Under the influence of fibroblast growth fac tors (FGFs) at the early blastocyst stage, cells in the embryoblast differentiate into epiblast and hypoblast cells (Fig. 1A). Initially, these cells are scattered in the embryoblast, but near the time of implantation, they segregate according to their specification to become a layer of epi blast cells dorsally and hypoblast cells ventrally, adjacent to the blastocyst cavity (blastocele; Fig. 1B). Thus, a dorsal—ventral polarity is established in the embryo. In addition, some of the hypoblast cells are specified to form the anterior Visceral endoderm (AVE), and these cells migrate to what will become the cranial end of the embryo (Fig. 1B). AVE cells are classified as endoderm (as is the entire hypo blast) and are responsible for secreting nodal antagonists, including cerberus and leftyl, that act on adjacent epiblast cells to specify the cranial end of the embryo. In the absence of these inhibitors, nodal establishes the primitive streak at the caudal end of the embryo. In this manner, the cranial—caudal embryonic aXis is established near the time ofimplantation (days 5.5 to 6).

Fig1. A. At the early blastocyst stage, cells are specified to form epiblast and hypoblast cells, which are scattered in the embryoblast. B. Near the time of implantation [5.5 to 6 days], hypoblast cells move to form a layer ventral to the epiblast and adjacent to the blastocyst cavity. In addition, some cells in the hypoblast form the anterior visceral endoderm [AVE], and these cells migrate to the future cranial end of the embryo. Here, they will signal nearby epiblast cells to form cranial structures. Notice that formation and positioning of the hypoblast and AVE establishes the dorsal—ventral and cranial—caudal embryonic axes, respectively.

مواضيع ذات صلة

التحريض الأولي (العصبي)

المعنى الكامل وراء تشكل الطبقات الجنينية

تكون الحبل الظهري عند الفقاريات

تشكل الطبقات الجنينية عند الثدييات

مقارنة تكون الطيور و البرمائيات

تكون المعيدة عند الطيور

تكون المعيدة عند أجنة البرمائيات

عملية تكون المعيدة