Department of Cardiology，Tangdu Hospital，Fourth Military Medical University，Xi'an710038，China;

    2 Department of Anatomy，Medical College，Xi'an Jiaotong University，Xi'an710061，China

    Keywords:myocardial sleeve;P-like cell;atrial fibrillation;dog

    Abstract:AIM To investigate the basis on which ectopic beats and focal atrial fibrillations originating from the pul-monary vein.METHODS We studied5adult dog hearts.A block of tissue encompassing left atrials and pulmonary veins was dissected.Serial sections from this tissue were stained with hematoxylin and eosin，Masson's trichrome.Then slides were examined by light microscope.RESULTS Car-diac muscles of left atriums extended onto pulmonary veins(myocardial sleeve).Myocardiums in the pulmonary veins course by various directions sometimes appeared internal cir-cular and external longitudinal，sometimes appeared only longtitudinal or oblique.Pulmonary vein walls of dogs were composed of3layers:the tunica intima，tunica media and tu-nica adventitia.P-like cells scattered in the myocardium sleeve，and mainly distributed in the middle part of myocardi-um sleeve and almost were located in the superior pulmonary veins.No apparent morphological differences were found be-tween myocardical cells in the pulmonary veins and myocar-dial cells in the left atriums.CONCLUSION Ectopic beats from pulmonary veins triggering atrial fibrillation may be the results of P-like cells in the myocardial sleeves in the pul-monary veins. ......