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DevelopmentW2010.Lecture8notes - Development BIO 120 Winter...

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Development BIO 120 Winter 2010 Jeremy Lee Eva Murdock Patrick Yuh Lecture 8 Neurulation I. Basic Process of Neurulation a. Neurulation involves the division of the ectoderm into 3 types of cells (Fig. #1) 1. Neural tube : forms neurons and glia of the central nervous system (CNS.) CNS = brain and spinal cord . 2. Epidermis : skin, hair, sweat glands, mammary glands, nails. 3. Neural crest: several types of cells, including: neurons and glia of peripheral nervous system (PNS); skin pigment cells; endocrine cells of various types; as well as cartilage, bones and connective tissues of face. b. Neurulation is induced in ectoderm by signals from cells in mesoderm; this causes ectoderm to proliferate, and the central cells to invaginate and form an internal neural tube. c. At neurulation and just following neurulation, most vertebrate animals appear surprisingly alike (Fig. #2); it is in later stages that significant divergence can be seen. II. Formation of the Neural Tube (Figs. #3, #4, #5) a. Cells of ectoderm change shape, due to inductive signals from underlying mesoderm. 1. Cells along midline (presumptive neural cells) elongate, forming the neural plate. 2. Cells flanking them (presumptive epidermis) flatten. 3. Concomitant elongation of central cells and flattening of the flanking cells causes the cells at the intersection of the two to rise, creating neural folds . Cells in the region of the folds will become the neural crest. b. Cells along the midline become median hinge point (MHP) cells. They are probably derived from neural plate cells at Hensen's node that undergo convergent extension. 1. These MHP cells attach to the underlying notochord. 2. Notochord induces MHP cells to shorten and become wedge shaped, forming a furrow above them. c. Cells laterally, called the dorsolateral hinge point (DLHP) cells, elongate and become wedge-shaped; the flanking epidermal cells continue to elongate, forcing the neural plate to bend. d. As neural tube forms, neural cells stop expressing E-cadherin, and begin to express N- cadherin and N-CAM; presumptive epidermal cells continue to express E-cadherin. 1. These changes cause the dorsal ends of the bending neural tube to adhere to each other. 2. They also cause the closed neural tube to separate from the surface ectoderm and eventually to lie internally to this developing epidermis. 3. Finally, closure depends on expression of certain genes ( Pax3, Sonic hedgehog , openbrain ) as well as dietary factors (folic acid, cholesterol). Defects caused by mutations in these genes or from dietary deficiencies include spina bifida , anencephaly (Fig. #6).
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e. Neural tube formation usually is initiated at the most anterior region of the embryo and
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DevelopmentW2010.Lecture8notes - Development BIO 120 Winter...

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