Ectopic Leydig cells of testis: An immunohistochemical study on tissue microarray

The incidence of ectopic Leydig cells (LCs) has been reported to be 40% to 90%. Several theories have been proposed to account for ectopic LCs, including in situ differentiation, migration from testicular interstitium, and trapping of peritubular LCs in the tunica propria of the seminiferous tubule during its thickening. To document the nature of ectopic LCs and to compare them with interstitial LCs, Sertoli cells, epididymal cells, mesothelial cells, and epithelial cells in rete testis, immunostainings with calretinin, CD10, inhibin, CK7, and CK20 were performed in 40 cases of orchiectomy specimens using tissue microarray sections. In addition, the frequency of ectopic LCs was evaluated. Of the 40 orchiectomy specimens, 14 cases demonstrated ectopic LCs with an incidence of 35%. Inhibin was positive in more than half of ectopic LCs (9/14, 64.3%) and in almost all interstitial LCs. Calretinin was positive in most of ectopic LCs (12/14, 85.7%) as well as in most of normally located interstitial LCs and mesothelial cells and some of rete testis epithelial cells. The ectopic and interstitial LCs as well as Sertoli cells were negative for CK7, CK20, and CD10. CK7 was positive in all epididymal cells and in most mesothelial cells and epithelial cells in rete testis. CD10 was positive in some of epididymal cells and epithelial cells in rete testis. CK20 was negative in all cells in the testis and epididymis. Ectopic LCs showed similar staining patterns to interstitial LCs with positive immunoreactivity for calretinin and inhibin. In this study, the frequency of ectopic LCs was 35%. The lower incidence in this study was most likely because of the limited sampling. Immunohistochemically, ectopic LCs showed identical immunohistochemical patterns with those of interstitial LCs. Calretinin appeared to be more sensitive but less specific than inhibin for LCs. Because calretinin is frequently positive in cells other than ectopic or interstitial LCs, a precaution is required to differentiate LCs from mesothelial cells and rete testis epithelial cells. Based on our study, we could not add anything else of what is known in regard to the histogenesis of ectopic LCs. The current theories including in situ differentiation, migration from testicular interstitium, and trapping of peritubular LCs in the tunica propria of the seminiferous tubules during its thickening seem to be valid histogenetic theories.