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Lecture III. Nervous System Embryology Bio 3411 Wednesday September 8, 2010 Slide 2 Reading NEUROSCIENCE: 4 th ed, pp 545-75 2Lecture III. Nervous System Embryology September 8, 2010 Slide 3 Biology Understanding the brain is THE major question in biology and science. Is it possible for the brain to understand itself? The brain like any organ has functions: input, output, thought, communication. 3Lecture III. Nervous System Embryology Summary from Lecture II September 8, 2010 Slide 4 Brain Diseases Interfere with brain functions as heart disease interferes with the circulation. Many diseases have a strong genetic component. Prevalence is high: 15 - 30% of the population. Cost is high: >> $2+ Trillion/year in care, lost income, social services, etc., in the US. Impact (personal, family, societal) is persistent, pervasive, enormous, incalculable. 4Lecture III. Nervous System Embryology September 8, 2010 Slide 5 Issues for thought Neurons (=nerve cells) 100 Billion Synapses (=clasp) 1/1,000,000 th of all stars & planets in the universe/person [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 genome are expressed only in Brain [70% of the balance are also expressed in the nervous system; the total is 85% of the genome] 5Lecture III. Nervous System Embryology September 8, 2010 Slide 6 Terrific scientific challenge. To understand the normal construction and operation of the embryonic (and adult) nervous system. To understand human neuronal diseases, and possible ways to repair and renew the nervous system. To understand neuronal plasticity, and learning and memory. To understand the evolutionary diversification of nervous systems. Developmental Neurobiology September 8, 20106Lecture III. Nervous System Embryology Slide 7 1. Origins: Where do neurons come from? 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? September 8, 20107Lecture III. Nervous System Embryology Major Questions Slide 8 pluripotent, stem cell differentiated Development proceeds by progressive developmental restrictions September 8, 20108Lecture III. Nervous System Embryology Slide 9 Developmental Restrictions may be: 1) Genetic (programmed by genes) or 2) Epigenetic (determined by environment) pluripotent, stem cell differentiated genetic environmental September 8, 20109 Slide 10 Constructing a Nervous System Axons & Synapses Cell Death & Life Experiences Proliferate Sort ShapeSpecify September 8, 201010Lecture III. Nervous System Embryology EGG BRAIN Slide 11 Embryology Egg Blastula Germ bands and cell types. Body axes. Folding, involutions and morphogenic movements. Origin, migration and differentiation of neurons. September 8, 201011Lecture III. Nervous System Embryology Slide 12 Vertebrate Embryology (Frog) An oocyte contains positional information (cytoplasmic determinants) contributed maternally. Some maternal RNAs are not uniformly distributed throughout the egg. The first informational axis is intrinsic to oocyte! Animal Vegetal (yolk) September 8, 201012Lecture III. Nervous System Embryology Slide 13 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 Vegetal (yolk) Sperm entry point Ca +2 Cortical granules September 8, 201013Lecture III. Nervous System Embryology Slide 14 The point of entry of the sperm determines a second positional axis, ventral (entry point) and dorsal (opposite to entry point). Cortical Rotation mixes cytoplasmic determinants and creates the dorsal-ventral axis. Animal Vegetal (yolk) Sperm entry point Animal Vegetal (yolk) Ventral Dorsal Gray Crescent (future blastopore) future Nieuwkoop Center September 8, 201014 Slide 15 Gray Crescent Formation in Xenopus Courtesy of Jeffrey Hardin University of Wisconsin September 8, 201015Lecture III. Nervous System Embryology Slide 16 Cortical Rotation redistributes maternal cytosolic determinants that are partitioned in different dividing embryonic cells Gray Crescent Nieuwkoop Center September 8, 201016Lecture III. Nervous System Embryology Slide 17 Early Cell Divisions in Amphibians Generate a Blastula Blastula blastocoel blastomere September 8, 201017Lecture III. Nervous System Embryology Slide 18 Fate Mapping the Blastula: 3 Major Spatial Axes Formed by Gradients of Signaling Molecules (Ventral) (Dorsal) future Blastopore (Vegetal) (Animal) Site of sperm entry 1. Animal/Vegetal (Maternal Determinants) 2. Dorsal/Ventral (Sperm Entry, Cortical Rotation) September 8, 201018Lecture III. Nervous System Embryology Slide 19 Fate Mapping the Blastula: 3 Major Spatial Axes Formed by Gradients of Signaling Molecules; Nieuwkoop Center Induces the Spemann Organizer 1. Animal/Vegetal (Maternal Determinants) 2. Dorsal/Ventral (Sperm Entry, Cortical Rotation) 3. Anterior/Posterior (Spemann Organizer) Blastopore Spemann Organizer Nieuwkoop Center (Anterior) (Posterior) September 8, 201019Lecture III. Nervous System Embryology Slide 20 Fate Map of the Blastula: 3 Principle Germ Bands (Layers) Generated Blastopore (Anterior) (Posterior) (Ventral) (Dorsal) (Vegetal) (Animal) Ectoderm (Skin, Neurons) Mesoderm (Notocord, Muscle, Kidney, Bone, Blood) Endoderm (Lining of Gut, Lungs, Placenta in Mammals) September 8, 201020Lecture III. Nervous System Embryology Slide 21 Gastrulation of the Amphibian Blastula A & B: September 8, 201021Lecture III. Nervous System Embryology tissue origami by large-scale cellular migrations and morphogenesis Slide 22 September 8, 201022Lecture III. Nervous System Embryology Gastrulation of the Amphibian Blastula C & D: tissue origami by large-scale cellular migrations and morphogenesis Slide 23 September 8, 201023Lecture III. Nervous System Embryology Gastrulation of the Amphibian Blastula E & F: tissue origami by large-scale cellular migrations and morphogenesis Slide 24 Gastrulation of Xenopus Blastula: 3-D Microscopic (Confocal) Reconstruction September 8, 201024Lecture III. Nervous System Embryology (Ewald, et al., 2004) Slide 25 Blastopore and yolk plug Mesoderm underlies (Neuro)Ectoderm, And induces the neural plate September 8, 201025Lecture III. Nervous System Embryology Endoderm and Mesoderm Involute with Gastrulation Neural plate (apposition of different germbands/ layers) Slide 26 Gastrulation and neurulation in Xenopus (Courtesy of Jeffrey Hardin, University of Wisconsin) September 8, 201026Lecture III. Nervous System Embryology Slide 27 Early cell divisions in the Zebrafish embryo (Courtesy of Paul Myers, University of Minnesota) September 8, 201027Lecture III. Nervous System Embryology Slide 28 Neurulation Closure of the neural tube. Ant Post September 8, 201028Lecture III. Nervous System Embryology Slide 29 Neurulation D V Formation of Neural Crest Cells (makes PNS, endocrine cells, pigment cells, connective tissue). Neural Crest September 8, 201029Lecture III. Nervous System Embryology Slide 30 Origin of Floor Plate and Neural Crest September 8, 201030Lecture III. Nervous System Embryology Slide 31 Origin of Floor Plate and Neural Crest Neural Crest September 8, 201031Lecture III. Nervous System Embryology Slide 32 Cephalization and Segmentation of the Neural Tube September 8, 201032Lecture III. Nervous System Embryology Slide 33 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. The earliest born neurons are found closest to the ventricular surface (thymidine pulse-chase labeling of dividing cells). (Rakic, 1974) September 8, 201033Lecture III. Nervous System Embryology Slide 34 Labeling of Single Precusors Reveal Shared Clonal Origins of Radial Glia and Neurons; and Direct Visualization of Radial and Tangential Migrations (Nadarajah, et al., 2002) (Nector, et al., 2001) (Cepko and Sanes Labs, 1991) September 8, 201034 Slide 35 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): 3. Telolecithal eggs (reptiles, birds, fish): (blastodisc) (epiboly) September 8, 201035Lecture III. Nervous System Embryology Slide 36 a) Blastula flattens into the inner cell mass. b) Endodermal cells form the trophoblast and placental structures. 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) September 8, 201036Lecture III. Nervous System Embryology Slide 37 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). 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 8, 201037Lecture III. Nervous System Embryology Slide 38 What this Lecture was About (cont): 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. September 8, 201038Lecture III. Nervous System Embryology Slide 39 Central Aspects of Embryogenesis 1. Oocyte possess maternal cytoplasmic determinants. 2. Fertilization triggers calcium influx, and creates dorsal-ventral axis by cortical rotation. 3. Cell divisions, synchronous at first, then asynchronous. 4. Blastula created. (Hollow ball of cells) 6. Germ bands (ectoderm, mesoderm, endoderm) created by molecular signals along the Animal/Vegetal axis. 5. Gastulation. (Involution of superficial cells through the blastopore). 6. Anterior-posterior axis created by Spemann organizer. September 8, 201039Lecture III. Nervous System Embryology Slide 40 Embryogenesis (cont.): 7. Apposition of future mesoderm with neuroectoderm induces the neural plate. 8. Neurulation. (Lateral neural folds bend over the midline and fuse into the neural tube.) 9. Neural crest cells derived from leading edge of neural folds, migrate into somites to form the PNS. 10. Segmentation, anterior enlargement (cephalization), cortical development and spinal specification. September 8, 201040Lecture III. Nervous System Embryology Slide 41 END