Expression of a complex developmental antigen in precise morphogenetic processes during chick embryogenesis.

In order to identify transitory molecules involved in specific events during development, monoclonal antibodies were raised against various structures of the chick embryo at stages exhibiting important tissue reorganization. A7H2 is a monoclonal antibody reacting with an epitope suddenly expressed in select embryonic tissues during precise morphogenetic processes. Splanchnomesoderm and most mesothelia exhibited a strong immunoreactivity during organogenesis whereas non-mesodermal A7H2-activities were usually confined to invaginating or folding epithelia. Most embryonic reactivities were transient but the amnion demonstrated a strong and persistent reactivity and therefore was selected as a source of material for A7H2-antigen characterization. The antigenic determinant was shown to be on a huge molecular complex having a pI of 5. Immunoblots obtained after SDS-PAGE demonstrated a highly disperse pattern in the high MW region of the gel that could be altered or abolished by proteolytic treatments. Fingerprint analysis of the immunoreactive peptide bands demonstrated a clear relationship between them. Using different fixation and permeabilization procedures on amnion cultured cells, we have observed that A7H2-antigen was associated with the cytoplasmic face of the plasma membrane and was resistant to treatments removing most cellular components and producing substratum-attached ventral membranes. We suggest that this macromolecular complex plays a role in specific subsets of cells involved in precise morphogenetic processes, possibly as a link with the external milieu or as a membrane associated structure.