Right ventricular outflow muscle in tetralogy of Fallot: Histologic and immunohistochemical monoclonal antibody analysis.

To evaluate progressive muscular right ventricular outflow tract (RVOT) obstruction in tetralogy of Fallot (TOF), we hypothesized that this tissue would demonstrate more prominent selected immunohistochemical markers of myogenous cell differentiation, growth factor/receptor, and extracellular matrix. Sections of formalin-fixed, paraffin-embedded myectomy tissue obtained from RVOT at the time of surgical correction of TOF (n = 32; ages = 3 months through 13 years) were compared with age-matched tissue from the RVOT of normal control hearts (n = 12) obtained at autopsy after non-cardiac death. Examining by light microscopy slides stained with a combination of hematoxylin and eosin and elastic trichrome revealed cardiomyocyte (CM) hypertrophy, extensive myofiber disarray, trabeculation, multinucleation (more than two nuclei per myocyte), fibrosis, and thick-walled coronary arteries within the myocardium of TOF tissue. The endocardium from TOF specimens was thickened and hypercellular with prominent fibrosis and elastosis. Mitosis was not observed. At the interface between the endocardium and myocardium, the TOF specimens demonstrated myocyte dispolarity (orientation of CMs perpendicular to the endocardial surface), vascularization, and fibrosis. Immunohistochemical studies were performed using monoclonal antibodies to vimentin, desmin, muscle-specific actin (MSA), epidermal growth factor (EGF), epidermal growth factor receptor (EGFR), and laminin. Compared to the tissue from controls, TOF tissue showed a pattern of upregulated expression of epitopes within the endocardium and adjacent subendocardial myocardium. Decoration for MSA, vimentin, desmin, and EGFR highlighted the zonal nature of this tissue hyperactivity. Laminin prominently outlined endocardial cells, subendocardial CMs, and interface vessels in TOF tissue compared to the remainder of the myocardium and tissue from controls. Overall, changes in TOF were age-related, with older patients showing less zonal myocardial reactivity. These findings provide evidence for an ongoing, complex remodeling of the RVOT muscle in TOF.