Introduction to Human General Embryology Developmental Genetics
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Neural Tube Formation The formation of the Neural Tube, begins with formation of the neural plate from the ectoderm (day 18-19), neural folds (day 20-21), and the neural tube (day 22-26); the latter will develop into the future brain and spinal cord The neural plate forms as a central strip of surface ectoderm cells, just above the notochord. It is induced to become neuroectoderm through changes in cell shape (cuboidal to columnar) and proliferation. The change in cell shape and a decrease in cell number cause the neuroectodermal cells to gradually fold inward, forming neural folds (day 20-21). The neural tube continues to grow in this manner, and eventually breaks free of the surface ectoderm, which will be sealed off. Up to this point, it was supplied continuously with amniotic fluid. As the neural tube forms, the closing process is critical, occurring from the cranial to the caudal end as the anterior neuropore closes around day 24, the posterior around day 26. This is a critical event, as defects in closure may result in spina bifida or other neural tube defects. The risk of a neural tube defect can be decreased by folic acid supplements, a campaign led by the March of Dimes. The neuroepithelial cells at this stage are bipotential, capable of forming neurons or neuroglial cells. Once the neural tube has closed completely, vertebral structures develop around it, as do meninges and finally, skin from the surface ectoderm. NB: The development of the neural tube is associated with the emergence of neural crest cells from the ectoderm and these cells are pluripotent in nature as they migrate to different parts of the developing embryo where they give origin to several structures. .. Somites As the notochord and neural tube form, the mesoderm alongside them forms longitudinal columns called paraxial mesoderm. These columns divide into paired cubical bodies called somites. The somites are responsible for the segmentation of the embryo during the fourth week. Each segment consists of a somites innervated by a segmental nerve derived from a segment of the neural tube. Somites develop in pairs; the first pair develops near the cranial end of the notochord around the end of the third week. Additional pairs of somites develop in a caudal direction from days 20 to 30 (period of somite development) and the number of somites is sometimes used as a criterion for determining an embryos age. The somites give rise to most of the axial skeleton (vertebral column, ribs, sternum, and skull base) and associated musculature, as well as to the adjacent dermis. A somite is divided into two parts: The Sclerotome is the ventro-medial part of the somite. It contains a cavity of loose cells. Cells from the sclerotome migrate medially to surround the notochord and neural tube and form the axial skeleton. b. The Dermomyotome is the dorso-lateral part of the somite. Cells from the dermomyotome migrate laterally and, as its name implies, gives rise to Skeletal muscles and dermis of the skin. NB: The concept of the myotome in gross anatomy is an embryological concept. Each anatomical myotome is derived from the embryological dermomyotome that is innervated by a segmental spinal nerve. Download 3.46 Mb. Share with your friends:
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