EVIDENCE OF ADIPOCYTE DIFFERENTIATION IN HUMAN ORBITAL FIBROBLASTS INPRIMARY CULTURE

Graves' ophthalmopathy (GO), in which orbital tissues are infiltrated with activated T lymphocytes and hyaluronan, can manifest an overabund ance of adipose tissue in the human orbit. Little is known about adipo genesis in this anatomic region. We have investigated whether orbital fibroblasts in culture possess the capacity to undergo adipocytic diff erentiation. Orbital tissue from patients with or without GO was place d in primary culture, and proliferating fibroblasts were sub-passaged Confluent fibroblasts were subjected to a differentiation protocol, in volving a serum-free defined medium supplemented with insulin, triiodo thyronine, carbaprostacyclin, thyrotropin, dexamethasone, and isobutyl methylxanthine. Control cells were maintained in serum-free medium sup plemented only with insulin. After approximately 14 days, light micros copy revealed characteristic morphologic changes of adipocyte differen tiation, including cell rounding and lipid droplet accumulation. Oil R ed O staining could be demonstrated in those cells. Scanning electron microscopy revealed that the cells undergoing adipogenesis contain mul tiple, discrete lipid droplets. The overall percentage of fibroblasts undergoing differentiation was somewhat variable, but no more than 5-1 0%. The adipocytes arise in close proximity to each other, and often a t the periphery of the culture surface. In contrast, dermal fibroblast s and perimysial fibroblasts from extraocular muscle fail to different iate. We conclude that a subpopulation of orbital fibroblasts is capab le of adipocyte differentiation. Theses in vitro observations may repr esent the in vivo process that contributes to excess orbital adipose t issue volume in GO.