Final answer:
Sustentacular (Sertoli) cells secrete Müllerian inhibiting substance during fetal development, causing regression of female reproductive structures in males and promoting male reproductive system formation.
Step-by-step explanation:
Sustentacular cells, also known as Sertoli cells, secrete Müllerian inhibiting substance (MIS) in fetal life, which leads to the regression of the Müllerian ducts in male embryos. The degradation of Müllerian ducts is crucial for the typical development of male internal reproductive structures such as the epididymis, ductus deferens, and seminal vesicles. If the secretion of MIS and testosterone does not occur, the fetal development will lead to the formation of female reproductive structures like the uterus, uterine tubes, and partly the vagina.
During fetal development, testosterone production is also important for the growth of the Wolffian duct, which further differentiates into male reproductive organs. Without testosterone, the Wolffian duct will also degrade. This balance of hormone secretion is essential for proper sexual differentiation and the formation of either male or female reproductive systems.