Ans. is 'a' i.e., Found in yolk sacStem CellsStem cells are characterized by their prolonged self-renewal Capacity by mitosis and by their assymetric replication,Asymetric replication is a special propey of stem cells, i.e. in every cell devision, one of the cells retain its self-renewing Capacity while the other enters a differentiating pathway and is Conveed to mature nondividing population.However, assymetry exists within a whole population of stem cells. Thus within a group of stem cells some self replicate and others differentiate.The two major type of stem cells are :?1. Embryonic stem cells (ES) Embryos contain pluripotent ES cells, which can differentiate into all the tissues of the human body.2. Adult stem cells -LI Many tissues in adult animals have been shown to contain reservoirs of stem cells, which are called adult stem cells.Li Compared to embryonic stem cells, which are pluripotent, adult stem cells have a more restricted differentiation capacity and are usually lineage specific, i.e are multipotent or unipotent.Adult Stem cells are of two types :?A. Bone marrow stem cellsThese are hematopoietic stem cells which reside in bone marrow and differentiate into various blood cells.Although residing mainly in the marrow,a subset circulates normally in the peripheral blood.From 3 weeks to 3 months of intraembryonic life, hematopoietic stem cells in the yolk soc are the primary source of blood cells.From 3 months to just before bih stem cells, migrate to liver, which is the major organ for hematopoiesis during that period.After bih bone marrow becomes the major source.A remarkable observation about hematopoietic stem cells is that they may capable of giving rise to neurons, hepatocytes, mycocardium, bile duct cells, endothelial cells and skeletal muscle.This change in stem cell differentiation from one cell type to another is called transdifferentiation.Other type of bone marrow stem cells are bone marrow stromal cells which can differentiate, depending on the tissue environment, into chondrocytes, osteoblasts, adipocytes, myoblasts, endothelial cell precursors. B. Tissue stem cellsLocated outside the marrow.Stem cells are located in sites called niches.'J For example in GIT they are located at the isthmus of stomach glands and at the base of crypts of the colon.Following information have been added in 8th/e of Robbins Adult stem cells are also known as somatic stem cells.Adult stem cells of the bone marrow, the skin and the lining of GI tract divide continuously.Adult stem cell of liver, pancreas and adipose tissue do not actively produce differentiated cells under normal conditions.Somatic stem cells in the small intestine epithelium may be continuously cycling.o Regardless of their proliferative activity, somatic stem cells generate rapidly dividing cells known as transit amplifying cells. These cells lose the capacity of self-perpetuation, and give rise to cells with restricted development potential known as progenitor cells.Uses of stem cells :?Studies of human embryonic stem cells may yield information about the complex vents that occur during human development. Some of the most serious medical conditions, such as Cancer and bih defects, are due to abnormal cell division and differentiation. A better understanding of the genetic and molecular control of these processes may yield information about how such diseases arise and suggest new stratgies for therapyTo test new drug, e.g. new drugs could be tested for safety on differentiated stem cells generated from human pluripotent cell lines.The most impoant use of human stem cells is generation of cells and tissues that could be used for cell-based therapies. Stem cells, directed to differentiate into specific cell types, offer the possibility of a renewable source of replacement cells to treat diseases including parkinsonism, Alzheimer's disease, spinal cord injury, diabetes, osteoahritis, RA, and hea diseases.Stem cells are used as bone marrow transplantation in various types of leukemia and lymphoma.They have made possible the production of knockout mice. To produce these mice, a specific gene is inactivated or deleted from cultured ES cells. These cells are injected into blastocysts, which are then implanted into the uterus of the surrogate mother. The genetically modified implanted blastocysts develop into full embroys, as long as thegene defect does not cause embryonic lethality. Knockout mice have become widely used models for the experimental study of human disease and provide essential information about gene function in vivo.I think following stem cell related terms require some introduction :Potency of stem cells is the potential to differentiate into different cell types :-i) Totipotent stem cells - These cells are produced from feilization of sperm and ovum and cells that are produced by first few division after feilization are also totipotent. These cell can differentiate into all the tissues of embryonic or extraembryonic cell types.ii) Pluripotent stem cells - These are descendants of totipotent stem cells and can differentiate into cells derived from any of the three germ layers.iii) Multipotent stem cells - These cells can differentiate only into cells of a closely related family, e.g. hematopoietic stem cells differentiate into RBC, WBC, platelets but not into other types.iv) Unipotent stem cells - Can differentiate only in to one cell type (e.g. muscle stem cell), but have propey of self-renewal which distinguiches them from non-stem cells.