S C I E N C E

 

Cells adhesion is the process of cells connecting with each other and with their matrix. This connection happens because of the existence of cell adhesion molecules (CAMs), which are proteins. (μόρια κυτταρικής προσκόλλησης). Some important functions that are regulated by cell adhesion include cell migration and tissue development.  If the process of cell adhesion changes (example : mutation) this can lead to diseases, like cancer.

 Cell adhesion molecules  are proteins within the membrane of the cell and they are classified in two main categories: 1)  Molecules  dependent on Calcium ions  which are Cadherins and selectins. 2) Molecules non- dependent on calcium ions which are : intergins and immunoglobulin (lg) superfamily. Cells are able to use different adhesion molecules according to their functions.

Integrins: Are mainly involved with the interaction of cells with their extracellular substance. Basically their role is to bind the intracellular cytoskeleton of the cell and the external substance.

Cadherins : These molecules bind cells with each other.  They are group of transmembrane proteins dependent on Calcium ions. If Calcium ions are missing cadherins can’t bond with each other. There are more than 40 different cadherins molecules and many groups of cells produce different kind of Cadherins. For example : In the previous article, the basic parts of embryogenesis were explained. During the final stages, when the neural tube is being formed, a bunch of group cells get detached from the ectoderm and end up in the mesoderm and create a different cell line (neural crest cells). This is a party of cells that wants to stop being connected to one specific group of cells ( the ectoderm cells) and wants to bind to another cell group (the mesoderm cells). The cadherin molecules that are binding the ectoderm cells with each other, get ruptured, their bonds are breaking of, while new cadherin molecules are created between the neural crest cells in the mesoderm.

Cadhering molecules  are categorized  based on their unique ability to bind. They develop homophilic binding ( N-cadherin with N-cadherin). They also, provide a lot of strength to cells and to the cellular connection of tissues. Once cadherins are attached to each other the binding site inside the cell, changes Essentially, the inside part of each protein is attached to the cytoskeleton. The cytoplasmic part of cadherins connects with actin through intracellular binding proteins , known was catenins. (catenin complex).

 

Celectins : they get attached to carbohydrates.

Immunoglobulin (lg) superfamily : they are commonly associated with roles in the immune system. 

 

Cell adhesion molecules create cell junctions.

 Cell junctions are essential for cell stability. They are symmetrical structures that are formed, between two adjacent  cells. There are three types of cell junction. 

Tight-occluding junctions , anchoring junctions and gap junctions.

Anhoring junctions : they are located above the tight junctions and they have three sub junctions : 1) zonula adherens or belt desmosome) , 2) macula adgerens or spot desmosome) and 3) hemidesmosome.

Gap junction: These structures are necessary to cells, since they give them the ability to create tissues and directly connect the cytoplasm of two cells (cell to cell junction).  Gap junctions are composed  of transmembrane proteins called connexin which are organized in groups. They all create a structure of a pore called the connexon which looks like a channel. Connexons bind to other connexons through their pores, their proteins bind to each other. This conformation results in molecules moving from the cytoplasm of one cell to the cytoplasm of another. And that’s how a tissue is being made, since many cells are able to function are one.

Tight junctions : Tight-occluding junctions have a very important function:  Τhey determine the polarity of the cells  by segregating the basal region from the apical region. Epithelial cells are polarized, which means that the polarity of one cell isn’t symmetrical, one half is different from the other.

Cell adhesion molecules	Many cell adhesion molecules create cell junctions	Cell junctions		Anchoring junctions	Belt desmosomes connects with	Actine
Cadherins		Tight junctions		Belt desmosome	Microfibers -actin (through cadherins)	Part of cytoskeleton
Selectins		Anchoring Junctions		Spot desmosome
Integrins		Gap Junctions		Hemidesmosome
immunoglobulin (lg) superfamily

 

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.