A stem cell is a cell that can divide to produce some descendant cells that continue as stem cells, and some cells that are destined to specialize in different types of tissue in an organism.
There is great interest, from a biomedical point of view, in stem cells since they have the potential to serve as a therapy to replace defective or damaged cells in a variety of disorders and injuries , including Parkinson’s disease, heart disease and diabetes.
There are two main types of stem cells: embryonic stem cells and adult stem cells. Stem cells generally have two options: to give rise to more stem cells, or to specialized cells that are targeted to form certain tissues.
Embryonic stem cell
Embryonic stem cells are stem cells that are derived from the inner cell mass of a mammalian embryo at a very early stage of development , when it is composed of a hollow sphere of dividing cells (a blastocyst). For this reason, the study of human embryonic stem cells raises certain ethical questions. Normally, those that have been used for study come from embryos that have been discarded, for example, in IVF ( in vitro fertilization). But the consent of the guardians of this organic material is needed for its use in biomedicine.
For all the above, the most studied embryonic stem cells are mouse embryonic stem cells, discovered in 1981. If laboratory-grown mouse embryonic stem cells are injected into a mouse embryo at the blastocyst stage, they will integrate in the embryo and will produce cells that are specialized in most or all types of tissue that the embryo will later develop. This ability to repopulate mouse embryos is the key characteristic that defines embryonic stem cells, and that is why they are considered pluripotent, that is, capable of giving rise to any type of cell in the adult organism. Instead, if embryonic stem cells are grafted into an adult mouse, they will develop into a type of tumor called a teratoma, which contains a variety of differentiated tissue types.
The epidermis of the skin contains layers of cells called keratinocytes. Some of these cells are stem cells. Keratinocytes slowly move outward through the epidermis as they mature, eventually dying and shedding on the surface of the skin.
The bone marrow contains cells called hematopoietic stem cells, which make all types of blood cells and the immune system. Hematopoietic stem cells are also found in small amounts in peripheral blood and in large amounts in umbilical cord blood. In the bone marrow, hematopoietic stem cells generate progeny that can develop into lymphocytes, granulocytes, red blood cells, and certain other types of cells.
Science has shown that there are also stem cells in the brain, neural stem cells. In mammals, very few new neurons form after birth, but some neurons are continually forming in the olfactory bulbs and in the hippocampus. These neurons arise from neural stem cells, which can be cultured in vitro in the form of neurospheres, small groups of cells that contain stem cells and part of their progeny. This type of stem cell is being studied for use in cell therapy to treat Parkinson’s disease and other forms of neurodegeneration or traumatic damage to the central nervous system.
Recently, stem cells have been used successfully to treat HIV patients, and also to treat COVID-19 in clinical trials.
