If fertilization and implantation occur, a gravid phase replaces the menstrual phase of the cycle

If fertilization and subsequent implantation occur, decline of the endometrium is delayed until after parturition. As the blastocyst becomes embedded in the uterine mucosa in the early part of the second week, cells in the chorion of the developing placenta begin to secrete hCG and other lu-teotropins. These hormones maintain the corpus luteum and stimulate it to continue the production of progesterone and estrogens. Thus, the decline of the endome trium is prevented, and the endometrium undergoes further development during the first few weeks of pregnancy.

Implantation is the process by which the blastocyst settles into the endometrium

The fertilized human ovum undergoes a series of changes as it passes through the uterine tube into the uterine cavity in preparation for becoming embedded in the uterine mucosa. The zygote undergoes cleavage, followed by a series of mitotic divisions without cell growth, resulting in a rapid increase in the number of cells in the embryo. Initially, the embryo is under the control of maternal informational macromolecules that have accumulated in the cytoplasm of the ovum during oogenesis. Later development depends on activation of the embryonic genome, which encodes various growth factors, cell junction components, and other macromolecules required for normal progression to the blastocyst stage.

The cell mass resulting from the series of mitotic divisions is known as a morula [L. morum, mulberry], and the individual cells are known as blastomeres. During the third day after fertilization, the morula, which has reached a 12-to 16-cell stage and is still surrounded by the zona pellu-cida, enters the uterine cavity. The morula remains free in the uterus for about a day while continued cell division and development occur. The early embryo gives rise to a blastocyst, a hollow sphere of cells with a centrally located clump of cells. This inner cell mass will give rise to the tissues of the embryo proper; the surrounding layer of cells, the outer cell mass, will form the trophoblast and then the placenta (Fig. 22.19).

Fluid passes inward through the zona pellucida during this process, forming a fluid-filled cavity, the blastocyst epithelium of endometrium blastocyst cavity cytotrophoblast

syncytio-trophoblast maternal blood trophoblastic uterine stroma lacunar network

extraembryonic somatic mesoderm chorion syncytiotrophoblast cytotrophoblast primary villus embryoblast

embryoblast epithelium of endometrium blastocyst cavity cytotrophoblast extraembryonic somatic mesoderm chorion chorionic cavity maternal blood trophoblastic uterine stroma lacunar network syncytio-trophoblast syncytiotrophoblast cytotrophoblast primary villus

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