A series of 14 primary and two metastatic rat
rhabdomyosarcomas (RMS) induced with
nickel sulfide was studied by light microscopy, transmission electron microscopy, indirect immunofluorescence,
avidin-
biotin-
peroxidase immunohistochemistry and two-dimensional gel electrophoresis. Monoclonal or affinity-purified polyclonal
antibodies were used for the immunohistochemical demonstration of
vimentin,
desmin, alpha-smooth muscle (alpha-sm) actin and alpha-sarcomeric (alpha-sr) actin. By histological and ultrastructural studies, four categories of RMS were diagnosed on the basis of the neoplastic cell types. These were: (1) well-differentiated RMS (n = 2), (2) pleomorphic RMS (n = 8), (3) embryonal RMS (n = 4), and (4) embryonal
myosarcomas (n = 2). Immunohistochemically, all these
neoplasms expressed
desmin and alpha-sr actin, reflecting their rhabdomyoblastic origin. Two dimensional gel electrophoresis performed on five
neoplasms demonstrated alpha, beta and gamma
actins spots in all cases. This study demonstrates that the alpha-sr actin antibody represents a good marker for rhabdomyoblastic differentiation is useful in the diagnosis of RMS since it was present in all morphologically confirmed RMS and in two ultrastructurally undifferentiated
sarcomas positive for
desmin. Neoplastic cells positive for alpha-sm actin were noted in 11 confirmed RMS. Double indirect immunofluorescence showed that all alpha-sm and alpha-sr positive cells also contained
desmin. Co-expression of alpha-sr and alpha-sm
actins was studied in serial sections of
formalin-fixed,
paraffin-embedded
tumor tissue. Both alpha-sm and alpha-sr
actins were localized in some rhabdomyoblasts. This study confirms our previous observations in human
tumors and shows, for the first time, that alpha-sr and alpha-sm
actins can be present in the same neoplastic cell in vivo.