Development of a human embryo and its brain comes about in three stages: blastulation, gastrulation, and neurulation. First, the egg and sperm fuse into a zygote which has an internal plasma membrane and an external zona pellucida. The zygote cleaves internally, maintaining the same volume inside the zona pellucida but multiplying the number of cells inside it, at which point it is called a morula. The cells inside the zona pellucida go through compaction and differentiation starts to occur. A sphere of trophoblasts develops just under the zona pellucida and around the mass of embryoblasts inside the embryo. The embryoblasts then accumulate at the top half of the embryo, leaving a space called a blastocoel under them. This whole process is known as blastulation and the resulting structure is a blastocyst. Gastrulation follows blastulation first by the loss of the zona pellucida. Then, the mass of embryoblasts forms an inner cavity called the amniotic cavity. The cells under the amniotic cavity become epiblasts and the cells under the epiblasts become hypoblasts. These two cell layers are collectively known as the bilaminar disc. The primitive streak forms along the middle of the bilaminar disc, on the epiblast layer. This primitive streak is composed of epiblast cells migrating down and forming a middle layer of cells between the two original layers of the bilaminar disc. This forms a trilaminar disc with three layers known as germ layers. The topmost layer is the ectoderm, the middle layer is the mesoderm, and the bottom layer is the endoderm. The ectoderm will form the nervous system and the skin. The mesoderm will make the muscles and bones. The endoderm will make the viscera. The formation of these layers is known as gastrulation.
Recent comments