Totipotent stem cell

Totipotent stem cells are obtained from embryos that are less than 5 days old. These cells have the full potential to develop into another individual and every cell type. 

A population of desired cell types that have the potential to produce new tissues should be generated. The potential of embryonic totipotent stem cells could be exploited in the transplantation of retinal pigment epithelium, myocardial progenitor cells capable of restoring cardiac function and contractility, dopaminergic neurons for the treatment of Parkinson s disease, pancreatic cells for the treatment of diabetes, and others.55... 

Metchnikoff (1883) recognized the role of cell types (phagocytes) which were responsible for the engulfinent and digestion of microorganisms. They are a major line of defence against microbes that breach the initial barriers described above. Two types of phagocytic cells are found in the blood, both of which are derived from the totipotent bone marrow stem cell. 

Stem cells are divided into three different categories totipotent, pluripotent, and multipotent. A description of the genesis of stem cells is shown in Fig. 4.11. 

Karin M, Mintz B. Receptor-mediated endocytosis of transferrin in developmen-tally totipotent mouse teratocarcinoma stem cells. J Biol Chem 1981 256(7) ... 

Stem cells—Unspecialized cells able to develop into specialized cells may have limited capacity (multipotent or pluripotent) or may be able to turn into any cell of the body (totipotent). 

Stem cells of totipotent, pluripotent, or multipotent nature supply new germ cells and other cells for multicellular organisms when needed... 

As an alternative to adult stem cells, embryonic stem cells can be used. These are totipotent and can be obtained from the internal blastocyst cell mass. Because of the capacity of these cells to generate any type of functional cell, their manipulation and differentiation have gained in significance. In spite of recent advances (Daley, 2003 Hwang et al., 2004), knowledge on the control of their differentiation and proliferation is still lacking, but will be necessary to make the exploitation of all their therapeutic potential turn into reality. Further discussion on cell therapy can be found in Chapter 20. 

Embryonic stem cells (ES cells) and homologous recombination are utilized to inactivate an endogenous gene from a host s genome. ES cell lines are derived from a 3-day embryo (ICM cells) and are undifferentiated but remain totipotent. Mouse... 

Stem cells. Embryonic and adult stem cells are distinguished. Embryonic stem cells are taken from an early stage of the embryo, such as from blastocytes. They are undifferentiated and totipotent. Their potential to differentiate and to form different cell lines is unlimited. Adult stemcells are taken from the blood forming bone marrow, from epithelial cells from the skin and other sources. They are pluripotent. Both, embryonic totipotent and adult pluripotent stem cells can replace functionally differentiated cells and tissues in the body. Stem cells can divide. After division, they may form again a stem cell or proceed to a final, fully differentiated state. 

Teratoma, occur most frequently in the testis and the ovary. They are tumours containing cells and tissues which do not belong to that particular organs. These dislocated tissues arise from totipotent embryonic stem cells. 

After about five days and several cycles of cell division, the totipotent cells form a hollow sphere of cells called a blastocyst. The blastocyst has an outer layer of cells surrounding clusters of cells. Those cells on the outside continue to divide and grow into the placenta and supporting tissues. The clusters of cells on the inside divide and form virtually all the cell types, except the placenta and supporting tissues, which give rise to a human being. These are the pluripotent stem cells, and they give rise to many different types of cells, but not a new individual. 

The fertilized egg, or zygote, is the ultimate totipotent cell because it has the capability to generate all the cell types of the body. Although not technically a stem cell because it is not self-renewing, the zygote does give rise to... 

FIGURE SC-2 Reprogramming a somatic nucleus. When transplanted into an oocyte, a somatic nucleus may respond to the cytoplasmic factors and be reprogrammed back to totipotency. These cytoplasmic factors erase the molecular memory of the somatic cells. Such cells can then be used to harvest pluripotent stem cells or to transfer a blastocyst into a carrier and develop an organism in vivo. (Taken from Suram, M. A. Nature, 414, 122-127[2001].)... 

---