How is temperature regulated in the testes for maximal spermatozoa production?

The temperature in testicles is controlled by their location and scrotum (its muscle layers)

Explanation:

The testicles in male body is located outside their body. This is so because it regulates the temperature requirement  of testicles to produce spermatozoa and male hormone testosterone is sufficient amount. Since testicle is located outside the body, its normal temperature is 4 to 5 degree lesser than the normal body temperature. Also the scrotum covering the testicles also regulate the temperature for optimum spermatozoa development. There are three layers in a scrotum that helps controlling the temperature –  

a)Tunica dartos (located inside the scrotum) – This layer is temperature sensitive. It relaxes when hot and contracts when cold

b)Cremaster muscle in the spermatic cord – Based on environmental temperature it regulates proximity of testicle to the body

c)Pampiniform plexus – It is the coil of testicular veins that cools the arterial blood entering the testicle