Continual maintenance of the blood-testis barrier during spermatogenesis: the intermediate compartment theory revisited.

Tight junctions occur between the lateral processes of neighboring Sertoli cells that divide the seminiferous epithelium into two compartments: basal and adluminal compartments. These tight junctions constitute the blood-testis barrier (BTB). The established theory that the BTB must open when spermatocytes translocate from the basal compartment to the adluminal compartment is marked by one contradiction, that is, normal spermatogenesis occurs in the testis because the BTB is expected to constantly seclude the adluminal compartment from the basal compartment in order to protect haploid germ cells from the autoimmune system. Subsequently, another concept was proposed in which two BTBs divide the seminiferous epithelium into three compartments: basal, intermediate and adluminal compartments. It has been suggested that the transition from the basal region to the adluminal region without the BTB open occurs through the agency of a short-lived intermediate compartment embodying some primary spermatocytes. In contrast, the results of recent findings in the molecular architecture of the BTB suggest that the BTB in the seminiferous epithelium must "open". In this paper, I re-examine the BTBs of boar and experimental cryptorchid mouse testes by transmission electron microscope (TEM). TEM analysis showed that an atypical basal compartment existed in the thin seminiferous epithelium of 14-day post-cryptorchid mice testes. In developmental boar testes, ectoplasmic specialization (ES) of the seminiferous epithelium showed dynamic behavior. The intermediate compartment was clearly observed between the basal and adluminal compartments of the mature boar seminiferous epithelium. ESs were observed between Sertoli cells and spermatids at all developmental stages, including early, late and mature. Furthermore, ESs were situated on the apical surface of the seminiferous epithelium. From these results, I propose that the BTB is continually maintained during spermatogenesis and suggest a model of ES circulation in the seminiferous epithelium.