Spermatogenesis occurs in the seminiferous tubules that form the bulk of each testis (see Figure). The process begins at puberty, after which time sperm are produced constantly throughout a man’s life. One production cycle from spermatogonia through formed sperm takes approximately 64 days. A new cycle starts approximately every 16 days although this timing is not synchronous across the seminiferous tubules. Sperm counts—the total number of sperm a man produces—slowly decline after age 35, and some studies suggest that smoking can lower sperm counts irrespective of age.