Gastrulation is a key process in embryonic development – it’s how a simple ball of cells begins to grow into a fully functioning being! Watch this lesson to learn about a key process in the development of almost all multicellular organisms.
Today we’re going to talk about how the early vertebrate embryo transforms itself from a very simple ball of cells into a more complex embryo with multiple layers of different cell types.
We’re going to talk about gastrulation, which is the process that results in the formation of three distinct germ layers in the early embryo.Gastrulation is a necessary process that occurs in all multicellular organisms except for sponges. Sponges are unique among multicellular animals in that they do not contain multiple tissue types; their entire body is composed of basically one type of cell. Because they do not need to differentiate their cells, they also do not need to go through the process of gastrulation. However, all other animals do need to differentiate their cells, and the first step in this process is gastrulation, which is accomplished in different ways in different species.
Gastrulation in Frogs
In some animals that lay eggs, such as frogs, gastrulation is accomplished when the hollow blastula invaginates to form an inner layer of cells called the endoderm and an outer layer of cells called the ectoderm. A third, middle layer of cells called the mesoderm is formed as cells begin to populate the space between the endoderm and the ectoderm.
These three layers, the endoderm, the ectoderm and the mesoderm, are called the primary germ layers.
So here’s a frog blastula. If you imagine that someone is poking the blastula and pushing one side of it in, that’s what the process of invagination looks like.
The hollow ball now looks like a cup with an inner layer of endoderm and an outer layer of ectoderm. This cup-like structure is called a gastrula. The mesoderm is formed when cells begin to form a middle layer between the endoderm and ectoderm. Eventually, the endoderm will push all the way through the bottom of the cup to form a tube with an opening on each end. The two openings will eventually become the mouth and anus of the animal, and the inner cavity of the tube will become the gut.
In mammals, gastrulation is achieved in a very different way.
This is because the outer layer of the mammalian blastocyst, which is roughly equivalent to the blastula, becomes the mammalian placenta and is not available to create the primary germ layers of the embryo. Instead, the inner cell mass, or ICM, has to form the three primary germ layers of the mammalian embryo after the blastocyst has implanted in the uterine wall. Instead of forming a hollow ball and invaginating to create the gut tube and the three cell layers, the ICM of the mammalian embryo forms a single layer of cells called the epiblast. As these epiblast cells proliferate, some of them migrate out of the epiblast to form the endoderm, which is this layer here, and then after that more cells migrate out to form the mesoderm, which is this cell layer here.
The cells that remain in the epiblast form the ectoderm, which is this cell layer up here.
What the Different Germ Layers Become
With the formation of the three primary germ layers, the embryo has begun the process of tissue differentiation. Each of the three primary germ layers will form a different set of specific organs and tissue types in the animal. The ectoderm will form the nervous system, the epithelial cells of the mammary glands, the lens of the eyes, hair, pigment cells and the epidermis of the skin.
The mesoderm will form skeletal muscle, smooth muscle, the heart, blood vessels, blood cells, kidney, spleen, fat cells, the skeleton, most connective tissues and most of the urogenital system. The endoderm will form the entire epithelial lining of the gastrointestinal tract, as well as the liver, pancreas, gall bladder, thyroid, the epithelial lining of the trachea and the respiratory surface of the lungs.
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