The major determinant of atrial fibrillation initiation is focal firing within the muscular portion of the pulmonary veins. We hypothesized that Interstitial Cajal cells, a known type of pace-maker cells, could underlie the pace-making activity of isolated pulmonary veins.

The aim of the study was to characterize the presence and the distribution of Interstitial Cajal cells (ICC) in human pulmonary veins.

Immunohistochemistry was performed on a transversal section of pulmonary vein of 8 adult human hearts obtained at autopsy from January 2005 to December 2005. Atrial fibrillation history was documented in 2 out of these 8 patients. Two immunostainings were performed on successive sections to differentiate ICC from mast cells (antibody cKit and antibody AA1). Morphological and distribution analysis were performed manually and automatically. Electron microscopy and immunostaining with HCN4 and smooth muscle alpha-actin antibodies were also used to further characterized Cajal cells.

ICC were found in the PV sections of 3 of the 8 patients and were mainly identified in sections with thick muscular sleeve. Two of these 3 patients had AF history. The mean distribution density of these cells was 0.6 ICC per 3mm² with the highest density reaching 14.6 ICC per 3mm² in a pulmonary vein of a patient with atrial fibrillation history. A positive immunostaining of Cajal Cells with HCN4 was also demonstrated.

Demonstration of the close contact of the cytoplasmic process with cells around particularly with muscular cells of the atrial muscular sleeve which extend in the circumference of the pulmonary vein.

Interstitial Cajal cells may be detected in human pulmonary veins, particularly in patients with atrial fibrillation. Given the electrophysiological attributes of these cells, their role as AF triggers deserves to be more documented.