Cell Differentiation · StemLab

Cell differentiation

The human body has more than two hundred kinds of cell, yet they all come from a single fertilised egg. The fertilised egg divides into many identical cells, and their descendants gradually come to differ, in a stable way, in shape, structure and function. This process is called cell differentiation. Differentiation makes cells specialised and forms different tissues.

After differentiation the genetic material has not changed; what changes is which genes are switched on. This is selective gene expression. Genes that every cell needs (such as the ATP synthase gene, a housekeeping gene) are expressed everywhere; genes like ovalbumin, β-globin and insulin are expressed only in particular cells.

First let the fertilised egg divide and differentiate, then choose a probe on the right and test the cells' DNA and mRNA.

For the molecular details of transcription and translation, see GeneLab.

Totipotency of plant cells

A cell that has already differentiated still has the potential to develop into a whole organism or to differentiate into other kinds of cell. This is called totipotency. In 1958 Steward grew complete new plants from cells of carrot phloem.

The isolated tissue first dedifferentiates, losing its special structure and function, and forms a loose callus; moved to a differentiation medium, it redifferentiates and grows shoots and roots. The ratio of auxin to cytokinin decides what grows: a high ratio favours roots, a low ratio favours shoots, and a balanced ratio only makes the callus grow.

A complete plantlet needs both roots and shoots. The whole process must be sterile: see what happens if you skip the disinfection.

Totipotency of the animal cell nucleus

In a highly differentiated animal cell the totipotency of the whole cell is restricted, but its nucleus is still totipotent. In 1996 scientists moved the nucleus of a mammary cell from a white-faced ewe into an enucleated oocyte from a black-faced ewe. After an electric pulse fused and activated it, it grew into an early embryo, was transferred into a surrogate ewe, and became the cloned sheep Dolly.

Use the buttons below to go step by step, or press Autoplay. On the right, change where the nucleus, the oocyte and the surrogate come from and see who the lamb looks like; then try skipping enucleation.

Stem cells and cell fate

Animals and humans keep a small number of cells that can still divide and differentiate. They are called stem cells. Embryonic stem cells can become almost every kind of cell in the adult body; adult stem cells can become only a few kinds. Blood stem cells in the bone marrow, for example, make red blood cells, white blood cells and platelets.

Waddington pictured differentiation as a ball rolling down a hillside: each valley is a stable state of gene expression, and the valleys get deeper the further down they go, so a differentiated cell usually does not turn into another kind. In 2006 Shinya Yamanaka used 4 genes to push mouse fibroblasts back up to the top and obtained induced pluripotent stem cells (iPS cells).

Release stem cells and watch where they go; add erythropoietin, or give a fibroblast reprogramming factors.

In-class testing

Look back over the four topics and answer the multiple-choice questions below.