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zygote that result in successive 2, 4, 8 and 16 cell stages. These divisions take place over a period of about 3 days as the developing conceptus passes down the uterine tube. At about the time the 16 cell morula reaches the uterine cavity, fluid penetrates between some of the celis and produces a cavity in the solid ball of cells. The conceptus is now calied a blastodermicvesicle (blastocyst). It consists of an outer layer of cells called the trophoblast, a cavity of the blastodermic vesicle (blastocoele) and an inner cell mass. After 3 days in the uterine cavity, the zona pellucida degenerates, and the sticky trophoblastic cells adhere to the endometrium.
Establishment of Ectoderm, Entoderm and Mesoderm. In the eighth day of development the inner cell masscavitates to form an ectodermally lined amniotic cavity. The ectoderm of the embryonic disk will eventually give rise to the neural tube, neural crest and epidermis. An inner entodermal layer of cells also differentiates from the inner cell mass. These cells proliferate to form the yoik sac. The dorsal portion of the yoIk sac later will become incorporated into the embryo as the primitive gut.Embryonic mesoderm arises from an elongate mass of cells called the primitive streak. The mesodermal cells turrí inward along the midline and move laterally, insinuating thernselves between the ectoderm and entoderm. In its forward, caudal and lateral movement, the embryonic mesoderm eventually joins the extraembryonic mesoderm that arises from the trophoblast. The notochord arises as a midlineforw%rd growth of cells from the primitive (Hensen's) node. In the third week of development, the embryonic mesoderm will have differentiated into paraxial (somite, dorsal), intermediate and lateral mesoderm. The paraxial mesoderm will develop further into paired somites that eventually give rise to vertebrae, ribs, skeletal muscle and connective tissues. The intermediate mesoderm will differentiateinto much of the urogenital system. The lateral mesoderm, like the extraembryonic mesoderm, splits to form a coelom. That lateral mesoderm adjacent to the ectoderm is somatic mesoderm, while that next to entoderm is splanchnic mesoderm. Somatic mesoderm later will give rise to body wall tissues. Splanchnic mesoderm further differentiates into the cardiovascular system, smooth muscle and connectivetissues in the walls of most visceral structures, mesenteries and the spleen.
Development of the Placenta. After the attachment of the blastodermic vesicle to the uterus at about the sixth postfértilization day, the trophoblast proliferates rapidly, and the conceptus begins to implant into the compacta layer of the endometrium. It is completely embedded in the uterine stroma by the eleventhpostfertilization day. In the rapid proliferation of cyto-

trophoblastic cells, cytokinesis has not kept pace with nuclear division, and an outer syncytial trophoblast appears over the single inner layer of cytotrophoblast. The coalescence of lacunae formed in the syncytial trophoblast leads to the formation of primary stem villi. These are most extensive in the trophoblast that faces the deeperlayers of the endometrium. It is in this region where most of the nutriments are being supplied to the trophoblast from invaded uterine glands and blood vesseis. The primary stem villi consist of a core of cytotrophoblast surrounded by syncytial trophoblast. Later, mesoderm. invades these villi, and they become secondary stem villi containing a core of connective tissue. By the end of the thirdweek, blood vesseis start to form in the secondary villi. With the vascularization of the trophoblast it is called the chorion. That part of the chorion that is the deepest in the uterine wall becomes the chorion frondosum portion of the placenta; the rest of it loses its villi and is called the chorion laeve. An anchoring villus and its free floating villi constitute a cotyledon. Septa of the...
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