α-Smooth muscle actin and contractile behavior of bovine meniscus cells seeded in type I and type II collagen-GAG matrices

Many types of injuries to the meniscus of the knee joint result in defects that do not heal, leading to pain and dysfunction. Several ongoing investigations are developing porous absorbable matrices to be used alone or seeded with cultured cells to facilitate regeneration of this tissue. The objective of this study was to evaluate in vitro the contractile behavior of meniscal cells seeded in type I and type II collagen matrices. In many connective tissues, fibroblasts that have assumed a contractile phenotype (myofibroblasts) have been found to play an important role in healing and in pathological conditions. This phenotype, if expressed by meniscal cells, could affect their behavior in cell‐seeded matrices developed for tissue engineering. In this study, the presence of a contractile actin isoform, α‐smooth muscle (α‐SM) actin, was assessed by immunohistochemistry in normal calf meniscal tissue and in meniscal cells in 2‐ and 3‐dimensional culture. Calf meniscus cells were seeded in type I and type II collagen‐glycosaminoglycan (GAG) matrices. The diameter of the matrices was measured every 2–3 days. Immunohistochemical staining of the 2‐dimensional cultures for α‐SM actin was performed after 1, 3, and 7 days and the staining of the seeded matrices was at 1, 7, 14, and 21 days. Transmission electron microscopy (TEM) was performed on selected samples. After 3 weeks the seeded type I matrices displayed a significant shrinkage of almost 50% whereas the type II matrix and both types of unseeded controls showed almost no contraction over the same time period. Positive staining for the α‐SM actin phenotype was seen in 10% of the cells of the normal tissue but was present in all cells seeded in monolayer and in both types of matrices. TEM of representative cell‐seeded matrices showed microfilaments approximately 7 nm thick, consistent with the myofibroblast phenotype. This is the first report of α‐SM actin containing cells in the knee meniscus. The finding that, under certain conditions, meniscal cells can express the myofibroblast phenotype warrants study of their role in meniscal healing and the tissue response to implants to facilitate tissue regeneration. © 1999 John Wiley & Sons, Inc. J Biomed Mater Res, 45, 157–166, 1999.