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Gastrulation: Formation of the Primary Germ Layers
Introductory Notes
Delamination precedes gastrulation separating ICM into
epiblast & hypoblast
Gastrulation occurs in epiblast (future embryo)
Gastrulation involves several types of movements and shape
changes
End product is three germ layers: ectoderm, endoderm and
mesoderm
The expression of certain genes regulates the formation
of the embryonic axes & the organization of the embryo
The Timing of Gastrulation and Subsequent
Events
Gastrulation begins about 15 days of development and is followed
by neurulation and the start of development of several major organ
systems as shown in the following figure
Formation of the Epiblast and Hypoblast
Once cleavage has resulted in the multi-celled blastocyst, morphogenetic
movements will come into play to reorganize the embryo into distinct
layers. The cells in the different layers will ultimately have
different developmental fates. As we will see in more detail later,
morphogenetic movements involve changes in the shapes of cells
and often involve active movements of individual cells and groups
of cells. As a result of these movements, new relationships will
be set up between different groups of cells. These new relationships
will allow new inter-cellular communications that are the cornerstone
of embryogenesis.
Most of our knowledge has been learned from mice and other mammalian
species but current inroads into human morphogenesis are being
made. The goal here is not to become proficient human embryologists
but to understand the means by which morphogenetic movements occur
and what their significance is to human embryogenesis.
Inner Cell Mass (ICM) delaminates to form hypoblast and
epiblast
Occurs just prior to implantation & gastrulation
Epiblast (green cells) is 2-layered (i.e., it is bilaminate)
disc of approximately cuboidal cells & will form the embryo
proper
Flatter hypoblast cells lie on top of epiblast and will
form yolk sac
Role of Gastrulation
Gastrulation will covert the bilaminate epiblast into
the three primary embryonic germ layers
Ectoderm: outside; this embryonic layer more or less surrounds
the other germ layers
Mesoderm: middle; this germ layer lies between the ectoderm and endoderm
Endoderm: inside; this germ layer lies at the most interior
of the embryo
Subsequently neurulation will form epithelial and neural
ectoderm from the ectoderm
Human Gastrulation: How Do We Know How
it Works?
Very little work has been done on human gastrulation
Most work on fixed and stained human embryos
No experimental work because it is not ethical
Difficulty getting specific stages
Originally used knowledge from chick gastrulation: general
morphology looks similar; more recently there has been lots
of work on mouse embryos
Mark embryos with particles & dyes to follow cell
movements
More Recently: Researchers have fluorescently label cells
& followed their movement by confocal microscopy (a special
laser-based microscopy which allows you to take optical sections
through tissues to construct 3-D images) and obtain sharper
resolution of stained material and its location.
Gastrulation in Human Embryo
Epiblast is bilaminate disk
Initially cells move along surface (blue arrows) but upon
reaching the center line will pile up and move internally (red
arrows).
The moving surface cells first pile up to form a prominant
bump known as the node. This occurs because the cells move along
the top faster than they can separate off and move internally.
It's sort of like crowds at a major concert. People stream into
the venue from all over the city but pile up at the door before
spreading out again once they're inside.
The node was discovered in mammals by Hensen and is appropriately
named Hensen's node in rabbits and other organisms but is only
referred to as the node in humans.
The cells that enter through the node will become the
notochord (see lecture on neurulation).
As the cells continue to move in the primitive groove
forms
The cells that migrate internally first will become the
endoderm which contains the presumptive notochordal tissue as
well
The cells move internally later will migrate over the
endoderm and form the mesoderm
The cells that remain on the surface will form ectoderm
These movements are shown in the following sequence of pictures of 16 and 18day old embryos.
Dynamics of Human Gastrulation
The following movie shows a dynamic representation of the general
movements of cells during human gastrulation. Just click on the
link. Download Movie
Cell Movements & Rearrangements
during Embryonic Development
During human gastrulation, cells move over the blastodisc surface,
enter the primitive streak and move internally. On the surface
they move in association with other cells but once they turn the
corner around the lip of the primitive streak the cells separate
as individuals to migrate internally to form the mesoderm and
endoderm. These are just some of the types of cell movements that
occur in animal embryos. Here's a full list. Later we'll examine
the shape changes that occur in the cells to carry out these movements.
Ingression: cells break away
from the tissue and migrate as individuals
Delamination: layers of cells
separate from each others more or less as sheets of cells
Intercalation: two cell layers
interlace with each other
Epiboly: a form of cell spreading
in which cells flatten out; this allows them to cover a much
larger surface area (1st detailed in frog development).
Invagination (Evagination):
a tissue layer folds in (out)
Involution: cells move over
a lip of tissue and into the interior
Convergent Extension: cells
reorganize to form less layers allowing the cells to extend
out from a point.
Not all of these morphogenetic movements have been detailed in
humans but they all have been shown to occur in other animals.
Also remember, one type of cellular rearrangement does not exclude
another with several different types of movement potentially occurring
at the same time.
Cross Section of Human Gastrula
Bottle Cells
Bottle cells were first observed in amphibian gastrulation
Occur during gastrulation in humans and many other species
Also seen during neurulation and during other types of
cellular rearrangements
Involves a shape change from an epithelial morphology
to bottle shape
Involves a loss of cell-cell adhesion so cells can move
as individuals; recent work has shown that this is due to the
loss of E-cadherin (Bursdall et al, 1993. Development 118: 829-844).
Experiments with frog embryos have shown that isolated
bottle cells show active movement in vitro
Cells move internally under own motive force
The way in which the bottle cells change shape and become
motile is detailed in the next lecture on neurulation
Early Cell Lineages in the Human Embryo
ICM delaminates to form the Epiblast & Hypoblast
The hypoblast will form the yolk sac
The epiblast will form embryo plus the amnion
Migration of cells of the epiblast through primitive streak
leads to formation of mesoderm and endoderm
Ectoderm is left behind
Thus gastrulation results in the formation of the three
primary embryonic germ layers: ectoderm, mesoderm and endoderm
During neurulation, the ectoderm will subdivided into the neural
tissue and epithelial tissue lineages which is the topic of our
next lecture.
Mammalian Pattern & Polarity
Humans, like most other living things, have a distinct organization.
Thus the head is at one end while the feet are at the opposite
end (Anterior-Posterior Axis; A-P axis). The face is one one side
(ventral) with the limbs and other body parts organized to function
in this direction as opposed to backwards (dorsal) thus defining
the dorsal-ventral axis. We also have right and left sides (Right-Left
Axis). These three axes become established during early development
and we are just beginning to understand how this comes about.
Eyal-Giladi (1997. Development 124:
2286-2296) has recently reviewed the literature dealing with the
establishment of the vertebrate axis.
It is very likely that the anterior-posterior axis is established
initially by implantation via mechanisms that remain to be elucidated.
The establishment of the A-P Axis would in turn define the orientation
of the primitive streak. In the mouse, the node (Hensen's node)
that appears at gastrulation at the anterior end of the primitive
streak contains information that oversees the construction of
the whole body form. After the endoderm has migrated internally
during gastrulation, the "anterior visceral endoderm" instructs
the formation of head components. These two signaling centres
regulate gene expression in the different regions of the embryo
to ensure that it develops with the relevant components in the
right places at the right time.
This is accomplished through the expression of genes appropriate
to each region. These genes were all discovered in other animals
(e.g., Drosophila, mouse) but their
precise roles in human development are under analysis. 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).
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.
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