Lipoma pathophysiology

Lipoma is formed from mature adipocyte cells and is histologically indistinguishable from fat tissue. Recent studies have observed cytogenetic abnormalities in 50-60% of lipomas, suggesting its role in the lipoma pathogenesis. Rearrangements of chromosome 12 are the most commonly cytogenetic abnormality. In a cross-sectional appearance, the lipoma is pale yellow to orange and has a uniform greasy surface with a lobular pattern. The subcutaneous lipoma is usually encapsulated and has a distinct lobulated pattern.

• Lipoma is formed from mature adipocytes and is histologically indistinguishable from fat tissue.
• Recent studies have observed cytogenetic abnormalities in 50-60% of lipomas, suggesting its role in the lipoma pathogenesis.^[1][2]
• Rearrangements of chromosome 12 is the most commonly cytogenetic abnormality.
• The DDIT3 gene, located on the long arm of chromosome 12, has been suggested to play a role in adipocytic differentiation.^[3]
• This gene encodes a protein family, known to be enhancer binding protein.
• Members of this protein family are highly expressed in fat and are engaged in the growth arrest of fat cells.

• Genetic abnormalities associated with lipoma pathogenesis include:^[4]
  • Translocations in 12q13-15
  • Interstitial deletions of 13q
  • Rearrangements in 6p21-23

• Gross pathology of lipoma may differ depending on anatomical location.^[5]
• Subcutaneous lipoma appear as a a soft, well-circumscribed rounded mass with different size ranging from millimeters to 5 cm or more.
• Lipomas larger than 10 cm are not common.
• In cross-sectional appearance, the lipoma is pale yellow to orange and has a uniform greasy surface with a lobular pattern.
• Lipoma is usually encapsulated.
• Focal hemorrhage or fat necrosis may also occur, but it is much less common than in liposarcomas.
• Intramuscular lipomas are usually larger than subcutaneous lipomas and may be encapsulated or diffuse.^[6]
• Grayish streaks of fibrous tissue may also be seen in fibrolipoma which has a firmer texture compared to ordinary lipoma.

• Microscopically, lipomas are formed from mature fat cells.^[8]
• There is no nuclear hyperchromasia.
• Subcutaneous lipoma is usually encapsulated and have a distinct lobulated pattern.
• Cystic like changes may also be seen in lipoma due to impaired blood supply.
• Infection or trauma may cause fat necrosis or local liquefaction of fat, which may appear with phagocytic cells and formation of lipid cysts.
• A possible benign finding in lipoma is the collection of histiocytes and the vacuolization of the nucleus.^[9]
• Intramuscular lipoma may be well-demarcated from the adjacent muscular tissue or have a diffuse pattern with alternating adipocytes and skeletal muscle cells.

• Lipomas may have following immunohistochemical markers:^[12]
  • Vimentin
  • S-100 protein
  • Leptin

• 
  Histopathologic slide of renal angiomyolipoma. Nephrectomy specimen. H & E stain.^[13]
• 
  Histopathologic slide of renal angiomyolipoma. Nephrectomy specimen. H & E stain.^[13]
• 
  Histopathologic slide of renal angiomyolipoma. Nephrectomy specimen. H & E stain.^[13]
• 
  Histopathologic image of renal angiomyolipoma. Nephrectomy specimen. HMB-45 immunostain.^[13]

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