The relationship between spontaneous cystic degeneration and pseudocapillarization in sinusoids in the liver of aged Sprague-Dawley rats

Abstract

Cystic degeneration (CD) in the liver is a cyst-like lesion composed of one or more pseudocysts lacking lining cells, occurring spontaneously in rats older than 12 months, with a male predilection. In this study, 32 CDs were identified in 23 out of 104 non-treated, control male Sprague-Dawley rats from two combined chronic toxicity and carcinogenicity studies with agrochemicals. They were examined histologically, histochemically, and immunohistochemically to assess the pathogenesis and pathological significance of CD, focusing on pseudocapillarization in aged rat liver. Pseudocapillarization refers to age-related capillarization of hepatic sinusoids and is distinct from sinusoidal capillarization observed in hepatic cirrhosis. Both CD and pseudocapillarization, characterized by factor VIII-related antigen expression, were primarily noted in the periportal regions of the rat liver. CD areas exhibited enhanced vimentin expression in a diffuse linear pattern in their septa with occasional focal linear α-smooth muscle actin expression and the fluid containing hyaluronic acid accumulated in their lumen that are thought to be formed by hepatocellular apoptosis. These findings suggest a series of reactive changes associated with hepatocellular apoptosis due to pseudocapillarization in the sinusoids. In conclusion, spontaneous CD in rat liver is not a degenerative lesion or cystic enlargement of stellate cells, but a structural abnormality in pre-existing liver tissue resulting from aging-related changes in sinusoidal endothelial cells and hepatocytes. Pseudocapillarization of sinusoids is considered a precursor lesion of CD in the rat liver.

Keywords: aging; cystic degeneration; liver; pseudocapillarization; rat; spongiosis hepatis.

Fig. 1.

(a) A typical histology of large cystic degeneration (CD) area. CD area was composed of many pseudocysts and some hepatocytes (arrows) were present within CD area. (b) High magnification of Fig. 1 (a). The pseudocysts without lining cells show various sizes and morphologies and contained fine granular to flocculent eosinophilic material. The septa of CD area were very thin and condensed erythrocytes (arrows) were noted. Two types of constituent cells could be recognized as hyperchromatic cells and hypochromatic cells (arrowheads) based on the amount of chromatin. (c) Apoptotic cells were observed within small CD area (thin arrows). CD area was surrounded by hypereosinophilic hepatocytes with condensed nuclei (thick arrows) and slit formation (arrowheads) was observed in the septa. (d) Small cystic spaces (arrows) were detected around the portal tract in non-CD-area. Some of those spaces contained apoptotic hepatocyte (arrowheads), a few inflammatory cells and eosinophilic materials. P: portal tract, Bar=(a) 200 μm, (b–d) 50 μm. HE stain.

Fig. 2.

(a) Alcian blue positive materials (arrows) accumulated in the cystic spaces around the portal tract (P), (b) Lipofuscin deposited in sinusoidal endothelial cells (arrows) but was unclear in Kupffer cells and hepatocytes. Bar=(a, b) 50 μm. (a) Alcian blue stain, (b) Schmorl reaction.

Fig. 3.

Immunostaining results of CD area. (a) Vimentin expression (arrows) was detected diffuse linearly in the septa of CD area. (b) α-SMA immunoreactivity (arrows) were observed multifocal linearly within CD area. (c) Capillarized sinusoids (arrows) were detected in CD area. (d) Elongated macrophages (arrows) in the septa of CD area and round-shaped macrophages (arrowheads) in the lumen of CD area. In non-CD areas, α-SMA expression (arrows) was detected in sinusoids around the portal tract and their expression patterns were associated with cellular reaction (e) or not (f). (g) FVIII-RAg expression (arrows) was observed in sinusoids in the perilobular region in non-CD area. Immunostainings with anti-vimentin (a), α-SMA (b, e, f), FVIII-RAg (c, g), and macrophage (clone name: ED-1) (d) antibodies. P: portal tract. Bar=(a, b, e, f) 50 μm, (c, d, g) 100 μm.

Fig. 4.

A shematic drawing showing the possible pathogenesis of CD. (a) Normal histological relationship between sinusoids, hepatic cord, and the space of Disse. Sinusoidal endothelial cells (SECs) have many fenestrations and quiescent stellate cells reside in the space of Disse. (b) Aging changes in the sinusoids and the space of Disse. Increased deposition of collagen fibers (blue lines) and secretion of HA (orange arrows) by activated stellate cells and BM formation (red lines) by thickened SECs without fenestrations in the space of Disse and hepatocellular apoptosis. (c) The space left by hepatocellular dropout owing to hepatocellular apoptosis was dilated by fluid retention after HA accumulation. The pseudocyst wall consists of SECs (blue cells) lined by BM and/or collagen fibers. Kupffer cells (orange cells) and stellate cells (green cells) are enclosed and stretched in the septa due to compression secondary to HA accumulation and fluid retention. SECs, sinusoidal endothelial cells; HA, hyaluronic acid.