Fig. 1

Histopathological analysis of normal vs. NOA human testes. A Schematic drawing on the left panel illustrates the polarized microtubule (MT). Each MT protofilament is formed by polymerization of GTP-bound α-/ß-tubulin heterodimers from the plus ( +), fast growing, end, that begins with the ß-tubulin, with the α-tubulin at the minus (-), flow growing, end. GTP-bound α-tubulin is non-exchangeable and never hydrolyzed. But GTP-bound ß-tubulin is rapidly hydrolyzed to become GDP-bound following assembly. A microtubule is a hollow cylindrical tube-like structure composed of 13 laterally associated protofilaments with 24-nm in diameter with an inner diameter of ~ 16 nm. The schematic drawing on the right panel illustrates MTs provide the structural support to Sertoli cells in the seminiferous epithelium that align perpendicular to the basement membrane of the tunica propria. MT cytoskeleton also localizes closely with the actin-based ectoplasmic specialization at the BTB known as basal ES, which is typified by the presence of actin filament bundles sandwiched between the apposing Sertoli cell plasma membranes and the endoplasmic  reticulum.  A similar ultrastructure known as apical ES is found at the Sertoli cell-elongate spermatid interface. The BTB is constituted by the actin-based TJ, basal ES and gap junction, and also intermediate filament-based desmosome, which also divides the seminiferous epithelium into the basal and adluminal (apical) compartments. The MTs serve as the tracks on which the MT-dependent motor proteins, such as dynein 1 and KIF15 (kinesin 15), move cargoes and developing spermatids to the MT minus (-) or plus ( +) end, respectively, conferring corresponding directional transport. B Images of cross-sections of testes from a normal human male (left panel) where germ cells of different types were noted. Using testes from human normal males, selected stages of tubules with different germ cell types were noted (three right panels). Scale bar, 200 µm (image in the left panel), 60 µm in the staged tubules, which apply to all other images in the three right panels. Keys to symbols were shown in the lower panel. C The top and bottom micrographs of the left panel illustrate the typical features of SCO (Sertoli cell only) testes. In the two cross-sections of SCO testes shown here, a few undifferentiated spermatogonia (black arrowheads) were routinely detected, besides Sertoli cells (red arrowheads). This observation is consistent with earlier reports, detecting few spermatogonia scattered around Sertoli cells in cross-sections of SCO testes [45,46,47]. In the two micrographs of the right panel, typical features of and MA (meiotic arrest) are noted wherein spermatocytes (purple arrowheads) failed to enter meiosis I/II to develop into haploid round spermatids and to undergo spermiogenesis to form elongate spermatids as noted in normal testes (see B). Scale bar, 250 µm in SCO testes encircled in blue box and in MA testes encircled in red box; and 60 µm in magnified image encircled in yellow box in the MA testis. Yellow dash line annotates location of the basement membrane in the tunica propria. Images shown herein are representative cross-sections of testes from n = 3 men in SCO and MA testes