After the ejaculation of semen during sexual intercourse, the sperm begins its path towards fertilization. The head of the sperm is called the acrosome. Once the sperm reaches the zone pellucida of the egg, the acrosome undergoes the acrosomal reaction, during which the acrosome fuses to the sperm's plasma membrane, resulting in the release of enzymes that were once contained in the acrosome. The acrosomal reaction allows for the sperm to break through the egg's plasma membrane in order to fuse with the egg. This fusion marks the beginning of fertilization. In order to prevent the egg being fertilized by more than one sperm, the egg undergoes the cortical reaction, which depolarizes the egg's plasma membrane, thereby avoiding polyspermy.
Once fertilized, the zygote begins cleavage. The fertilized egg undergoes mitosis and DNA synthesis of the sperm and egg, followed by the division into two separate cells. This mitotic division results in the creation of a blastula, a ball of cells surrounded by a liquid cavity, known as a blastocoel.
Through gastrulation, the blastula divides into a multi-layered embryo, creating a gastrula. The gastrula is comprised of four main parts. The outermost layer is the ectoderm, which is responsible for the creation of tissues associated with the skin, hair, sweat glands, epithelium, as well as the brain and nervous cord. The middle layer is the mesoderm, which is responsible for the formation of connective tissue, muscle, cartilage, bone, blood, kidneys and reproductive organs. The innermost layer, the endoderm, is responsible for the creation of the organs involved in the digestive system, respiratory system, as well as the liver, pancreas, gall bladder, thyroid and parathyroid glands. The inner cavity of the gastrula is the archenteron, which forms the alimentary canal responsible for carrying food through digestion.
The formation of the organs in each layer of the gastrula is known as
organogenesis.
