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.
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
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.
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.
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...