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Embryonic Flexion and Folding Bilaminar embryo End of gastrulation Remember placement of heart primordium The lateral plate mesoderm splits. Each leaf will fold toward midline. Narrowing yolk sac and gut & enclosing coelom. Note how amnion follows embryonic folding. Narrowing of gut tube and yolk sac. Folding complete: midgut remains open to vitelline duct Foregut is initially suspended by dorsal and ventral mesentery. Ventral mesentery reabsorbed. Dorsal remains. Organ in intraperitoneal. Intraembryonic coelom continues anterior of the heart primordium. What is different about the embryo after flexion? Embryonic sources of adult diaphragm The ectoderm: neurulation, neural tube, neural crest Neural tissue is said to be induced by mesodermal tissue Signals divert midline ectoderm from an ectodermal fate. Shaping the neural plate PRIMARY NEURULATION Neural induction, formation of the neural plate Formation of of the neural groove and neural folds Closure of neural Neural crest folds, formation of neural tube and neural crest Initially, the neural tube is composed of a single layer of neuroepithelial cells Dorsal view Ventral view Days 21-22 Day 23 REGIONS OF NEURAL TUBE CLOSURE How are billions of CNS cells (neurons and glia) generated? The neuroepithelium is a layer of rapidly dividing stem cells. What are the mechanisms for dispersal of cells from the ventricular layer? NEUROGENESIS IN THE CEREBRAL CORTEX Cerebral cortex 1. Developing post-mitotic neuroblasts use special glial cells and their processes as migration paths. 2. Neurons born at early stages migrate to the deepest layers of the cortical plate. 3. Neurons born at later stages form the more superficial layers of the cortex. 4. There are other mechanisms for lateral dispersal. 5. For each region neurons tend to be born before glia. REGIONALIZATION OF THE CNS PRIMARY AND SECONDARY VESICLES AND FLEXURES Cephalic Rhombomere flexure Cervical flexure Pontine flexure Ectomesenchyme The neural crest slug THE REGION OF THE NEURAXIS FROM WHICH A CREST CELL MIGRATES DETERMINES THE TARGET REACHED BY ITS DERIVATIVES cranial truncal cardiac vagal truncal sacral .

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The Genetic Basis of Mammalian Neurulation
REVIEWS THE GENETIC BASIS OF MAMMALIAN NEURULATION Andrew J. Copp*, Nicholas D. E. Greene* and Jennifer N. Murdoch‡ More than 80 mutant mouse genes disrupt neurulation and allow an in-depth analysis of the underlying developmental mechanisms. Although many of the genetic mutants have been studied in only rudimentary detail, several molecular pathways can already be identified as crucial for normal neurulation. These include the planar cell-polarity pathway, which is required for the initiation of neural tube closure, and the sonic hedgehog signalling pathway that regulates neural plate bending. Mutant mice also offer an opportunity to unravel the mechanisms by which folic acid prevents neural tube defects, and to develop new therapies for folate-resistant defects. 6 ECTODERM Neurulation is a fundamental event of embryogenesis distinct locations in the brain and spinal cord .By The outer of the three that culminates in the formation of the neural tube, contrast, the mechanisms that underlie the forma- embryonic (germ) layers that which is the precursor of the brain and spinal cord. A tion, elevation and fusion of the neural folds have gives rise to the entire central region of specialized dorsal ECTODERM, the neural plate, remained elusive. nervous system, plus other organs and embryonic develops bilateral neural folds at its junction with sur- An opportunity has now arisen for an incisive analy- structures. face (non-neural) ectoderm. These folds elevate, come sis of neurulation mechanisms using the growing battery into contact (appose) in the midline and fuse to create of genetically targeted and other mutant mouse strains NEURAL CREST the neural tube, which, thereafter, becomes covered by in which NTDs form part of the mutant phenotype7.At A migratory cell population that future epidermal ectoderm.
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Clonal Dispersion During Neural Tube Formation 4097 of Neuromeres
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And Krox-20 and on Morphological Segmentation in the Hindbrain of Mouse Embryos
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Cephalic Neurulation in the Mouse Embryo Analyzed by SEM and Morphometry
THE ANATOMICAL RECORD 203:375-396 (1982) Cephalic Neurulation in the Mouse Embryo Analyzed by SEM and Morphometry ANTONE G. JACOBSON AND PATRICK P.L. TAM Department of Zoology. Uniuersity of Texas, Austin, TX 78712 (A.G.J.) and Department of Anatomy, (‘hinese University of Hong Kong, Shatin, N.T., Hong Kong IP.PL.T) ABSTRACT A detailed account of mouse neurulation is given based mostly on SEM analysis over 20 hr of development. Many observations and measure- ments were made on staged living embryos and on embryos prepared for scanning and light microscopy to help deduce what mechanisms may contribute to neural tube formation. Each lateral half of the early cephalic neural plate makes a convex bulge, opposite to the way it must fold to form a tube. Underlying mesenchyme and matrix are reported to have a role in forming these bulges. Processes that form the tube must overcome this opposed folding and the forces that produce it. Crani- al flexure begins long before tube formation. The flexure commences at the rostra1 tip of the cephalic neural plate, then the apex of the flexure migrates caudally to the mesencephalic region. Early appearance of this flexure imposes a mechanical impediment to tube closure in forebrain and midbrain regions. Tube closure begins in the cervical region exactly where the neural plate is reflected dorsally by a bend in the embryo. This bend may mechanically assist closure in this region. Cells of the mouse neural plate are reported to contain organized microfilaments and mi- crotubules, and the plate cells appear to change shape (reduce apical area and in- crease cell height) in the same manner as that suggested in embryos of some other species to contribute to neural tube formation.
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CENTRÁLNÍ a PERIFERNÍ NERVOVÝ SYSTÉM Mikroskopická Stavba A
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Sonic Hedgehog Signaling Directs Patterned Cell Remodeling During
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Neural Crest Formation in the Head of the Mouse Embryo As Observed Using a New Histological Technique
J. Embryol. exp. Morph. Vol. 64, pp. 105-120, 1981 \ Q5 Printed in Great Britain © Company of Biologists Limited 1981 Neural crest formation in the head of the mouse embryo as observed using a new histological technique By DAVID H. NICHOLS1 From the Department of Anatomy, School of Medicine, University of Virginia SUMMARY A histological technique is described which results in the differential staining of neural crest cells. This is used to describe the formation and early migration of crest cells in the head of the mouse embryo. The first indications of crest formation are seen in the midbrain/ anterior hindbrain at 3-4 somites where crest cells accumulate in the basal surface of the ectodermal epithelium near the future margin of the neural plate. Shortly thereafter (4-6 somites) these cells disrupt the basal surface of the epithelium and escape as mesenchyme. The apical epithelial cells in this region become the surface ectoderm adjacent to the neural plate. Subsequently, crest is formed from neural plate rather than surface ectoderm. In addition, mesenchyme is formed from presumptive surface ectoderm in a groove in the lateral portion of the fold between the forebrain and the midbrain. By 5-7 somites, crest mesenchyme is formed at all levels of the midbrain, hindbrain, and from the margins of the forebrain adjacent to the optic pits. Because of the bending of the embryonic axis, forebrain crest cells appear to migrate dorsally over the presumptive eye where they are met by ventrally migrating midbrain crest cells. Crest formation continues in the region of the midbrain and hindbrain during, and for an undetermined period after closure of the head folds at between 8 and 16 somites.
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