Totipotency and Stem Cells

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. 

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. 

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. 

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

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... 

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... 

Well, it s only human nature to ask, If sea anemones can clone themselves without hardly trying, and the natural cloning powers of plants are so easily exploited, why can t we begin learning how to clone ourselves The answer to that question is hidden in the secrets of the cell. All plants have one kind of cell that remains forever in the embryonic condition. These cell layers of embryonic tissue are totipotent and can give rise to new differentiated cells. Cut the stem of a plant and these cells will produce root, stem, and leaf tissues. In some animals, totipotent cells—as in the foot of a sea anemone—when cut, will dedifferentiate, and return to an embryonic condition. Then they... 

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. 

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. 

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].)... 

For definitive testing of the totipotency concept, single cells or protoplasts must be picked axenically from the culture medium and placed on a suitable growing medium then through manipulation of the mineral, vitamin, amino acid, and hormonal components of the medium cause development of a tissue (usually callus) that gives rise to embryoids that in turn develop roots, stems. 

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