Download Lecture 17-Embryology of the Nervous System KEY

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.
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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.
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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.
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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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(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