Every cell in your body came from one fertilized egg, and almost all of them carry the same DNA. Yet a nerve cell can stretch from your spine to your toe, a red blood cell is a tiny flexible disc, and a muscle cell can contract with great force. How can cells with the same instructions turn out so different? The answer is differentiation: as an embryo develops, each cell switches on only the instructions it needs for one particular job and keeps the rest switched off. The result is a specialized cell whose shape and contents fit its function.
Structure fits function: specialized cells
| Specialized cell | Special features | How the features help |
|---|---|---|
| Red blood cell | Disc shape dented on both sides; no nucleus in mature mammalian cells; packed with hemoglobin | Large surface area for oxygen to diffuse in and out; more room for hemoglobin; flexible enough to squeeze through capillaries |
| Nerve cell (neuron) | Long extension (axon), some over a metre long; many short branches | Carries electrical signals long distances and connects with many other cells |
| Skeletal muscle cell | Long, with several nuclei and many mitochondria; filled with protein fibres | Fibres slide past each other to contract; mitochondria supply the energy |
| Sperm cell | Tail (flagellum) and many mitochondria | Swims to the egg, powered by the mitochondria |
| Skin (epithelial) cell | Flat and tightly packed | Forms a protective barrier |
| Root hair cell (plant) | Long, thin extension into the soil | Huge surface area for the absorption of water and minerals |
| Guard cell (plant) | Pair of bean-shaped cells around each stoma | Change shape to open or close the pore that lets gases in and out of the leaf |
| Palisade mesophyll cell (plant) | Column-shaped and packed with chloroplasts | Captures as much light as possible for photosynthesis |
Stem cells
A stem cell is an unspecialized cell that can do two things: divide to make more stem cells (self-renewal) and, when signalled, differentiate into specialized cells. Stem cells are how your body keeps replacing blood, skin and gut lining throughout life. Once most cells fully specialize, they usually cannot go back.
| Type of stem cell | Where it comes from | What it can become |
|---|---|---|
| Embryonic stem cells | The inner cells of an embryo a few days old, usually donated after fertility treatment | Almost any cell type in the body (pluripotent) |
| Adult (tissue) stem cells | Bone marrow, skin, the intestinal lining and other tissues | A limited range: for example, blood stem cells make all the types of blood cell (multipotent) |
| Umbilical cord blood stem cells | Blood collected from the umbilical cord after birth | Blood cells; used like bone marrow in transplants |
| Induced pluripotent stem cells (iPSCs) | Ordinary adult cells, such as skin cells, reprogrammed in the lab (first done in 2006) | Almost any cell type, without using an embryo |
Meristematic cells: plants keep growing
Humans and most other mammals and birds stop growing once they reach adult size, but many plants grow for their whole lives. They can because they keep pockets of unspecialized, constantly dividing cells called meristematic cells, found in meristems. Apical meristems at the tips of roots and shoots make the plant longer. In woody plants, a ring of meristem tissue in the stem (the cambium) adds new xylem and phloem each year, making the stem thicker; that yearly growth is what produces tree rings. New cells from a meristem elongate and then differentiate into xylem, phloem, epidermis and the other plant tissues.
Regeneration
Regeneration is the regrowth of lost or damaged body parts. Planarian flatworms can regrow a whole worm from a small piece; axolotls (a type of salamander) can regrow legs; sea stars can regrow arms. Many plants can grow a new plant from a stem or leaf cutting, because their cells can be triggered to divide and differentiate into roots. Human regeneration is limited: skin heals, broken bones mend and the liver can regrow a large part of its mass, but we cannot regrow a whole finger or a limb. Studying animals that regenerate well may one day help repair human tissue.
Medical uses and ethical questions
Blood stem cell transplants (from bone marrow, blood or cord blood) are an established treatment for leukemia and some other blood disorders. Many other uses, such as repairing spinal cord injuries, heart damage or the insulin-making cells lost in type 1 diabetes, are still being researched in clinical trials. Clinics that sell unproven stem cell treatments are a real concern, because a therapy has to be tested for safety and effectiveness first.
- Embryonic cells can become almost any cell type, so they may treat many diseases
- Research uses embryos left over from fertility treatment that would otherwise be discarded, with donors' consent
- Studying them teaches how human development works
- Obtaining the cells destroys an early human embryo, which some people believe should be protected
- Transplanted cells could be rejected by the immune system or grow uncontrollably and form tumours
- Alternatives such as adult stem cells and iPSCs may avoid using embryos