Background:
Tissue engineered dermal replacements such as Integra™ and Alloderm™ require host vascular invasion in order to become permanently integrated into the patient. The long interval required for vascular invasion (at least three weeks) necessitates multiple dressing changes, immobilization of the affected part and predisposes these constructs to the risk of infection and loss. We have designed a natural hydrogel scaffold to identify in vivo the ideal geometrical, chemical, and mechanical characteristics that promote optimal vascularization.
Methods:
Acellular hydrogel scaffolds composed of either 4% alginate or 2% collagen were fabricated and sterilized. One hundred micrometer pores were either present or absent, and when present, were either empty or filled with 0.3% collagen. These scaffolds were then subcutaneously implanted on the dorsum of 12 male C57/BL6 mice (4 scaffolds per mouse for a total of 48 scaffolds). The scaffold and surrounding tissue were harvested at 3, 7, and 14 days, processed for histology, and stained with hematoxylin and eosin. Immunohistochemical staining with CD31 was also performed.
Results:
Histologic analysis revealed that in all porous scaffolds, native cells exhibited rapid, directional migration into the pores, with complete cellular lining of all pores at 3 days. Furthermore, lateral migration through the substance between the pores of the scaffold occurred in all scaffolds by 14 days (Figure; note early presumptive vascular channels containing erythrocytes and lined by CD31+ cells). The rate and degree of pore invasion and lateral migration was further increased in porous scaffolds that were composed of 2% collagen and those that contained 0.3% collagen in the pores, such that porous invasion occurred by 3 days and lateral migration by 7 days, with a more pronounced, "dendritic" pattern. Neovascularization, evidenced by the presence of red blood cells within channels lined by presumptive endothelial (CD31+) cells within the interporous regions of the scaffolds was evident in constructs that contained collagen in the substance and/or the pores, but not in those that did not contain any collagen.
Conclusions:
The results of this study indicate that given the appropriate inductive cues, cellular invasion of acellular scaffolds in vivo can occur in as few as 3 days. The presence of pores increases the depth of cellular penetration, and the presence of collagen (either in the substance of the scaffold or filling the pores), increases the rate of cellular invasion to the degree that neovascularization is only observed in the presence of collagen. This study provides important information regarding the roles of geometrical, mechanical, and chemical cues that drive cellular invasion of hydrogel scaffolds, and this information will be clinically applicable to the design of improved, more rapidly-vascularizing biologic scaffolds.