estrogen, FSH, and luteinizing hormone (LH) blood levels peak.

The denuded surface of the endometrium becomes reepithelialized, the functionalis layer becomes thickened ( 3 mm thick), and its helical arteries are reestablished and begin to become coiled; uterine glands are not as yet coiled but begin secretion.

Helical arteries and uterine glands of the functionalis become highly coiled; the functionalis reaches its full thickness ( 5 mm thick); the uterine glands are filled with their secretory products; cells of the stroma undergo decidual reaction and accumulate glycogen and lipids that provide nutrients for the blastocyst embedding itself in the endometrium.

blood flow dislodges the blood-filled necrotic functional layer, which becomes sloughed as the hemorrhagic discharge, so that only the basal layer of the endometrium remains as the lining of the uterus.

During pregnancy, the smooth muscle cells of the myometrium undergo estrogen-induced hypertrophy and hyperplasia, increasing the thickness of the muscle wall of the uterus. The smooth muscle cells increase from the 50-mm length of the nonpregnant uterus to as much as 500 mm in the gravid uterus.

•These smooth muscle cells acquire gap junctions that facilitate their coordinated contractile actions.

•At parturition, oxytocin and prostaglandins cause the uterine muscles to undergo rhythmic contractions that assist in expelling the fetus.

Subsequent to delivery, the lack of estrogen is responsible for apoptosis of many of the smooth muscle cells with a consequent reduction in the thickness of the myometrium.

Cervix of the Uterus

The cervix is the inferior aspect of the uterus and it protrudes into the vagina. The lumen (canal) of the cervix is continuous with the lumen of the uterus (superiorly) and the vaginal canal (inferiorly).

•The wall of the cervix is thick and is composed of a dense irregular fibroelastic connective tissue housing some smooth muscle cells and branched cervical glands.

The cervical glands produce a serous secretion that lubricates the vagina.

After fertilization, these glands produce a thick, viscous mucous that impedes the entry of spermatozoa and microorganisms into the uterine lumen.

•Its lumen is lined by a simple columnar epithelium whose cells secrete a mucous substance.

•The inferior aspect of the lumen is lined by a stratified squamous nonkeratinized epithelium, which is continuous with the vaginal epithelium.

•The thick cervical wall becomes thinner and less rigid at parturition due to the effects of the hormone oxytocin.

FERTILIZATION, IMPLANTATION, AND THE PLACENTA

Fertilization and Implantation

The union of the haploid sperm pronucleus with the pronucleus of the haploid ovum is known as fertilization, whereby a new diploid cell, the zygote, is formed. Fertilization usually occurs in the ampulla of the oviduct.

•As the zygote travels along the oviduct, it undergoes mitotic cell division, known as cleavage, to form a solid cluster of cells, known as the morula. Approximately