The development of a multicellular organism such as a human being begins with a fertilized oocyte, which contains genetic information about the construction of all the tissues and organs. This fertilized egg (zygote) is the basic, "most important" stem cell. It is a totipotential cell, which means that it can develop (by divisions and differentiation) into different sorts specialized cells, of which are more than 100 kinds, and their number reaching 80 billion.

As a result of the merging of a sperm cell and an oocyte, a primal, totipotential stem cell is created. First divisions of this cell are symmetrical, which means that all its offspring cells (named blastomers, their number is 32 or 64 and they are created in 4 or 5 divisions) contain all the genetic information and are totipotential. This means those cells can develop into any kind of specialized cells of the organism.

Fig. 1. The development of a fertilized egg cell

Next stage a blastocyst is formed, in which some cells lose their "totipotentiality". In the next stage of an embrional development called gastrulation cells differentiate and organize into three separate areas and become multipotential cells. Those cells can develop into many, but not all kind of cells of the organism.

Many cells live significantly shorter than the whole organism. The group of short-living cells consists among others of : blood cells (101 to 100 days), epidermis cells (7 to 10 days), epithelium of respiratory and digestive tracts (7 to 10 days). Therefore a need of constant replacement of short-living cells exists. This is possible due to stem cells, which can be called "tissue" stem cells. These cells, are a part of a constant quantity population, which means that their divisions are non-symmetrical o one offspring cell remains a stem cell and the second one differentiates (specializes) becoming a mature epithelium, blood cell, etc. Examples of this process are blood stem cells, which are present mostly in bone marrow, but also in peripheral blood and in umbilical cord blood.

Fig. 2. The development of somatic cells from stem cells

Stem cell divides into two offspring cells: one that retains its stem cell properties and the other which after a few divisions creates specialized cells that circulate in the vascular system: erythrocytes, leukocytes and thrombocytes (platelets). Besides blood stem cells bone marrow consists of a different kind of stem cells, named mesenchymal, that can differentiate into bone tissue cells (osteocytes), fat tissue cells (adipocytes), muscle tissue and also cells that form the bone marrow matrix.