Exp. Eye Res. (1979) 29,331-351 Contact Inhibition and the Regulation of Endocytosis in the Cornea1 Endothelium: Correlation with a Restricted Surface Receptor Lateral Mobility and the Appearance of a Fibronectin Meshwork D. GOLDMINZ, I. VLODAVSKY, L. K. JOHNSON AND D. GOSPODAROWICZ The Cancer Research Institute and the Department of Medicine, University of California, Medical Center, San Francisco, Ca. 94143, U.S.A. (Received 7 July 1978 and in. revised form 22 December 1978, New York) The correlation between adsorptive endocytosis, surface receptor lateral mobility, and the production of fibronectin was analyzed using cornea1 endothelial cells maintained in tissue culture as a model. These properties were studied as a function of the cell densit,y and organization using sparse, subconfluent, and confluent cultures. Sparse, actively growing cultures bind, internalize, and degrade low density lipoprotein (LDL), whereas confluent. endothelial cell cultures, although still capable of binding LDL, no longer internalize it and hence show no degradation of the lipoprotein. Disruption of the cell monolayer by means of wounding was associated with a gain of LDL uptake activity only in cells located at the edges of the wound or migrating into the wound. Cap formation induced by Concanavalin A was observed in sparse, but not in confluent cultures, indicating a lower degree of receptor lateral mobility concomitant with the formation of a confluent cell monolayer. Lactoper- oxidase catalyzed iodination of sparse and confluent cultures revealed an accumulation of fibronectin primarily in the pericellular matrix produced by confluent cultures. In contrast,, no extracellular matrix and almost no fibronectin associated with the cell surface were detected in actively growing cells which were not yet in contact. The formation of a confluent cell monolayer is therefore associated with the inhibition of LDL uptake, possibly via membrace changes that lead to a restriction of surface recept.or lateral mobility. This might be brought about by a direct or indirect interaction of the extracellular meshwork of fibronectin with various surface components and subjacent cgtoskeletal elements. It, is suggested that the organization of cells located as a single layer at the interface between fluid and tissue can dictate, both in vitro and in vivo, metabolic functions such as adsorp- tive endocytosis and thus be a prominent fartor in determining their physiological role ~1s a barrier. Key w&s: cornea1 endotheliutn; low-density lipoprotein; receptor rcdistribut,iolr ; fibronertin: cellular organization. 1. Introduction Previous studies with cultured vascular endothelial cells derived from the big vessels have shown that the formation of a highly confluent and fully organized cell mono- layer is associated with changes in the cell surface membrane which impede the process of adsorptive endocytosis and allow the cells to form an efficient block against low density lipoprotein (LDL) in the medium (Vlodavsky, Fielding, Fielding and Gospodarowicz, 19’78; Gospodarowicz, Vlodavsky, Fielding and Birdwell. 1978 ; Gospodarowicz? Vlodavsky, Greenburg and Johnson, 1979; Fielding, Vloclavsk~, Gospodarowicz and Fielding, 1979). Th e b arrier formed by the confluent endothelium is correlated with a massive accumulation, mainly in the pericellular matrix, of disulfide cross-linked fibronectin (Birdwell, Gospodarowicz and Nicolson, 1978 ; 001-i-4835/79~100331$31 $01.00/0 c) 1979 Academic Press Inc. (London) Limited 331