受精卵の発生と幹細胞の形成

A zygote is a diploid cell formed from the fusion of haploid sperm and egg cells. It is totipotent, meaning it can develop into a complete organism. The zygote undergoes mitotic cell division to form an eight-cell mass, then continues dividing to reach the 64-cell stage called the blastocyst, which contains the inner cell mass that produces the embryo.

Totipotent stem cells are cells capable of developing both embryonic and extraembryonic tissues. Found in the early eight-cell stage, these cells are spherical, loosely attached, and can reproduce and specialize into different tissues. Their unique capacity to form a complete organism distinguishes them from later-stage stem cells with more limited developmental potential.

The inner cell mass (ICM) produces embryonic tissues and forms the three germ layers that give rise to neural cells, epithelial cells, muscle cells, and blood cells. The trophectoderm (TE) forms the outer epithelial layer and differentiates into extraembryonic tissues such as the placenta, amnion, and chorion that support the growing embryo.

The blastocyst is a 64-cell stage structure that forms after the zygote undergoes multiple rounds of mitotic division. It consists of two distinct cell populations: the outer trophectoderm and the inner cell mass. At this stage, cells differentiate into two developmental pathways and become self-renewing but capable of differentiating only to limited cell types.

Adult stem cells retain limited potency and can only form cells of the specific tissue in which they reside, such as hematopoietic stem cells in bone marrow or epidermal stem cells in skin. Unlike embryonic stem cells, adult stem cells cannot develop into multiple tissue types. They function primarily to replace damaged or dead cells and maintain tissue integrity throughout life.

Adult tissues including skin, muscles, and gut retain a fraction of stem cells called adult stem cells. These tissues also include bone marrow, brain, lung, and liver, each containing specialized stem cell populations. Adult stem cells in these tissues continuously work to replace damaged or dead cells and maintain the structural and functional integrity of their respective tissues.

After blastocyst implantation on the uterine wall, cells undergo further embryonic development. The inner cell mass differentiates into the three germ layers—mesoderm, endoderm, and ectoderm—which generate all embryonic cell types. The trophectoderm continues forming extraembryonic tissues that support fetal growth until birth, when adult stem cells take over tissue maintenance roles.