Human prostate, coloured scanning electron micrograph (SEM) of a section through the prostate. The prostate is a small gland found only in men. It's about the size of a walnut and gets a little bigger with age. The prostate produces a thick, white fluid that mixes with the sperm produced by the testicles to make semen. It also produces a protein called prostate-specific antigen (PSA) that turns the semen into liquid. The pseudostratified columnar epithelium (purple) is covered in microvilli (small hair-like projections). Magnification: x 1000 when printed at 10 centimetres wide.