## 4 Discussion  
This study histologically described and 3D reconstructed an intrahepatic collection of white adipocytes in a black barbel. Besides the structural evidence for such identification, the negativity for PAS staining indirectly supported that the major cytoplasm content in vivo should have been lipids; using paraffin, direct lipid staining was impossible. The oval-shaped and well-demarcated focus of unilocular fat cells was deeply nested into the liver. The histological characteristics of the adipocyte focus, including the absence of lipoblasts, increased mitosis, scarcity of vasculature and association with lymphocytes, nicely matched detailed lipoma descriptions [32]. Objectively, the focus fulfilled the basic structural definition of a lipoma [1] and therefore was presumptively diagnosed as such. The good circumscription of the cell agglomerate favours a priori this diagnosis over another possibility: ectopic adipocytes. The presence of ectopic fat and ectopic adipocytes in unexpected locations is usually reported in human and experimental animal pathology in strict association with caloric intake (high-fat diets) exceeding expenditure, obesity, insulin resistance and type 2 diabetes mellitus [33,34]. Still, despite the fact the liver may accumulate ectopic fat (within steatotic hepatocytes), it is not among the organs where ectopic adipocytes typically appear under triggering conditions [35,36]. In the case revealed here, it is virtually impossible that just one wild barbel would have been consistently subjected to a high-fat diet, which, even so, would have sparked steatosis (not observed here) well in advance of ectopic adipocyte differentiation and proliferation. Therefore, the structural and circumstantial evidence favours the lipoma diagnosis over a pocket of ectopic adipocytes. Anyway, it could be questioned if such a small focus of fat cells can be diagnosed as a lipoma in barbels. However, the literature does not specify a minimal size for an abnormal agglomerate of fat cells to be deemed as a lipoma. In humans, the size of (clinically detected) lipomas varies between 2 and 10 cm in diameter [36]. Yet, such benign neoplasms are very slow progressing. Therefore, the macroscopic tumours derive from earlier and much smaller (micro)lipomas, as illustrated well in intramucosal lipomas [37]. According to the predominant histological morphology, the World Health Organization classification of tumours of soft tissue considers the following lipoma types: conventional lipoma (the most common by far), fibrolipoma, angiolipoma, lipomatosis of nerve and lipoblastoma [38]. Since the liver lipoma in the studied black barbel consisted predominantly of mature adipocytes, without an excess of fibrous tissue or of substantial vascular involvement, it should be classified as a conventional lipoma. Lipomas are rarely reported in fish, and, as far as we know, it seems that a lipoma has only been reported once in the liver [25]. Unlike the present findings in the black barbel, where the lipoma was noted only after histological examination, the liver lipoma found in the marine coly was visually apparent and noticed in the lower lobe of the liver, having a smooth surface and greenish colour [25]. Additionally, this liver lipoma was found in a seven-year-old specimen. Lipomas are generally noticed in older individuals. Their incidence increases with age in domestic animals [39] and in humans [40]. Contrarily, in the current case, the lipoma was noticed in a two-year-old female, an adult but not a particularly old fish. It is common to find specimens of _B. balancius_ aged over two years [41,42], and, in general, barbels can live for more than 20 years [43]. The cause(s) of lipoma development in animals is (are) far from clear, but several theories were advanced, mostly from studies in humans. Some authors proposed genetic causes while favouring trauma as the aetiology [44], along with a diversity of risk factors, such as obesity, ethanol abuse, glucose intolerance and liver disease [40].