Unlike other rat veins of comparable diameter, the smooth muscle cells of the extrahepatic segment of the portal vein are arranged in two distinct mutually perpendicular layers. The relatively narrow inner layer is nearly circumferential and devoid of vasa vasorum; the cells resemble those seen in other veins. In the longitudinal outer layer, vasa are abundant and the cells contain unusual numbers of mitochondria and abundant pinocytotic vesicles, indicating relatively increased metabolic activity. Between the layers, the connective tissue contain; nerve fibers, suggesting coordinated reflex activity.During a study of the repercussions of hepatic injury on the ultrastructure of hepatic blood vessels in the rat, we observed that the normal architecture of the extrahepatic segment of the portal vein is distinctly different from that of other rat veins of comparable diameter. Distinguishing features of portal vein structure, discernable by light microscopy, were noted in human material several years ago by Li ('40); there have been no reports of electron microscopic studies of this vessel. We believe that the special architectural features of the portal vein are worthy of note. Details concerning differences in ultrastructure of various veins could provide insight into the relation of structure to function in vessel walls. Knowledge of the special morphology of the normal portal vein could facilitate interpretation of the hyperplastic and sclerotic portal venous changes associated with cirrhosis or chronic congestion of the liver (Li, '40; Hou and McFadzean, '65). It will be shown that the portal vein of the rat consists of two distinct smooth muscle layers separated by fibrous and neural elements. The inner circumferential layer is shallow and devoid of vasa vasorum; the outer, thicker, longitudinal layer is richly vascularized and its component cells show evidence of markedly increased metabolic activity. MATERIALS AND METHODSAdult Sprague-Dawley rats weighing 220-280 gm and of both sexes were studied. Dissection and preparation of various veins was performed through a ANAT. REC., 166: 529-540.longitudinal midline incision with the animals anesthetized by ether inhalation. The portal vein was exposed by lifting and retracting the liver gently. A no. 21 guage syringe needle was then inserted through the vein wall and the lumen flushed slowly for one minute with 10 ml of a 5% phosphate-buffered (pH 7.3) gluteraldehyde solution at 2-4°C. The vein segment was then resected, and cut into longitudinal and transverse strips 0.1 cm wide and 0.3 cm long while immersed in cold gluteraldehyde fixative. Fixation was then continued in cold gluteraldehyde at 4°C for two hours. After three washings in buffered phosphate solution, supplementary fixation in chromium buffered osmium tetroxide solution at pH 7.3 was continued for 75 minutes. Dehydration was performed in graded ethanol solutions and Fropylene oxidme; epon was used for infiltration and embedding was carried out in flat embedding molds. Similar preparations we...

In all mammalian species examined thus far, the media of the adult mammalian portal vein consists of two mutually perpendicular smooth muscle zones separated by a fibrous layer. The cells of the muscle zone nearest the intima are arranged circumferentially and resemble smooth muscle of other vessels; the muscle cells of the thicker, vascularized outer zone are longitudinal and contain mitochondria and pinocytotic vesicles in great abundance, suggesting relatively high metabolic activity. The adult configuration is not present at birth and only develops during the first three postnatal weeks in the rat. Partitioning of the media commences at about three days and contrasting orientation and composition of the cells of the two muscle zones is not established until seven to ten days after birth. Vasa vasorum appear in the outer muscle layer at about two weeks and nerve fibers appear even later. Post-natal establishment of a special, double layered neuromuscular structure may be related to the adaptation of the portal circulation to the effective closure of the ductus venosus.