Human development starts with the merging of the sperm cell and the ovum in fertilization. The fertilized egg begins to divide so that the embryo has two, four, eight, sixteen and eventually hundreds of cells. How can a single cell develop into a functioning, generally healthy person? In short, the genes that we inherit drive our individual growth and development like the clockwork.

All cells in the embryo are originally stem cells. They are undifferentiated and have the potential to evolve into any cell. This is why they are called all-powerful or totipotent cells. In many primitive animals and plants, cells retain this differentiation ability for a longer time than human stem cells do. Thus, for example, a viviparous lizard is able to grow a new tail, and a new earthworm can develop from a cut-off piece of another earthworm. An example of stem cell division in humans is the healing of wounds and ossification of fractured bone. These processes show how our cells are able to differentiate as needed. Stem cells can now be artificially grown and differentiated in laboratory conditions.

Stem cells begin to diversify guided by their genes and location. By differentiating, stem cells can transform into various cell types, such as bone cells, muscle cells, blood cells, etc.

Similar cells form tissues. Different tissues have different properties. For example, muscle tissue can contract, nerve tissue transmits electrical impulses, bone tissue is hard and strong, and epithelial tissue protects us by being flexible. Connective tissues attach muscles to the bones and blood vessels to the muscles in a durable and flexible way.

Below, you can see cells from various tissues. Can you guess their functions just by looking at them?