For our discussion, I have identified nine key embryological events:
We discussed the process of mitosis at the beginning of the course. You already know how essential this process is to the maintenance of healthy, normal cells in organs such as the skin. Now think about what happens during development.
At the end of oogenesis, a female pronucleus containing 23 single-stranded chromosomes is ready to unite with a male pronucleus which contains the same number of homologous chromosomes. After their union, a segmentation nucleus is produced. This segmentation nucleus is the beginning of the embryo. Isn't it humbling to realize that we began as a single cell! This cell divides, giving us the 2 cell stage. Since these cells are both so large, they are often called blastomeres. Mitosis continues until the morula is formed. When a cavity called the blastocoele (or, alternately, blastocele) forms in the morula, the blastocyst appears and implantation begins.
Constant cell division during development yields enough cells for an entire body!
The single cell formed at the end of fertilization contains the genetic information for every cell in the body. That cell divides and produces cells that also carry the entire genome. Yet, while we view the progression from segmentation nucleus through the development of the blastocyst, we cannot identify a muscle cell or a skin cell. In fact the cells that are present might become any cell in the body.
This equal ability of cells to form any cell in the body is maintained until we can begin to see a difference in cells.
Notice from your reading that when the blastocyst develops, two distinct cellular masses form. These are called the inner cell mass aka embryoblast and the trophoblast. The destinies of these two cell groups differs. The embryoblast will develop into the embryo, whereas the trophoblast will form the syncytiotrophoblast and the cytotrophoblast which become part of the embryonic membrane called the chorion.
The syncytiotrophoblast secretes enzymes that aid implantation--the attachment of the blastocyst to the endometrial wall of the uterus. This ability is not shared by any other part of the embryo, and represents the first example of differentiation.
The embryoblast also changes as, first the bilaminar, and then the trilaminar, embryo develop. Cells that become ectoderm have restricted options available to them. These cells may become skin cells in the epidermis or nerve cells in any part of the CNS or PNS; but they have lost the ability to become skeletal muscle cells which arise from mesoderm or epithelial cells of the stomach which arise from endoderm.
Differentiation goes hand-in-hand with the loss of the equal ability of cells to form any tissue. I view differentiation as a gradual process that starts here in the blastocyst.
Through a sequential series of gene expressions, ectodermal cells will progress to become neurons. Some of these neurons will form the brain rather than the spinal cord or the ganglia of the PNS. Some neurons in the brain will form the cerebral cortex as opposed to the thalamus or pons. Finally, some cortical neurons become pyramidal cells and others become stellate cells (another type of neuron in the cerebral cortex).
The sequential process I have described is an example of a process that becomes more and more specific at each step. Finally, we reach the point where these cells are committed to becoming neurons and have lost the ability to become glial cells, for example.
The process of committing to a cell line cannot occur with differentiation. Through extensive, cellular proliferation of the newly differentiated cells at each point, large groups of committed cells are formed.
In some cases, differentiation cannot occur without some triggering event. The development of the nervous system is an excellent example of this principle.
Some mesodermal cells of the trilaminar embryo form a structure called the notochord. Let me repeat that the notochord is mesoderm. However, the presence of the notochord causes the overlying ectoderm to thicken and form the neural plate, the first stage in the development of the nervous system. Once the neural plate is formed, continued development of the nervous system will proceed without the notochord which eventually becomes resorbed into the nucleus pulposus of each vertebra.
Continue to trace the development of the nervous system in this week's assigned URL. Pay particular attention to the role that each of the following structures plays in this development: neural groove, neural folds, and the neural tube.
In describing the process of the differentiation of cells, I inferred that cells moved or migrated from one position to another. Cells in the blastocyst move to form the inner cell mass. Some cells in the differentiated, trilaminar embryo move to an area that becomes the midline of the embryo. When cells destined for this location group together or aggregate, they form a structure called the primitive knot from which a primitive streak extends.
Once the primitive streak appears, the embryo contains right and left sides. The stage is set for bilateral symmetry of organs and structures to begin. Patterning the segmentation of the body plan is a process that is accomplished when mesodermal cells migrate to form the somites. These structures set up the pattern that we discussed as we considered the vertebral column, the spinal nerves and the dermatomes of the body, just to name a few examples of segmentation. Skeletal muscles in the body develop from special masses in the somites; whereas the muscles of the head region develop from one of four branchial arches.
Again, the key events of cellular proliferentiation, differentiation, migration, and aggregation are repeated. Whether cells migrate before they differentiate or differentiate before they migrate is not just a brain teaser. It is an important consideration in the development of some parts of the body.
There is so much cellular proliferation, that a beginning student might assume that it doesn't matter when most structures of the body appear. In fact, a sequence that is absolutely rigid for the development of some structures can be less so for others. The reason may lie in the fact that, often, cells must migrate in order to aggregate. Cells moving from special regions of some somites form the limb buds. The limb buds appear at the beginning of the second month post-fertilization. Once these structures appear, complete limbs form quickly. If some event causes the newly aggregated cells of the limb buds to die, the limbs may not form. There is no catch-up later.
I lived through the thalidomide tragedy in my lifetime. Thalidomide, alcohol and cigarette smoke are three examples of teratogens. A teratogen is any substance that interferes with the nomal development of the embryo. The presence of these substances can prolong the period of cell death (next paragraph) or trigger apoptosis in cells that should not degenerate. Teratogens can definitely make a difference in development.
During my discussion of "the cell" early in the course, I gave you a textbook reading assignment that included a discussion of apoptosis. Apoptosis is the process by which cells commit suicide; it is repeated in the embryo a number of times.
When the digits appear, they are webbed. Spread your fingers of one hand and notice that the webs have disappeared. Some trigger causes cells in the web to die. This pattern is often referred to as "programmed cell death" because it happens in all human embryos.