ENDOMETRIAL CYCLE
In the early proliferative phase of the menstrual cycle the endometrium is thin, and relatively homogeneous. The glands are simple and straight, leading directly from the base to the surface. Under high-power magnification, an occasional mitosis may be seen in the low columnar epithelial cells. A similar picture is found in prepubertal or postmenopausal endometrium, except that the endometrium is even thinner, consisting only of the basalis layer resting on the myometrium, and the glands are wholly inactive, the shrunken nuclei are pyknotic, and no mitoses appear to be present.
The late proliferative stage demonstrates marked growth in glands and
stroma, with tortuosity of the glands and corkscrew convolutions. The stroma
cells of the superficial layer may be separated by edema, and mitoses are
frequently seen. The epithelium is higher and more columnar, and the nuclei are
disorderly placed, both centrally and peripherally, at different levels in the
cells.
Within 2 or 3 days after ovulation, the early signs of the secretory
phase induced by progesterone are clearly visible. The endometrium shrinks
slightly in thickness as the edema of the superficialis is lost. In the
epithelial glands, the nuclei are now rounded and are arranged more or less in
line in the middle of the cell. The cyto- plasm of these cells is condensed toward
the lumen by the accumulation of glycogen-rich secretions basally, but with
hematoxylin and eosin staining this appears as a subnuclear empty space or
vacuole. Mitoses are less common and disappear entirely by about day 20 of the
cycle.
From day 21 through day 25, the endometrium is in active secretion; edema
is now grossly apparent in the midlayer, so that the total thickness of the
endometrium reaches a maximum. The glands take on a distinctive jagged or
saw-toothed appearance. The round nuclei sink to a basal location, while
secretions form vesicles at the luminal margin, which are disgorged into the
gland lumen and leave the impression of a frayed and shaggy cellular edge.
Arterioles exhibit a major increase in growth and tortuosity. Capillaries are
prominent in the superficial layers, and contiguous stromal cells are first noted
to become swollen and pale-staining.
Through the last 2 or 3 days (late secretory phase) of the cycle,
regressive changes are found to coincide with a decrease and, finally, cessation
of function of the corpus luteum. Endometrial intracellular edema is resorbed,
causing shrinkage in total thickness of the endometrium. Superficially, the
stroma cells accumulate cytoplasm in a dense layer called predecidua. The
sectioned glands are widely dilated and filled with secretion and cellular
debris. The glandular epithelium appears inactive, the cells are low columnar
or cuboidal, and the nuclei are often pyknotic. With impending menstruation, an
extensive diapedesis of red and white blood cells is
seen in the stroma.
The process of menstruation first begins by a pooling of blood cells in
intercellular spaces beneath the surface epithelium. Breaks in the surface
occur, and pieces of stroma and broken glands are lifted off. Desquamation of the
top layers down to the basalis takes place in 2 or 3 days. There are increased
numbers of lymphocytes and polymorphonuclear leukocytes; the epithelial cells
are characterized by pyknosis and fragmentation.
If conception has occurred, the secretory activity of the endometrium is maintained and increased by the corpus luteum of pregnancy. In a full-thickness specimen the thick, well-developed, true decidua extends well down into the midlayers, which are crowded with “saw-toothed” glands. The functioning cells are large, with basal, round, vesicular nuclei, filled with coarse granules except at the translucent luminal margin, where a dis- tinct cell membrane balloons out into the gland lumen.
