Jeremy has a master of science degree in education.
What is Gastrulation?
Gastrulation occurs after fertilization of an egg cell by a sperm cell. It's one of the processes that readies an organism for more complex processes.
During gastrulation, we see an arrangement of the cells in order to begin formation of our organs. It's an important time in the cycle of human development and one we'll explore, after we understand how we arrive at gastrulation.
Fertilization to Gastrulation
When a sperm cell finally reaches and burrows into an egg cell, we have fertilization, the fusion of gametes (egg and sperm) that initiates the start of the development of an organism. The first few divisions of the zygote (the fertilized egg cell) occur within a few days as it travels to the uterus.
The cells are loosely gathered together until they go through compaction, where they gather and stick tightly to one another. All of this occurs in one week. We then start to see the cells gather around a hollow center, which is the blastocyst stage of development. The bundled cells finally reach the uterus, where the blastocyst attaches to the endometrium (uterine lining) and will develop further. The hollow center of this blastocyst (or blastula, as it will come to be known), is called the blastocoel.
The extraembryonic membranes will develop here and create the chorion and amnion. The chorion is a layer that develops in the fetus, and the amnion is a membrane that covers the embryo, filling with fluid and creating a sac. The amnion helps protect the developing embryo. The next step is gastrulation.
The Process of Gastrulation
Gastrulation rearranges the cells of the blastula and ultimately forms the three germ layers of the embryo. The primary layers that will form all of the major body and organ systems of the organism, these germ layers include the ectoderm, mesoderm, and endoderm. They are the outer, middle, and inner layers, respectively.
Let's look at the gastrulation process in a bit more detail.
At the beginning of gastrulation, the blastula forms a small tuck or inward movement in its side. The cells around the tuck start pushing further and further inward, including the cells that will become endoderm and mesoderm cells. The cells that will end up becoming ectoderm cells begin to stretch as they move toward the tuck, covering the outside of the embryo. Picture in your head a ball of clay that you push in on itself while at the same time pulling the outer clay around it.
As gastrulation progresses, the cells that got pulled inside arrange themselves in such a way that the innermost layer is the endoderm, with cells that are the mesoderm covering them, and finally ending with the ectoderm cells that got pulled and extended over the entire embryo. The newly formed gastrula, or 3-layered embryo, is now ready to start organogenesis, the process whereby all the major organs and organ systems form in the organism.
A newly fertilized egg cell goes through many changes within its first few hours and days of existence. The loosely collected cells go through compaction, forming a blastocyst, which in turn has a blastocoel, or hollow center, and becomes the blastula. The blastula then begins to rearrange its cells in order to start gastrulation. A small tuck forms in the side of the blastula, and some cells get pulled inward, arranging themselves as the endoderm and mesoderm. The ectoderm cells get pulled toward the tuck, stretching out over the entire embryo. The resulting gastrula is ready to start organogenesis.
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