The implanting embryo should be limited in its invasiveness so that it remains within the endometrium and does not reach the myometrium. The uterus plays a vital role in providing a maternal side of the mature placental architecture, including the basal plate where the fetal side attaches, and large intervillous spaces that become filled with maternal blood after the first trimester. The uterus also grows and expands with the growing fetus, creating an aqueous nonadhesive environment for its development. Additionally, the uterus provides strong muscular contractions to expel the fetus and placenta at term.

To understand the function of the uterus and the changes that occur during nonfertile menstrual cycles, it is important to review the fine structure of the endometrium and its relationship with the uterine blood supply. The endometrium's luminal surface is covered with a simple cuboidal/columnar epithelium that is continuous with mucosal glands known as uterine glands. These glands extend deep into the endometrium. The mucosa is vascularized by spiral arteries, which are branches of the uterine artery that run through the myometrium. The terminal arterioles of the spiral arteries project just beneath the surface epithelium, giving rise to a subepithelial plexus of capillaries and venules that contain venous lakes or lacunae. The lamina propria, the connective tissue beneath the epithelium, is densely cellular and its stromal cells play important roles during both pregnancy and menstruation.

During menstruation, about two-thirds of the luminal side of the endometrium, known as the functional zone or stratum functionalis, is shed. The remaining basal third is called the basal zone or stratum basale. The basal zone is fed by straight arteries that are separate from the spiral arteries, and it contains all the cell types found in the endometrium, including epithelial cells from the remaining tips of glands, stromal cells, and endothelial cells.

The hormonal regulation of the uterine endometrium during the menstrual cycle involves monthly oscillations in ovarian steroids. When the dominant follicle is selected and begins producing elevated levels of estradiol, the uterine endometrium is just ending menstruation, with only the stratum basale remaining. The rising levels of estrogen during the mid to late follicular phase induce the proliferative phase of the uterine endometrium. Estrogen promotes cell growth and division in all cell types of the stratum basale through its cognate receptors, ER-α and ER-β. It also indirectly controls uterine growth through the production of growth factors. Furthermore, estrogen stimulates the expression of progesterone receptors, preparing the uterine endometrium to respond to progesterone during the luteal phase of the ovary.
limit the invasiveness of the implanting embryo so it stays in the endometrium and does not reach the myometrium 811 •Fig. 44.26 Diagramoftheorganizationofglandsandbloodflowwithintheuterineendometrium.(FromStrausIII.In:YenSSCetal[eds].Reproductive Endocrinology. 4thed.Philadelphia:Saunders;1999.)Radial branch Spiral artery Uterine gland ENDOMETRIUM MYOMETRIUM Uterine artery Functional Zone Basal Zone Venous lakes Arcuate
CHAPTER 44 The Male and Female Reproductive Systems 4.
provide a maternal side of the mature placental architecture, including the basal plate to which the fetal side attaches, and large intervillous spaces that become filled with maternal blood after the first trimester 5.
grow and expand with the growing fetus so it develops within an aqueous nonadhesive environment 6.
provide strong muscular contractions to expel the fetus and placenta at term To understand the function of the uterus and uterine changes during nonfertile menstrual cycles, the fine structure of the endometrium and the relationship of uterine blood supply to the endometrium will be reviewed (Fig. 44.26). The luminal surface of the endometrium is covered with a simple cuboidal/columnar epithelium. The epithelium is •Fig. 44.27 Themenstrualcycleoftheuterineendometrium.(ModifiedfromWhiteBA,PorterfieldSP.Endocrine Physiology. 4thed.Philadelphia:Mosby;2013.) continuous with mucosal glands (called uterine glands) that extend deep into the endometrium. The mucosa is vascularized by spiral arteries, which are branches of the uterine artery that run through the myometrium. The terminal arterioles of the spiral arteries project just beneath the surface epithelium. These arterioles give rise to a subepithelial plexus of capillaries and venules that have ballooned thin-walled segments called venous lakes or lacunae. The lamina propria itself is densely cellular. The stromal cells of the lamina propria play important roles during both pregnancy and menstruation.
About two-thirds of the luminal side of the endometrium is lost during menstruation and is called the functional zone (also called the stratum functionalis) (see
Fig.
44.26 ). The basal third of the endometrium that remains after menstruation is called the basal zone (also called the stratum basale). The basal zone is fed by straight arteries that are separate from the spiral arteries, and it contains all the cell types of the endometrium (i.e., epithelial cells from the remaining tips of glands, stromal cells, and endothelial cells).
Hormonal Regulation of the Uterine Endometrium During the Menstrual Cycle
Monthly oscillations in ovarian steroids induce the uterine endometrium to enter different stages. At the time of selection of the dominant follicle and its elevating production of estradiol, the uterine endometrium is just ending menstruation. The stratum functionalis has been shed and only the stratum basale remains (
Fig. 44.27).
The rising levels of estrogen during the mid to late follicular phase of the ovary induce the proliferative phase of the uterine endometrium. Estrogen induces all cell types in the stratum basale to grow and divide. In fact the definition of an “estrogenic” compound has historically been one that is “uterotropic.” Estrogen increases cell proliferation directly through its cognate receptors (ER-α and ER-β), which regulate gene expression (
Fig. 44.28 ). Estrogen also controls uterine growth indirectly through local production of growth factors. In addition, estrogen induces expression of progesterone receptors, thereby “priming” the uterine endometrium so it can respond to progesterone during the luteal phase of the ovary.