During fertilization, the young embryo goes through some rapid growth in the first few days after conception. Among this rapid growth is the beginning of the mesoderm, one of three of the important layers that give rise to our organs. We will discuss the mesoderm here.
Germ Layer Formation
After fertilization, a new soon-to-be organism goes through a few changes. One of those changes is gastrulation. Gastrulation is an early phase of embryonic development. During this phase, the blastula (the embryo when it’s still a hollow ball of cells) rearranges itself into the gastrula (the embryo with three germ layers).
What does this have to do with the topic of our lesson- the mesoderm?Well, the mesoderm is the embryonic tissue layer that sits between the ectoderm and the endoderm in the gastula. So to understand how the mesoderm forms, we have to understand how gastrulation occurs. During gastrulation, the blastula will form a small tuck in the side of it. The cells around the tuck start pushing further and further inward. As this occurs, the cells that will become endoderm and mesoderm cells move further and further in. At the same time this is occurring, the cells that eventually will be ectoderm cells begin to stretch and move towards the tuck, covering the outside of the embryo.As we progress further, the cells that got pulled inward will arrange themselves so the innermost layer is the endoderm, covered by the mesoderm, covered by the ectoderm that got pulled and extended over the entire embryo.
The germ layers that form during gastrulation will arrange themselves on the outside, inside, and the middle of the newly formed gastrula.
On the outside of the embryo, we see the ectoderm. This layer will eventually form the outer layer of skin, linings of the esophagus, and many other linings and nerve tissue. On the inside of the gastrula is the endoderm, which serves as the digestive tract of the embryo.
It will eventually form the digestive tract of the organism, as well as many of the associated larger organs.As we mentioned, the mesoderm is the layer of embryonic tissue directly between the ectoderm and the endoderm. The mesoderm is just as important as the ectoderm and endoderm in that it develops many of our muscle cells and organs, including our skeletal system, muscle system, and the main parts of our nervous system. Now let’s talk about how our bodies form these parts in a process called organogenesis.
Organogenesis is the process that causes the organs to form in each of the layers. During this process, we see each of the layers above starting to coalesce, divide, and fold into their respective organs.
The actual organism will determine what actually forms, but most are essentially the same – at least at the start.The mesoderm first forms the notochord and neural tube of the organism, which are among the most important parts of the animal. These will be the beginnings of the central nervous system in animals.
The notochord essentially becomes the spinal cord and around it we see somites (strips of mesoderm cells that arrange around the notochord) forming. These somites will condense to become the vertebrae of the backbone in vertebrates.The mesoderm will also give rise to a number of other systems, such as the lymphatic system, reproductive system, and adrenal cortex, just to name a few. How each of these different systems forms from the mesoderm could be a course in and of themselves.
A fertilized egg will form a blastula, or hollow ball of cells. This blastula will then go through gastrulation, where it forms the gastula, or 3-layered embryo. The three layers that form here are the endoderm, ectoderm, and mesoderm.
The mesoderm is the middle layer of the three. It forms during gastrulation where a small tuck will form in the blastula. The cells that will become the endoderm and mesoderm push their way further into the blastula, while the ectoderm cells move around and cover the outside of it. Eventually, the ectoderm will cover the entire embryo with the inside having the two layers, mesoderm and endoderm.
The mesoderm will give rise to the notochord and in vertebrates, the individual vertebrae. It also is responsible for the skeletal and muscular system, as well as a variety of other important organs and organ systems.