The human body comprises more than 200 types of cells, and every one of these cell types arises from the zygote, the single cell that forms when an egg is fertilized by a sperm. Within a few days, that single cell divides over and over again until it forms a blastocyst, a hollow ball of 150 to 200 cells that give rise to every single cell type a human body needs to survive, including the umbilical cord and the placenta that nourishes the developing fetus.

Each cell type has its own size and structure appropriate for its job. Skin cells, for example, are small and compact, while nerve cells that enable you to wiggle your toes have long, branching nerve fibers called axons that conduct electrical impulses.

Cells with similar functionality form tissues, and tissues organize to form organs. Each cell has its own job within the tissue in which it is found, and all of the cells in a tissue and organ work together to make sure the organ functions properly.

Regardless of their size or structure, all human cells start with these things in common:

Stem cells are the foundation of development in plants, animals and humans. In humans, there are many different types of stem cells that come from different places in the body or are formed at different times in our lives. These include embryonic stem cells that exist only at the earliest stages of development and various types of tissue-specific (or adult) stem cells that appear during fetal development and remain in our bodies throughout life.

Stem cells are defined by two characteristics:

Beyond these two things, though, stem cells differ a great deal in their behaviors and capabilities.

Embryonic stem cells are pluripotent, meaning they can generate all of the bodys cell types but cannot generate support structures like the placenta and umbilical cord.

Other cells are multipotent, meaning they can generate a few different cell types, generally in a specific tissue or organ.

As the body develops and ages, the number and type of stem cells changes. Totipotent cells are no longer present after dividing into the cells that generate the placenta and umbilical cord. Pluripotent cells give rise to the specialized cells that make up the bodys organs and tissues. The stem cells that stay in your body throughout your life are tissue-specific, and there is evidence that these cells change as you age, too your skin stem cells at age 20 wont be exactly the same as your skin stem cells at age 80.

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Stem Cell Basics - ISSCR

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