The ontogeny of acid alpha-naphthyl acetate esterase activity in the thymus

The acid alpha-naphthyl acetate esterase (ANAE) activity was examined in tissue sections of thymus from 24 fetuses, infants and children. ANAE activity was first detected in thymocytes at 11 wk of gestation, and the fraction of stained cells increased gradually until 18 wk of gestation. Thymocytes with circumscribed deposits of ANAE activity were observed as early as at 11 wk, whereas thymocytes with a diffuse, granular staining pattern were not observed before 16 wk of gestation.