From One Cell to Many Kinds: Cell Specialization, Stem Cells and Meristems

Same instructions, different jobs

You began as one cell. That cell divided by mitosis again and again, and today you have trillions of cells. Here's the puzzle: mitosis makes identical copies, so almost every cell in your body has the same DNA. Yet a nerve cell can be over a metre long, a red blood cell has no nucleus at all, and a muscle cell can contract. How can cells with the same instructions be so different?

Cell specialization

Think of the DNA as a giant cookbook. Every cell has the whole book, but each cell type only uses certain recipes. A muscle cell switches on the genes for contracting proteins; a cell in your pancreas switches on the gene for insulin. Which genes are "on" is called gene expression.

As an embryo grows, its cells receive chemical signals from neighbouring cells and their surroundings. These signals switch genes on and off, and cells gradually differentiate (specialize). Once most animal cells have specialized, they usually stay that type.

Two processes, working together, build every tissue and organ:

1. Cell division makes more cells.
2. Cell specialization gives those cells the right structure for their job.

New muscle tissue, for example, needs both: cells must divide to increase the number, and they must specialize into muscle cells.

Stem cells

A stem cell is an unspecialized cell that can:

• divide to make more stem cells (self-renewal), and
• differentiate into one or more kinds of specialized cell.

Types of stem cells:

• Embryonic stem cells come from a very early embryo (a few days old). They are pluripotent: they can become almost any cell type, such as nerve, muscle, blood or skin cells.
• Adult (tissue) stem cells are found in tissues such as bone marrow, skin and the lining of the intestine. They are usually multipotent: a blood stem cell can make red blood cells, white blood cells and platelets, but not nerve cells. They replace cells that wear out.
• Umbilical cord blood contains blood stem cells and can be donated at birth.
• Induced pluripotent stem (iPS) cells: in 2006, Japanese scientist Shinya Yamanaka showed that ordinary specialized cells (such as skin cells) can be reprogrammed to behave like embryonic stem cells. He shared the 2012 Nobel Prize for this work.

Why they matter in medicine: stem cells could one day repair damaged hearts, spinal cords or brains. One stem cell treatment is already common: bone marrow (stem cell) transplants, which replace the blood-forming cells of patients with some blood cancers, such as leukemia.

Plants: growing all their lives

Most animals stop growing when they become adults. Many plants keep growing for as long as they live, because they keep zones of unspecialized dividing cells called meristems (meristematic tissue). Cells made by the meristem then elongate and differentiate into the plant's specialized tissues: xylem, phloem, epidermis and so on.

• Apical meristems are at the tips of roots and shoots. They make the plant longer (primary growth). You saw the root apical meristem in the mitosis lab.
• Lateral meristems form rings inside stems and roots and make the plant thicker (secondary growth). The vascular cambium makes new xylem on the inside (which becomes wood) and new phloem on the outside. In places with seasons, like Ontario, the wood made in spring and summer looks different, so the trunk shows annual growth rings.

Plant cells are also flexible: a cutting from a houseplant can grow brand-new roots, because some of its cells can divide and specialize into root tissue.