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Lecture 11

DEV2011: Lecture 11 summary

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Monash University

LECTURE 11 Epiblast: The epiblast is the inner cell mass in mammals. It lies above the hypoblast. In mammalian embryogenesis, the columnar cells of the epiblast are adjacent to the trophoblast, while the cuboidal cells of the hypoblast are closer to the blastocoele. The epiblast, whilst referred to as the primary ectoderm, differentiates to form all three layers of the trilaminar germ disc in a process called gastrulation. Hypoblast: The hypoblast is a tissue type that forms from the inner cell mass. [1It lies beneath the epiblast and consists of small cuboidal cells. Extraembryonic endoderm (including Yolk sac) is derived from the epiblast, which displaces the hypoblast cells. The absence of hypoblast results in multiple primitive streaks in chicken embryos. Gastrulation: Gastrulation is a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a trilaminar ("three- layered") structure known as the gastrula. These three germ layers are known as the ectoderm, mesoderm, and endoderm. Gastrulation takes place after cleavage and the formation of the blastula and primitive streak. Gastrulation is followed by organogenesis, when individual organs develop within the newly formed germ layers. Each layer gives rise to specific tissues and organs in the developing embryo. The ectoderm gives rise to epidermis, and to the neural crest and other tissues that will later form the nervous system. The mesoderm is found between the ectoderm and the endoderm and gives rise to somites, which form muscle; the cartilage of the ribs and vertebrae; the dermis, the notochord, blood and blood vessels, bone, and connective tissue. The endoderm gives rise to the epithelium of the digestive system and respiratory system, and organs associated with the digestive system, such as the liver and pancreas. [4]Following gastrulation, cells in the body are either organized into sheets of connected cells (as in epithelia), or as a mesh of isolated cells, such as mesenchyme. In amniotes, gastrulation occurs in the following sequence: (1) the embryo becomes asymmetric; (2) the primitive streak forms; (3) cells from the epiblast at the primitive streak undergo an epithelial to mesenchymal transition and ingress at the primitive streak to form the germ layers. Loss of Symmetry: In preparation for gastrulation, the embryo must become asymmetric along both the proximal-distal axis and the anterior-posterior axis. The proximal-distal axis is formed when the cells of the embryo form the “egg cylinder,” which consists of the extraembryonic ti
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