['Although gonadotropins and testosterone are high in the fetal/early postnatal periods, Sertoli cells remain immature and spermatogenesis does not progress. We hypothesized that Sertoli cells do not respond to testosterone because they do not express the androgen receptor.', 'The objective of the study was to describe the precise ontogeny of androgen receptor expression in the human testis from fetal life through adulthood.', 'This was an immunohistochemical study on testicular biopsies from fetal, neonatal, prepubertal, pubertal, and adult human testes.', 'Quantification of androgen receptor expression in Sertoli cells was measured. Evaluation of androgen receptor expression in peritubular and interstitial cells as well as anti-Müllerian hormone and inhibin-alpha was also performed.', 'Androgen receptor expression was first observed in the nuclei of few Sertoli cells at the age of 5 months. Labeling was weak in 2-15% of Sertoli cells until 4 yr of age and progressively increased thereafter. High levels of androgen receptor expression were observed in more than 90% from the age of 8 yr through adulthood. Androgen receptor was positive in peritubular cells and variable in interstitial cells. Anti-Müllerian hormone immunolabeling was strong in all Sertoli cells from fetal life throughout prepuberty and weakened progressively as spermatogenesis developed. Inhibin-alpha expression was detected in all Sertoli cells from fetal life through adulthood.'] Is physiological androgen insensitivity of the fetal , neonatal , and early infantile testis explained by the ontogeny of the androgen receptor expression in Sertoli cells?