Lecture 14 Notes - Vandan Desai BIOL 442Developmental...

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Vandan Desai BIOL 442—Developmental Biology 1 Lecture #14—Neurulation and Neural Crest I. Organogenesis :: Case Study = The BRAIN A. Induction— brain’s outermost layer (ectoderm) => skin + nervous tissue; early signals tell it to be neural = induction B. Morphogenesis (Neurulation)— how does ectodermal tissue fold to form neural tube ; neural tube forms brain anteriorly and spinal cord posteriorly C. Growth— ex. neural tube has to expand to form brain and fill its ventricles w/ cerebrospinal fluid (CSF) D. Patterning— brain is different from spinal cord; it’s specialized/patterned to perform distinct functions, such as: vision or olfaction E. Wiring —establish synaptic connections b/w neurons II. Neurulation :: process of forming the neural tube => brain & spinal cord A. Neural Tube is Shaped during Primary and Secondary Neurulation i. Primary Neurulation —occurs in head and trunk ( anterior regions) Ectoderm can be divided into three sections: Internally positioned neural tube (forms brain & spinal cord ) Externally positioned epidermis (forms skin ) Neural crest cells Form in region connecting neural tube and epidermis Migrate to new locations Generate peripheral neurons , glia , pigment cells , etc. Characteristics Cells are epithelial Organized in layers Apical region = tight junctions + adherens junctions Closely connected; prevent diffusion of substances from the environment Adherens junctions composed of catenins, cadherins, and microfilaments Basal region = attached to a basement membrane (extracellular matrix) Bending & Closure Cells delaminate below to form tube like structure Head will expand while tail will narrow ii. Secondary Neurulation —occurs in caudal/ posterior regions Characteristics— Cells are mesenchymal Migratory Tissues are generally not highly organized Condensation—mesenchymal cells slowly coalesce Cavitation—to form a cord like structure; looks same as primary but means of formation different B. Stages of Primary Neurulation i. Summary of Events: Shortly after neural plate formation, edges thicken and move upward to form neural folds
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Vandan Desai BIOL 442—Developmental Biology 2 U-shaped neural groove appears in center of plate Neural folds migrate to the midline of the embryo Fuse and form neural tube beneath the overlying epidermis ii. Embryonic Axes Anterior-posterior (A-P) = head to tail Medio-lateral (M-L) = becomes dorso-ventral (D-V) once neurulation is complete Lateral becomes dorsal & medial becomes ventral Narrowing of the M-L axis by cell movement results in lengthening along the A-P axis ( convergent extension ) iii. Details : 4 Stages of Primary Neurulation ( SEM images of chick embryo ) Stage 1 & 2 = Formation and Shaping of the Neural Plate Formation of the neural plate— Apicobasal thickening of neural plate (red arrow) relative to non-neural ectoderm Same cell surface area but different shape Short/Fat => Tall/Skinny
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