In this connective tissue component, the orbit becomes inflamed, and infiltrated with T and B lymphocytes and mast cells [38]. the possibility that they might be useful targets against which therapeutic agents might be aimed. Keywords:connective tissue, Graves’ disease, inflammation, molecular biology, ophthalmopathy == Introduction == Fibroblast heterogeneity has been appreciated for several decades but its biological significance and the basis for cellular diversity remain uncertain. The question of why fibroblasts from distant anatomical regions should exhibit phenotypic divergence is unanswered. Moreover, a mechanism that might account for dissimilar Gilteritinib hemifumarate fibroblasts inhabiting specific tissues has not been Gilteritinib hemifumarate identified. Three connective tissue depots from which fibroblasts have been studied with considerable rigour include lung, joint and orbital connective tissue [14]. The origins and phenotypic characteristics of the fibroblasts found in these tissues have become increasingly important as investigation into the nature of organ-specific autoimmune diseases proceeds. The concept that localization of systemic diseases could result, at least in part, from the peculiarities exhibited by fibroblasts in affected tissues continues to appeal to substantial discussion. However, significant advances have been made recently in our ability to distinguish between similarly appearing cells with fibroblast-like morphologies. Despite these new insights, substantial imprecision persists in identifying the diverse biological roles of cells that resemble each other. At the heart of the problem lingers the absence of a single, specific marker that could distinguish fibroblasts from all other cells. Once characterized, such a protein would undoubtedly prove invaluable in elucidating more clearly the molecular mechanisms and cellular interactions that underlie normal and pathological tissue remodelling. == The orbital fibroblast: an example of cellular diversity == Orbital fibroblasts comprise a heterogeneous population of cells that can be separated into discrete subsets based on their display of surface markers [5]. The most frequently studied of these is Thy-1, which has been used by several investigators to discriminate between those fibroblasts that can differentiate into myofibroblasts (Thy-1+) and those capable of becoming adipocytes (Thy-1-) [6,7]. This assignment is also true for fibroblasts from lung [8,9]. When Thy-1+fibroblasts are exposed to transforming growth factor (TGF)-, they differentiate into myofibroblasts. In contrast, Thy-1-fibroblasts terminally differentiate into adipocytes when proliferator-activated receptor (PPAR) is activated with prostaglandin J2or thiazolidinediones such as rosiglitazone. Whether these distinctions hold true for cellsin vivois not yet known. The basis for the Gilteritinib hemifumarate Gilteritinib hemifumarate cellular diversity observed in these connective tissue depots has yet to be determined, but may ultimately explain the patterns of tissue remodelling observed in both anatomic regions. With regard to the orbit, the potential for Thy-1-fibroblasts to differentiate into adipocytes might help to explain the apparent expansion of fat found in Graves’ disease. == Infiltration of fibrocytes into tissues might contribute to apparent fibroblast diversity == Fibrocytes represent circulating bone-marrow derived monocyte lineage cells that present antigen efficiently to lymphocytes, prime naive T cells and can enter sites of tissue injury [10,11]. They are distinct from fibroblasts, T and B lymphocytes, monocytes, MDNCF epithelial, endothelial and dendritic cells and can differentiate into mature fat cells, osteoblasts and myofibroblasts. Their ultimate fate depends, at least in part, upon the signals they receive from their microenvironment and which of the intracellular signalling pathways become activated [12]. In this regard, fibrocytes resemble fibroblasts. Fibrocytes were first described by Bucallaet al. in 1994 as possessing a CD34+vimentin+collagen+phenotype [10], They were found capable of circulating as members of a population of peripheral blood mononuclear cells and were shown to enter wound chambers implanted in subcutaneous tissue. They were identified in connective tissue scars. Once fibrocytes have infiltrated injured target tissues undergoing remodelling, they assume a fibroblast-like morphology. Moreover, they appear to lose their surface expression of CD34 as they develop into fibroblasts [13], suggesting that this protein behaves as a progenitor marker. Fibrocytes are believed to interact with other mononuclear cells that have also been recruited from the Gilteritinib hemifumarate circulation. They can also cross-talk with residential fibroblasts. Currently it is uncertain exactly what roles fibrocytes play in tissue regeneration or how they might participate in the formation of fibrosis. Moreover, the mechanisms and signalling pathways through which they exchange molecular information with other cells are only partially identified. A major hurdle in characterizing fibrocytes and distinguishing them from fibroblasts continues to result from the absence of specific surface markers. Identification of fibrocytes as a distinct cell type has resulted.