Lipomas are benign tumors of fat cells (adipocytes) that present as soft, painless masses most commonly seen on the trunk, but can be located anywhere on the body. Lipomas usually range from 1- >10 cm. They are mesenchymal tumors and are found anywhere in the body where normal fat cells are present. They are benign and have many histologic subtypes.
Source : NIH Stat Pears, sept 2022
Though they have been documented and examined for centuries, the exact pathology which leads to the formation of lipomas remains unknown. There is some (disputed) evidence that they can be triggered by injuries, and it's generally thought that there could be genetic factors involved, as they do tend to 'run in families'.
One unusual indetifying characteristic of the tumours is that they're 'mobile', i.e. easily moved - because they're not attached to thefasciaplugin-autotooltip__plain plugin-autotooltip_bigThe Fascia
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Fascia are whitish bands or sheets of connective tissue - mainly composed of collagen and fat, which stabilise, enclose, and separate muscles and other internal organs.
An exact definition has been much disputed over the years. Aside from 'compartmentalising' the various organs etc, its functions are poorly described and not well understood.
Around 1 - 2 % of the global human population develops a lipoma at some stage. They're also found in other mammals.
As the paper cited above mentions, they are almost always entirely benign - although they can cause obstruction problems if they happen to form at crucial locations on internal organs. Treatment is usually in the form of minor surgery to remove them.