This article, aims to explain the basic levels of embryogenesis and create the foundation for understanding the concept of EMT which will be analyzed in another paper.

 Everything begins with a  zygote. The zygote is made out of the zona pellucida, an outer glycoprotein coat , a plasma membrane (inside of the zona pellucida) and two genetic materials, one of each parent. The zygote starts dividing into multiple cells, basically cutting into two cells, which is a process called cleavage. Over time, more and more dividing occurs , resulting in the creation of many more  cells. This is called a morula. (It’s a Greek word that means mulberry because it somehow looks like it) . Those cells, which are called stem cells without any differentiation , aren’t very adherent with each other but they are all very similar. During the next step of this procedure, the cells begin to get  more and more closer together and start to differentiate, which produces two populations of cells. The one  population is called trophoblasts and the second one embryoblasts. Embryoblasts are the cells on the inside while trophoblasts are formed on the outside layer.

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The embryoblasts start to cluster all together in the middle forming a mass called inner cell mass. This actually leaves an empty “bursa” which is called blastocoel.

This whole structure now is called a blastocyst. At this point, the zona pellucida starts to fall apart until it is gone completely while the blastocyst remains. Another bursa forms in the inner cell mass while those cells start to differentiate even more resulting in different type of cells in two layers, the hypoblasts and the epiblasts.

Hypoblasts and epiblasts form a structure of two layers which is called the bilaminar disk. On the edge of the bilaminar disk, another formation is created and that is called primitive streak.  The primitive streak is  the place where the cells begin to migrate.

This causes a change in the structure of the disk. Τhree layers (germ layers) are formed and that is called a trilaminar disk. The first layer is called ectoderm, the second is called mesoderm and the third one, endoderm.

Το summarize the whole process until this point, everything started of with a bunch of cells that were all the same , existing in the whole area without them  clinging to each other. While the process started to grow, the cells appear to differentiate and start bonding with each other. The bonding which developed between them is actually the adhesion of the epithelium cells.

The cells of the mesoderm start to differentiate even more creating a formation called the notochord ( will later form some part of the intervertebral disks).

This formation, will generate another structure in the upper layer, the ectoderm, called the neural plate. The neural plate will fall down to the mesoderm and will create another formation, known as the neural tube. For this structure to be created, the cells of the ectoderm lose a percentage of their adhesion which they have between each other, so they can “dive” away from the surface.

Lastly, the cells of endoderm , mesoderm and ectoderm are diverse with different characteristics. The mesoderm cells, will create the mesenchyme and the connective tissue. Because of that, those cells are very distinctive from the cells of endoderm and ectoderm. They aren’t very adherent to one another but more attached with their matrix ,  they have a better ability of movement and they aren’t polarized the way the cells of the other two layers are. As a consequence, epithelial cells are developed from all of the three layers while the connective tissue is built only from the mesoderm cells.