Ontogeny begins from a single cell, the zygote. It is generally believed that this restriction in developmental potential indicates irreversible differentiation and specialization of early embryonic cells into the first two lineages, the inner cell mass (ICM) that includes cells that will give rise to the fetus and the trophectoderm (TE), and an outer layer of cells that is destined to an extraembryonic fate ( Fig. As embryo development progresses to the 8-cell stage and beyond depending on the species, the individual blastomeres that comprise the embryo gradually lose their totipotency. In the latter case, we confirmed the ability of isolated blastomeres from 2- and 4-cell stage, IVF produced embryos of the rhesus monkey to support term pregnancies and to produce live animals. This approach was pioneered in rats and has been realized in several mammalian species including nonhuman primates. Experimentally, totipotency can be demonstrated by the isolation of a single blastomere from a preimplantation embryo and subsequently monitoring its ability to support a term birth following transfer into a suitable recipient. This is said to be true for the zygote and for early embryonic blastomeres up to at least the 4-cell stage embryo (see Fig. Thus, each totipotent cell is a self-contained entity that can give rise to the whole organism. Consistent with the definition, the zygote is totipotent, meaning that this single cell has the potential to develop into an embryo with all the specialized cells that make up a living being, as well as into the placental support structure necessary for fetal development. Mammalian development commences when an oocyte is fertilized by a sperm forming a single celled embryo, the zygote. For example, a plant cutting or callus can be used to grow an entire plant. In some organisms, cells can dedifferentiate and regain totipotency.

Totipotent cells formed during sexual and asexual reproduction include spores and zygotes. Totipotency is defined in Wikipedia as the ability of a single cell to divide and produce all the differentiated cells in an organism, including extraembryonic tissues. In this review, we summarize two major approaches to reprogramming: (1) somatic cell nuclear transfer and (2) direct reprogramming using genetic manipulations. Reprogrammed somatic cells may have an even more important role in cell replacement therapies since the patient’s own somatic cells can be used for reprogramming thereby eliminating immune based rejection of transplanted cells.

Pluripotent cells resembling ESCs can also be derived experimentally by the nuclear reprogramming of somatic cells. Since many human diseases result from defects in a single cell type, pluripotent human ESCs represent an unlimited source of any cell or tissue type for replacement therapy thus providing a possible cure for many devastating conditions. ESCs retain their ability to differentiate into cells representing the three major germ layers: endoderm, mesoderm or ectoderm or any of the 200+ cell types present in the adult body. Pluripotent cells can be isolated, adapted and propagated indefinitely in vitro in an undifferentiated state as embryonic stem cells (ESCs). As development unfolds, cells of the early embryo proliferate and differentiate into the first two lineages, the pluripotent inner cell mass and the trophectoderm.

Mammalian development commences with the totipotent zygote which is capable of developing into all the specialized cells that make up the adult animal.

Somatic stem cells definition