The six stages of human embryo development are gamete formation, fertilization, cleavage, gastrulation, organ formation and tissue specialization, as Indiana University — Purdue University Indianapolis Department of Biology describes. Each stage can only progress successfully if the previous stage completed successfully.
Gamete formation refers to the stage in embryonic development when the egg cell and the sperm cell form independently in specialized tissues via the process of meiosis, explains IUPUI Department of Biology. Fertilization occurs when these two cells fuse inside the fallopian tubes, producing a zygote containing genetic information from both the mother and the father. During cleavage, the zygote begins undergoing mitotic division, initiating the transformation into a multicellular being. This stage takes place during the first days and weeks following the formation of the zygote.
Following cleavage, gastrulation is the stage when the cells reorganize into three germ layers, called the ectoderm, endoderm and mesoderm, states IUPUI Department of Biology. The ectoderm consists of the nervous system and skin, while the endoderm contains the organs and gut lining. The mesoderm contains the bones, heart and muscles. During organ formation, the cells become designated for different functions such that, for example, the cells making up the brain are distinct from liver cells. Lastly, the cells in the different organs specialize further into different tissues.