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Abstract
Stem cells are un differentiated cells characterized by their prolonged self renewal capacity and by their asymmetric replication.
There are two main types of stem cell, embryonic and adult type. Many tissues in adult animal have been shown to contain reservoir of stem cells.
Hematopoietic stem cells can be collected directly from bone marrow, from umbilical cord blood, and from circulating blood of individual receiving cytokines.
Adult and embryonic stem cells have different characteristic and any therapeutic application developed from them would have different strengths and weakness.
Most embryonic stem cells currently used in research are derived from embryos that were created in the course of infertility treatments by means of in vitro fertilization (IVF) procedures. Embryonic stem cells can be also derived from IVF embryos created specifically for research purposes. Some scientists are investigating a third source of embryonic stem cells, involving the production of embryos by means of a technique variously known as somatic cell nuclear transfer (SCNT) while.