The testis is composed of a tightly packed collection of tubules. In the prepubertal testis, the tubules are lined with spindly, radially arranged Sertoli cells and rare spermatogonia . After puberty, spermatogenesis begins, and the tubules are dominated by the developing spermatocytes. Maturation is completed near the lumen of the tubules, where you can see tiny sesame-seed-like spermatids (which grow tails to become spermatozoa). Polygonal pink Leydig cells in the interstitium produce testosterone (Figure 14.1).

Sperm leaves the testis via the rete testis (Figure 14.2), a collection of epithelial-lined slit-like channels at the hilum of the testis, which lead into the epididymis, which eventually feeds into the vas deferens. The epididymis is lined by a pseudostratified and ciliated epithelium (Figure 14.3).

In the setting of an undescended testis, the testis may be removed to prevent the development of a germ cell neoplasm. The typical cryptorchid testis shows small atrophic seminiferous tubules, fibrosis, and widened interstitial spaces (Figure 14.4). A related finding is the “vanishing testis syndrome ” in which, upon surgical retrieval of the undescended testis, there is nothing but a nub of fibrosis and dystrophic calcification attached to an epididymal remnant. These conditions are generally signed out descriptively.

A testis biopsy may be indicated in the workup of a persistent low sperm count (male infertility). From the pathologists’ perspective, the options are the following:

Tumors of the testis are staged by their extent of invasion beyond the testis. If you imagine the testis as a lime, the outer green rind is the tunica vaginalis. Like the pleura, this is a mesothelial layer that has both a visceral side, on the testis, and a parietal side, lining the scrotal wall. As in the lime, which has a white pith underneath the green rind, under the tunica vaginalis is a fibrous layer called the tunica albuginea (“white membrane”). It is important to sample tumor that appears to erode into these layers.

The vast majority of testicular tumors are of germ cell origin (Table 14.1). The remainder, aside from lymphoma, are tumors of mesothelial, epithelial, or connective tissue origin which will not be covered here.