Ultrastructural and immunocytochemical evidence for the presence of polarised plasma membrane H(+)-ATPase in two specialised cell types in the chick embryo chorioallantoic membrane.

The chick embryo, confined in the eggshell, has to dispose/buffer the acid generated by its metabolism, as well as to release calcium from the shell which is used for growth. To localise H(+)-ATPase, electron microscope and immunocytochemical studies were conducted on chorioallantoic membranes of 15-17 d chick embryos. Ultrastructural studies of the villus cavity (VC) cells in the chorionic epithelium demonstrated that their apical plasma membrane, juxtaposed with the shell membranes, contains microvilli as well as microplicae which possess 9-10 nm studs at a density of 16,700 particles/micron2, a characteristic feature of the polarised H(+)-ATPase pump. Immunocytochemical staining, using a monoclonal antibody to the 31 kDa subunit of H(+)-ATPase, confirmed the presence of large amounts of the vacuolar H(+)-ATPase in the VC shells with a distribution highly polarised towards the eggshell membranes. Immunoelectron-microscopic localisation studies using a rabbit antiserum to whole bovine H(+)-ATPase and immunogold technique, confirmed the localisation of H(+)-ATPase at the apical microvilli/microplicae as well as in the subapical vesicles. In the allantoic epithelium, the presence of mitochondria-rich (MR) cells was confirmed; it was shown that these cells extend through the full thickness of this epithelium. The MR cells also contained large numbers of 9-10 nm studs, typical of proton secreting cells, in their apical plasma membrane. This was confirmed by immunocytochemical staining which showed abundant localisation of H(+)-ATPase in these cells; this localisation was, however, diffuse rather than apical.(ABSTRACT TRUNCATED AT 250 WORDS)