BV, blood vessels Gl, glands

M, myometrium SB, stratum basale

SEp (inset, Fig. 1), surface epithelium SF, stratum functionale

After estrogen brings about the uterine events designated the proliferative stage, another hormone, progesterone, influences additional uterine changes that constitute the secretory stage of the uterine cycle. This hormone brings the endometrium to a state of readiness for implantation, and as a consequence of its actions, the thickness of the endometrium increases further. There are conspicuous changes in the glands, primarily in the stratum functionale, where the glands take on a more pronounced corkscrew shape and secrete mucus that accumulates in sacculations along their length.

The vasculature of the endometrium also proliferates and degenerates in each menstrual cycle. Radial arteries enter the stratum basale of the endometrium from the myometrium and give rise to small, straight arteries that supply the stratum basale and continue into the endometrium to become the highly coiled spiral arteries. Arterioles derived from the spiral arteries supply the stratum functionale. The distal portion of the spiral arteries and the arterioles are sloughed with the stratum functionale during menstruation. Alternating contraction and relaxation of the basal portions of the spiral arteries prevent excessive blood loss during menstruation.

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