Cardiac musculature ofthecranial vena cavainthecommon treeshrew(Tupaia glis)

Cardiac musculature ofthecranial venacavainthecommontree shrew(Tupaia glis) wasexamined bylight andtransmission electron microscopy. Thecommontree shrew haswell developed cardiac myocyte layers in thetunica mediaofthecranial venacava, extending fromtheright atrium totherootofthesubclavian vein. Because thecommontree shrew belongs toaprimitive groupofmammals, theoccurrence ofcardiac musculature inthecranial venacavamaybeacommonfeature inlower mammals. Thedevelopment ofthis musculature indicates that active contraction ofthecranial venacavawalloccurs inthis species. Electron micrographs showed thetypical ultrastructure ofmyocytes andnerve endings. Theseobservations suggest that this musculature mayserve asaregulatory pumpforthereturn ofvenous blood totheright atrium and asablood reservoir system under conditions ofrapid heart rate. Additionally, thepresence ofatrial natriuretic polypeptide (ANP)wasalso demonstrated inthemyocytes ofthevenacavaimmunohistochemically. These findings showthat thecardiac endocrine organforANPdevelops eveninthe principal veins including thecranial venacava.