The morphology of the canine (Beagle) liver

In spite of an extremely large volume of literature concerning the microscopic and sub-microscopic morphology of the mammalian liver, most investigations have been carried out with only a few ‘representative' species and there has been no definite statement of the nature of the canine liver. In the present study, the liver of the purebred Beagle was examined by means of light microscopy, graphic reconstruction, and corrosion cast techniques to: (1) test the validity of the ‘lobule' concept of liver structure; (2) describe the relationships band patters of distribution displayed by the hepatic sinusoids and vasculature; (3) enable comparison of the canine liver with that of other species, especially those which have been used as models for mammalian liver structure; (4) establish the structural (architectural) principle of the liver. Serial histological sections were examined and an attempt was made to produce a plastic reconstruction of a liver lobule. However, the absence of anatomically-definable boundaries mad accurate reconstruction impossible. It was determined that the lobular pattern observed on section of liver tissue is an illusion resulting from blood pressure gradients and the regular interdigitation of the afferent and efferent vessels at uniform distances. This latter feature is clearly evident in the corrosion cast preparations. Graphic reconstructions of the channels for the hepatic vasculature support the theory that the attainment of the most regular interdigitation of the afferent and efferent vessels is the true principle of hepatic architecture. This concept was determined to be consistent with the functional aspect of maintaining uniform blood distribution in the hepatic parenchyma. The corrosion cast studies suggested the occurrence of arteriovenous anastomoses, but were inconclusive. Graphic reconstructions of the hepatic sinusoids from serial sections showed the sinusoids form a highly complex and anastomosing labyrinth which is continuous throughout the hepatic parenchyma. Electron microscopy studies of the ultrastructure of the perinsinusoidal space of Disse and the endothelial lining of hepatic sinusoids in several non-canine species have shown that specie differences do exist among mammals. In the present study, biopsy and necropsy specimens of Beagle livers examined via electron microscopy showed the endothelial lining of the hepatic sinusoids to be predominantly continuous. However, many openings occur in the lining, most of which are patent channels allowing direct communication between the sinusoidal lumen and the Disse space. Openings occur both as intercellular gaps and as intracellular fenestrations composing ‘sieve-plate' formations in the cytoplasm of the endothelial cells. Diaphragms were occasionally observed occluding the intracellular fenestrations, but the majority is patent. A definitive basement membrane was not observed beneath the endothelial cells. The Disse space is almost completely occupied by the microvilli of the hepatic cells. Bundles of collagen fibrils are frequently encountered in the perinsinusoidal space, becoming more numerous toward the periphery of the ‘classical' hepatic field. "Fat-storing cells" of Ito (Folia anat. Jap. 24:243-258, 1952) occurred only occasionally in the Disse space.