After fertilization, the zygote embarks on a journey to the uterus, during which it undergoes a process known as cleavage. Cleavage consists of rapid mitotic divisions that transform the zygote into a morula, a solid ball of cells. It's crucial to understand that as the zygote divides, the resulting cells, called blastomeres, become progressively smaller rather than maintaining the size of the original zygote.
There are two types of cleavage: indeterminate and determinate. Indeterminate cleavage allows the resulting cells to develop into complete organisms, which is how fraternal twins can arise from separate cells. In contrast, determinate cleavage results in cells that are committed to specific differentiation pathways, influenced by gene expression and transcriptional differences.
A significant concept in development is induction, where differentiated cells can affect the fate of neighboring cells through chemical signals. These signals, released by one cell, diffuse to nearby cells that possess the appropriate receptors, guiding their differentiation.
Cytoplasmic determinants play a vital role in this process. These regulatory molecules are unevenly distributed within the egg, leading to specialized cell formation during cleavage. For instance, if certain determinants are concentrated in specific regions of the egg, the resulting blastomeres will follow different developmental paths based on the determinants they inherit.
Once the morula is formed, the next stage is blastulation, where the morula develops into a blastocyst. In mammals, this hollow ball of cells is referred to as a blastocyst, which contains a fluid-filled cavity known as the blastocoel. Surrounding the blastocoel are trophoblast cells, which contribute to the formation of the chorion and placenta—support structures essential for the developing organism's survival but not part of its body.
Within the blastocyst, the inner cell mass consists of cells that will ultimately give rise to the organism itself. Thus, the trophoblast cells provide necessary support, while the inner cell mass is responsible for the development of the actual organism.