Developmental aspects of the male reproductive system.

THE DEVELOPMENT OF THE MAMMALIAN REPRODUCTIVE SYSTEM INVOLVES: (1) an indifferent or ambisexual stage, in which both the male (Wolffian) and female (Mullerian) duct systems are present; (2) sexual differentiation, in which the phenotypic sex is expressed by the enhancement of Mullerian structures in the female and Wolffian structures in the male and reciprocal suppression of the opposite duct system; (3) cytodifferentiation, in which the epithelial, stromal and muscular features are regionally established; and (4) actual response to endogenous hormones, especially in mammals, such as the human and guinea pig, in which the differentiated tissues respond according to their capability. Specifically in the male, the onset of sexual differentiation is signaled by the elaboration of an androgenlike material (possibly testosterone) and a Mullerian-inhibiting factor from the testis. In the absence of these two influences, or one of them, the reproductive tract remains essentially female in configuration, a normal situation in the female and in abnormal males in which the urogenital sinus and Wolffian structures are incapable of hormonal responses due to the lack of specific enzymes or receptors. Male differentiation in particular involves enlargement of the penis and its canalization by the urethra, scrotal development and descent of the testis, and the formation of accessory glandular structures from the urogenital sinus or Wolffian ducts (bulbourethral gland, seminal vesicles, ampulla, prostate). Remnants of the Mullerian system may persist at the upper pole of the testis and are normally present (uterus masculinus) in relation to the prostatic part of the urethra.The emergence of the characteristic parts of the male reproductive system in higher mammals in relation to phylogeny and the detailed origin of these in individual ontogeny are described. The use of the guinea pig as a model animal system for the study of transplacental effects of hormones (diethylstilbestrol, estradiol) and teratogens is briefly discussed.