Gastrulation is the first stage in forming the body plan
Gastrulation occurs by
movement of cells from
the exterior of the
blastula to the interior.
Forms the 3 germ layers
Movement occurs
through a region called
the blastopore.
Protostomes: in many
animals the blastopore
develops into the mouth
(most animals).
Gastrulation in the sea urchin
Deuterostomes: in others
it forms the anus and the
mouth develops else
where (vertebrates).
How is the characteristic body plan for any organism developed?
Gastrulation: the first step in the process of body formation. It transforms a
complex sphere into 3 basic germ layers from which all other tissues develop.
Ectoderm is the outer layer = forms epidermis and nervous system.
Mesoderm is in the middle and forms a many structures (i.e., heart, muscles).
Endoderm is the inner layer and forms the ‘gut’ and related organs.
The first change is to generate the rudiment of the digestive tract, hence the
name gastrulation (gastric = stomach).
2 Questions:
1. How does
gastrulation occur
and how does it
differ in different
animals?
2. What are the
molecular and
cellular mechanisms
that cause
movement and
rearrangement?
Gastrulation is the first step of morphogenesis
Morphogenesis is the process whereby individual cells undergo complex
movements that generate the organ rudiments. Gastrulation generates the
three basic germ layers from which organs arise.
How do sheets of cells (epithelia) move during gastrulation? 5 methods.
Invagination is the local inward movement of cells from a cavity
Involution is similar, but more dramatic. It is an inward expansion of epithelial
cells around an edge such as the blastpore.
Convergent extension is elongation of an epithelium in one direction while it
shortens in the other direction (stretching taffy). The cells can keep their
relative positions and elongate or they can interdigitate.
Epiboly is spreading movement of an epithelium to a deeper or thinner layer.
Delamination is the splitting of one layer into 2 different layers.
Different combinations of these basic movements yield a variety of changes
that characterize gastrulation
How do individual cells move during gastrulation?
Seven basic types of cell movement lead to the changes in epithelial sheets
that characterize gastrulation.
1. Migration is the movement of an individual cell over other cells or a
substrate.
2. Intercalation is wedging of cells between their neighbors. Lateral
intercalation involves lateral movements of cells in the same layer between
one another = convergent extension. Radial intercalation involves wedging of
2 different layers. This process often leads to epiboly, the surface area of the
epithelium increases while the thickness decreases.
3. Ingression is the movement of individual cells from an epithelium into an
embryonic cavity.
4. Shape changes are coordinated changes in cell shape that cause an
epithelium to invaginate, buckle or undergo convergent extension.
5. Cell division without growth increases the total number of ‘building blocks’
6. Changes in adhesiveness: cells adhere to one another and to extracellular
material via different types of specialized attachments. Loss of these
attachments can facilitate cell movement.
7. Programmed cell death (apoptosis): cells undergo death in a programmed
manner in order to sculpt organs into their final appearance (fingers).
What happens during gastrulation in sea urchins?
Sea urchins are studied commonly
because they are transparent
creatures, and it is easy to see
what is happening inside.
Sea urchins develop from a
blastula containing mesomeres,
macromeres, and micromeres.
Each of these layers has a specific
fate during gastrulation and they
always form the same tissue.
Gastrulation begins when the
blastula contains about 1000 cells
and has a large fluid filled
blastocoel.
Embryonic cells are broadly classed as epithelial or mesechymal
Epithelial cells are well-differentiated. They compose skin and line the body
cavities (ie, the digestive tract). They are polarized. Their apical surface
faces out and their basal surface rests on the basement membrane
(extracellular matrix that supports cells). Epithelial cells are closely
connected with adjacent cells by specialized attachments including tight
junctions, gap junctions, and desmosomes.
Mesenchymal cells are
poorly differentiated and
have the potential to
develop into many
different tissues, including
epithelial cells. They have
a leading edge with
lamellipodia, and a trailing
edge. They are not
connected to adjacent
cells but they are in
contact with the
extracellular matrix.
Vegetal plate: the first step in sea urchin gastrulation is formation of the
vegetal plate, a thickening of epithelial cells in the vegetal pole.
Primary mesenchymal cells: these cells
change adhesive properties and the large
micromeres start to migrate into the
blastocoel as free mesenchymal cells
(ingression). Mesenchymal cells are loose
cells that can differentiate into many
different organs.
Archenteron: the primitive gut. The
archenteron is formed in several stages. 1.
the vegetal plate invaginates into the
blastocoel, 2. It elongates by convergent
extension. 3. It hooks up with the front and
is pulled forward, and 4. Involution occurs
with movement of cells around the
blastopore and into the archenteron.
What forces drive the process of
gastrulation?
Two models for vegetal plate invagination.
1. Apical constriction model: the epithelial cells of the vegetal plate change
morphology. The apical surface constricts, causing the vegetal plate to bend
inward.
2. Gel swelling model: the
outside surface of the sea urchin
blastula is covered by a hyaline
layer which consists of several
distinct laminations.
Cells at the vegetal plate secrete
large amounts of chondroitin
sulfate, a water absorbing
material. The inner hyaline layer
starts to become thickened by
absorption of excess water, and
it causes the vegetal plate to
bend inward.
Both methods may contribute to
invagination of the vegetal plate.
The second phase of gastrulation is caused by
convergent extension of cells into the blastocoel
The invaginated cells of the vegetal plate extend to form a long thin
archenteron. It is unclear as to how this occurs.
Stretching model: the extension could result from the cells changing shape to
become long and thin.
Cell movement model: invaginated cells could move to extend the length of
the tube.
How does the developing archenteron know which way to grow?
Secondary mesenchymal cells: cells at the tip of the archenteron guide the
progress. They send filopodia (thin extensions) to find the correct area of the
roof. The roof cells send back other filopodia to direct the archenteron where
to go.
This allows the mouth to hook up with the gut.
Gastrulation in Amphibians
Amphibian gastrulation is much more complex than sea urchins, due in part
to the large amount of yolk. There are also more layers in the blastula
Different areas of the gastrula show distinct behavior:
Animal cap: about 3 layers deep and derived from the animal hemisphere of
the egg. It expands by epiboly until it covers about half of the surface of the
gastrula.
Non involuting marginal zone: a wide belt of cells 4 to 5 layers deep. It
expands by convergent extension to cover the lower part of the gastrula
Involuting marginal zone: involutes during gastrulation and comes to lie in
the interior. The limit of involution reaches the edge of the blastopore by
the end of gastrulation.
The superficial layer of the involuting zone forms the roof of the
archenteron (the lining of the future gut).
The deep layer of the involuting zone forms the future trunk mesoderm
(muscles of the body)
Deep zone: a ring of cells that migrate towards the animal pole. Deep zone
cells become head mesoderm and heart.
Vegetal base: formed by the large yolky cells in the vegetal hemisphere.
During gastrulation this base is tilted and displaced ventrally. It will form
the floor of the archenteron (gut).
Ectoderm = animal cap and non involuting marginal zone
Mesoderm = deep zone and deep layer of involuting marginal zone
Endoderm = vegetal cells and superficial layer of involuting marginal zone
What causes gastrulation?
What factors are responsible for all of the complex and orchestrated
movements that occur during gastrulation? There are at least 2 hypotheses
and they are not mutually exclusive.
1. Cellular behaviors observed in each gastrula region are caused by local
accumulations of gene products that are already present in the fertilized
oocyte
2. One region of the
blastula could be
determined to organize
the behavior of all other
regions.
The dorsal lip of the
blastopore appears to be
particularly important in
regulating the fate of
other areas. 2 genes,
goosecoid and noggin
seem to mediate this
effect.
Chickens gastrulate differently: they develop from
an ingression of cells from the blastoderm
Blastoderm: a thin layer of blastomeres that are located on top of the yolk.
Area pellucida: the central portion is clear as it is raised from the yolk.
Area opaca: the outer area is opaque because it rests on the yolk.
Gastrulation starts in
the rear of the area
pellucida. Cells move
into a mound which
elongates anteriorly to
become the primitive
streak. The center
opens to form the
primitive groove
between 2 primitive
ridges. The primitive
pit is the most anterior
region and it is
surrounded by cells
called Henson’s node.
The primitive groove and pit are the site of gastrulation in birds
Crossection of blastoderm (blastula in birds).
Epiblast is the upper layer of epithelial cells, blastocoel is the space below the
epiblast, and hypoblast is the lower layer of epithelial cells.
Epiblast cells
roll over the
primitive ridge
and involute
into the groove.
The cells lose
contact with
one another and
migrate inwards
by ingression =
Mesoderm.
3 germ layers
are established
Germ layers spread to form extra embryonic membranes
Extra embryonic ectoderm: ectoderm spreads by epiboly to form a wide sheet
that surrounds the entire yolk mass.
Extra embryonic mesoderm spreads out below the ectoderm to form blood
vessels
As development continues, the embryo proper is pinched off from the extra
embryonic membranes.
How does gastrulation occur in humans?
Mammalian embryos have not been studied as extensively as others because
they are difficult to maintain in culture after the blastula stage (this is because
they implant in the uterus).
Surprisingly, mammals undergo gastrulation very similar to birds!!
After about 5 days the
blastula appears as an
inner cell mass and a
tropoblast
12 day old human embryo
This is very similar to a blastula in birds before primitive streak formation.
The blastoderm is very small (only 0.2 mm in diameter).
Gastrulation in a 16 day old human embryo
• The primitive streak and Henson’s node form just as in birds.
• The cell movements are similar
• Cells roll over the primitive ridge and into the groove
• They ingress individually and move out to form discs with the 3 germ layers
• The 3 layers also move laterally to form the extra embryonic endoderm and
mesoderm even though there is no yolk to digest. This is surprising
because mammalian embryos could gastrulate easily by invagination as sea
urchins (they have a placenta and no yolk).
• Instead, gastrulation appears to recapitulate a pattern established by birdlike ancestors and reptiles.