Phagocytic properties of human keratocyte cultures

The uptake of polystyrene latex beads (approximately 0.75 micrometers) and glutaraldehyde-treated erythrocytes by human corneal stromal keratocytes maintained in culture has been studied. Combined phase-contrast and scanning electron microscopic observations on individual cells after exposure to either beads or erythrocytes demonstrated that the majority of these particles were present intracellularly. Transmission electron microscopy revealed that the beads were membrane-bound within the cytoplasm of these cells. Thus the human keratocytes in culture are phagocytic and able to internalize particles ranging in size from approximately 0.75 to 6 micrometer. Kinetic studies showed continuous uptake of the polystyrene latex beads for at least 72 hr, with an approximate linear uptake rate between 4 and 48 hr. The extent of bead uptake was dependent on the initial bead concentration. The human keratocyte cultures were markedly more phagocytic than human skin fibroblasts or rabbit chondrocytes. It was also found that after extensive bead uptake the normal growth pattern of the keratocytes was affected. It is suggested that the phagocytic ability of the human keratocytes is involved in the turnover of the corneal stromal matrix as well as in the initial response of this avascular tissue to injury or bacterial infection.