Changes of the liver histology and ultrastructure in aged Beagle dogs

Abstract

Objective To investigate the changes of liver histology and ultrastructure in aged Beagles dogs. Methods A total of fifteen healthy Beagles, 1.5 to 10.6 years old, were divided into middle-aged group (n=8, average (2.21±0.94) years old) and aged group (n=7, average (8.26±1.52) years old) according to age. After intravenous anesthesia, the liver tissues of dogs were taken to compare number and morphology of hepatocytes and extra cellular matrix (ECM) of two groups under light microscope with hematoxylin-eosin staining. The number density, morphology and area fraction of mitochondria, the morphology and area fraction of endoplasmic reticulum, the changes of number density and morphology of microbody, the thickness of hepatic sinusoidal endothelial cell as well as the density and the diameter of fenestrae, the changes of morphology and number of lipofuscin in two groups were compared under electron microscope. Independent sample t-test was performed for comparison between two groups. Results Under light microscope, hepatocytes were arranged radially around the central veins, especially obvious in the middle aged group. Hepatocytes were circular or oval, most were mononuclear or binuclear. The cytoplasma of hepatocyte in middle-aged group was dense eosinophilic pink. Vacuolar lipid droplets were small and scanty. The cell boundary was clear. In aged group, cytoplasmic vacuolization of hepatocyte was more obvious, bigger vacuolated lipid droplets were observed, eosinophillic granules were scattered in the cytoplasma, cellular boundary was blurry and the number of hepatocytes significantly decreased when compared with middle-aged group ((75.80±5.76)/high power field (HPF) vs (97.80±6.45)/HPF, t=–2.510, P=0.026). There were no obvious infiltration of inflammatory cells in the liver tissue of Beagles dogs in both middle-aged group and aged group. Compared with middle aged group, cell gap and ECM increased in aged group. Under electron microscope, mitochondria of normal hepatocyte was oval, round or horseshoe-shaped with double membranes, ridge was observed inside and distributed around nucleus. Compared with middle-aged group, the shape of hepatocyte mitochondria in aged group was varied, the number density of mitochondria significantly decreased (0.202±0.021 vs 0.248±0.040, t=–2.683, P=0.020) and the area fraction remarkably reduced (0.136±0.015 vs 0.202±0.019, t=–6.083, P<0.01). Endoplasmic reticulum was mesh laminated arranged, scattered flake distributed and was mostly seen around nucleus in flat or stacked shape. Compared with middle aged group, the area fraction of endoplasmic reticulum of hepatocyte in aged group was obviously reduced (0.006±0.001 vs 0.011±0.004, t=–3.338, P=0.005). Lipofuscin was shaped as round or irregular structure without membrane and pigment unevenly distributed inside. The number of lipofuscin in aged group was significantly higher than that in middle-aged group (0.028±0.025 vs 0.001±0.000, t=2.880, P=0.014). The number of microbody in aged group was slightly less than that of middle aged group and the difference was not statistically significant (P=0.603). The thickness of the liver sinusoidal endothelium in aged group significantly increased compared with that in middle-aged group ((0.242±0.047) μm vs (0.114±0.075) μm, t=3.881, P=0.002). The diameter and density of endothelial fenestration in aged group ((1.895±0.439) μm and (0.260±0.120)/μm) obviously decreased compared with those in middle-aged group ((3.186±0.332) μm and (0.723±0.184)/μm, t=6.345 and –4.511, both P<0.01). Conclusions There were significant degradations of liver in quality and quantity among aged individuals. These degradations were the structure basis of decreased hepatic reserve function and involved directly or indirectly in high prevelance of hepatic diseases among elders. Key words: Aging; Liver; Histology; Ultrastructure; Models, animal