Lecture III. Nervous System Embryology Bio 3411 Wednesday September 8, 2010

Reading NEUROSCIENCE: 4 th ed, pp Lecture III. Nervous System Embryology September 8, 2010

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

Brain Diseases Interfere with brain functions as heart disease interferes with the circulation. Many diseases have a strong genetic component. Prevalence is high: ≈ % 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

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

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

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

pluripotent, stem cell differentiated Development proceeds by progressive developmental restrictions September 8, 20108Lecture III. Nervous System Embryology

Developmental Restrictions may be: 1) Genetic (programmed by genes) or 2) Epigenetic (determined by environment) pluripotent, stem cell differentiated genetic environmental September 8, 20109

Constructing a Nervous System Axons & Synapses Cell Death & Life Experiences Proliferate Sort ShapeSpecify September 8, Lecture III. Nervous System Embryology EGG BRAIN

Embryology Egg – Blastula Germ bands and cell types. Body axes. Folding, involutions and morphogenic movements. Origin, migration and differentiation of neurons. September 8, Lecture III. Nervous System Embryology

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, Lecture III. Nervous System Embryology

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, Lecture III. Nervous System Embryology

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,

Gray Crescent Formation in Xenopus Courtesy of Jeffrey Hardin University of Wisconsin September 8, Lecture III. Nervous System Embryology

Cortical Rotation redistributes maternal cytosolic determinants that are partitioned in different dividing embryonic cells Gray Crescent Nieuwkoop Center September 8, Lecture III. Nervous System Embryology

Early Cell Divisions in Amphibians Generate a Blastula Blastula blastocoel blastomere September 8, Lecture III. Nervous System Embryology

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, Lecture III. Nervous System Embryology

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, Lecture III. Nervous System Embryology

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, Lecture III. Nervous System Embryology

Gastrulation of the Amphibian Blastula A & B: September 8, Lecture III. Nervous System Embryology “tissue origami” by large-scale cellular migrations and morphogenesis

September 8, Lecture III. Nervous System Embryology Gastrulation of the Amphibian Blastula C & D: “tissue origami” by large-scale cellular migrations and morphogenesis

September 8, Lecture III. Nervous System Embryology Gastrulation of the Amphibian Blastula E & F: “tissue origami” by large-scale cellular migrations and morphogenesis

Gastrulation of Xenopus Blastula: 3-D Microscopic (Confocal) Reconstruction September 8, Lecture III. Nervous System Embryology (Ewald, et al., 2004)

Blastopore and yolk plug Mesoderm underlies (Neuro)Ectoderm, And induces the neural plate September 8, Lecture III. Nervous System Embryology Endoderm and Mesoderm Involute with Gastrulation Neural plate (apposition of different germbands/ layers)

Gastrulation and neurulation in Xenopus (Courtesy of Jeffrey Hardin, University of Wisconsin) September 8, Lecture III. Nervous System Embryology

Early cell divisions in the Zebrafish embryo (Courtesy of Paul Myers, University of Minnesota) September 8, Lecture III. Nervous System Embryology

Neurulation Closure of the neural tube. Ant Post September 8, Lecture III. Nervous System Embryology

Neurulation D V Formation of Neural Crest Cells (makes PNS, endocrine cells, pigment cells, connective tissue). Neural Crest September 8, Lecture III. Nervous System Embryology

Origin of Floor Plate and Neural Crest September 8, Lecture III. Nervous System Embryology

Origin of Floor Plate and Neural Crest Neural Crest September 8, Lecture III. Nervous System Embryology

Cephalization and Segmentation of the Neural Tube September 8, Lecture III. Nervous System Embryology

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, Lecture III. Nervous System Embryology

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,

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, Lecture III. Nervous System Embryology

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, Lecture III. Nervous System Embryology

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, Lecture III. Nervous System Embryology

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, Lecture III. Nervous System Embryology

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, Lecture III. Nervous System Embryology

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, Lecture III. Nervous System Embryology

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