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
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.