Histological evidence of increased apoptosis and deposition of human IgG in conduction tissue of a 20-week fetus with congenital heart block (CHB). (A) A longitudinal section through the septal tissue of a 20-week fetus with CHB, counterstained with hematoxylin and eosin. Apoptotic cells are identified by TUNEL peroxidase and are scattered among healthy cells. (B) The same tissue section stained with alkaline phosphatase conjugated to anti-human IgG. (C) The same tissue section double-stained with TUNEL peroxidase and alkaline phosphatase conjugated to anti-human IgG to demonstrate colocalization of apoptosis and of IgG, respectively.