## 3 Results  
One agglomerate of well-differentiated unilocular adipose cells was noticed when studying the serial histological sections from one black barbel's liver. Taking into account that 658 captured fish were analysed--in the context of a biomonitoring study--the estimated prevalence of the intrahepatic oddity in the Bregalnica black barbel population was 1.5%. The case fish was a two-year-old female with a body weight (BW) of 27.95 g, total length (TL) of 135 mm and liver weight (LW) of 0.68 g. The condition factor of the fish (K = 100 \(\times\) BW/TL\({}^{3}\)) was 1.36, and the hepatosomatic index (HSI = 100 \(\times\) LW/BW) was 2.43. During the necrops, neither the external nor the internal examinations revealed gross changes or deformities that could signal pathological conditions (Figure 1). In particular, the liver had a normal appearance, with the typical reddish-brown colour and no gross lesions. However, the microscopic examination of the liver revealed the presence of one \"island\" made of distinctively empty-looking cells (Figure 2A). The cell mass was encircled by normal liver parenchyma. When sectioned, in the microscopic slides, the cell agglomerate was oval-shaped. It did not protrude the surface of the liver because it was buried deep in the organ, and it was tiny, reaching a maximum diameter of \(\sim\)0.5 mm. The nodular lesion was perfectly circumscribed and consisted of well-differentiated and large adipocytes. Their individual size varied little from one another. They contained an unambiguous single roundish empty space that filled most of the unilocular fat cell's  
Figure 1: View of the necropsy procedure on the black barbel from the River Bregalnica. Gross lesions were not found on the body surface or in internal organs and cavities. Figure 2: Representative histological sections (**A–C**) across the intrahepatic adipose cell agglomerate. There is a well-marked frontier between the adipocytes (*) and the liver parenchyma (LP). The flattened nuclei of the adipocytes are perceivable at the cell peripheries (dashed arrows). At specific locations around the adipose cell agglomerate, lymphocytic-like infiltrates are noted (block arrows) in the close vicinity of venules (V). cytoplasm. Indeed, it should be stressed that during the preparation of the tissue for histology, which includes dehydration in ethanol and clearing in xylene, free lipids are expected to dissolve. Therefore, the cytoplasmic location where the major lipid droplet stayed, in vivo, appeared empty in histological sections. The cytoplasm was negative for PAS, excluding the idea that carbohydrates could have caused the empty space. The nucleus was squeezed towards the cell's periphery. The intercellular limits were prominent. At times, an interstitial connective tissue matrix could be seen. Lipoblasts and mitotic figures were not observed. The internal vasculature was made of a few erratic thin-walled capillaries. In specific smaller profiles of the adipose cell agglomerate, and at its margins, there were scattered groups of small roundish cells, with a morphology compatible with a leucocytic (mainly lymphocytic) infiltrate (Figure 2B,C). When the agglomerate profile was evidently smaller, we noted a relatively more extended infiltrate in the vicinity of a venue (Figure 2C). We saw no microparasites of any sort at the margin or inside the agglomerate. The agglomerate of adipocytes was peripherally associated with scant small venules (Figure 2C), but not any major blood vessels or intrahepatic pancreatic tissue, and it was situated far from the liver hilum. Thus, the agglomerate was not a pocket made of an extension of the extrahepatic (peritoneal) fat. It was perfectly well demarked from the normal hepatic parenchyma but not encapsulated by a distinct connective tissue band. The surrounding hepatocyte elements caused no prominent peripheral compression of the agglomerate.