Embryology is the study of embryo development in the uterus from the fertilization of the zygote to fetus growth in a period of nine months (38 weeks). Embryogenesis begins with the fusion of the male and female sex gamete, which then the parent cell is replicated to produce identical cells in mitosis - this forms a hollow ball structure called the blastula. The blastula is characterized as having an animal pole, where there are rapidly dividing cells and the vegetal pole that has slowly dividing cells. Then a blastocyst forms, which is an inner cell mass in the blastula.
Gastrulation is when the cells in the blastula migrate inwards, producing a pore that eventually becomes the anus – humans are classified as deuterostomes from this distinction. In gastrulation the blastula reorganizes itself to have three germ layers that encapsulate the blastopore: ectoderm, mesoderm and endoderm. In organogenesis these three layers develop into different organs systems, the ectoderm is the outer lining which forms the nervous system and skin. The mesoderm is the middle layer that produces the musculoskeletal and blood system, whilst the endoderm is the inner layer develops into the digestive organs, lungs and bladder. The next stage is neurulation, this is when the ectoderm folds itself to produce the notochord, which becomes the central nervous system (CNS).
There are many different terms used in embryology. Zygote is the initial diploid cell produced from the fusion of the male and female sex gametes, whilst the embryo is multi-cellular diploid mass that is embedded into the endometrium and undergoing stages of development. The embryo is then renamed ‘fetus’ during the ninth week of development.