Examination of the tubule epithelium reveals two kinds of cells: a proliferating population of spermatogenic cells and a nonproliferating population, the sustentacular, or Sertoli, cells. The Sertoli cells are considerably fewer and can be recognized by their elongate, pale-staining nuclei (Sn) and conspicuous nucleolus. The Sertoli cell cytoplasm extends from the periphery of the tubule to the lumen.
The spermatogenic cells consist of successive generations arranged in concentric layers. Thus, the spermatogonia (Sg ) are found at the periphery. The spermatocytes (Sc), most of which have large round nuclei with a distinctive chromatin pattern (because of their chromatin material being reorganized), come to lie above the spermatogonia. The spermatid population (Sp) consists of one or two generations and occupies the site closest to the lumen. The tubules in this figure have been identified according to their stage of development. The tubule at the upper right can be identified as stage VI. At this stage, the mature population of spermatids (identified by their dark-blue heads and eosinophilic threadlike flagella protruding into the lumen) are in the process of being released (spermiogenesis). The younger generation of spermatids is composed of round cells and exhibits round nuclei. Moving clockwise, the tubule indicated as stage VII is slightly more advanced. The mature spermatids are now gone. Progressing to stage VIII, the tubule at the bottom of the micrograph reveals that the spermatid population is undergoing a change in nuclear shape. Note the tapered nuclei (arrows). Further maturation of the spermatids is reflected in the tubule at the top of the micrograph, stage XI. Finally, the tubule marked stage II, on the left, reveals slightly greater maturation of the luminal spermatids, and with the start of the new cycle (stage I), a newly formed spermatid population is now present. By examining the spermatid population and assessing the number of generations present (i.e., one or two) and the degree of maturation, it is possible with the aid of a chart to approximate the stage of a tubule.
KEY BV, blood vessels L, lobule LC, Leydig cells LP, lamina propria S, connective tissue septa Sc, spermatocytes Sg, spermatogonia Sn, Sertoli nuclei Sp, spermatids TA, tunica albuginea X, tangential section of tubule with lumen obscured arrows, spermatid nuclei displaying early shape change
While the mature testis is characterized by seminiferous tubules, the immature testis is characterized by cords of cells consisting of an epithelium of sustentacular (Sertoli) cells surrounding occasional gonocytes, precursors of spermatogonia that are derived from the primordial germ cells that invaded the developing gonad in the embryo. At puberty, these cords become canalized, and the gonocytes begin the multiple divisions that give rise to the spermatogonia that, in turn, will divide and differentiate into the mature sperm. The seminiferous tubules terminate as straight tubules (tubuli recti) that are lined only by Sertoli cells. The tubuli recti lead to the rete testis, a complex series of anastomosing channels in the mediastinum testis that is the termination of the intratesticular tubule system.
Prepubertal testis, newborn human, H&E 180; inset 360.
The various germ cell types representative of spermatogenesis in the mature seminiferous tubules are not present in the testis before puberty or in the postpubertal undescended testis. Instead, the “tubules” are represented by cords of cells in which a lumen is lacking. The seminiferous cords display the same tortuosity as in the adult; the tunica albuginea (TA) of the testis, though thinner, is of the same relative thickness.