A primary culture refers to cells that have been taken directly from the tissue of interest, and then placed in an artificial environment supported with essential nutrients and growth factors to facilitate proliferation. Since primary cells closely resemble the tissue of origin they are commonly used in basic research to investigate the normal physiology and biochemistry of cells, and in drug discovery to analyze the effects of therapeutic and toxic compounds on cells. Common primary cells used in research include epithelial cells, endothelial cells, hematopoietic cells, fibroblasts, keratinocytes, muscle cells and mesenchymal stem cells.