התפתחות זיגוטית ויצירת תאי גזע

All multicellular organisms arise from a single fertilized egg cell called the zygote. The zygote divides mitotically to form an eight-cell mass.

At this stage, the cells are called totipotent stem cells that can reproduce, renew, and specialize into different tissues upon implantation.

The dividing stem cells form a 64-cell mass called the blastocyst, with an outer layer called the trophectoderm or TE, and a core of loosely attached cells called the inner cell mass or ICM.

The TE comprises a single layer of cells firmly adhered to each other, forming the first epithelium. These cells later differentiate to form extraembryonic tissues, while the ICM produces the embryo.

As a baby grows into an adult, tissues such as skin, muscles and gut retain some stem cells. Adult stem cells can only form the specific tissue in which they reside and replace damaged or dead cells.