Download Neurulation: Making the Brain and Spinal Cord

Neurulation: Making the Brain and Spinal Cord
Lecture Outline
• The Primary Germ Layers
• Notochord Formation
• The Events of Neurulation
• Bottle Cells
• Bottle Cells & Neurulation
• Induction of Neural Tissue by Chordamesoderm
• Hensen's Node Induces Neural Axis
• Genes and Head Development
• Hox and the A-P Axis
• Cell Lineages at Neurula Stage
• Late Neurula
The Primary Germ Layers
The following diagram shows the general arrangement of tissues in a cross-section of the human embryo after
gastrulation.
•
•
•
After gastrulation, three germ layers are evident: Ectoderm, mesoderm, endoderm
Next the presumptive notochordal tissue will separate from the endoderm to form the notochord
As notochord formation is occuring, neurulation will separate the ectoderm into the epithelial ectoderm
and the neural ectoderm with the latter being transformed into the neural tube by the end of the process
Notochord Formation
At this point the presumptive notochordal tissue will separate from the endoderm and re-organize as the
notochord below the neural ectoderm as the neural ectoderm begins folding to form the neural tube (Sulik et al,
1994. Developmental Dynamics 201: 260-278).
•
•
•
The future neural ectoderm overlies notochordal process (chordamesoderm)
Folding of the neural ectoderm results in the neural tube
The neural tube is the precursor of the brain & spinal cord
The Events of Neurulation
While the neural tube is forming, other embryonic tissues are undergoing changes too as the embryo itself is
elongating and undergoing torsion. The following gives an idea of some of these changes and, more
importantly, sets the stage for some of the developmental processes that we will cover in future lectures.
Figure 4-4 from Larsen’s Human Embryology. A. Drawings of human embryos from day 21 to 25 (top to bottom). B. Photos (left
side) and computer models (right) of human embryos. D. Scanning EM of human embryo.
•
•
•
•
Neural ectoderm thickens
Neural folds form producing Neural Groove
Neural folds contact
Cells rearrange forming Neural Tube with overlying Ectoderm
These events have been well characterized in the chick embryo as seen in the following sets of scanning
electron micrographs.
Fig. 4-5 from Larsen’s Human Embryology. The key regions are the neural plate (NP) that thickens and folds to produce the neural
folds (NF) and neural groove (NG) before closing to form the neural tube. The status of the notochord (N) can also be observed as
well as changes in surrounding tissue an surface ectoderm (SE).
Bottle Cells
Special cells: Bottle Cells act as driving force of neurulation
|
•
•
•
•
•
Bottle cells appear at time of neurulation
Change in shape of epithelial to bottle cell is believed to be one of driving forces for event
Cytoskeletal changes lead to bottle cell shape
Microfilaments (F-actin) at apex
Microtubules down long axis of cell
F-Actin (Microfilaments)
•
•
•
•
•
•
•
Long polymers of G-actin
Appear like two twisted strands of pearls
Contractile filaments that work with myosin for contraction
Bind to cell membrane and affect cell shape
Cytochalasins (B, C, D) inhibit F-actin formation & thus their function
Phalloidin binds to intact microtubules
Detect with anti-actin antibodies or Rhodamine-phalloidin
•
Use these drugs to study their functions
Microtubules
•
•
•
•
•
•
Polymers of tubulin
Detect with anti-tubulin antibodies
Define cell shape, mediate chromosome separation, roles in motility
Also act as "tracks" for movement of cytoplasmic contents
Inhibited by Colchicine
Stabilized by Taxol
Cytoskeleton of Bottle Cells & Neurulation
•
•
•
Inhibition of either F-Actin or microtubules inhibits neurulation
F-Actin acts like purse strings to pull apex of cell
Microtubules elongate cell to give it polarity for movement
Induction of Neural Tissue by Chordamesoderm
•
•
•
During gastrulation special region of mesoderm underlies overlying ectoderm
Chordamesoderm is future notochord
Signals emitted by chordamesoderm have not been identified
Hensen's Node Induces Neural Axis
In the previous lecture on gastrulation, the role of the node (Hensen's Node) in organizing the overall body plan
was discussed briefly. Historically, this special region of tissue has been shown to establish the vertebrate body
plan in everything from frogs to zebra fish to mice (Nieto, 1999. Cell 98:417-425). Experimentally this
organizing ability is shown by the ability of transplanted nodal tissue to induce a second embryonic axis as
revealed in the next diagram.
•
•
•
•
These classic experiments were done in chicken embryos
Remove HN: No neural tissue development
Transplant HN to host embryo: 2nd Neural Axis induced
Primary Induction: Induces primary embryonic axis
•
•
These experiments have also been done in the mouse showing that the node is an embryonic organizer in
mammals (Beddington, 1994. Development 120: 603-612. Beddington & Robertson, 1998. Trends
Genet. 14: 277-284).
More recent studies have shown however, that the mammalian node is not a classic organizer because it
can't act alone to induce the embryonic axis but requires other anterior germ tissues to be fully effective
(Tam & Steiner, 1999. Development 126: 5171-5179). Thus progressive cellular interactions are more
important rather than a single inductive event that characterizes the organizer activity of the Hensen's
Node region in lower vertebrates.
Genes and Head Development
The node expresses genes called Noggin and Chordin that are not expressed by the anterior visceral endoderm. On the
other hand, the anterior visceral endoderm expresses genes called Lim-1, Hesx-1 and Otx-2 which are essential for head
development. The picture below shows the effects of knocking out the Lim-1 gene in mice.
A normal mouse embryo is shown in the top panel, the Lim-1 KO mouse in the lower panel. Lim-1-deficient mice are
essentially headless with the most anterior structure observed being the pinnae of the ears (arrows; Shawlot & Behringer,
1995. Nature 374: 425-430; Figure 4-4 from Larsen’s Human Embryology).
HOX and the A-P Axis
The A-P axis of all animals appears to be specified by the expression of Hox genes that were first discovered in
Drosophila. These genes are highly conserved between animals. These genes are highly organized on chromosomes and
appear to be expressed sequentially in the same order that they are arranged in the chromosomes. The Human Hox
complex (HOXA-HOXD) is present in four sets on four separate chromosomes in each haploid set of chromosomes.
Retinoic acid is a natural morphogenetic agent that also acts as a teratogen when it is present at high levels. Retinoic acid
affects A-P axis formation and HOX gene expression. Because of time limitations, we will only discuss the role of HOX
genes and retinoic during the lectures on limb development.
Cell Lineages At Neurula Stage
For a detailed review of neurulation check out: Smith & Shoenwolf, 1997. Neurulation: Coming to Closure. Trends in
Neurosci. 20: 510-517.
Late Neurula
•
•
•
•
Neural Tube: One layer of cells surrounding a lumen & covered by external limiting membrane
Neural Crest have separated out--begun to migrate
Neural Tube thickens & folds to form brain
New layers of nerve cells will appear in brain & spinal cord (neurogenesis)
© Copyright 1998-2009 Danton H. O'Day