CONTEXT: CD31 (platelet-endothelial cell adhesion molecule-1; PECAM-1), an 
adhesion molecule involved in the process of angiogenesis, is used as a marker 
of normal and neoplastic vascularization. During the assessment of angiogenesis 
and vascular invasion in a thymic carcinoid tumor, we observed unexpected 
immunostaining for CD31 in perithymic brown fat nests.
OBJECTIVE: To determine whether CD31 is expressed by normal and neoplastic cells 
of brown fat, a tissue whose thermogenetic activity depends heavily on high 
perfusion.
DESIGN: Formalin-fixed, paraffin-embedded archival tissues were immunostained by 
the labeled avidin-biotin method using antibodies against CD31 (clones JC70A and 
1A10) after retrieval of heat-induced epitopes. Archival tissues included 
perithymic, periadrenal, axillary, and neck adipose tissue in which were 
embedded nests of brown fat (n = 15), hibernoma (n = 3), lipoma (n = 6), 
well-differentiated liposarcoma (n = 4), and myxoid liposarcoma (n = 4).
RESULTS: Invariably, multivacuolated and univacuolated adipocytes of normal 
brown fat and hibernomas were intensely positive for the CD31 antigen. The 
immunostaining "decorated" cell membranes and the membranes of intracytoplasmic 
vacuoles. No expression of CD31 was found in normal adipocytes of white fat, in 
neoplastic cells of lipomas, or in multivacuolated lipoblasts of 
well-differentiated and myxoid liposarcomas.
CONCLUSIONS: The spectrum of cell types that express CD31 is expanded to include 
normal and neoplastic brown fat cells. We speculate that the expression of CD31 
may play a role in the development and maintenance of the vascular network 
characterizing this specialized adipose tissue. Moreover, CD31 may inhibit the 
Bax-mediated apoptosis of brown fat cells. For practical purposes, CD31 may be 
used as an immunohistochemical marker for distinguishing between white and brown 
fat and for diagnosing hibernoma in paraffin sections.