Final answer:
The steroid hormone testosterone, produced by Leydig cells in the testes starting at the seventh week of fetal development, is responsible for masculinization of the male brain as well as the development of male sexual organs and secondary sexual characteristics.
Step-by-step explanation:
In the male fetus, the steroid hormone primarily responsible for the masculinization of the brain is testosterone. This androgen is produced by the Leydig cells in the testes as early as the seventh week of development, with peak concentrations reached during the second trimester. Testosterone is crucial for the development of male sexual organs and the expression of secondary male characteristics during adolescence, such as the deepening of the voice and the growth of facial, axillary, and pubic hair. Additionally, testosterone and its derivatives play a fundamental role in the masculinization of the brain, influencing behaviors and cognitive functions that are often sexually dimorphic.