Definition

Oval fat bodies are cells with birefringent fat droplets within their cytoplasm. Under low power magnification, oval fat bodies are often seen as large brown spots (sometimes almost black). This coloration is due to the yellowish brown pigmented fat making the droplets. These cells are usually seen in a context of heavy proteinuria.

The lipiduria manifests itself as lipid droplets:

• free as the birefringent fat droplets
• intracellular in the oval fat bodies
• imbedded within a cast matrix in the fatty cast.

Origin of the fat droplets

Fat droplets can originate from a vacuolar fatty degeneration of intracytoplasmic membranes. This fatty degeneration phenomenon is frequent. Thus, cells kept for several days in a urine develop all kinds of vacuoles. In some cases, droplets are quite similar to those found within the oval fat bodies, except for the birefringence. The intracytoplasmic droplets can also originate from phagocyted material followed by an intensive lysozome digestion.

True oval fat bodies show a typical "maltese cross" interference pattern when viewed under polarized light. This interference is a crystalline structure property, and is due to the presence of esterified cholesterol in a liquid crystal state. Naturally, cholesterol-free droplets are not birefringent. It is possible to stain the fat droplets with a suitable fat stain like Sudan, Fat Red 7B, and others. To our view, staining is of little use, since the birefringent criteria is easy to observe and that staining has a tendency to stain degenerated cells that are erroneously interpreted as oval fat bodies.

Some fat droplets lose their birefringent characteristics. It is sometimes possible to correct the crystalline disorganization by gently heating the slide, followed by a rapid cooling.

Clinical value

The nature of oval fat bodies is debated. For Schumann, the cell is an oval renal proximal tubular cell with a fat droplets filled cytoplasm. For Stamey, oval fat bodies are in fact macrophages also known as foam cells. The latter has proposed the use of the term oval fat macrophages. This apparent controversy is probably due to the fact that both possibilities exist.

Oval fat bodies, in a high proteinuria context, are associated with the nephrotic syndrome (nephrosis). The link between the nephrotic syndrome and lipiduria is not known. Lipiduria seems to be related to the proteinuria, and not to plasmatic lipids level. Some have suggested that the lipiduria could be a consequence of a specific apolipoprotein accessing, like the albumin, to the urinary space through the glomerule. Filtrated free fatty acid adsorbed to the urinary albumin could play a role in the intracytoplasmic accumulation of fat by the renal proximal tubular cell. (Oval fat bodies are often seen with the droplets filling one side of the cell)

Oval fat bodies are not specific to the nephrotic syndrome. These cells are sometime seen in specimens with a normal proteinuria. This situation can be explained by the presence of fatty macrophages often seen in chronic inflammation sites.

Stamey has reported the presence of oval fat macrophages in seminal fluid with patients having prostatitis.
Foam cells can be seen in many human fluids: bile, bronchial...

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