Mustansiriyah University College of science Biology Dept Zoology
Mustansiriyah University College of science Biology Dept. Zoology th 4 class Embryology LAB. (3) NAME : 1
• 3 rd week: Further development • The embryo develops further by forming three germ layers, the process being known as gastrulation. Two layers have already formed: the epiblast and the hypoblast. These two together are termed the bilaminar embryonic disc. • From this point : • the epiblast becomes known as the ectoderm. • the hypoblast as the endoderm. • The ectoderm gives rise to the third layer that comes to lie between the two original germ layers, the intra-embryonic mesoderm.
• As a useful generalization, • 1. The ectoderm (the outer skin) forms the covering of the body (the epidermis) as well as the nervous system. • 2. The endoderm (the inner skin) forms the lining of the gastrointestinal and respiratory systems. • 3. The mesoderm (the middle skin) forms the skeletal, connective and muscle tissues of the body.
• Primitive streak formation • By the end of the second week of development there is a groove-like midline depression in the caudal end of the bilaminar embryonic disc. This marks the appearance of the primitive streak. • By the beginning of week 3 the streak deepens. At the cephalic end of the streak the primitive node develops. • The Ectoderm • Cells of the ectoderm layer migrate towards the streak and then detach from it, spreading out laterally beneath it. This migration forms a new germ layer, the intraembryonic mesoderm.
• Notochord formation • Cells derived from the primitive node migrate cranially towards the buccopharyngeal membrane. • This results in the appearance of the notochordal plate, which in turn folds in to form the solid cylinder of the notochord. The notochord thus comes to underlie • 1. the future neural tube (the future brain and spinal cord). and form • 2. the longitudinal axis for the embryo.
• Neurulation • The process of the formation of the brain and spinal cord is known as neurulation. The ectoderm germ layer gives rise to neuroectoderm, which gives rise to most of the major components of the nervous system. • The mesoderm • Differentiates further into paraxial, intermediate and lateral plate mesoderm.
• The paraxial mesoderm forms the segmented somites , which give rise to muscles, skeletal structures and dermis. • The intermediate mesoderm contributes to the urogenital system. • the lateral plate mesoderm forms the parietal and visceral layers of the serous membranes of the body. • The endoderm: • In addition to the • lining of the gastrointestinal and respiratory systems, the endoderm also gives rise to the parenchymal cells of the liver and pancreas, and of the thyroid and parathyroids, as well as the lining of the urinary bladder.
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