Immuno-labelling for freeze-fracture: application to a cell surface attachment antigen

The cell-surface attachment (CSAT) antigen was investigated by using a technique that combines freeze-fracture with thin-sectioning. Chick heart cells were labelled using colloidal gold secondary antibodies, then fractured and shadowed with platinum vapor. The cells were dehydrated and embedded with their gold labels and replicated surfaces still intact. Sectioning revealed gold particles superimposed with external (E) fracture faces. The gold was definitely not coincident with E face particles and did not appear to be associated with pits. Therefore, it can be concluded that the CSAT glycoproteins is not represented by an E face particle. Whether it may be seen as an E face pit, or is not visible at all in freeze-fracture, is still an open question.