Endocytosis in the embryonic chick chorionic epithelium.

The chorionic epithelium of the developing chick embryo consists of two major cell types, villus cavity (VC) and sinus-covering (SC) cells. Ultrastructurally, VC cells contain apical microvilli with numerous underlying vesicles, whereas SC cells possess thin cytoplasmic processes (containing occasional vesicles) overlying adjacent blood sinuses. This study was performed to determine whether vesicles observed in VC and SC cells are endocytic in nature and whether endocytosis is limited to either VC or SC cells. Exposure of air space chorionic epithelium in ovo to microperoxidase (MRP-11) resulted in endocytic uptake of marker by the thin cytoplasmic processes of 70% of SC cells. Vesicles containing MRP-11 reaction product were also observed in approximately 20% of VC cells. Exposure of the chorionic epithelium of shell-less cultured embryos to horseradish peroxidase (HRP) resulted in endocytic uptake of HRP in approximately 80% of SC cells and in 33% of VC cells. Thus, VC and SC cells demonstrate endocytic activity both in ovo and in shell-less culture. In ovo, endocytosis may be involved in the recovery of water and/or ions from the eggshell or shell membranes.