Gastrulation Producing the Body Plan

The blastula is typically a fluid-filled ball of cells. How does this simple ball of cells become an embryo, made up of multiple tissue layers, with head and tail ends and dorsal and ventral sides? Gastrulation is the process whereby the blastula is transformed by massive movements of cells into an embryo with multiple tissue layers and visible body axes. The resulting spatial relationships between tissues make possible the inductive interactions that trigger differentiation and organ formation.

During gastrulation, the animal body forms three germ layers (also called cell layers or tissue layers):

► Some blastomeres move together as a sheet to the inside of the embryo, creating an inner germ layer called the endoderm. The endoderm will give rise to the lining of the digestive tract, respiratory tract, and circulatory system and make up other internal tissues such as the pancreas and liver.

► The cells remaining on the outside of the embryo become the outer germ layer, the ectoderm. The ectoderm will give rise to the nervous system, the skin, hair, and nails, sweat glands, oil glands, and milk secretory ducts.

► Other cells migrate between the endoderm and the ectoderm to become the middle germ layer, or mesoderm. The mesoderm will contribute tissues to many organs, including blood vessels, muscle, bones, liver, and heart.

Some of the most challenging and interesting questions in animal development have concerned what directs the cell movements of gastrulation and what is responsible for the resulting patterns of cell differentiation and organ formation. In the past 25 years, scientists have answered many of these questions at the molecular level. In the discussion that follows, we'll consider the similarities and differences among gastrulation in sea urchins, frogs, reptiles, birds, and mammals. We'll also review some of the exciting discoveries about the mechanisms underlying these phenomena.

Invagination of the vegetal pole characterizes gastrulation in the sea urchin

The sea urchin blastula is a simple, hollow ball of cells that is only one cell thick. The end of the blastula stage is marked by a dramatic slowing of the rate of mitosis, and the beginning of gastrulation is marked by a flattening of the vegetal hemisphere (Figure 20.8). Some cells at the vegetal pole bulge into the blas-tocoel, break free, and migrate into the cavity. These cells become primary mesenchyme cells—cells of the middle germ layer, the mesoderm. (Mesenchyme cells are unconnected to one another and act as independent units, in contrast to epithelial cells, which are tightly packed into sheets or tubes.)

The flattening at the vegetal pole results from changes in the shape of the individual blastomeres. These cells shift from being rather cuboidal to become wedge-shaped, with constricted outer edges and expanded inner edges. As a result of these shape changes, the vegetal pole bulges inward, il The vegetal pole of the blastula flattens.

2| Some cells change shape and move inward to form the archenteron.

3} Other cells break free, becoming primary mesenchyme.

4} More cells break free, il The vegetal pole of the blastula flattens.

2| Some cells change shape and move inward to form the archenteron.

3} Other cells break free, becoming primary mesenchyme.

4} More cells break free,

Human Gastrulation

Secondary mesenchyme

Ectoderm

Endoderm

Archenteron

Primary mesenchyme

Vegetal hemisphere

20.8 Gastrulation in Sea Urchins During gastrulation,cells move to new positions and form the three germ layers from which differentiated tissues develop.

Vegetal hemisphere

20.8 Gastrulation in Sea Urchins During gastrulation,cells move to new positions and form the three germ layers from which differentiated tissues develop.

Secondary mesenchyme

Ectoderm

Endoderm

Archenteron

Primary mesenchyme or invaginates, as if someone were poking a finger into a hollow ball. The cells that invaginate become the endoderm and form the primitive gut, the archenteron. At the tip of the archenteron more cells break free, entering the blastocoel to form more mesoderm, the secondary mesenchyme.

The early invagination of the archenteron is due to the changes in cell shapes, but eventually it is pulled by the secondary mesenchyme cells. These cells, attached to the tip of the archenteron, send out extensions that adhere to the overlying ectoderm and contract. Where the archenteron eventually makes contact with the ectoderm, the mouth of the animal will form. The opening created by the invagination of the vegetal pole is called the blastopore; it will become the anus of the animal.

What mechanisms control the various cell movements of sea urchin gastrulation? The immediate answer is that specific properties of particular blastomeres change. For example, some vegetal cells migrate into the blastocoel to form the primary mesenchyme because they lose their attachments to neighboring cells. Once they bulge into the blastocoel, they move by extending long processes called filopodia along an extracellular matrix of proteins that is laid down by the ec-todermal cells lining the blastocoel.

A deeper understanding of gastrulation requires that we discover the molecular mechanisms whereby certain blas-tomeres develop properties different from those of others. Cleavage divides up the cytoplasm of the egg in a very systematic way. The sea urchin blastula at the 64-cell stage is radially symmetrical, but it has polarity. It consists of tiers of cells. As in the frog blastula, the top is the animal pole and the bottom the vegetal pole.

If different tiers of blastula cells are separated, they show different developmental potentials (see Figure 19.7). Only cells from the vegetal pole are capable of initiating the de velopment of a complete larva. It has been proposed that the reason for these differences is an uneven distribution of various transcriptional regulatory proteins in the egg cytoplasm. As cleavage progresses, these proteins end up in different combinations in different groups of cells. Therefore, specific sets of genes are activated in different cells, determining their different developmental capacities. Let's turn now to gastru-lation in the frog, in which a number of key signaling molecules have been identified.

Gastrulation in the frog begins at the gray crescent

Amphibian blastulas have considerable yolk and are more than one cell thick; therefore, gastrulation is more complex in amphibians than in sea urchins. Furthermore, there is considerable variation among different species of amphibians. In this brief account, we will mix results from studies done on different species to produce a generalized picture of amphibian development.

Amphibian gastrulation begins when certain cells in the gray crescent change their shape and their cell adhesion properties. The main bodies of these cells bulge inward toward the blastocoel while they remain attached to the outer surface of the blastula by slender necks. Because of their shape, these cells are called bottle cells.

The bottle cells mark the spot where the dorsal lip of the blastopore will form (Figure 20.9). As the bottle cells move inward, they create this lip, over which successive sheets of cells will move into the blastocoel in a process called involution. The first involuting cells are those of the prospective en-doderm, and they form the primitive gut, or archenteron. Closely following are the cells that will form the mesoderm. As gastrulation proceeds, cells from the animal hemisphere move toward the site of involution in a process called epiboly.

Animal pole

|Gastrulation begins when cellsjust below the center of the gray crescent move inward to form the dorsal lip of the future blastopore.

Animal pole

Frog Crescent Blastopore

Blastocoel

Bottle cells

Dorsal lip of blastopore

Blastocoel displaced

^ Cells of the animal pole spread out, pushing surface cells below them toward and across the dorsal lip. These cells involute into the interior of the embryo, where they form the endoderm and mesoderm.

Archenteron

Involution creates the archenteron and destroys the blastocoel. The blastopore lip forms a circle, with cells moving to the interior all around the blastopore; the yolk plug is visible through the blastopore.

Neural plate T of brain Neurula

Endoderm Mesoderm Ectoderm

| Gastrulation is followed by neurulation, which is marked by the development of the nervous system from ectoderm.

Blastocoel

Bottle cells

Dorsal lip of blastopore

Blastopore

Blastocoel

Dorsal lip of blastopore

Blastocoel displaced

Blastocoel

Dorsal Lip Endoderm

Archenteron Mesoderm

Dorsal lip of blastopore

Endoderm

Blastopore Lip

Ectoderm

Mesoderm (notochord)

Dorsal lip of blastopore

Yolk plug

Ventral lip of blastopore

Neural plate T of brain Neurula

Endoderm Mesoderm Ectoderm

| Gastrulation is followed by neurulation, which is marked by the development of the nervous system from ectoderm.

Archenteron

Neural plate

Blastopore

Dorsal lip of blastopore

Archenteron Mesoderm

Dorsal lip of blastopore

Endoderm

Ectoderm

Mesoderm (notochord)

Dorsal lip of blastopore

Yolk plug

Ventral lip of blastopore

Notochord

Neural plate

20.9 Gastrulation in the Frog Embryo The colors in this diagram are matched to those in the frog fate map (Figure 20.6).

Blastopore

20.9 Gastrulation in the Frog Embryo The colors in this diagram are matched to those in the frog fate map (Figure 20.6).

The blastopore lip widens and eventually forms a complete circle surrounding a "plug" of yolk-rich cells. As cells continue to move inward through the blastopore, the archenteron grows, gradually displacing the blastocoel.

As gastrulation comes to an end, the amphibian embryo consists of three germ layers: ectoderm on the outside, endoderm on the inside, and meso-derm in the middle. The embryo also has a dorsal-ventral and anterior-posterior organization. Most importantly, however, the fates of specific regions of the endoderm, mesoderm, and ectoderm have been determined. The discovery of the events whereby determination takes place in the amphibian embryo is one of the most exciting stories in animal development.

The dorsal lip of the blastopore organizes embryo formation

In the 1920s, the German biologist Hans Spemann was studying the development of salamander eggs. He was interested in finding out whether the nuclei of blastomeres remain totipotent—capable of directing the development of a complete embryo. With great patience and dexterity, he formed loops from a single human baby hair to constrict fertilized eggs, effectively dividing them in half.

When Spemann's loops bisected the gray crescent, both halves of the zygote gastrulated and developed into complete embryos (Experiment 1 in Figure 20.10). But when the gray crescent was on only one side of the constriction, only that half of the zygote developed into a complete embryo. The half lacking gray crescent material became a clump of undifferentiated cells that Spemann called the "belly piece" (Experiment 2 in Figure 20.10). Spemann thus hypothesized that cytoplasmic determinants in the region of the gray crescent are necessary for gas-trulation and thus for the development of a normal organism.

To test his hypothesis, Spemann and his student Hilde Mangold conducted a series of delicate tissue transplantation experiments. They transplanted pieces of early gastrulas to various locations on other gastrulas. Guided by fate maps (see Figure 20.6), they were able to take a piece of ectoderm

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Responses

  • stephanie fried
    Is gastrulation in humans similar to sea urchins, frogs, or birds?
    5 years ago
  • marvin
    Does primary mesenchyme become mesoderm?
    5 years ago
  • maxima
    Where is the vegetal pole in a frog gastrula?
    5 years ago
  • Maximilian Diederich
    Which germinal layers give rise to digestive, circulatory, and nervous system?
    5 years ago
  • doreen
    What do the gastrula layers become?
    5 years ago
  • rufus
    What is the end of gastrulation marked by?
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  • simon
    Why cells grow inside to blastocoel in gastrulation?
    5 years ago
  • sesuna
    Why frog blastula animal and vegetal hemisphere/?
    5 years ago
  • LANNY
    What directs an organisms body plan?
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  • dinodas
    What tissue forms the blood vessels in sea urchin?
    5 years ago
  • Rosie
    Does the endoderm of sea urchin form the future digestive surface?
    5 years ago
  • niamh gordon
    Which side is the dorsal in frog zygote?
    5 years ago
  • james
    Do the main parts form from animal or vegetal poles in gastrulation?
    4 years ago
  • kerttu
    What does the process of gastrulation produce?
    3 years ago
  • miika
    What does gastrulation produces?
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