A side population (SP) in

flow cytometry Flow cytometry (FC) is a technique used to detect and measure 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 flo ...

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 differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type o ...

-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 very first definitive HSCs arise from the ventral endothelial wall of the embryonic aorta within t ...

by Dr.

Margaret Goodell Margaret (“Peggy”) A. Goodell (born March 23, 1965) is an American scientist working in the field of stem cell research. 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. It occurs in t ...

s and may be the cells that efflux

chemotherapy Chemotherapy (often abbreviated to chemo and sometimes CTX or CTx) is a type of cancer treatment that uses one or more anti-cancer drugs (chemotherapeutic agents or alkylating agents) as part of a standardized chemotherapy regimen. Chemotherap ...

drugs, accounting for the resistance of cancer to chemotherapy. Recent studies on

testicular A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testost ...

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 testis. There are three subtypes of spermatogonia in humans: * ...

, 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