The development of all multicellular organisms starts with the fusion of haploid cells called sperm and egg to form a diploid zygote. A zygote is a totipotent cell that can develop into a complete organism. The zygote undergoes cell division or cleavage to form an 8-cell mass. Until this stage, the cells are spherical, loosely attached, and remain totipotent. Totipotent cells are capable of developing both the embryonic and the extraembryonic tissues. However, as they continue to divide, they reach the 64-cell stage, called the blastocyst. The cells differentiate into two distinct developmental pathways and form the inner cell mass (ICM) and the trophectoderm (TE) cells. ICM is the source of embryonic stem cells or ES cells. ICM and TE are self-renewing but can differentiate only to limited cell types.
The ICM next differentiates into the embryonic tissues and forms the three germ layers (mesoderm, endoderm, and ectoderm) that give rise to all cell types of the embryo (neural cells, epithelial cells, muscle cells, and blood cells). In contrast, TE cells form the extraembryonic tissues such as the placenta, amnion, and chorion that cover the growing embryo. Once the blastocyst gets implanted on the uterine wall, they undergo further embryonic development.
After birth, each adult tissue retains a fraction of stem cells called the adult stem cells. Adult stem cells can form cells of a particular tissue type to help replace damaged or dead cells and maintain tissue integrity. Adult stem cells include hematopoietic stem cells of the bone marrow and epidermal stem cells of the skin, gut, brain, lung, or liver.
