Changes

The human body is made of billions and billions of cells, which have specific shapes, particular structures, and different functions. Cells of same types make tissues, and tissues make organs. Cells, tissues, organs and systems make the human body. No matter how simple or complex an organism is, the cell remains the structural and functional basic units of life (there are even unicellular organisms, like bacteria.)

Some cells (but not all) have the ability to multiply by division. Most of the diseases known to medical science are due to malfunction of cells, to premature cell death, or to cells' wrong multiplication rates.

Stem cells are a particular kind of cells, with two major characteristics: