One cell, many jobs: what is differentiation?
An adult human has about 37 trillion cells. They look very different: a nerve cell is long and branched, a muscle cell is long and thick, a red blood cell is a small round disc. Yet they all began from one cell.
Differentiation is the process in which a young, unspecialised cell changes into a specialised cell with a special shape and job.
- Every body cell has the same DNA (the same genes).
- A cell switches some genes ON and keeps the others OFF.
- The proteins made by the ON genes decide the shape and the job.
So differentiation is not a change in DNA. It is a change in which genes are being used.
Totipotency: the cell that can become everything
A totipotent cell can grow into a whole new organism. The zygote, made when an egg and sperm join, is totipotent. The first few cells after it (up to about the 8-cell stage) are also close to totipotent.
Plants are special: many plant cells stay totipotent. A small piece of a carrot or a leaf, kept in the right food and hormones in a lab, can grow into a whole plant. This is called tissue culture.
In animals, differentiation is usually one-way. A nerve cell does not turn back into a skin cell by itself.
Levels of potency, from most to least power:
- Totipotent: can form a whole organism (zygote).
- Pluripotent: can form any body cell, but not a whole organism (embryonic stem cells).
- Multipotent: can form a few related cell types (bone marrow stem cells make different blood cells).
- Specialised: one fixed job, usually cannot divide much.
How do cells choose? Gene switches
Think of each gene as a light switch. In a nerve cell the switches for nerve proteins are ON. In a muscle cell the switches for muscle proteins (actin and myosin) are ON. In a red blood cell the switch for haemoglobin is ON.
Some genes are ON in every cell, for example the genes for making energy. They are called housekeeping genes.
What flips the switches? Chemical signals from neighbouring cells and from the body, and the position of the cell in the embryo. Signals reach the nucleus and change which genes are read.
Special features follow the switches: a nerve cell grows a long fibre to carry signals, a muscle cell has fibres that can shorten, and a mature red blood cell loses its nucleus to make room for haemoglobin.
Stem cells
A stem cell is an unspecialised cell that can (1) divide again and again, and (2) turn into one or more specialised cell types.
When a stem cell divides, it can give one stem cell + one cell that will specialise. This is called self-renewal. It keeps a supply of stem cells for life.
- Embryonic stem cells: from a very young embryo. Pluripotent.
- Adult (tissue) stem cells: found in bone marrow, skin, gut and other places. Multipotent. They repair and replace worn-out cells.
- Induced pluripotent stem cells (iPS): body cells that scientists have reset in a lab so that they act like embryonic stem cells.
Uses of stem cells and care needed
Uses: bone marrow transplants for blood diseases, growing skin for serious burns, growing a cornea for the eye, and testing new medicines on lab-grown tissue.
Care needed: using embryonic stem cells raises ethical questions, because an embryo is used. Stem cell treatment must be done only in approved hospitals. Many clinics online promise cures that are not proved, so people must be careful.
Try it
In the 3D: go to the gene switches step. Before you tap a cell type, predict which switches will be ON. Then check. Notice that switch 1 is ON in every cell.
At home: take one set of 6 LEGO or paper cards (the "genes"). Give 3 friends the same 6 cards. Ask each one to keep only 3 cards "ON" for a job (runner, singer, cook). Same cards, different jobs: that is differentiation.
Key formulas and definitions
- Differentiation = same DNA + different genes switched ON
- Zygote (totipotent) → embryonic stem cell (pluripotent) → adult stem cell (multipotent) → specialised cell
- Stem cell division = 1 stem cell + 1 cell that will specialise (self-renewal)
Worked examples
1. A nerve cell and a muscle cell of the same person are very different. Do they have different DNA?
No. Both have the same DNA because both came from the same zygote by cell division. They differ because different genes are switched ON.
2. A scientist grows a whole carrot plant from one root cell in a dish. Which power of the cell does this show?
Totipotency. The single plant cell kept the power to form every cell type and the whole plant. This method is called tissue culture.
3. A stem cell divides 4 times, and each time it gives one stem cell and one specialised cell. How many stem cells and specialised cells are there at the end?
Each division keeps 1 stem cell and adds 1 specialised cell. After 4 divisions: 1 stem cell and 4 specialised cells.
4. Why can bone marrow stem cells make red cells and white cells but not nerve cells?
They are multipotent, not pluripotent. They can form only a limited group of related cell types (blood cells).
Common mistakes
- Thinking that different cell types have different DNA. They have the same DNA; only the ON/OFF genes differ.
- Mixing up totipotent (a whole organism) and pluripotent (any body cell but not a whole organism).
- Believing a stem cell always becomes specialised. It can also copy itself and stay unspecialised.
- Saying red blood cells keep a nucleus. Mature human red blood cells have none.