Background: There is a need to identify the existence of large fenestrations and to evaluate the type and pattern of the distribution of sinusoidal fenestrations in the hepatic lobule.
Methods: The sinusoidal fenestrations in the rat liver lobule were investigated by scanning electron microscopy of perfusion-fixed specimens and by transmission electron microscopy of freeze-fracture replicas from immersion-fixed specimens. Ringer solution used for precleaning was equilibrated with 84%/16% O 2 /CO 2 at 37ยฐC, and the perfusion pressure was maintained at 5-10 mmHg in the portal vein.
Results: The fenestrations were recognized on the luminal surface of sinusoids on the basis of regional difference in size and distribution within the lobule and divided into three types: small, medium-sized, and large. The small fenestrations were numerous in each region. The number of medium-sized fenestrations was large in the pericentral and intermediate regions, while that of large fenestrations was predominant in the intermediate region. Based on the occurrence of fenestrations, the sinusoids were also divided into three types. The type I sinusoid mainly showed small fenestrations and was predominant in the periportal region. The type II sinusoid showed small and medium-sized fenestrations and was predominant in the pericentral region. The type III sinusoid showed all the types of fenestrations including large ones and was predominant in the intermediate region. The images showing dumbbell-shaped or irregularly shaped margins were observed between small or medium-sized fenestrations on the freeze-fracture replica.
Conclusions: 1) The fenestrations are divided into three types according to the size and pattern of distribution within the lobule. 2) The sinusoids are also divided into three types based on the distribution pattern of each type of fenestrations. 3) The small fenestrations are evenly distributed throughout the entire lobule, while the medium-sized and large fenestrations show the regional difference within the lobule. 4) The large fenestrations exist as real structures of the endothelial lining cells of sinusoids and may result from the fusion of contiguous smaller ones.