Retroprosthetic membrane histopathology. (A) Portion of retroprosthetic fibrous membrane showing myriad vacuoles. Note the absence of a lymphocytic infiltrate. (B) Higher power of the spaces in the retroprosthetic membrane. The inset reveals peripherally displaced nuclei and thin strands of compressed cytoplasm. (C) Transmission electron micrograph of the fibrous portion of the membrane containing collagen (Co) and fibroblasts (F) with profiles of rough-surfaced endoplasmic reticulum (arrow). Note the absence of basement membrane formation around the fibroblast. (D) Transmission electron micrograph demonstrating dissolved-out lipid extracellular locules (L) and surrounding compressed fibroblasts with stretched-out cytoplasm and flattened nuclei (arrow). Co, membranous collagen. (A) Methylene blue, ×100. (B) Methylene blue, ×400; inset, ×600. (C, D) Transmission electron micrographs, ×11,000 and ×3,500.