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TR056/PG1007 Lecture 4 Deriva7ve of the Ectodermal Germ Layer (Neurula7on) Dr. Neil Docherty My Teaching Objec/ves • To describe the formation of the neural tube from the emergent ectoderm • To state the origin and fate of neural crest cells • To discuss how failure to close the neuropores leads to anencephaly or spina bifida The Germ Layers Derived From Gastrulation ECTODERM WEEK 3 MESODERM ENDODERM RE: Lecture 3 ECTODERM gives rise to; • CNS • PNS • Special senses epithelia • Integumentary system Organs and structures that maintain contact with the outside world CONSTANT INTERNAL ENVIRONMENT FLUCTUATING EXTERNAL ENVIRONMENT Ectodermal Broadening During Week 3 Broadening of cephalic end during week 3 precedes the the specification of cells in the ectoderm to become neural (neuroectoderm) 1.25mm 0.68mm Bone Morphogenic Protein (BMP) Inhibition Regulates Neural Induction in Cranial Region • BMPs (BMP-4) induce production of epidermis from the ectoderm • When this is opposed the ectoderm is neuralised Fibroblast growth factor Neuroectoderm formation + BMP inhibitor gene expression (chordin, noggin, follistatin) BLOCKADE OF BMP SIGNALLING Neurulation-1 Induction of the ectoderm results in the sequential development of the -neural plate -neural folds -neural groove Neural Tube Formation Folding proceeds until the lateral edges of the neural folds meet at the midline and fuse This intially occurs in the cervical region and proceeds cranially and caudally Note that communication with the amniotic sac persists at the anterior and posterior extremities via the neuropores Closes day 25 Brain vesicles Spinal cord Closes day 28 The Neural Crest Cells • Cells at the lateral border of the elevated neural folds undergo a phenotypic alteration termed EPITHELIAL-MESENCHYMAL TRANSITION Acquire motility and invasiveness like observed previously in The epiblast during gastrulation The cells migrate to the mesoderm to give rise to a variety of cell popoulations contributing to structure and connectivity in the developed embryo; -Melanocytes -Neurons -Glia -Skeletal tissues Formation and Migration of Neural Crest Cells Intermediate levels of BMPs important The Placodes and Pharyngeal Arches Arise from cranial derived neural crest cells. STRUCTURE AND SENSES Neural Tube Defects cranial Failure to close neuropores caudal ANENCEPHALY (lethal defect) SPINA BIFIDA (split spine) Incidence varies 1:200-1:1000 Ireland has a high rate internationally 70% of cases preventable by folate supplementation MUST START BEFORE CONCEPTION Look up what folate is used for in foetal development Today’s Learning Objec/ves Your learning from today should focus on being able to; 1) Describe the formation of the neural tube from the emergent ectoderm 2) List the origin and fate of neural crest cells 3) Highlight how failure to close the neuropores leads to anencephaly or spina bifida