Embryology

After sexual intercourse sperm must travel through the female genitalia. Once a sperm cell finds an egg, the acrosomal reaction occurs in the head of the sperm (the acrosome) to break the follicular cells surrounding the oocyte. Then, the cortical reaction makes it so that the egg becomes impenetrable by any other sperm, so that only one male gamete nuclei can fuse with the female one, thus beginning fertilization. These two haploid gametes form one diploid zygote. Through cleavage, that single-celled zygote can become a multicellular embryo through mitosis. This creates the blastula, a ball of cells surrounding a liquid cavity. The blastula becomes the gastrula once those cells move to form specific layers. The ectoderm is the outermost layer, responsible for forming skin, hair sweat glands, epithelium, the brain and nervous system structures. The middle layer is known as the mesoderm, responsible for muscles, cartilage, bone, blood, connective tissue, reproductive organs and kidneys. The innermost layer is the endoderm, which forms the digestive and respiratory organs, the endocrine glands, liver, pancreas and gall bladder. At the center of the gastula is the archenteron, a cavity which later shapes the alimentary canal used for food to pass through for digestion. The creation of these specific tissues and organs is known as organogenesis, and is a major distinction between the embryo and the fetus.



http://embryo.soad.umich.edu/index.html