Stem cells are undifferentiated cells found in multi-cellular organisms that can proliferate by mitosis to produce either more stem cells or specialized cells that differentiate into the different organs and tissues. In scientific research, stem cells have been obtained and developed from different sources which include:
Embryonic Stem cells
Embryonic stem cells (ESC) are pluripotent cells that have the potential of proliferating indefinitely into any of the 3 germ layers of cells - endoderm (which differentiate further to specialized cells of the interior stomach lining, gastrointestinal tract, the lungs), mesoderm (which differentiate further to specialized cells of the muscle, bone, blood, urogenital), or ectoderm (which differentiate further to specialized cells of the epidermal tissues and nervous system) – which eventually give rise to specialized tissues and organs. These stem cells can be obtained from the inner cell mass of the blastocyte by isolating the inner cell mass, 4 – 5 days after fertilization. Studies have shown that these cells are able to renew their population for a long time and have the ability to remain in an undifferentiated state in culture once special requirements such as presence of feeder cells, serum, or cytokines1 are met, and thus these cells have been used in most scientific research.
Figure 1: Sources of Embryonic stem cells. Embryonic stem cells obtained from inner cell mass of the Blastocyst 5 days post fertilization
Adult/Somatic Stem Cells
Adult stem cells are cells that are found in the Bone marrow, Mammary glands, Intestine, Lungs, Brain and Skin humans and also in the umbilical cord of new born that are capable of differentiating into specific cell types from which they originate. Although these cells are not in a pluripotent state, they are able to replenish damaged cells and also self-renew. Despite these cells been able to differentiating into cells of the particular organ they were harvested form, Clarke D L et al have suggested that adult stem cells might have the ability to differentiate into cell types from different germ layers. For instance, neural stem cells from the brain, which are derived from ectoderm of the embryonic stem cell, can differentiate into ectoderm, mesoderm, and endoderm2 this has been referred to as trans-differentiation. Adult stem cells are forced to express some embryonic transcription factors which Stadtfeld and Hochedlinger in 2010 showed that these transcription factors play a key role in determining the state of these cells and also highlighted the fact that these transcription factors whose genes are preserved from embryonic stage to adult stage stem cells, when forced to be expressed, can change one cell type to another3and thus these cells can be used in place of embryonic stem cells.
Figure 2 Sources of Adult/Somatic stem cells. Found throughout the body undifferentiated and divide by mitosis to replenishes dying cells and regenerate ding...