The ultrastructure of an IOL "cocoon membrane".

A white cocoon membrane emanating from an inferotemporal focus in the iris stroma totally ensheathed a Binkhorst four-loop iris plane lens 18 months after implantation. The membrane was removed and studied by scanning (SEM) and transmission (TEM) electron microscopy. By SEM the external surface of the membrane was characterized by a fibrillar meshwork, with only an occasional scattered spindle-shaped or rounded inflammatory cell occupying the surface. One-micron plastic sections revealed both spindle and polyhedral heavily pigmented cells in the outer aspect of the membrane, and compressed elongated nonpigmented spindle cells scattered throughout the central portions. By TEM cells clinging to the outermost aspects were bipolar iris stromal melanocytes with small melanosomes, inactive fibroblasts, or histiocytes with cytoplasmic ruffles. The larger rounded pigmented cells in the superficial region were viable or degenerated iris pigment epithelial cells or macrophages. Most of the substance of the membrane was composed of collagen fibrils 100-500 A in diameter with and without 640 A periodicity; these fibrils were generally oriented parallel to the spindle cells but did not display distinct bundling or a lamellar architecture. The spindle cells in the center of the lesion were active fibroblasts with abundant rough-surfaced endoplasmic reticulum and scattered actin myofilaments; they contained rare small and large melanin granules indicative of phagocytosis. It has been concluded that the membrane was produced predominantly by iris stromal fibroblasts probably activated by the haptics of the IOL or McCanell suture.