What are Stem Cells?

Stem cells are undifferentiated biological cells that have the ability to differentiate into specialized cells. They are basically the initial building blocks of the body of a living organism. Furthermore, they can divide, through the process of mitosis, to produce more stem cells. They are found in most multicellular organisms. There are two broad types of stem cells in mammals: embryonic stem cells, which are isolated from the inner cell mass of blastocysts, and adult stem cells, which various tissues are endowed with.

Stem cells have the potential to develop into many other diverse types of cells found in the body. Additionally, in many developed tissues, they can serve as an internal repair system, dividing substantially without limit to replenish other cells as long as the organism lives.

Stem cells are distinguished from other cells by two important characteristics. Firstly, they are unspecialized cells that have the capacity of renewing themselves through cell division, sometimes even after extended periods of inactivity. Second, under certain physiologic or experimental conditions, they can be induced to develop into tissue-specific or organ-specific cells that contain special functions. In some organs, such as the bone marrow and the gut, stem cells routinely divide in order to repair and/or replace worn out or damaged tissues. In other organs, however, such as the heart and pancreas, stem cells only divide under certain special circumstances.

There are several reasons why stem cells are crucial for the proper functioning of living organisms. In the three- to five-day-old embryo called the blastocyst, the inner cell mass (ICM) can give rise to the complete body of the organism, which includes all of the varied specialized cell types and organs including the heart, skin, lungs, eggs, sperm as well as other tissues. In some adult tissues, such as the bone marrow, muscle, and brain, distinct populations of adult stem cells initiate and create replacements for the cells that have been lost through regular wear and tear, disease, or injury.

Whenever a stem cell divides, each new cell created has the potential to either remain as a stem cell or to become another type of cell with a more specialized function, such as a brain cell, a red blood cell, or a muscle cell.


Microscopic view of a colony of undifferentiated human embryonic stem cells
Source: James Thomson's research lab, University of Wisconsin-Madison


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