The male reproductive system consists of the testes, genital excurrent ducts, accessory sex glands, and penis (Fig. 22.1). The accessory sex glands include the seminal vesicles, the prostate, and bulbourethral glands. The two primary functions of the testis are spermatogenesis (the production of sperm, called male gametes), and steroidogenesis (synthesis of androgens, also called sex hormones). Androgens, mainly testosterone, are essential for spermatogenesis, play an important role in embryonic development of the male embryo into the phenotypic male fetus, and are responsible for sexual dimorphism (male physical and behavioral characteristics).
The events of cell division that occur during production of male gametes, as well as those of the female, the ova, involve both normal division, mitosis, and reduction division, meiosis.
A brief description of mitosis and meiosis is included in Chapter 3. A basic understanding of these processes is essential to understanding the production of gametes in both men and women.
The adult testes are paired ovoid organs that lie within the scrotum, located outside the body cavity. Each testis is suspended within the end of an elongated musculofacial pouch, which is continuous with layers of the anterior abdominal
FIGURE 22.1 • Schematic diagram demonstrating the components of the male reproductive system. Midline structures are depicted in sagittal section; bilateral structures including the testis, epididymis, ductus deferens, and seminal vesicle are shown intact.
wall and projects into the scrotum. Testes are connected by the spermatic cords to the abdominal wall and tethered to the scrotum by scrotal ligaments, the remnants of the gubernaculum (see further in the chapter).
Gender Determination and Development of the Testis
Gender differentiation is accomplished through a cascade of gene activations.
Genetic sex is determined at fertilization by the presence or absence of the Y chromosome. The testes, however, do not form until the seventh week of development. Gonadal sex is determined by the presence of the SRY gene located in the sex-determining region of the short arm of the Y chromosome. The SRY gene expression in early embryonic development is responsible for gender determination.
The genetic information encoded in the Y chromosome itself is not sufficient to guide a complex development of the male gonads. Instead, the SRY gene operates as a master switch that controls the cascade of several gene activations on autosomes 9, 11, 17, and 19 and the X chromosome. A transcription factor called the testis-determining factor (TDF), encoded by the SRY gene, has a molecular domain that binds into a specific region of DNA and alters it structure. The affected DNA forms a loop that permits binding of other transcription factors. They in turn cause the expression of other genes that initiate formation of not only the testes but also other male sex organs. Several other genes are expressed about the same time as the SRY gene, including these:  WT-1 gene, that is required for the development of the urogenital system and for regulation of the SRY transcrip tion. Mutations of the WT-1 gene are found in children with familial Wilms’s tumor and in children with accompanied genitourinary malformations.
 SOX-9 gene found in the genital ridges activates the AMH gene that is responsible for Mlerian-inhibiting factor synthesis. The mutation of the SOX9 gene is linked to a reversal of the gender of a male individual (46, XY).
 SF-1 gene (steroidogenic factor-1 gene) that regulates the expression of a number of steroidogenic genes.  DAX-1 gene that encodes nuclear receptor DAX-1. Acti vation of this receptor suppresses the SRY gene during gonadal sex differentiation and its mutation is responsible for congenital adrenal hypoplasia.
The testes develop on the posterior wall of the abdomen and later descend into the scrotum.