Large, rounded, smooth projections of the apical plasma membrane, which have been identified in the uterine epithelium of many species during early pregnancy, from rodents to humans.
These structures are seen during the receptive phase for blastocyst implantation in humans, and are important indicators of normal endocrine progression and of uterine receptivity for blastocyst implantation. While their function is unknown, they represent a transformation of a microvillous plasma membrane into a smooth and flattened plasma membrane during early gestation.
Reference Cell Research (2004) 14, 259–267. doi:10.1038/sj.cr.7290227