Quantitative Analysis of Unidirectional 2-D Tissue Formation of Endothelial Cells

Significance of porosity and compliance of microporous, polyurethane-based microarterial vessel on neoarterial wall regeneration

The microporous structure of artificial vascular grafts, which increases compliance and porosity simultaneously, may enhance neoarterial regeneration. In order to differentiate these effects, three models of segmented polyurethane grafts (inner diameter, 1.5 mm; wall thickness, 100 microns) with or without micropores fabricated using an excimer laser ablation technique, were prepared, and their neoarterial regenerative potentials were studied upon implantation: Model I (microporous, permeable, compliant); Model II (smooth-surfaced, impermeable, compliant); and Model III (smooth-surfaced, impermeable, noncompliant). In Models I and II, the pore or groove size (diameter, 100 microns) and pore or groove arrangement were fixed, and consequently their compliances were almost identical. Irrespective of model, the luminal surfaces were coated with benzophenone-derivatized gelatin and subsequently photocured. Twenty grafts (length, 20 mm) of each model were implanted in the aortas of rats. Predetermined implantation periods were 4, 12, and 24 weeks. Total patency rate decreased in the order Model I (100%), II (87%), and III (59%) grafts. All patent grafts were completely endothelialized after 12 weeks of implantation, irrespective of model. After 12- and 24-week implantations, in Model I grafts, the neoarterial wall was thin, and smooth muscle cells (SMCs) were of the contractile phenotype. In Model II grafts, the neoarterial wall exhibited considerable thickening. In Model III grafts, the neoarterial wall exhibited marked thickening, and SMCs were of the synthetic phenotype. The neoarterial wall thickness at the midportion of the grafts after 24 weeks of implantation increased in the order Model I (48 +/- 8 microns), II (146 +/- 87 microns), and III (385 +/- 21 microns) grafts. These results strongly suggest that compliance matching and porosity synergistically resulted in neoarterial wall restoration without appreciable thickening.

Cell-substrate and cell-cell interactions differently regulate cytoskeletal and extracellular matrix protein gene expression

In this study, cellular events during tissue formation were investigated at the mRNA level using the Northern blot technique. The levels of expression of mRNAs encoding specific proteins (beta-actin, fibronectin, and laminin) during tissue formation on tissue culture dishes were quantitatively assessed using a Northern blot technique with autoradiography. The level of beta-actin mRNA increased with incubation time and reached a maximal level near the confluent state, followed by reduced beta-actin mRNA expression at a later stage of tissue formation. The time course of beta-actin mRNA expression corresponded well to the time course of morphologic changes and cytoskeletal organization in adherent cells. Expression of the mRNAs encoding the extracellular matrix proteins fibronectin and laminin was initiated at the proliferation stage. After maximum expression levels of these two mRNAs were reached at the confluent stage, a gradual decrease in their expression levels was seen during long-term culture. Expression patterns of mRNAs encoding cytoskeletal and extracellular matrix proteins strongly depended on the type of artificial substrates used; a mRNA expression pattern similar to that observed during tissue formation on tissue culture dishes was observed on a cell-adhesive substrate during tissue formation, whereas reduced expression was seen during tissue formation on a less adhesive substrate. Thus, the dynamic changes occurring during tissue formation were quantified to investigate the roles of artificial substrates in tissue formation at the mRNA level.

Control of cell adhesion, migration, and orientation on photochemically microprocessed surfaces

Using surface-photochemistry-driven microprocessing, striped patterns of cell-adhesive and nonadhesive domains were prepared on tissue-culture dishes. The width of striped patterns ranged from 20 to 130 microns. When endothelial cells were cultured on such dimensionally well-defined surfaces, cells adhered, migrated, and proliferated only on cell-adhesive domains. Migration potentials such as tracks of moving cells and migration rates were determined using a time-lapse video recording apparatus under a phase-contrast microscope and a computer-assisted image analyzer. The migration track in the direction of the width of the stripe-pattern was limited to the size of the width, and effective migratory distance over 400 min of observation was considerably reduced, to almost half that for a nontreated surface, whereas migratory rate was not changed by surface processing, irrespective of the stripe-pattern width. After a 2-day culture, oriented patterned cellular sheets were obtained. Cells were elongated and aligned along the axis of the striped pattern. The degrees of orientation and elongation were enhanced with a decrease of the line width. At the narrowest surface domain, cells only migrated back and forth, and eventually they became highly elongated and oriented along the axis of the domain. These results indicated that the adhesion area, migrating direction, and orientation of cells can be controlled by this method with micron-order precision. This method provides quantitative information on the kinetics of the migration process and the morphogenesis of the microprocessed surface.

Surface microarchitectural design in biomedical applications: in vitro transmural endothelialization on microporous segmented polyurethane films fabricated using an excimer laser

We describe the preparation of segmented polyurethane (SPU) films with round micropores and present a quantitative assay method of endothelial cell (EC) migration through micropores of and growth on microprocessed SPU films as an in vitro model of transmural endothelialization in open-cell-structured small-diameter vascular grafts. The micropored films, pores of which ranged from 9 to 100 microns in diameter, were microfabricated using an excimer laser. Time-dependent processes of EC ingrowth through micropores of SPU films with different pore sizes, which have a confluent monolayer sheet on one face and are cell free on the other, and subsequent endothelialization were quantitatively studied. The circular cellular sheet centered at the micropores expanded as incubation proceeded. Markedly retarded migration was found for the smallest pore size (9 microns in diameter). The larger the pore, the higher was the endothelialization rate. The endothelialization characteristics were studied on multiply microspored films of different pore sizes and densities, each of which was prepared so as to provide a fixed total pore area per unit area (0.01 mm2 per mm2). The highest endothelialization rates in an early incubation period were found on films with microspores between 18 and 50 microns in diameter.

Mathematical simulation of unidirectional tissue formation: in vitro transanastomotic endothelialization model

In vitro transanastomotic endothelialization was studied using a mathematical model with the Fisher equation. The Fisher equation is a nonlinear parabolic equation which has a cell migration term and a population growth term. The mathematical model was used to simulate recent experiments performed to investigate quantitatively the unidirectional formation of a bovine endothelial cell monolayer in vitro. The two parameters included in the equation were estimated using a trial and error method in which the calculated solutions of the various values of the two parameters are compared with the experimental data and the best fit pair is adopted: one parameter, D, which represents the unidirectional cell migration rate and the other, k, which represents the population growth rate. The calculated solutions fit the experimental data well. We also simulated the healing of a mechanically disrupted endothelial monolayer sheet. The significance of cellular biomechanics in tissue formation and design of tissue-engineered devices is discussed.