Tip of villi,
are lined by a columnar epithelium, having a clear
cytoplasm.
High power
view, enterocytes are stuffed with clear vacuoles.
Closer view
of the intestinal epithelium
The
epithelium gradually becomes normal towards the crypts.
Closer view
on the previous field: clear cytoplasmic vacuoles, are
negative to special stains in the processed tissues.
DIAGNOSIS
Enterocytic
lesions with preserved intestinal villous architecture
consistent with abetalipoproteinemia.
Abetalipoproteinemia
is a rare
group of inherited disorders of childhood characterized
by an absence or a deficiency of apoprotein B, which is
required to synthesize and transport very low density
lipoproteins and chylomicrons. This results in normal
absorptive cell uptake of fatty acids but inability to
synthesize them into chylomicrons. The absorptive cells
become stuffed with fat . Ultimately, fat is unable to
enter the absorptive cells and steatorrhea ensues.
Pathogenesis
The mechanism
of chylomicron formation and secretion in the gut is
complex. Following lipolysis of dietary fat in the gut
lumen, there is passive diffusion of fatty acids into the
endoplasmic reticulum of enterocytes. Here chylomicron
assembly occurs in the presence of phospholipids,
cholesterol, and a number of apoproteins. The
chylomicrons are then assembled in the Golgi apparatus
and extruded by exocytosis into the intercellular spaces
or intestinal lymphatics. To date, abnormalities have
been described in apoprotein production and from a defect
in the final assembly of chylomicrons in the mechanism of
exocytosis.
The small
bowel mucosa has a characteristic appearance. The upper
two-thirds of the villi contain epithelial cells that
appear vacuolated and clear-staining.
There is one
case report of a patient with a generalized neutral lipid
disorder in whom there were jejunal changes similar to
abetalipoproteinemia. However, this patient also had
lipid accumulation in other parts of the gastrointestinal
tract, such as the gastric glands, and in many other
tissues, such as liver, striated muscle, and granulocytes.
Another
potential source of diagnostic confusion arises when an
infant with suspected abetalipoproteinemia has a biopsy
taken shortly after feeding. In the postfeeding state
with a meal containing fat, vacuoles of absorbed fat may
normally be seen in villous tip epithelium. This
confusion can usually be resolved easily by taking an
appropriate history and by avoiding feeding the infant
for at least 6 hours prior to biopsy. Vacuolar change is
also found in severe mucosal lesions, for example, celiac
sprue. In these cases, the morphologic features of a
severe mucosal lesion are clearly seen and should not
pose any diagnostic problem.
The other
manifestations of this disorder are retinitis pigmentosa,
acanthocytosis of red blood cells, ataxia, and
mental retardation. Acanthocytosis noted in smear of the
peripheral blood.
Variation in
size and form of RBC in the vessels noted in the duodenal
biopsy (left) are due to fixative agents, are also
observed in biopsies without acanthocytosis (right,
Giemsa stain)
