HETEROTOPIC CARDIOMYOCYTES IN THE WALL OF PULMONARY VESSELS IN EXPERIMENTAL ANIMALS

Heterotopic cardiomyocytes (CMC) have been described in the wall of pulmonary veins including intraparenchymatous branches, in various species of laboratory animals. Brief note is given here of their morphology including electron microscopy and immunohistological findings. From the current literature hints concerning arrhythmogenic activity of heterotopic CMC in pulmonary vessels in the possible initiation of atrial tachyarrhythmia and fibrillation are also mentioned.


INTRODUCTION
In previous experimental studies we have repeatedly observed the heterotopic occurence of cardiomyocytes (CMC) in the walls of pulmonary vessels in various species of laboratory animals, more especially white mice, including genetic lines of athymic nude mice -Nu/Nu, and in rabbits.In these cases, however, the heterotopic occurrence of CMC in vessel walls also included their deep intraparenchymal ramification in regions far from the pulmonary hilus and the venoatrial junction.These findings were verified histologically, immunohistologically and by ultrastructural electron microscopic examination.In view of the current clinical relevance of heterotopic CMC in pulmonary veins as possible foci of arrhythmogenic activity leading to atrial tachyarrhythmia and fibrillation, our findings are briefly mentioned in this communication.

MATERIAL AND METHODS
The laboratory animals, that were used from several experimental projects and various breedings were as follows: The tissues were fixed in 10% formalin for histologic studies using various routine and special staining methods (Hematoxylin and eosin stain, Phosphorwolfram hematoxylin, Masson' trichrom stain, modified Heidenhain iron hematoxylin etc.).Selected tissue blocks positive for CMC were examined by immunohistological methods for the expression of specific markers of muscle tissue (Desmin, Sarcomeric actin, Smooth muscle actin, Calponin); furthermore, after proper osmification the tissues were examined using an electron microscope.

RESULTS AND DISCUSSION
The occurence of cellular elements of myocardial tissue in the wall of the large pulmonary veins in man and animals was considered in the past to be a continual extension of cardiomyocytes (CMC) from the left atrial myocardium into the vessels wall in the region of the veno-atrial junction 1 .In recent years the histological and experimental findings of CMC (autonomous rhythmogen and/or contractile ones) in the walls of pulmonary veins have also been reported by several authors 2,3,4 .At present, this problem assumes particular interest in that the heterotopic CMC may be the source of arrhythmogenic activity and subsequently of atrial fibrillation 5,6,7 .
The findings of heterotopic CMC in pulmonary veins have been most of them described in the region reaching from the left veno-atrial junction in a distal direction to the lung hilus.From our observations, heterotopic CMC in some laboratory animals were also present in the intraparenchymatous ramification of the pulmonary vessels.This fact has permitted us to make some assumptions about their dysontogenetic origin.It was suprisingly noted, that CMC were present focally in certain areas only in the ramification of pulmonary blood      vessels.In some cases it was quite difficult to differentiate morphologically lesions of the intraparenchymatous segment of the venous and arterial wall.Immunohistology showed marked positivity of CMC in the reactions for sarcomeric actin and desmin, whereas expression of smooth muscle actin (SMA) concerned the smooth muscle cells in vessel wall and bronchial structures.These markers (including calponin) were used previously for grading the CMC maturation in prenatal rat hearts 8 .
Electron microscopy showed normal endothelium of pulmonary vessels and typical morphology of CMC in their walls -i.e. the myofilaments, cross striation, Zbands and especially the intercalar discs, characteristic of myocardial tisuue.
The extent of CMC, which were present in the intraparenchymatous ramification of pulmonary blood vessels were not quantified or stereotopically studied.Apart from typical CMC in the wall of large pulmonary veins, some thin ("piloid") myocytes in the intima were also noticed; these probably smooth muscle elements contained some myofilaments and showed also strong coexpression of desmin (v.s. possible dual differentiation).
According to the literature, the heterotopic CMCs in the walls of pulmonary veins may represent a source of arrhythmogenic activity with the possibility of consequent atrial tachyarrhythmia or fibrillation.Chen et al. 3 reported in 40% of isolated CMC in dogs a rhythmogenic ("pacemaker") activity.The heterotopic occurence of CMC in the walls of pulmonary veins is at present a matter of studies, oriented toward the clinical problem of atrial fibrillation.

Fig. 6 .
Fig. 6.Ultrastructural feature of a cardiomyocyte in the wall of a pulmonary vein in experimental mouse.Myofibrilar structures and characteristic periodicity of fibers are clearly discernible.E-14283, EM-23016, Orig.magn.8000×.

Fig. 5 .
Fig. 5. Electron microscopic feature of the inner portion of a branch of pulmonary vein in experimental white mouse.Endothelial cell and longitudinal and circular layer of heart muscle cells are documented.E-14283, EM-23010, Orig.magn.8000×.