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Ectodermal Derivatives: Neurulation Gilbert - Chapter 12 pp. 373-380 Today’s Goals • Conclude Unit 2 - Cleavage and Gastrulation • Overview of Germ Layer Derivatives/Body Plan of Vertebrates • Identify Derivatives of the Ectoderm, and Neural Ectoderm • Define Neurulation • Define Primary versus Secondary Neurulation • Identify different mechanism of Neurulation in mammals • Define “Neural Crest” & identify the adult structures formed by these cells Ectoderm • Surface Ectoderm: – Epidermis, hair, nails, olfactory epithelium, mouth epithelium, tooth enamel, lens, cornea • Neural Crest: – PNS (sympathetic and parasympathetic), adrenal medulla, melanocytes, facial cartilage, dentine of teeth • Neural Tube: – Brain, neural pituitary, spinal cord, motor neurons, retina Neurulation • During gastrulation, a population of the dorsal ectoderm is specified to become neural ectoderm – Involves signals: Noggin, chordin, follistatin – Cells become columnar in their appearance: Neural plate – The neural plate will be induced to form a hollow neural tube = spinal cord, brain, etc. Formation of the Neural Tube • 2 mechanisms • Primary Neurulation – Cells that flank neural plate induce the neural plate to proliferate, invaginate, pinch off to form neural tube • Secondary Neurulation – Mesenchyme cells of the ectoderm coalesce to form a solid cord – Cord then hollows – This happens further posteriorly Primary Neurulation • Divides the ectoderm into 3 populations – Neural tube, overlying ectoderm, neural crest • Edges of neural plate thicken – Move up to form neural folds – Between folds, neural groove forms – Forms the midline of the embryo • Neural folds come together at midline – Fuse to form neural tube – Neural crest cells migrate out of dorsal-most neural tube Primary Neurulation • The neural plate is induced to form by signals from the underlying mesoderm (notochord) • Occurs in a rostro-caudal (anteriorposterior) direction Neural Tube Closure: Xenopus Model • Neural tube separates from surrounding ectoderm and seals to form hollow tube – Mediated by expression of adhesion molecules – N-cadherin and N-CAM are expressed in neural plate – E-cadherin is expressed in remaining ectoderm – Thus, surface ectoderm and neural plate can’t adhere to each other Neural Tube Closure: Xenopus model • If N-cadherin is overexpressed in the surrounding surface ectoderm, neural tube closure is impeded – This is achieved by injecting N-cadherin mRNA into the embryo at the 2-cell stage Neurulation in Mammals • Neural tube closes in several places along axis (not in A to P direction) • If any one of those closures fails, neural defects result Mammalian Neural Tube defects • Spina Bifida – Posterior Neural tube does not fuse – Spinal cord remains exposed • Anencephaly – Anterior Neural tube does not fuse – Forebrain ceases to develop – Lethal • Occur in 1 of 500 births in humans • Preventable through diet (over 50%) – Folic Acid Secondary Neurulation • Mesenchyme cells condense to form solid medullary cord • Cord hollows out • This occurs in the posterior end of the embryo (lumbar, tail regions) Neural Crest • Originate in dorsal neural tube • Migrate extensively to populate the body with a variety of cell types – Neurons, glia, adrenal medulla, pigment cells of the skin (melanocytes), skeletal components of head Neural Crest Formation • Neural crest cells form at the junction of the neural plate and the epidermal ectoderm – At the boundary, cells are induced to express neural crest specific transcription factors: Slug, FoxD3 Slug and FoxD3 in Neural Crest Formation • FoxD3 – If inhibited, no neural crest cells form – Thus, necessary for neural crest specification • Slug – If inhibited, neural crest cells cannot migrate – Thus, necessary for neural crest migration Neural Crest: 4 domains • Cranial (cephalic) neural crest – Cartilage, bone, cranial neurons, glia and connective tissues of the face, bones of middle ear, jaw, tooth primordia • Trunk neural crest – Dorsal root ganglia and sensory neurons, sympathetic ganglia, melanocytes - ALL pigment cells of body! • Vagal and sacral neural crest – Parasympathetic nerves of gut • Cardiac neural crest – Melanocytes, neurons, division between aorta & pulmonary artery SO Complex • You could take a whole course on developmental neurobiology • We can’t even scratch the surface in this class! But wait!. . . . • Neural crest cells are individual migrating cells Difficult to see in the embryo • How did scientists find out where they were located, and what they became later on? • Ex. Nicole Le Douarin and the Quail-Chick Chimera • http://www.sdbonline.org/archive/dbcinema/ledouarin/ ledouarin.html - anchor Example of a Quail-Chick Chimera Not Le Douarin, Not Neural crest, but somite