Semen is a fluid of complex composition, produced by the male sex organs. There is a cellular component, spermatozoa, and a fluid component, seminal plasma. An average ejaculate is 3 to 4 ml containing 70 to 150 million sperm.  Sperm are the male reproductive cells. Each consists of a head, tail and mid-piece. In humans, the head is a tiny disc, about 4.5 µm long and 2.5 µm wide. The tail is about 40 µm long, and is rapidly lost in ejaculates. The head is where the DNA is preserved. Ape sperm are similar in size and shape. Dogs have similarly shaped sperm but about one third the size of human sperm. Other animals have differently shaped sperm.

Seminal plasma contains proteins, salts, organics (including flavins which are the source of its UV fluorescence, and choline), and some cellular material. The components originate from several sources, including seminal vesicles and the prostate gland.  The prostate is the source of the enzyme acid phosphatase and the protein prostate specific antigen, or p30 protein.

Vasectomy severs or ligates the ducts carrying sperm to the penis. Thus vasectomized men will have no sperm but will have the plasma components present in their ejaculate.

After ejaculation during intercourse, semen is lost by drainage and by biochemical change. Microscopical examination of vaginal swab samples shows a sequence of changes with time, since there is some biochemical evidence for the persistance of tails as long as heads. Tails are lost first - the damage begins immediately and about 25% will have no tails by 6 hours.  By 12 hours, there will be few sperm with intact tails and by 24 hours there will be mainly heads left.  These proportions and times are highly variable.  Sperm survival in stains outside the body depends on environmental conditions, but a small stain that has dried quickly may have intact sperm preserved for months or even years.

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