The actual mechanism in which testosterone elicits these changes is somewhat complex. When free in the blood stream, the testosterone molecule is available to interact with various cells in the body. This includes skeletal muscle cells, as well as skin, scalp, kidney, bone, central nervous system, and prostate tissues. Testosterone binds with a cellular target in order to exert its activity, and will, therefore, effect only those body cells that posses the proper hormone receptor site (specifically the androgen receptor). This process can be likened to a lock and key system, with each receptor (lock) only being activated by a particular type of hormone (key). During this interaction, the testosterone molecule will become bound to the intracellular receptor site (located in the cytosol, not on the membrane surface), forming a new “receptor complex.” This complex (hormone + receptor site) will then migrate to the cell’s nucleus, where it will attach to a specific section of the cell’s DNA, referred to as the hormone response element. This will activate the transcription of specific genes, which in the case of a skeletal muscle cell will ultimately cause (among other things) an increase in the synthesis of the two primary contractile proteins, actin and myosin (muscular growth). Carbohydrate storage in muscle tissue may be increased due to androgen action as well.