An Overview of Biological Basics - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 39

means. They are classified on the basis of their motion. The amoebae move through ameboid
motion, whereby the cytoplasm of the cell flows forward to form a pseudopodium (false
foot), and the rest of the cell flows toward this lobe. The flagellates move using their flagella.
Trypanosomes move by flagella and cause a number of diseases in humans. The ciliates move
through motion of a large number of small appendages on the cell surface called cilia. The
sporozoans are nonmotile and contain members that are human and animal parasites. These
protozoa do not engulf food particles but absorb dissolved food components through their
membranes. Protozoa cause some diseases, such as malaria and dysentery. Protozoa may
have a beneficial role in removing bacteria in biological wastewater treatment processes.

2.2. STEM CELL

Stem cells are unspecialized cells (in multicellular organisms) that have two defining prop-
erties: 1) potency or the ability to differentiate into diverse specialized cells including differ-
entiated stem cells and 2) reproductivity or the ability to duplicate.
Stem cells are found in all multicellular organisms. In mammals, there are two broad types
of stem cells: embryonic stem cells and adult stem cells. Adult stem cells, also known as
somatic (fromGreek, “of the body”) stem cells and germline (giving rise to gametes) stem cells,
are found in various tissues and in children as well as adults. In adult organisms, stem cells
and progenitor cells act as a repair system for the body, replenishing adult tissues. Embryonic
stem cells are isolated from the inner cell mass of blastocysts. In a developing embryo, stem
cells can differentiate into all the specialized cells (these are called pluripotent cells) but also
maintain the normal turnover of regenerative organs, such as blood, skin, or intestinal tissues.
The ability to differentiate is the potential to develop into other cell types. Table 2.4 shows
the potency of the stem cells. In a normal development of a multicellular organism, a totipo-
tent stem cell (e.g. spores and fertilized egg) is the stem cell having the highest potency, which
can differentiate into all cell types including the embryonic membranes, pluripotent cells,

TABLE 2.4 Potency or the Differentiation Potential of the Stem Cell

Stem cell type Potency: types of stem cells can be produced.

Totipotent (or omnipotent) Can differentiate into embryonic and extraembryonic cell types. Such cells can

construct a complete, viable multicellular organism, for example, spores and
fertilized egg. These cells are produced from the fusion of an egg and sperm cell.
Cells produced by the first few divisions of the fertilized egg are also totipotent.

Pluripotent Descendants of totipotent cells. Can differentiate into nearly all cells, i.e., cells

derived from any of the three germ layers.

Multipotent Can differentiate into a number of stem cell types, but only those of a closely

related family of cells.

Oligopotent Can differentiate into only a few stem cell types, such as lymphoid or myeloid

stem cells.

Unipotent Can produce only one stem cell type, their own, but have the property of self-

renewal, which distinguishes them from nonstem cells (e.g. muscle stem cells).