The basement-membrane-like matrix of the mouse EHS tumor: III. Immunodetection of the amyloid P component in basotubules.

Immunohistochemical methods were applied to the ultrastructural localization of the amyloid P component in the EHS tumor matrix. First, the preembedding approach was used by exposing frozen sections of tumor to antiserum against the mouse amyloid P component followed by the peroxidase-antiperoxidase sequence. Second, using the postembedding approach, Lowicryl K4M sections of the tumor were exposed to antiserum against the amyloid P component and subjected to the protein A-gold procedure. In both cases, the immunostaining was restricted to structures which appeared in longitudinal section as fairly straight rods and in cross section as 7- to 10-nm pentagonal or roughly circular profiles outlining a lumen with a central dot. Since these features are characteristic of basotubules, it is concluded that the basotubules of the tumor matrix possess the antigenicity of the amyloid P component and presumably contain this substance itself. Similar experiments carried out on the thick basement membrane known as Reichert's membrane demonstrated that its basotubules also possessed amyloid P-component antigenicity. It is likely, therefore, that the amyloid P component is a constituent of basotubules.