Immunohistochemical Identification of Aggregated Actin Filaments in Formalin-Fixed, Paraffin-Embedded Sections

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Abstract
Some authors have claimed that actin is not immunostained in characteristic intracytoplasmic inclusions of infantile digital fibromatosis, whereas others have claimed that it is. Formalin-fixed specimens were used in the former studies; specimens fixed in alcohol used in the latter studies. Actin at other sites, such as the rim of the inclusions, was distinctly immunostained even in the formalin-fixed specimens. Such phenomena make it difficult to accept the loss of antigenicity of actin as a result of formalin fixation. The use of usual pretreatment with trypsin provided the same results. We were able to im-munostain actin distinctly and strongly in the inclusions for the first time in formalin-fixed specimens by combining KOH in 70% ethanol and trypsin. This successful staining results from the adequate etching effect of trypsin, which occurs because of a loosening of proteins in the inclusions due to KOH. These phenomena suggest that steric hindrance of antigen determinant has occurred only in the inclusions as a result of intramolecular cross-linkage, because of extremely dense accumulation of actin filaments in the inclusions.

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