Gastrulation is the process during embryonic development that changes the embryo from a blastula with a single layer of cells to a gastrula containing multiple layers of cells. In gastrulation, the blastula typically folds in or divides, creating two layers of cells.
Organisms that do not form a third layer are called diploblastic organisms. This includes jellyfish and related animals. Triploblastic organisms contain a third layer, the mesoderm, which is created from one of the first two layers. Triploblastic organisms make up the majority of higher animals.
Gastrulation is a formative process by which the three germ layers, which are precursors of all embryonic tissues, and the axial orientation are established in embryos. During gastrulation, the bilaminar embryonic disc is converted into a trilaminar embryonic disc. The embryo during this stage is called a gastrula.
Gastrulation is the process during embryonic development that changes the embryo from a blastula with a single layer of cells to a gastrula containing multiple layers of cells. Gastrulation typically involves the blastula folding in upon itself or dividing, which creates two layers of cells.
The layers created by the gastrulation process become germ layers that differentiate the embryo into three types of special tissue. The ectoderm is the outermost layer that makes the skin and nervous system. the mesoderm, from which connective tissue, circulation, muscles, and bones develop; and the endoderm, which makes up the digestive system, lungs, and urinary system
Before gastrulation, these layers are not defined. Gastrulation proceeds differently depending on the organism and the type of blastula from which it originates. The picture below shows an embryo before and after gastrulation.
Before gastrulation, a unicellular zygote must divide many times to form a ball of cells called a blastula. This process is known as fission and has different patterns in different organisms. The blastula of different organisms can take many different shapes. Some blastulas are a hollow ball of cells, others are a solid ball of cells, while other types retain yolks as an energy source and form around or on the yolk. The gastrulation of these different types of blastula is discussed below.
Gastrulation occurs at three embryonic sites:
- 1. Primitive Streak
- 2. Node
- 3. Tail Bud
Gastrulation at the Primitive Streak
The epiblast is a simple circular plate of cells. Cells multiply and converge by migrating at one pole of the epiblast. For reasons of space, these cells are columnar. This defines the back of the embryo. Eventually, convergence leads to cells being shifted forward. As more cells converge, the primitive stripe elongates.
Cells in the primitive groove change from columnar to wedge-shaped, creating two bumps on either side of a groove. Epiblastic cells of the primitive streak begin to invade ventrally to form the new germ layers. As the primitive stripe extends forward, it enters certain “areas” of the epiblast. These are regions that are organized from posterior to anterior and are designed to become certain germ layers.
Prospective endoderm cells are the first to invade. Upon penetration, bonds to neighboring cells of the epiblast are broken attachments and moved ventrally. Cells that have detached from the epiblast form an epithelial to the mesenchymal transition.
This is achieved by breaking the basal lamina, followed by breaking the intercellular connections. Cells that go through this transition change from regularly shaped to irregularly shaped. When the cells reach the hypoblast, they intercalate with the cells of the hypoblast and return to their original epithelial state.
As the streak elongates, it enters the prospective lateral plate mesoderm region (LPM) of the epiblast. Cells of the prospective LPM converge and ingress, making an epithelial to mesenchymal transition. They do not intercalate with the endoderm but remain mesenchymal. They then migrate laterally and anteriorly.
As the streak continues to expand, it enters the central and paraxial areas of the mesoderm, which penetrate in a similar manner. The cells of the LPM are most lateral, then intermediate, followed by the paraxial mesoderm. As the penetration continues and the endoderm continues to intercalate, the hypoblast is pushed sideways. The remaining epiblast forms the ectoderm.
Gastrulation at the Node
The primitive streak stops elongating about 75% of the way along the length of the posterior-anterior axis. Cells condense at this front point of the primitive streak and continue to multiply; called Hensen’s Node. They then ingress and replace the anterior hypoblast and form the foregut endoderm.
Cells then migrate anteriorly, leaving the node. These cells will form all of the head mesoderms. The node then recedes backward, destroying the primitive streak and forming the notochord (axial mesoderm). The notochord begins at the level of the midbrain. Notochord regression also creates a paraxial mesoderm. The anterior cells develop before the posterior cells undergo gastrulation.
Gastrulation at the Tail Bud
Cells at the posterior of the primitive streak are allocated to form the tailbud. When the primitive streak elongates, these cells do not follow, but migrate backward and form a condensed cell ball called the tailbud. The tail bud exists to make cells of the hindlimb. Tail notochord and somites are formed by the regression of the tail bud.
No endoderm exists in the tailbud. Production of the tailbud is called secondary gastrulation, production of the primitive streak, node, and notochord is called primary gastrulation. Without the tail bud, the notochord would run out of cells before the hindlimb could be made. In animals with tails, cells are gastrulated at an additional site
Types of Gastrulation
Although gastrulation patterns exhibit enormous variation throughout the animal kingdom, they are unified by the five basic types of cell movements that occur during gastrulation:
Gastrulation of a Coeloblastula
A coeloblastula is a hollow cell sphere one cell thick. Gastrulation in a blastula of this type involves invagination, ingression, or delamination. Invagination involves the blastula folding in on itself, creating a pocket with an opening. These are known as the archenteron and blastopore and become parts of the intestine.
The in-fold becomes the endoderm, while the outer later becomes the ectoderm. The resulting gastrula is called a coelogastrula because it remains hollow. This can be seen in the above image. The blastocoel is simply the empty space inside of the blastula.
Coeloblastulae can also undergo a type of gastrulation that creates a solid gastrula known as a stereogastrula. During this form of development, the cells of the blastula divide and migrate into the blastocoel, eventually filling the space. The movement of cells in this form of gastrulation is known as ingression.
The cells on the inside will develop as endoderm, while the surface cells will develop as ectoderm. This can be seen in many cnidarians, such as sea anemones and jellyfish. The final form of gastrulation is called delamination and occurs when the cells in a coeloblastula divide and form a layer of cells that surrounds the original blastula. This form is seen only in some groups related to jellyfish.
Gastrulation of a Stereoblastula
A stereoblastula is a blastula that is present as a solid cell mass. Gastrulation in stereoblastulae differs from gastrulation in a coeloblastula because there is no internal space in which cells can divide. Instead, cells on the surface of the sphere divide faster until the surface of the sphere is covered with a new layer of cells. This layer acts as the ectoderm, while the solid ball in the center forms the endoderm, as in the previous form of the stereogastrula. The archenteron will later develop out of the solid mass of the cells.
Gastrulation of a Discoblastula
In contrast to the other forms of blastulae, a discoblastula does not form a cell ball around the original cell. Rather, the cells are arranged in a disk at one end of the blastula, and each has access to a yolk reservoir at the other end. During gastrulation in a discoblastula, the ends of the intervertebral disc curve and grow towards each other. The lower layer develops as the endoderm while the upper layer, farther away from the yolk, develops as the endoderm. This is known as involution.
In addition to these standard forms of gastrulation, there are many others in nature. It is mostly a combination of these different forms. Scientists can study the gastrulation of organisms as a characteristic that helps distinguish between related organisms. Related organisms, like other traits, tend to develop in similar ways.