The influence of heparin on the wound healing response to collagen implants in vivo

The biologic response to fibrillar collagen (collagen) and fibrillar collagen plus heparin (collagen/heparin) implants have been compared in the rat subcutaneous and guinea pig dermal wound models. The reconstituted bovine dermal collagen implants were injected subcutaneously in rats at concentrations ranging from 18 to 30 mg/ml and in volumes ranging from 0.5 to 1.0 ml. The biologic response to the collagen implants alone was characterized by a transient invasion of a modest number of inflammatory cells within the first three days of implantation that was followed by limited fibroblast invasion into the peripheral 1/3 of the implant during the course of the next three to four weeks. Occasionally, blood vessels were observed to invade the peripheral regions of the implant. The degree (number) and extent (depth) of cell invasion were inversely related to initial collagen implant concentration. Addition of heparin (0.3-20 micrograms/mg collagen) to these implants resulted in a significant dose-dependent increase in the degree and extent of fibroblast invasion. Radiolabeling studies showed that the collagen and collagen/heparin implants were cleared from the subcutis at identical rates. Implantation of these formulations in a guinea pig dermal wound model was also performed, using a semi-occlusive wound dressing (Opsite) to maintain the implant in the wound site. The fibrillar collagen implant alone was pushed upward by developing granulation tissue at the base of the wound and served as a support for epidermal cell migration, proliferation, and differentiation as wound closure proceeded. The implant was slowly invaded and turned over as granulation tissue developed from the base and margins of the wound bed. The inclusion of heparin in these implants resulted in a significantly different pattern of wound healing. The collagen/heparin implants histologically presented a more broken-up or porous appearance following implantation, which was associated with a greater degree of penetration of developing granulation tissue into the implant itself as compared to the collagen implants. Radiolabeling studies revealed that clearance rates of implants with and without heparin from wound sites were similar, as noted in the rat subcutis. Laser doppler flowmetry studies suggested that the heparin--containing implants were more vascular than control wound sites or sites treated with collagen alone.