VISCOSITY AND ELASTICITY DURING COLLAGEN ASSEMBLY IN-VITRO - RELEVANCE TO MATRIX-DRIVEN TRANSLOCATION

In order to better understand the gelation process associated with col lagen assembly, and the mechanism of the in vitro morphogenetic phenom enon of ''matrix-driven translocation'' [S.A. Newman et al. (1985) Sci ence, 228, 885-889], the viscosity and elastic modulus of assembling c ollagen matrices in the presence and absence of polystyrene latex bead s was investigated. Viscosity measurements at very low shear rates (0. 016-0.0549) s(-1)) were performed over a range of temperatures (6.9-11 .5 degrees C) in a Couette viscometer. A magnetic levitation sphere rh eometer was used to measure the shear elastic modulus of the assemblin g matrices during the late phase of the gelation process. Gelation was detected by the rapid increase in viscosity that occurred after a lag time t(L) that varied between 0 and similar to 500 s. After a rise in viscosity that occurred over an additional similar to 500 s, the coll agen matrix was characterized by an elastic modulus of the order of se veral Pa. The lag time of the assembly process was relatively insensit ive to differences in shear rate within the variability of the sample preparation, but was inversely proportional to the time the sample spe nt on ice before being raised to the test temperature, for test temper atures >9 degrees C. This suggests that structures important for fibri llo-genesis are capable of forming at 0 degrees C. The time dependence of the gelation process is well-described by an exponential law with a rate constant K similar to 0.1 s(-1). Significantly, K was consisten tly larger in collagen preparations that contained cell-sized polystyr ene beads. From these results, along with prior information on effecti ve surface tension differences of bead-containing and bead-lacking col lagen matrices, we conclude that changes in matrix organization contri buting to matrix-driven translocation are initiated during the lag pha se of fibrillo-genesis when the viscosity is less than or equal to 0.1 Poise. The phenomenon may make use of small differentials in viscosit y and/or elasticity, resulting from the interaction of the beads with the assembling matrix. These properties are well described by standard models of concentrated solutions. (C) 1997 John Wiley & Sons, Inc.