Pathologists involved in the examination of small rodent tissues are familiar with the presence of striated muscle in walls of intrapulmonary veins. Because these striated muscles express cardiomyocyte markers and are involved in rhythmical contraction during systole, this morphologic arrangement was named "pulmonary myocardium". Striated cardiac muscles of intrapulmonary veins have also been found in numerous species, including non-human primates. Great attention has been given to animal striated pulmonary veins because in humans, the presence of cardiomyocytes in the pulmonary veins (myocardial sleeves) is a major origin of paroxysmal atrial fibrillation. It is unequivocally suggested that direct extension/migration of cardiomyocytes from the left atrium into pulmonary veins is the origin of "pulmonary myocardium". This model sounds logical in regard to humans and large mammals because in these cases, striated myocytes extend into pulmonary veins only by 10-30 mm from the left atrium. However, the cardiomyocyte direct extension/migration model becomes less parsimonious when applied to the numerous small intrapulmonary veins with striated muscles in their walls in small mammals. In 1972, Alfred Sherwood Romer, renowned for his contributions to the study of vertebrate evolution, suggested that visceral smooth muscle cells developed a striated phenotype due to the need for more efficient musculature. Romer theorized that the primary anatomical division of the vertebrate muscular system should be not into smooth and striated types but into somatic and visceral systems, because the line of division along the gut between striated and smooth musculature is not a fixed point. Romer's work offered a parsimonious model for the presence of striated muscle in the walls of the intrapulmonary veins-visceral smooth muscle cells, which are capable of differentiating into striated muscle cells, are always present; whether they differentiate depends on functional demands. From Romer's hypothesis, it also could be foreseen that striated muscle could appear in visceral compartments other than the blood circulatory system and pharyngeal muscles to facilitate functional needs. Indeed, it was shown that 1) the lymphatic system of many low vertebrates acquired a lymphatic heart with striated muscle cells; 2) some vertebrates also acquire striated muscles in tunica Muscularis of the stomach and intestine. The facts above undoubtedly corroborate Prof. Romer's notion on the evolution of the vertebrate muscular system.
Keywords: Vertebrates, evolution, visceral organs, smooth muscles, striated muscles, intrapulmonary veins, lymphatic hearts, Alfred Sherwood Romer