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Bio 3411, Fall 2006
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.
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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.
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
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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.
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:
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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.
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]
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Fall 2006
Bio 3411 (A. Wei)
Lecture 17 - Embryology
Key Stages of Embryogenesis
1) Unfertilized Oocyte
2) Fertilization
3) Cortical Rotation
(Ca+2Wave)
(A/V axis)
4) Cell Divisions
5) Blastula
(Three germ bands)
(Zygotic transcription
begins)
(Spemann Organizer)
(A/P axis)
7) Neurulation
(Mesoderm induces
Neuroectoderm to
form Neural Plate)
(Neural folds)
(Induction of floor plate
by notochord)
(Closure of neural tube)
(Migration of Neural Crest)
(Gray Crescent/
future Nieuwkoop Center )
(D/V axis)
6) Gastrulation
(Mesoderm and Endoderm
Involute)
(Ectoderm Spreads)
8) Cephalization
and Segmentation
(Neuronal precusors born
in ventricular layer, migrate
radially on Bergmann glia)