First embryonic manifestations
Fertilization occurs with the penetration of the sperm into the egg, and the consequent formation of the zygote. The zygote, by the process of mitosis, will be divided until it forms an individual with the same genetic heritage contained in its initial form. However, throughout development, cells undergo a process of cell differentiation, in which some genes are activated (coordinating cellular functions) and others are deactivated (no longer acting in the organism). In this way, tissues are formed from different types of cells, which, together, make up the organs that form the systems that ultimately complete the organism.
Embryonic development (which begins with the first mitotic division of the zygote) can be divided into three consecutive stages: segmentation , gastrulation, and organogenesis.
The segmentation, or cleavage phase, is the step from the first division of the zygote to the formation of the blastula. As for its development, it can be divided into two phases: the morula phase and the blastula phase.
The cells that form from cell divisions (cleavages) are called blastomeres. In this phase, there is an increase in the number of cells, but the total volume of the embryo hardly increases, because the cleavages occur very quickly. Thus, the embryo is a cluster of cells, and this stage of development is called a morula.
The blastula is a developmental stage that occurs after the morula stage. It is characterized by having a rounded shape, with a cavity (blastocoel) filled with fluid.
In the next step, called gastrulation , the embryonic cells continue to multiply and begin to organize themselves to form the gastrula. At this stage, cell differentiation begins, with the formation of embryonic layers (ectoderm, mesoderm and endoderm).
In the stage of organogenesis , the differentiation of tissues and organs that will be part of the organism as an adult will occur from the embryonic leaflets.
In mammals, the segmentation is holoblastic and equal, that is, the zygote divides completely and the blastomeres formed are practically the same size. The development of the zygote begins before the embryo implants in the uterus and takes place during the course of the zygote towards it.