piano key appearance 

medium power image of endometrial glands at 17 days with piano keys appearance

HISTOPATHOLOGY

A term of art referring to the appearance of the even rows of subnuclear vacuoles–fancifully likened to piano keys–seen by low-power light microscopy in endometrial biopsies on day 17 in women with a normal menstrual period. 

Reference Endometrial Biopsy Interpretation, Brigham and Women’s Hospital, Boston, 22-23.11.2003