Occurrence of cardiac muscle in the hepatic portal vein wall of the mouse and rat.

Cardiac myocytes have been shown to occur in the tunica media and adventitia at the region near the hepatic end of the portal vein of the mouse and rat, and have been studied by electron microscopy in the mouse portal vein. They measured 3-10 microns in breadth at their nuclear level, possessed centrally located nuclei, and were connected with each other by the intercalated disk. In these myocytes in the mouse portal vein, sarcoplasmic reticulum was represented by a rather simple and loose network of the anastomosing tubules. The membrane-bound granules, which closely resemble the atrial specific granules, were found in many of the mouse portal vein myocytes. Transverse tubules, 40-200 nm in diameter, were sometimes detectable at the Z line level. The nexus occupied about 3-5% of the whole junctional area between cardiac myocytes in the tunica media, whereas in the tunica adventitia the corresponding value was about 17%. Blood capillaries with fenestrated endothelium supplied the cardiac myocytes in the adventitia of mouse portal vein. The closest relationship between the adrenergic axon and portal vein cardiac myocytes was observed to be ca. 0.3 micron apart. The significance of these findings is discussed in relation to pulsations of the portal vein.