At the beginning of a new life, DNA from a spermatozoon ­ ( sperm cell ­) enters an oocyte ­ ( egg cell ­). ­ This results in a single zygote, a cell which will eventually divide through mitosis to form every single cell of every tissue in the body. ­ The DNA of each cell will be the same, and yet the function, form, size and shape will vary massively depending upon the tissue the cell finds itself within. ­ In order to permit this one cell to produce so many different cell-types the process termed differentiation is used. ­ In short, cell differentiation is the way in which cells become different more specialised types of cell.

Totipotent Cells

From the zygote which divides to form an embryo springs forth every type of cell in the body, of which there are over 200, and all the tissues required for the placenta. ­ Because of this, these very early cells which, in humans, occur just in the first four days following fertilisation, are termed totipotent. ­ Toti- is from the latin totus, meaning whole ­ ( the same root of the word total ­) , and potent being from the latin potens meaning to be able ­; thus the cell is able to produce any other cell. ­

Pluripotent Cells

In humans, after the first four days, the cells are no longer able to produce every type of tissue required ­; they have already begun to differentiate. ­ At this stage they can already be broadly grouped into the three primary germ layers ­; the ectoderm ­ ( covering tissues such as skin and retina ­) , endoderm ­ ( the gastrointestinal tract, respiratory tract, the liver and the pancreas ­) and the mesoderm ­ ( mostly connective and circulatory, such as bones, heart, blood vessels and muscles ­). ­ As these tissues are able to produce multiple cell-types but are not able to produce all cell-types they are termed pluripotent, from the latin pluralis meaning more.

Stem Cells

The totipotent and pluripotent cells where sourced from an embryo are a classified as embryonic stem cells, cells within the embryo from which other types of cell originate. ­ Other such stem cells may be acquired from mature tissues later in life, and these are identified by their source, such as the adipose stem cells which originate from the adipose ­ ( fat ­) tissues.

Stem cells, in addition to being able to produce many types of cell, have the remarkable property of not becoming old. ­ The mechanism of cell death via telomere shortening is avoided by the presence of the enzyme telomerase which rebuilds telomere portions following each division. ­ This means that a single stem cell can divide an infinite number of times, providing a source of new cells even late in life. ­ As an example of this, stem cells close to an incision act to rapidly produce new cells for rapid healing.

Mature Cells

When a cell has differentiated into its final form then it is termed a mature cell. ­ As every cell contains the same genetic information, i.e. they are the same genotype, they are described by a phenotype. ­ The range of phenotypes describes all the functional cells of the body, the cells which form tissues. ­ Familiar examples of these cells include erythrocytes in the blood, chondrocytes in cartilage, vascular endothelial cells on artery walls, and fibroblasts within skin.

A mature cell may be capable of further differentiation in order to reflect a change of role within a tissue, an example of which is the bone-secreting osteoblast which becomes a tissue-maintaining ­ ostyocyte.

Another important way in which mature cells can differentiate is when their physical environment changes. ­ This is termed dedifferentiation and means the loss of a phenotype, whether or not a new phenotype is acquired. ­ Of particular interest to tissue engineering is the dedifferentiation of chondrocytes, cartilage-based cells which lose their chondrocytic phenotype and become fibroblastic ­ ( like a fibroblast ­) if they are cultured on a flat plate ­ ( monolayer culture ­) rather than a three-dimensional structure such as that in cartilage itself. ­ For this reason, were one to expand chondrocytes in monolayer for use in Tissue Engineering the resulting cells would no longer produce cartilage.