TYPES OF ADULT STEM CELLS

Adult stem cells of the nervous system: Scientists belive that stem cells inthe fetal and adult brain divide and give rise to more stem cells or to several types of precursor cells.Neuronal precursors (also called neuroblasts) divide and give rise to nevre cells.

Stem cells in the Bone Marrow:There are two types of bone marrow cells.1) bone marrow hemapoietic stem cells 2) bone marrow stromal cells.
Hemopoietic stem cells (HCS) are responsible for forming all types of blood cells in the body.
Bone marrow stromal cells generate bone,cartilage, fat, fibrous connective tissues.

Endothelial Progenitor Cells: Endothelial cells line the inner surface of blood vessels throughout the body, and it has been difficult to identify specific endothelial stem cells in either the embryonic or the adult mammal.During embryonic development,just after gastrulation, a kind cell called hemangioblast,which is derived from mesoderm, is presumed to be the precursor of both the hemopoetic and endothelial cell lineages.
Skeletal Muscle Stem Cells; skeletal muscle cells called satellite cells. Satellite cells mediate muscle growth and although satellite cells are normally non dividing cells, they can be triggered to proliferate as a result of injury, or weight bearing exercise.

Epithelial Cell Precursors in the Skin and Digestive System: Epithelial cells,which constitute 60 percent of the differantiated cells in the body are responsible for covering the internal and external surfaces of the body.The epithelial cells are replaced constantly.The population that renews the epithelium of the small intestine occurs in the intestinal crypts,deep invaginations in the lining gut. The crypt cells are often regarded as stem cells.
The skin of mammals contains at least three populations of epitelial cells; epidermal cells,hair follicular cells, glandular cells.

Another population of stem cells in skin occurs in the basal layer of the epidermis.These stem cells proliferate in the basal region, and differantiate as they move toward the outersurface of the skin.
Stem Cells in the Pancreas and Liver;In adult mammals,both the pancreas and the liver contain multiple kinds of differantiated cells that may be repopulated or regenerated by multiple types of stem cells.In the pancreas,endocrine (hormone-producing) cells occur in the islets of Langerhans.They include the beta cells(which produce insulin),the alpha cells (which secrete glucagon), and cells that release the hormones somatostatin and pancreatic polypeptide.

Recent studies in rodents indicate that HSCs (derived from mesoderm) may be able to home to liver after it is damaged and demonstrate plasticity in becoming into hepatocytes.But the question remains as to whether cells from the bone marrow normally generate hepatocytes in vivo.It is not known whether this kind of plasticity occurs without severe damage to the liver or whether HSCs from the bone marrow generate oval cells of the liver.Although hepatic oval cells exist in the liver,it is not clear whether they actually generate new hepatocytes. Oval cells may arise from the portal tracts in liver and may give rise to either hepatocytes and to the epithelium of the bile ducts. Indeed, hepatocytes themselves,may be responsible for the well-know regenerative capacity of liver.