Keratocytes of the corneal stroma produce a specialized extracellular matrix responsible for corneal transparency. Corneal keratan sulfate proteoglycans (KSPG) are unique products of keratocytes that are down-regulated in corneal wounds and in vitro. This study used cultures of primary bovine keratocytes to define factors affecting KSPG expression in vitro. KSPG metabolically labeled with [35 S]sulfate decreased during the initial 2-4 days of culture in quiescent cultures with low serum concentrations (0.1%). Addition of fetal bovine serum, fibroblast growth factor-2 (FGF-2), transforming growth factor ␤, or platelet derived growth factor all stimulated cell division, but only FGF-2 stimulated KSPG secretion. Combined with serum, FGF-2 also prevented serum-induced KSPG down-regulation. KSPG secretion was lost during serial subculture with or without FGF-2. Expression of KSPG core proteins (lumican, mimecan, and keratocan) was stimulated by FGF-2, and steady state mRNA pools for these proteins, particularly keratocan, were significantly increased by FGF-2 treatment. KSPG expression therefore is supported by exogenous FGF-2 and eliminated by subculture of the cells in presence of serum. FGF-2 stimulates KSPG core protein expression primarily through an increase in mRNA pools.The corneal stroma is a disc of connective tissue that constitutes about 90% of the mammalian cornea. This tissue consists of a unique transparent extracellular matrix populated by keratocytes, flattened mesenchymal cells responsible for production and maintenance of this matrix. In healing corneal wounds keratocytes become activated, begin mitosis, and migrate to the wound location, where they secrete nontransparent scar components (1, 2). Cells in the healing wound are characterized by secretion of pro-inflammatory cytokines such as interleukin-1␣ and proteolytic enzymes involved in tissue remodeling, collagenase, gelatinase, and stromelysin (3, 4). This remodeling (fibroblastic) phenotype is simulated in vitro when keratocytes are cultured in medium containing fetal bovine serum and subcultured by trypsinization (5).The extracellular matrix of the normal corneal stroma is characterized by a unique class of molecules known as the corneal keratan sulfate proteoglycans (KSPG). 1 These consist of three structurally related proteins modified with N-linked keratan sulfate chains (6). The three proteins (lumican, keratocan, and mimecan) are found in a number of connective tissues but are expressed at much higher levels in the cornea compared with noncorneal tissues (7-9). In noncorneal tissues, these proteins are not modified with keratan sulfate (10). The high level of expression of these three proteins combined with a specialized glycosylation is a property unique to keratocytes and constitutes an essential feature of the role of the keratocyte in maintenance of corneal transparency. This conclusion is supported by the recent demonstration that mice bearing null mutations in the lumican gene lose corneal transparency, whereas mice with a similar...