Gastrulation

Gastrulation is an important stage in the early development of most animals. During this stage, a simple ball of cells called a blastula, or in mammals a blastocyst, changes and reorganizes. This process turns the single layer of cells into an embryo with two or three layers.

Before gastrulation, the embryo is just one sheet of cells. But after gastrulation, the cells start to become different kinds of cells and the basic shape of the body begins to form. This includes setting up the back-to-front and top-to-bottom directions of the body.

Gastrulation is guided by special signals in the cells. These include BMP, Wnt, and Nodal. They help cells move to the right places, decide what kind of cells to become, and set up the body’s main axes. This stage is crucial because it lays the foundation for all the organs and tissues that will form later. embryonic development animals blastula cells blastocyst epithelial differentiation cell lineages dorsal–ventral anterior–posterior gut

Gastrula layers

In animals that develop into three layers, called triploblastic organisms, the gastrula has three main layers. These layers are the ectoderm (outer layer), mesoderm (middle layer), and endoderm (inner layer). These layers form when cells move through a special path called the primitive streak.

After gastrulation, the next step is organogenesis, where the body’s organs begin to form. Each layer becomes different parts of the body. The ectoderm becomes the skin’s outer layer and the nervous system. The endoderm becomes the lining of the digestive and breathing systems, as well as organs like the liver and pancreas. The mesoderm becomes muscles, bones, and connective tissue, and in vertebrates, it also forms structures like the heart, blood, and cartilage.

Basic cell movements

Gastrulation is an important step in how animals grow from a tiny ball of cells into more complex shapes. During this step, cells move in five main ways to help form the body’s layers and structure.

These movements include rolling a layer inward to make an indentation, rolling a layer over an edge to cover more space inside, cells moving from the outside to the inside, splitting one layer into two, and stretching a layer to cover deeper parts. These actions help create the different parts of the body as the embryo develops.

Invagination
Involution
Ingression
Epiboly

Etymology

The words "gastrula" and "gastrulation" were first used by a scientist named Ernst Haeckel in 1872. He wrote about them in his book called "Biology of Calcareous Sponges". The word "gastrula" means "little belly" and comes from an ancient Greek word for "belly".

Importance

One important idea in science comes from Lewis Wolpert, a scientist who studied how animals grow and develop. He said that of all the big events in life—like being born, getting married, or growing old—the time when a tiny baby animal changes shape inside its mother is very special. This change is called gastrulation, and it helps decide how the baby's body will look and work.

Model systems

Gastrulation looks different in various animals, but it shares some basic ideas. Scientists study this process in animals that develop outside their mother, making it easier to observe. Some of the animals used for these studies include the mollusc, sea urchin, frog, and chicken. There is also a special model called the gastruloid that helps scientists understand human development.

Protostomes versus deuterostomes

The difference between protostomes and deuterostomes is how the mouth forms during early growth. In protostomes, the first opening becomes the mouth, while in deuterostomes, that opening becomes the anus.

During development, protostomes have a special type of cell division called spiral cleavage, where the cells’ roles are set early. Deuterostomes have radial cleavage, where cells remain flexible.

Sea urchins

Further information: Sea urchin § Development

Sea urchins have been important model organisms in developmental biology since the 1800s. They help scientists understand how animals grow and develop.

During a very early stage of growth, special signals help organize the cells. The first cells to move inside form parts that will become the animal's skeleton. Later, more cells move in to help form the gut and other body parts. This process helps shape the young sea urchin and sets up its body structure.

Amphibians

The frog genus Xenopus is often studied to learn about how embryos develop. During a very early step called gastrulation, the embryo changes shape to form different layers of cells that will become various parts of the body.

In frogs, a special area called the "organizer" forms to guide this process. This area helps decide which parts of the embryo will develop into different tissues. Signals such as retinoic acid, along with other signals like Wnt and BMP, help control how cells form specific structures like the pancreas or intestines.

Amniotes

In animals like reptiles, birds, and mammals, a process called gastrulation helps shape the early embryo. During this time, the embryo changes from a simple ball of cells into layers that will become different parts of the body. An important part of this process is the formation of a structure called the primitive streak, which helps cells move and change to create new layers.

Before gastrulation, the embryo needs to become asymmetrical, meaning it looks different on one side compared to the other. This helps set up the basic layout of the body. Special signals help guide this change and the formation of the primitive streak. As cells move through the primitive streak, they change their shape and behavior, allowing them to form new layers that will eventually become different tissues and organs in the body.

Cell signaling driving gastrulation

During gastrulation, cells change to form different layers that will become parts of the body. Special signals help these changes happen. One important signal is called nodal signaling, which is part of a bigger group of signals. These signals tell certain receptors to start a chain of actions that finally turn on genes needed for the body’s layers to form.

Other signals, like Wnt, fibroblast growth factors, bone morphogenetic protein, and retinoic acid, also help guide how the body develops. They play roles in forming organs like the liver and lungs, and in setting up early body plans.

Gastrulation in vitro

Scientists have worked to understand the process of gastrulation by using special lab techniques. They grow cells from early embryos in controlled settings, which helps them study development without using animals as much. These methods let scientists add exact amounts of substances to the cells at just the right time, which can be hard to do inside a living embryo.

For example, mouse cells grown in the lab can form structures that look like early development stages. Human cells can also be guided to form patterns similar to early body layers. These lab studies help scientists learn more about how embryos develop, though they must be careful to connect these findings back to what happens inside a real embryo.