Cultured chick yolk sac epithelium: structure and IgG surface binding

The chick yolk sac endoderm transports maternal immunoglobulin G (IgG) from the yolk into the embryo during development, providing the newly hatched chick with passive immunity until it becomes immunocompetent. To study this transport process, chick yolk sac endodermal cells isolated from embryos of 6 to 18 days of incubation were grown in vitro on a collagen substrate. The cultured cells possessed a remarkable structural similarity to the in vivo tissue and reformed a polarized confluent epithelium with tight junctions and desmosomes joining the cells at their apical margins. In addition, the cells exhibited apical microvilli, numerous phagolysosomes in the cytoplasm and retained the expression of the yolk sac endoderm-specific enzyme marker, cysteine lyase. Importantly, the cultured cells retained the ability to specifically bind IgG as demonstrated by indirect immunofluorescence. Chicken IgG bound to the cultured cells at 4 degrees C in a diffuse pattern that clustered into a punctate pattern when a second antibody was used. Cultures from yolk sacs of day 6 through day 18 of development all demonstrated this immunofluorescent labeling for at least 14 days in culture. These results demonstrate that cultured yolk sac endoderm maintains its differentiated morphology and ability to bind IgG.