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Smooth muscle cells from the aortic media of adult pigs and monkeys have been grown in primary culture by plating cells enzymatically dissociated from the intact aorta. During the first 6 d these cells are in the "contractile" phenotype. That is, they contract slowly in response to angiotensin II and their cytoplasm is filled with both thick and thin myofilaments . In this state they do not incorporate [3H]thymidine into DNA or proliferate in response to normolipemic or hyperlipemic whole blood serum (WBS) . After 7 d in culture the cells undergo a spontaneous modulation of phenotype to a "synthetic" state where they cannot be stimulated to contract and their cytoplasm is filled with organelles usually associated with synthesis of secretory protein . Thick myosin-containing filaments can no longer be demonstrated . When challenged with normolipemic or hyperlipemic WBS the cells incorporate [3H]thymidine into DNA and undergo logarithmic growth . It is suggested that when smooth muscle is in the contractile phenotype (as normally exists for most cells in the aortic media of adult animals) it does not divide when challenged with serum mitogens but can undergo a change of phenotype to a synthetic state in which division can be stimulated .Differentiation of a cell involves transition from an initially multipotential state to the specialized form typical of the adult . A fully differentiated cell is one therefore that has lost all potentiality to develop into another cell type . The concept of cell modulation encompasses the fact that a fully differentiated cell can assume a different function (with associated morphological changes) in response to an altered environment without any change in its type-specific character. Modulation is therefore reversible.The smooth muscle cell is the only cell type present in the media of mammalian arteries (19). It must therefore be responsible not only for maintaining artery wall tension via contraction-relaxation but also for vascular remodeling, repair, and growth . These latter functions require the retention in the adult of certain basic mesenchymal properties, namely the abilities to synthesize extracellular matrix and to divide (33). This multiplicity of functions requires a whole spectrum of variation in morphology or phenotype. At one end of the spectrum is the smooth muscle cell whose major function is contraction . The cytoplasm of this cell is largely filled with thick and thin myofilaments and it has been described as being in the "contractile" state (6) . The vast majority of cells in the media of the adult aorta are in this phenotype. At the other end of the spectrum is the smooth muscle cell engaged in the synthesis of extracellular matrix and/or division. The cytoplasm of this cell

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When smooth muscle from the aortic media of adult pigs, monkeys and rabbits are dispersed into single cells and seeded into primary culture they are in the contractile phenotype and do not undergo cell division when challenged with 5 or 10% whole blood serum. However, after 6-8 days the isolated cells undergo a spontaneous change in phenotype to a synthetic state whereby their contractility is lost and they become responsive to serum mitogens. This change in function is accompanied by distinct morphological changes and is reversible if the cells are seeded sufficiently dense (5 × 104 -1 × 105/ml) that a confluent monolayer of cells is achieved within 5-7 days of the original change in phenotype. If however the cells are seeded so sparsely (1 × 103 - 5 × 103/ml) that 2-3 weeks of proliferation is necessary for confluence to be achieved then the cells remain permanently in the synthetic phenotype and immediately responsive to serum mitogens upon subculture. While the cells are in the synthetic phenotype, whether proliferating or quiescent, they have a considerably decreased production of prostaglandins E2, F2α and I2. They also have a decreased ability to metabolize low density lipoprotein, a decrease in the specific activity of cholesteryl esterase and an increase in cholesterol ester deposits.The spontaneous change in phenotype from the contractile to synthetic state in culture can be prevented by seeding the contractile smooth muscle sufficiently dense (1 × 106/ ml) that a confluent monolayer is present from day 1. It can also be inhibited by a confluent monolayer of contractile smooth muscle or endothelial cells in co-culture with the sparsely-seeded smooth muscle such that the two cell layers are not in contact but bathed by the same nutrient medium. A factor which inhibits smooth muscle phenotypic change can also be extracted from pig and rabbit aortic tissue and its effect mimicked by sodium heparin at 50 units/ml.It is suggested that smooth muscle phenotype and its control are important but unrecognized factors in atherogenesis.