Three separate cell layers begin to form from the blastula that allow for the differentiation of cells that ultimately form the being. These germ layers or sheets are referred to as the ectoderm, the mesoderm, and the endoderm. Each type of germ layer forms multiple unique cells that together form the embryo, fetus, and infant. This is the first type of specification that forms in the growing blastula. Besides the three germ layers, cells that begin to create the mammalian placenta begin to form, which is the structure than nourishes the embryo. The placenta does not come from any of the germ layers.
As the embryo develops, some cells turn on specific genes, while other cells turn off genes, which results in the specification of cells. The upregulation of specific genes makes proteins in the cytoplasm that do different things inside the cell, creating the molecules and organelles necessary for cell function. The result of the upregulation of certain genes and the downregulation of certain genes is the formation of different cell types that go from the simplest cell types—the ectodermal cells, endodermal cells, and mesodermal cells—to the more complex cells, like nerve cells, muscle cells, and epidermal cells. As the process of differentiation of the blastula occurs, the ectodermal cells arrive on the outside of the blastula, the mesodermal cells arrive at the middle of the blastula, and the endodermal cells arrive at the inside of the blastula/embryo. These cells communicate with one another, beginning the process of intracellular communication, which will be discussed in chapter 8. The shape of the embryo begins to form with a caudal aspect, mesial aspect, and cephalad aspect, giving the tail, trunk, and head of the embryo, respectively. Rather quickly, the embryo begins to grow because different DNA molecules are turned on in certain cells that change the growth rate of each specific type of cell. Some cells grow faster than others, which creates the unique shape of the embryo. The head is larger than the tail because these cells grow faster and bigger than the caudal cells so the embryo takes on the shape of a larger head, a medium-sized middle, and a smaller tail. No one knows how these cells communicate with one another to create this unique shape.
Cell Biology Basics The cellular unit consists of numerous different molecules that make organelles—each of which is enclosed by a membrane of some sort. The organelles are separate from one another but are connected by an intracellular matrix that allows them to stay basically in the same place unless they are needed to travel to one or another part of the cell.
