Stem cells are unspecialized cells that have the capability to constantly self-renew, with unlimited divisions and to differentiate into various types of specialized cells of the human organism, such as blood cells, heart muscle cells brain cells or bone cells. Most of the time, they stay inactive but under the right condition in the body, by physical or chemical factors they get prompted to proliferate and differenziate into cells with specialized functions.
STEM CELLS PROPERTIES
The ability to undergo an unlimited number of replication cycles, without changing its differentiation status.
The ability to generate progenies for different cell types.
Hierarchy of Cell Potency
Totipotent Stem Cells
The zygote, that derives from the fusion of the gametes, is a single totipotent cell, which can divide into numerous identical cells. Totipotent stem cells can give rise to any cell types found in an embryo and they originate all fetal tissues (placental cells and body cells).
Pluripotent stem cells
Pluripotent stem cells can give rise to all cell types of the body and can originate all body tissues: ectoderm (neurons, skin), mesoderm (muscles, cartilage,bones, blood, fat), endoderm (endocrine glands). Pluripotent stem cells cannot develop into extra-embryonic tissue (the placenta or yolk sac, for example) and cannot generate the whole organism.
Multipotent Stem Cells
They are cells that can only differentiate into a limited and specialized series of cell lines, present in a specific tissue or organ. This category includes adult stem cells, also called tissue-specific or somatic stem cells; for example, hematopoietic stem cells that can generate only blood cells. They remain quiescent until they perceive signals to differentiate into specific type of cells, to replenish damaged and dead cells in the body (tissue maintenance role). They are found in specific microenvironment called stem cell niche.
Unipotent Stem Cells
They can differentiate into only one cell type.
STEM CELL CLASSIFICATION BASED ON ORIGIN
Embrionic Stem Cells
Embryonic stem cells are pluripotent cells, that are derived from the inner cell mass of a blastocyst and that give rise to the fetus. They are pluripotent stem cells, so they can divide into more stem cells and can become any type of cell in the body.
Fetal Stem Cells
Fetal stem cells can be isolated from the fetal blood and bone marrow and from other fetal tissues, including liver, pancreas, spleen and kidneys. After birth they will become unipotent adult stem cells.
Fetal Adnexa Derived Stem Cells
Present in the umbilical cord, in the placenta and in the amniotic sac. The embryonic and fetal stem cells have ethical and scientific issues that hinder their use; in contrast, stem cells recovered postnatally from the umbilical cord, including amnion/placenta are capable to forming many different cell types because they are multipotent and they possess the characteristics of both embryonic and mesenchymal stromal stem cells.
Adult Stem Cells
Adult stem cells are derived from adult tissue and are multipotent or unipotent cells. They are capable of maintaining cellular homeostasis of a mature organism. Furthermore, unlike embryonic and fetal stem cells, are free from ethical or religious controversies.