂuence. This staining pattern of the edematous stroma is termed pseudodecidual because of its similarity to the pattern that occurs in pregnancy. Approximately 2 days before menses, there is a dramatic increase in the number of polymorphonuclear lymphocytes that migrate from the vascular system. This leukocytic infiltration heralds the collapse of the endometrial stroma and the onset of the menstrual ﬂow.