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Bio 3411 Lecture III. Nervous System Embryology September 5, 2012 Reading Lecture III. Nervous System Embryology NEUROSCIENCE: 5th ed, pp. 477-506 NEUROSCIENCE: 4th ed, pp. 545-575 Bio 3411 Wednesday September 5, 2012 September 5, 2012 Lecture III. Nervous System Embryology 2 Summary from Lecture II Biology Brain Diseases • Interfere with brain functions as heart disease • Understanding the brain is THE major question interferes with the circulation. • Many diseases have a strong genetic component. in biology and science. • Prevalence is high: ≈ 15 - 30% of the population. • Is it possible for the brain to understand itself? • Cost is high: >> $2+ Trillion/year in care, lost income, social services, etc., in the US. • The brain like any organ has functions: input, • Impact (personal, family, societal) is persistent, output, “thought”, communication. September 5, 2012 Lecture III. Nervous System Embryology pervasive, enormous, incalculable. 3 September 5, 2012 Lecture III. Nervous System Embryology 4 Developmental Neurobiology Issues for thought Major scientific challenge. Neurons (=nerve cells) ≈ 100 Billion • To understand the normal construction and operation of the embryonic (and adult) nervous system. [the number of stars in our galaxy – the Milky Way!!] Synapses (=clasp) 1/1,000,000th of all stars & planets in the universe/person • To understand human neuronal diseases, and possible ways to repair and renew the nervous system. [less than the total of human synapses of people living in the St.L area!!] vs Genes 50% of ≈ 20,000-25,000 genes in the human • To understand neuronal plasticity, and learning and memory. genome are expressed only in Brain [70% of the balance are also expressed in the nervous system; the total is 85% of • To understand the evolutionary diversification of nervous systems. the genome] September 5, 2012 Lecture III. Nervous System Embryology 5 September 5, 2012 Lecture III. Nervous System Embryology 6 1 Bio 3411 Lecture III. Nervous System Embryology September 5, 2012 Major Questions Development proceeds by progressive developmental restrictions 1. Origins: Where do neurons come from? pluripotent, stem cell 2. Identity: How does a cell become a neuron? 3. Specificity: How does a neuron make the right connections? 4. Plasticity: Does experience modulate the above? differentiated September 5, 2012 Lecture III. Nervous System Embryology 7 September 5, 2012 Lecture III. Nervous System Embryology 8 Constructing a Nervous System Developmental Restrictions may be: EGG 1) Genetic (programmed by genes) or 2) Epigenetic (determined by environment) Sort Proliferate pluripotent, stem cell Shape Specify genetic environmental September 5, 2012 differentiated Lecture III. Nervous System Embryology 9 Axons & Synapses September 5, 2012 BRAIN Cell Death & Life Experiences Lecture III. Nervous System Embryology 10 Vertebrate Embryology (Frog) • Egg – Blastula An oocyte contains positional information (cytoplasmic determinants) contributed maternally. Some maternal RNAs are not uniformly distributed throughout the egg. • Germ bands and cell types. The first informational axis is intrinsic to oocyte! Embryology • Body axes. Animal • Folding, involutions and morphogenic movements. • Origin, migration and differentiation of neurons. September 5, 2012 Lecture III. Nervous System Embryology Vegetal (yolk) 11 September 5, 2012 Lecture III. Nervous System Embryology 12 2 Bio 3411 Lecture III. Nervous System Embryology September 5, 2012 The point of entry of the sperm determines a second positional axis, ventral (entry point) and dorsal (opposite to entry point). Fertilization triggers an influx of calcium, that sweeps across the egg. This causes rapid release of cortical granules to form the fertilization envelope, blocking polyspermy. Animal Cortical granules Sperm entry point Animal Sperm entry point Cortical Rotation mixes cytoplasmic determinants and creates the dorsal-ventral axis. Ca+2 Vegetal (yolk) Animal Gray Crescent (future blastopore) Ventral Vegetal (yolk) Dorsal future Nieuwkoop Center Vegetal (yolk) September 5, 2012 Lecture III. Nervous System Embryology 13 September 5, 2012 Lecture III. Nervous System Embryology 14 Cortical Rotation redistributes maternal cytosolic determinants that are partitioned in different dividing embryonic cells Gray Crescent Formation in Xenopus Gray Crescent Nieuwkoop Center Courtesy of Jeffrey Hardin University of Wisconsin September 5, 2012 Lecture III. Nervous System Embryology 15 September 5, 2012 Lecture III. Nervous System Embryology 16 Fate Mapping the Blastula: 3 Major Spatial Axes Formed by Gradients of Signaling Molecules Early Cell Divisions in Amphibians Generate a Blastula (Animal) (Ventral) blastomere blastocoel Blastula future Blastopore Site of sperm entry (Dorsal) (Vegetal) 1. Animal/Vegetal (Maternal Determinants) 2. Dorsal/Ventral (Sperm Entry, Cortical Rotation) September 5, 2012 Lecture III. Nervous System Embryology 17 September 5, 2012 Lecture III. Nervous System Embryology 18 3 Bio 3411 Lecture III. Nervous System Embryology September 5, 2012 Fate Mapping the Blastula: 3 Major Spatial Axes Formed by Gradients of Signaling Molecules; Nieuwkoop Center Induces the Spemann Organizer (Anterior) Fate Map of the Blastula: 3 Principle Germ Bands (Layers) Generated Ectoderm (Skin, Neurons) (Anterior) (Animal) (Posterior) (Posterior) (Ventral) Spemann Organizer Blastopore Blastopore (Dorsal) Mesoderm Nieuwkoop Center (Notocord, Muscle, Kidney, Bone, Blood) (Vegetal) 1. Animal/Vegetal (Maternal Determinants) 2. Dorsal/Ventral (Sperm Entry, Cortical Rotation) Endoderm (Lining of Gut, Lungs, Placenta in Mammals) 3. Anterior/Posterior (Spemann Organizer) September 5, 2012 Lecture III. Nervous System Embryology 19 Gastrulation of the Amphibian Blastula A & B: Lecture III. Nervous System Embryology Lecture III. Nervous System Embryology 20 Gastrulation of the Amphibian Blastula C & D: “tissue origami” by large-scale cellular migrations and morphogenesis September 5, 2012 September 5, 2012 21 “tissue origami” by large-scale cellular migrations and morphogenesis September 5, 2012 Lecture III. Nervous System Embryology 22 Gastrulation of Xenopus Blastula: 3-D Microscopic (Confocal) Reconstruction Gastrulation of the Amphibian Blastula E & F: “tissue origami” by large-scale cellular migrations and morphogenesis (Ewald, et al., 2004) September 5, 2012 Lecture III. Nervous System Embryology 23 September 5, 2012 Lecture III. Nervous System Embryology 24 4 Bio 3411 Lecture III. Nervous System Embryology September 5, 2012 Gastrulation and neurulation in Xenopus (Courtesy of Jeffrey Hardin, University of Wisconsin) Endoderm and Mesoderm Involute with Gastrulation Mesoderm underlies (Neuro)Ectoderm, And induces the neural plate Neural plate (apposition of different germbands/ layers) September 5, 2012 Blastopore and yolk plug Lecture III. Nervous System Embryology 25 September 5, 2012 Lecture III. Nervous System Embryology 26 Early cell divisions in the Zebrafish embryo (Courtesy of Paul Myers, University of Minnesota) Neurulation Ant Closure of the neural tube. Post September 5, 2012 Lecture III. Nervous System Embryology 27 Neurulation September 5, 2012 Lecture III. Nervous System Embryology 28 Origin of Floor Plate and Neural Crest D Formation of Neural Crest Cells (makes PNS, endocrine cells, pigment cells, connective tissue). September 5, 2012 Neural Crest Lecture III. Nervous System Embryology V 29 September 5, 2012 Lecture III. Nervous System Embryology 30 5 Bio 3411 Lecture III. Nervous System Embryology September 5, 2012 Cephalization and Segmentation of the Neural Tube Origin of Floor Plate and Neural Crest Neural Crest September 5, 2012 Lecture III. Nervous System Embryology 31 Lecture III. Nervous System Embryology 32 Labeling of Single Precusors Reveal Shared Clonal Origins of Radial Glia and Neurons; and Direct Visualization of Radial and Tangential Migrations Cortical Development: Laminar structure of the cortex is constructed from the inside-out Neurons are born in the ventricular layer and migrate radially along glia to their differentiated adult cortical layer. September 5, 2012 The earliest born neurons are found closest to the ventricular surface (thymidine pulse-chase labeling of dividing cells). (Rakic, 1974) (Nadarajah, et al., 2002) (Cepko and Sanes Labs, 1991) (Nector, et al., 2001) September 5, 2012 Lecture III. Nervous System Embryology 33 The amount of yolk determines the symmetry of early cleavages and the shape of the blastula 1. Isolecithal eggs (protochordates, mammals): 2. Mesolecithal eggs (amphibians): September 5, 2012 Lecture III. Nervous System Embryology 34 Mammalian eggs have no yolk, so early divisions resemble isolecithal eggs (protochordate-like). However, later stages resemble the blastodisc of telolecithal eggs (reptile/bird/fish-like) a) Blastula flattens into the inner cell mass. 3. Telolecithal eggs (reptiles, birds, fish): b) Endodermal cells form the trophoblast and placental structures. (epiboly) (blastodisc) September 5, 2012 Lecture III. Nervous System Embryology 35 September 5, 2012 Lecture III. Nervous System Embryology 36 6 Bio 3411 Lecture III. Nervous System Embryology September 5, 2012 What this Lecture was About 1. Three germbands (layers), ectoderm (skin and neurons), mesoderm (muscle, blood and internal organs) and endoderm (lining of the gut). 5. Guideposts: a) Fertilization/Cortical Rotation. b) Blastula (hollow ball of cells). c) Gastrulation (inside-out involution of surface cells to the interior, through the blastopore). d) Neurulation (neural tube formation). e) Segmentation/Cephalization. f) Birth of neurons from the ventricular zone. Radial, then tangential migration to final destination. 2. Development proceeds from pleuripotency (stem cells) to the differentiated state (adult neuron). 3. Neuronal induction requires specific contact between groups of cells; embryonic morphogenesis allows this to occur. 4. Positional information is created early by asymmetric distribution of molecules. These form axes (Animal/Veg, D/V, Ant/Post) that guide the movement of embryonic cells. September 5, 2012 Lecture III. Nervous System Embryology What this Lecture was About (cont): 37 September 5, 2012 Lecture III. Nervous System Embryology 38 Central Aspects of Embryogenesis Embryogenesis (cont.): 1. Oocyte possess maternal cytoplasmic determinants. 2. Fertilization triggers calcium influx, and creates dorsal-ventral axis by cortical rotation. 7. Apposition of future mesoderm with neuroectoderm induces the neural plate. 3. Cell divisions, synchronous at first, then asynchronous. 4. Blastula created. (“Hollow” ball of cells) 8. Neurulation. (Lateral neural folds bend over the midline and fuse into the neural tube.) 6. Germ bands (ectoderm, mesoderm, endoderm) created by molecular signals along the Animal/Vegetal axis. 9. Neural crest cells derived from leading edge of neural folds, migrate into somites to form the PNS. 5. Gastulation. (Involution of superficial cells through the blastopore). 10. Segmentation, anterior enlargement (cephalization), cortical development and spinal specification. 6. Anterior-posterior axis created by Spemann organizer. September 5, 2012 Lecture III. Nervous System Embryology 39 September 5, 2012 Lecture III. Nervous System Embryology 40 END 7