This study describes the morphological behaviour of spermatogonia foll
owing recovery from two doses of busulfan treatment in the rat. Twenty
days after the second intraperitoneal injection of busulfan, the test
es lost most of their spermatogenic cells and there were fewer dispers
ed singly surviving spermatogonia. These surviving cells were in close
contact with the basal portions of adjacent Sertoli cells and the shr
unken basal lamina, and were the source for repopulating the depleted
seminiferous epithelium. During the initial stage of repopulation (48
days later), surviving spermatogonia underwent a phase of active proli
feration: type A spermatogonia underwent symmetric and asymmetric divi
sions; type B spermatogonia underwent asynchronous differentiation. At
day 96, normal spermatogenesis was fully recovered in many seminifero
us tubules, represented by 80% of the rats regaining various degrees o
f fertility at day 120. These data provide an additional model for the
study of self-renewal of stem spermatogonia and suggest that the asym
metric division of type A spermatogonia and their close contact with b
oth the basal lamina and the Sertoli cells may be involved in regulati
ng the number of stem spermatogonia and the delicate process of normal
spermatogenesis.