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Bio 3411, Fall 2005 Aguan Wei 979 McDonnell Sciences Building, Medical School Campus, WUMS. Office: 747-3306 email: [email protected] Lecture 17-Embryology of the Nervous System KEY CONCEPTS: Condensed time-line: a) Animal/Vegetal axis present in oocyte. b) Fertilization. c) Calcium wave to block polyspermy. d) Cortical rotation to make Ventral/Dorsal axis. e) Formation of future Nieuwkoop Center (Gray Crescent). f) Cell divisions to create Blastula. g) Creation of three germ bands along the Animal/Vegetal axis of the blastula. h) Induction of Spemann Organizer by the Nieuwkoop center, and future site of the blastopore. i) Spemann Organizer determines Anterior/Posterior axis. j) Gastrulation. Mesoderm and Endoderm involute through the blastopore, while Ectoderm/Neuroectoderm spreads to cover the entire surface of the embryo. k) Mesoderm induces overlying Neuroectoderm to become the neural plate. l) Neurulation. Thickenings at the edges of the neural plate become the neural folds. Neural folds bend back towards midline and fuse to form the neural tube. m) Neural Crest cells derived from the tips of the neural folds, migrate laterally to form the peripheral nervous system. n) Cephalization. Enlargement of three cephalic vesicles form the main divisions of the future brain. o) Segmentation of the neural tube proceeds from anterior to posterior regions. p) Neurons are born in the ventricular zone of the neural tube and migrate radially along Bergmann glia, then tangentially to populate cortical layers of the maturing nervous system. Detailed time-line: 1. Unfertilized oocytes possess positional information. This information is created from molecules (RNAs and proteins) contributed by the mother, which are distributed asymmetrically in the cytoplasm. These informational molecules are termed maternal cytoplasmic determinants. 2. The Animal/Vegetal axis is the first informational axis of the egg. It is inherited from the mother through the unequal distribution of maternal cytoplasmic determinants in the oocyte. 1 3. Fertilization by a sperm cell triggers a rapid influx of calcium from the external bath into the egg. This calcium wave spreads over the surface of the egg and causes the release of cortical granules docked under the surface of the plasma membrane. Upon release, the contents of the cortical granules rapidly polymerize to form the fertilization envelope. Deployment of the fertilization envelope prevents fertilization by multiple sperm cells (polyspermy). 4. Cortical rotation occurs as plasma membrane, associated cytoskeleton and cytoplasm, rotate ~30o toward the site of fertilization. This movement mixes cytoplasm determinates between the animal and the vegetal poles of the egg and generates a second asymmetric axis of positional information. The newly created wedge of mixed cytoplasm is called “the gray crescent” (for diluted pigmentation), which determines the location of the future “Nieuwkoop Center” in the blastula. The future ventral pole (site of fertilization) and dorsal pole of the embryo is thus defined even before the first cell division. 5. Cell divisions occur. Early divisions are synchronous. Synchrony becomes increasingly uncoupled over the embryo with later divisions. 6. Through early cell divisions, the embryo is instructed solely by maternal RNAs and proteins (contributed by the mother). With later cell divisions, zygotic transcription begins (from the newly created genome of the embryo). 7. The blastula is formed, which resembles a hollow ball of cells. The fluid-filled cavity inside the blastula is called the blastocoel. 8. Three germ bands are established by the completion of the blastula stage, the ectoderm, mesoderm and endoderm. These germ bands are positioned along the animal/vegetal axis, and are likely determined largely by the gradient of maternal cytoplasmic determinants creating this axis. 9. Ectoderm becomes future epidermis and nervous system. Mesoderm becomes notocord, muscles, blood, bones, and internal organs. Endoderm becomes the lining of the gut. 10. The Nieuwkoop center (the gray crescent) induces the dorsal adjacent patch of cells to become the Spemann Organizer. 11. The Spemann Organizer is so named because when transplanted to the ventral side of an egg, can organize a “siamese-twin” embryo, with a complete second nervous system. The Spemann organizer induces formation of the blastopore, and becomes located at the dorsal lip of the blastopore. It also establishes a third informational axis, the anterior (distal from the blastopore) and the posterior (proximal to the blastopore) poles of the future embryo. 2 12. Gastrulation results from movement of cells from the surface of the blastula into the interior. Mesodermal and endodermal cells involute into the blastocoel, while ectodermal cells spread over the entire surface of the blastula. The original blastocoel is forced ventrally and eventually collapses. A new internal cavity (the archenteron) is created by expansion of the involuted mesoderm and endoderm, which will become the future gut of the embryo. 13. Gastrulation allows further induction to proceed, by creating the physical apposition of blocks of tissue derived from different germ bands. 14. At the end of gastrulation, mesoderm (interior) comes to lie underneath neuroectoderm (exterior). Mesoderm induces formation of the neural plate from overlying neuroectoderm. 15. Neurulation. The lateral edges of the neural plate thicken to form the neural folds. The neural folds bend up and back toward the midline. The ventral midline of the neural tube is induced by underlying notocord to become the neural floor plate. The neural floor plate buckles and assists the tip of the neural folds to join and fuse along the dorsal midline. This forms the neural tube. 16. Neural crest cells derived from the tips of the neural folds, migrate laterally away from the midline, to populate mesoderm and form the peripheral nervous system. 17. Cephalization. Proliferation leads to thickening and enlargement of the anterior neural tube. Three cephalic vesicles form which become the telencephalon, diencephalon and hindbrain of the future brain. 18. Neuronal precusors are born in the ventricular layer of the neural tube. They migrate radially towards the surface of the neural tube, along Bergmann glia. At appropriate cortical layers, they exit tangentially and undergo final neuronal differentiation to create the laminar organization of the brain. Evolutionary Specializations: 1. Three classes of vertebrate oocytes, grouped by amount and distribution of yolk: a) Isolecithal (Protochordates and Mammals). Uniform distribution of yolk between animal and vegetal poles. Little or no yolk. b) Mesolecithal (Amphibians). Medium amount of yolk and medium asymmetry of distribution of yolk between animal and vegetal poles. c) Telolecithal (Reptiles, Birds and Fish). Large amount of yolk, very asymmetrically distributed. 3 2. Isolecithal embryos divide to produce uniform blastomeres, which form a nearly symmetric spherical blastula. Gastrulation like mesolecithal embryos. 3. Telolecithal embryos divide to produce a thin blastodisc, which sits on top of the large yolk sac. Gastrulation begins by involution of the posterior region of the blastodisc, underneath the edge of the blastodisc. Gastrulation proceeds by anterior extension of the involuting edge to form the primitive streak (analogous to the amphibian blastopore). The organizer region, at the anterior end of the advancing primitive streak is called “Hensen’s Node” (analogous to the amphibian Spemann Organizer). 4. Mammalian embryos develop initially like protochordates, then later like reptiles, birds and fish. With no yolk, early cell divisions and blastula formation are like other isolecithal embryos. After blastula formation, a subpopulation of endodermal cells differentiate into the trophoblast (amniotic sac) and placental structures. The remainder of cells that continue to form the embryo, flatten into a thin disk within the trophoblast. This disc is called the inner cell mass, and resembles the blastodisc of telolecithal embryos. Gastrulation of mammalian embryos proceeds like telolecithal embryos (Reptiles, Birds, Fish). References: 1. A. S. Romer. The Vertebrate Body, 2nd Edition. (1955) Saunders Company, Philadelphia. [especially Chapter 5] 2. S. F. Gilbert and A. M. Raunio, editors. Embryology, Constructing the Organism (1997) Sinauer Associates, Inc. Publishers, Sunderland. [especially Chapter 20] 3. S. F. Gilbert. Developmental Biology, 3rd Edition. (1997) [especially Chapters 6 and 7] 4 (Mesoderm induces Neuroectoderm to form Neural Plate) (Spemann Organizer) (A/P axis) 5) Blastula (Ca+2Wave) 2) Fertilization (Neural folds) (Induction of floor plate by notochord) (Closure of neural tube) (Migration of Neural Crest) (Neuronal precusors born in ventricular layer, migrate radially on Bergmann glia) 8) Cephalization and Segmentation (Mesoderm and Endoderm Involute) (Ectoderm Spreads) 6) Gastrulation (Gray Crescent/ future Nieuwkoop Center ) (D/V axis) 3) Cortical Rotation Key Stages of Embryogenesis 7) Neurulation (Three germ bands) (Zygotic transcription begins) 4) Cell Divisions (A/V axis) 1) Unfertilized Oocyte Fall 2005 Bio 3411 (A. Wei) Lecture 18 - Embryology