By Gary C. Schoenwolf PhD, Steven B. Bleyl MD PhD, Philip R. Brauer PhD, Philippa H. Francis-West PhD

Larsen's Human Embryology works as a well-organized, effortless consultant to this hugely complicated topic, putting an emphasis at the medical program of embryology and providing it in an simply digestible demeanour. excellent for visible scholars, this up-to-date scientific textbook encompasses a stronger artwork software, brand-new on-line animations, and high quality photographs all through; transparent descriptions and motives of human embryonic improvement, according to all the newest clinical discoveries and knowing, hold you abreast of the newest wisdom within the box.

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Larsen's Human Embryology

Larsen's Human Embryology works as a well-organized, simple advisor to this hugely complicated topic, putting an emphasis at the medical program of embryology and providing it in an simply digestible demeanour. excellent for visible scholars, this up-to-date scientific textbook contains a improved artwork application, brand-new on-line animations, and top of the range photographs all through; transparent descriptions and reasons of human embryonic improvement, in response to all the most modern medical discoveries and realizing, continue you abreast of the most recent wisdom within the box.

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An extracellular basement membrane is laid down between the two layers as they become distinct. The resulting two-layered embryoblast is called the bilaminar embryonic disc, or bilaminar blastoderm. With formation of the bilaminar embryonic disc, the 47 In the Research Lab Initiating Endoderm Formation The hypoblast, or primitive endoderm, is the first layer to form from the inner cell mass. Studies mainly in Xenopus and zebrafish suggest that a series of factors initiate endoderm formation. These include a T-box–containing transcription factor (VegT), which activates nodal (a member of the Tgfβ family of growth factors), which in turn induces expression of downstream transcriptional regulators (mixer, a paired-homeobox–containing transcription factor; Gata, a zinc finger GATA-binding transcription factor; and Fox, a forkhead box transcription factor).

This response is called the decidual reaction (covered in Chapter 6). The endometrial glands in the vicinity also enlarge, and the local uterine wall becomes more highly vascularized and edematous. It is thought that secretions of the decidual cells and endometrial glands include growth factors and metabolites that support growth of the implanting embryo. The uterine lining is maintained in a favorable state and is kept from sloughing partly by the progesterone secreted by the corpus luteum. In the absence of an implanted embryo, the corpus luteum normally degenerates after about thirteen days.

These trophoblast layers contribute to the extraembryonic membranes, not to the embryo proper. The syncytiotrophoblast, cytotrophoblast, and associated extraembryonic mesoderm, together with the uterus, initiate formation of the placenta. During this process, the fetal tissues establish outgrowths, the chorionic villi, which extend into maternal blood sinusoids. Many events occur in twos during the second week. Thus, a “rule of twos” constitutes a handy mnemonic for remembering events of the second week.

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