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The medial collateral ligament (MCL) and anterior cruciate ligament (ACL) from New Zealand white rabbits ages 2, 12, and 36 mos were resected and utilized for analysis by light microscopy and electron microscopy, as well as for determinations of water content, collagen concentration, collagen crosslink (i.e., reducible, nonreducible) analysis, and collagen synthesis rates in vitro. Both modes of microscopy revealed substantial differences between the various age groups. Water content, as well as collagen concentration, decreased significantly from the 2-mo to the 12- or 36- mo groups. The concentration of the reducible crosslinks, associated with less mature tissue, was significantly greater in the younger tissues relative to the aged tissues. The nonreducible crosslink increased with maturation of the collagen in the 36-mo rabbits. Collagen synthesis rates fell from a highest value in the 2-mo group to the lowest in the 36-mo group.

The integrins are a family of adhesion-mediating cell surface receptors that play critical roles in cell-extracellular matrix interactions and have been shown to be important in the healing response in several tissues. We have studied integrin expression in normal human and rabbit anterior cruciate (ACL) and medial collateral (MCL) ligaments of the knee as a preamble to studies of beta 1-integrin expression in healing ligaments. Histologic sections of human and rabbit ACL and MCL were probed for integrin expression utilizing integrin-specific monoclonal antibodies (mAbs) followed by immunoperoxidase detection. Staining of human specimens with mAbs revealed the presence of beta 1-, alpha 1-, and alpha 5-integrin chains on the tissue fibroblasts of both ACL and MCL, while staining of rabbit specimens with rabbit integrin-reactive monoclonals revealed the presence of beta 1- and alpha 5-integrin on these ligaments. Equivalent amounts of the integrins studied were present on normal ACL and MCL. We conclude that the rabbit is an appropriate model for analyzing the expression and functional role of integrins in ligament wound healing.