arias-stella reaction arias-stella reaction arias-stella reaction, also arias-stella phenomenon, is a benign change in the endometrium associated with the presence of chorionic tissue. micrograph showing arias-stella reaction in endometrium arias-stella reaction is due to progesterone primarily. cytologically, it looks like a malignancy and, historically, it was diagnosed as endometrial cancer. significance it is significant only because it can be misdiagnosed as a cancer. it may be seen in a completely normal pregnancy. diagnosis it is characterized by nuclear enlargement and may also have any of the following: an irregular nuclear membrane, granular chromatin, centronuclear vacuolization, and pseudonuclear inclusions. five subtypes are recognized: minimal atypia. early secretory pattern. secretory or hypersecretory pattern. regenerative, proliferative or nonsecretory pattern. monstrous cell pattern. history it was first described by javier arias stella, a peruvian pathologist, in 1954.