Cell

The Unit of Biological Life (adapted from Wikipedia)

The cell is the structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building brick of life. Some organisms, such as most bacteria, are unicellular (consist of a single cell).

Each cell is at least somewhat self-contained and self-maintaining: it can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary. Each cell stores its own set of instructions for carrying out each of these activities.

All cells have several different abilities:

  • Reproduction
  • Use of enzymes and other proteins coded by DNA genes.
  • Metabolism (taking in raw materials, building cell components, converting energy, molecules and releasing by-products (excretion).
  • Response to external and internal stimuli such as changes in temperature, pH or levels of nutrients.
  • Cell contents are contained within a cell surface membrane that is made from a lipid bilayer with proteins embedded in it.

Stem cells can renew themselves through mitotic cell division and differentiation into a diverse range of specialized cell types. The two broad types of mammalian stem cells are: embryonic stem cells that are isolated from the inner cell mass of blastocysts, and adult stem cells that are found in adult tissues. In a developing embryo, stem cells can differentiate into all of the specialized embryonic tissues. In adult organisms, stem cells and progenitor cells act as a repair system for the body, replenishing specialized cells, but also maintain the normal turnover of regenerative organs, such as blood, skin or intestinal tissues.

Stem cells can be grown and transformed into specialized cells with characteristics consistent with cells of various tissues such as muscles or nerves through cell culture. Highly plastic adult stem cells from a variety of sources, including umbilical cord blood and bone marrow, are routinely used in medical therapies. Embryonic cell lines and autologous embryonic stem cells generated through therapeutic cloning have also been proposed as promising candidates for future therapies

 
Cell Potency specifies the differentiation potential (the potential to differentiate into different cell types) of the stem cell.
  • Totipotent (a.k.a omnipotent) stem cells can differentiate into embryonic and extraembryonic cell types. Such cells can construct a complete, viable, organism. 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 stem cells are the descendants of totipotent cells and can differentiate into nearly all cells, i.e. cells derived from any of the three germ layers.
  • Multipotent stem cells can differentiate into a number of cells, but only those of a closely related family of cells.
  • Oligopotent stem cells can differentiate into only a few cells, such as lymphoid or myeloid stem cells.
  • Unipotent cells can produce only one cell type, their own, but have the property of self-renewal which distinguishes them from non-stem cells (e.g. muscle stem cells).

The definition of a stem cell - the functional definition - "a cell that has the potential to regenerate tissue over a lifetime." For example, the gold standard test for a bone marrow or hematopoietic stem cell (HSC) is the ability to transplant one cell and save an individual without HSCs. In this case, a stem cell must be able to produce new blood cells and immune cells over a long term, demonstrating potency. It should also be possible to isolate stem cells from the transplanted individual, which can themselves be transplanted into another individual without HSCs, demonstrating that the stem cell was able to self-renew.