Morphology (left panel) and percent (right panel) of atretic follicles. Follicles were exhibiting histologic evidence of atresia (pyknotic, unadhered granulosa cells in the antrum; left panel) following administration of the hCG bolus in COS protocols in rhesus monkeys. Although relatively few (<10%) appear atretic prior to hCG injection (0 hr; small number below bar indicates sample size), the number increases significantly by 36 hrs post-hCG (40%; controls, CTRL). Moreover, the percentage of atretic follicles was influenced by the steroid milieu, since administration of a steroid synthesis inhibitor (trilostane, TRL) produced a cohort of 70% atretic follicles and co-administration of a progestin (R5020) reduced the percentage to pretreatment (0 hr hCG) levels of 10%. Thus, oocytes collected prior to follicle rupture could originate from degenerating, as well as healthy (i.e., luteinizing, ovulatory) follicles. See ref  for further details, including statistical (X2) analyses. Different letters above bars indicate significant differences over time within controls (CTRL; hCG alone). Asterisk, or NS indicate significant or nonsignificant differences between treatment groups at one timepoint.