FIGURE 22.7 • Electron micrograph of Leydig cells. This electron micrograph shows portions of several Leydig cells. The cytoplasm contains an abundance of sER, a characteristic of Leydig cells. Other features characteristic of the Leydig cell seen in the lower-power micrograph are the numerous lipid droplets (L), the segmented profiles of the Golgi apparatus (G), and the presence of variable numbers of lysosomes (Ly). Occasional profiles of rER are also seen. Note also the presence of microvilli along portions of the cell surface (arrows). M, cytoplasm of adjacent macrophage. 10,000. Inset. sER at higher magnification. The very dense particles are glycogen. 60,000.
 Spermatocyte phase (meiosis), in which primary sper matocytes undergo two meiotic divisions to reduce both the chromosome number and amount of DNA to produce haploid cells called spermatids Spermatid phase (spermiogenesis), in which sper
At the end of spermatogenesis, spermatids undergo their final maturation and are released during a process called spermiation from the supporting Sertoli cells into the lumen of the seminiferous tubule.
In the spermatogonial phase, stem cells divide to replace themselves and provide a population of committed spermatogonia.
Spermatogonial stem cells undergo multiple divisions and produce spermatogonial progeny that display differences in nuclear appearance in routine H&E preparations.
FIGURE 22.8 • Electron micrograph of a Reinke crystal. This electron micrograph shows the internal structure of a Reinke crystal in the cytoplasm of a human Leydig cell. Also note the sER (arrows) and a lipid droplet (L) in the cytoplasm. 16,000. (Courtesy of Dr. Don F. Cameron.)
Human spermatogonia are classified into three types based on the appearance of the nuclei in routine histologic preparations:  Type A dark (Ad) spermatogonia have ovoid nuclei with intensely basophilic, finely granular chromatin. These spermatogonia are thought to be the stem cells of the seminiferous epithelium. They divide at irregular intervals to give rise to either a pair of type Ad spermatogonia that remain as stem cells or to a pair of type Ap spermatogonia.
 Type A pale (Ap) spermatogonia have ovoid nuclei with lightly staining, finely granular chromatin. Ap spermatogonia are committed to the differentiation process that produces the sperm. They undergo several successive mitotic divisions, thereby increasing their number.
 Type B spermatogonia have generally spherical nuclei with chromatin that is condensed into large clumps along the nuclear envelope and around a central nucleolus (see Fig. 22.6).
An unusual feature of the division of an Ad spermatogonium into two type Ap spermatogonia is that the daughter cells remain connected by a thin cytoplasmic bridge. This same phenomenon occurs through each subsequent mitotic and meiotic division of the progeny of the original pair of Ap spermatogonia (Fig. 22.9). Thus, all of the progeny of an initial pair of Ap spermatogonia are connected, much like a strand of pearls. These cytoplasmic connections remain intact to the last stages of spermatid maturation and are essential for the synchronous development of each clone from an original pair of Ap cells.
After several divisions, type A spermatogonia differentiate into type B spermatogonia. The appearance of type B spermatogonia represents the last event in the spermatogonial phase.
In the spermatocyte phase, primary spermatocytes undergo meiosis to reduce both the chromosome number and the amount of DNA.