Side Population
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A side population (SP) in
flow cytometry Flow cytometry (FC) is a technique used to detect and measure the physical and chemical characteristics of a population of cells or particles. In this process, a sample containing cells or particles is suspended in a fluid and injected into the ...
is a sub-population of cells that is distinct from the main population on the basis of the markers employed. By definition, cells in a side population have distinguishing biological characteristics (for example, they may exhibit
stem cell In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can change into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type of cell ...
-like characteristics), but the exact nature of this distinction depends on the markers used in identifying the side population.


Examples

Side populations were first identified in
hematopoietic stem cells Hematopoietic stem cells (HSCs) are the stem cells that give rise to other blood cells. This process is called haematopoiesis. In vertebrates, the first definitive HSCs arise from the ventral endothelial wall of the embryonic aorta within the ...
by Dr.
Margaret Goodell Margaret ("Peggy") A. Goodell (born March 23, 1965) is an American scientist working in the field of stem cell research. Dr. Goodell is Chair of the Department of Molecular and Cellular Biology at Baylor College of Medicine, Director of the Stem ...
. SPs have been identified in
hepatocellular carcinoma Hepatocellular carcinoma (HCC) is the most common type of primary liver cancer in adults and is currently the most common cause of death in people with cirrhosis. HCC is the third leading cause of cancer-related deaths worldwide. HCC most common ...
s and may be the cells that efflux
chemotherapy Chemotherapy (often abbreviated chemo, sometimes CTX and CTx) is the type of cancer treatment that uses one or more anti-cancer drugs (list of chemotherapeutic agents, chemotherapeutic agents or alkylating agents) in a standard chemotherapy re ...
drugs, accounting for the resistance of cancer to chemotherapy. Recent studies on
testicular A testicle or testis ( testes) is the gonad in all male bilaterians, including humans, and is homologous to the ovary in females. Its primary functions are the production of sperm and the secretion of androgens, primarily testosterone. The r ...
stem cells indicate that more than 40% of the SP (defined in this case as cells that show higher efflux of DNA-binding dye Hoechst 33342) was undifferentiated
spermatogonia A spermatogonium (plural: ''spermatogonia'') is an undifferentiated male germ cell. Spermatogonia undergo spermatogenesis to form mature spermatozoa in the seminiferous tubules of the testicles. There are three subtypes of spermatogonia in human ...
, while other differentiated fractions were represented by only 0.2%. SP cells can rapidly efflux lipophilic fluorescent dyes to produce a characteristic profile based on fluorescence-activated flow cytometric analysis. Previous studies have demonstrated SP cells in bone marrow obtained from patients with acute myeloid leukemia, suggesting that these cells might be candidate leukemic stem cells, and recent studies have found a SP of tumor progenitor cells in human solid tumors. These new data indicate that the ability of malignant SP cells to expel anticancer drugs may directly improve their survival and sustain their clonogenicity during exposure to cytostatic drugs, allowing disease recurrence when therapy is withdrawn. Identification of a tumor progenitor population with intrinsic mechanisms for cytostatic drug resistance might also provide clues for improved therapeutic intervention. The molecules involved in effluxing Hoechst 33342 are members of the ATP-binding cassette family, such as MDR1 (P-glycoprotein) and ABCG2.


References

Cell biology Flow cytometry {{cell-biology-stub