Context: The pathogenesis and mechanisms behind the degeneration of the seminiferous tubules in testes of Klinefelter syndrome (KS) subjects are yet unknown.

Objective: To characterize the testicular degeneration process during puberty in boys with KS by describing the immunoexpression of some developmentally regulated markers of testis maturation in relation to serum levels of reproductive hormones.

Design: Prospective clinical study.

Setting: University central hospital, pediatric referral endocrinology outpatient clinic.

Patients: 14 boys with KS, aged 10.1-14.0 yr.

Intervention: None.

Main outcome measures: Immunoexpression of germ cell differentiation markers (AP-2, CHK2, OCT-3/4, NY-ESO-1, MAGE-A4) and androgen action-related proteins (androgen receptor AR, anti-Müllerian hormone AMH, MIC2, inhibin B; - and B- subunits) in testicular biopsies of KS boys in relation to serum reproductive hormone levels.

Results: In KS boys, gonocytes differentiated to the spermatogonium stage, but no spermatocytes were visible. Despite this, down-regulation of AMH expression in the Sertoli cells occurred concomitantly with decreasing serum AMH levels. Expression of inhibin -and B-subunits appeared in the biopsies even when circulating inhibin B levels were undetectable. In the KS boys compared to age-matched controls, the proportion of Sertoli cell nuclei expressing AR was smaller, and cytoplasmic staining of Sertoli cells was constantly present.

Conclusions: We showed with several testis-specific markers that in KS gonocytes differentiate to spermatogonia and that the degeneration of the testes accelerates at the onset of puberty. Altered immunoexpression of AR indicates that a relative androgen deficiency at least at the testicular level develops in KS boys during puberty.