An endogenously deposited fibrin scaffold determines construct size in the surgically created arteriovenous loop chamber model of tissue engineering

BackgroundAn arteriovenous loop (AVL) enclosed in a polycarbonate chamber in vivo, produces a fibrin exudate which acts as a provisional matrix for the development of a tissue engineered microcirculatory network.ObjectivesBy administering enoxaparin sodium - an inhibitor of fibrin polymerization, the significance of fibrin scaffold formation on AVL construct size (including the AVL, fibrin scaffold, and new tissue growth into the fibrin), growth, and vascularization were assessed and compared to controls.MethodsIn Sprague Dawley rats, an AVL was created on femoral vessels and inserted into a polycarbonate chamber in the groin in 3 control groups (Series I) and 3 experimental groups (Series II). Two hours before surgery and 6 hours post-surgery, saline (Series I) or enoxaparin sodium (0.6 mg/kg, Series II) was administered intra-peritoneally. Thereafter, the rats were injected daily with saline (Series I) or enoxaparin sodium (1.5 mg/kg, Series II) until construct retrieval at 3, 10, or 21 days. The retrieved constructs underwent weight and volume measurements, and morphologic/morphometric analysis of new tissue components.ResultsEnoxaparin sodium treatment resulted in the development of smaller AVL constructs at 3, 10, and 21 days. Construct weight and volume were significantly reduced at 10 days (control weight 0.337 ± 0.016 g [Mean ± SEM] vs treated 0.228 ± 0.048, [P < .001]: control volume 0.317 ± 0.015 mL vs treated 0.184 ± 0.039 mL [P < .01]) and 21 days (control weight 0.306 ± 0.053 g vs treated 0.198 ± 0.043 g [P < .01]: control volume 0.285 ± 0.047 mL vs treated 0.148 ± 0.041 mL, [P < .01]). Angiogenesis was delayed in the enoxaparin sodium-treated constructs with the absolute vascular volume significantly decreased at 10 days (control vascular volume 0.029 ± 0.03 mL vs treated 0.012 ± 0.002 mL [P < .05]).ConclusionIn this in vivo tissue engineering model, endogenous, extra-vascularly deposited fibrin volume determines construct size and vascular growth in the first 3 weeks and is, therefore, critical to full construct development.

Clinical RelevanceTissue engineering is an expanding field and ultimately has the potential of growing new organs and tissues for replacement of damaged or lost organs. The Bernard O'Brien Institute of Microsurgery (Melbourne, Australia) has developed an in vivo model in which a surgically created arteriovenous loop (AVL) created on the rat femoral vessels and placed in an empty poly-carbonate chamber under the groin skin exudes a fibrin matrix that fills the chamber in the first day. Capillaries begin sprouting from the femoral vein portion of the AVL 3-5 days later, supported by the fibrin matrix (Lokmic et al).5 The chamber is filled to about three-quarters full with vascularized connective tissue in about 3 weeks. The endogenous fibrin scaffold is an integral part of the construct. Many tissue engineering models use biological or synthetic scaffolds to support and maintain the construct. This study looks at the significance of this endogenous fibrin scaffold by partially inhibiting its formation using enoxaparin sodium (an inhibitor of fibrin polymerization) and subsequently comparing with untreated controls, construct weight, and volume and vascularization over the following 3 weeks. Utilizing endogenous fibrin scaffolds has enormous potential for in vivo tissue engineering, particularly in growing large three dimensional vascularized tissues. Endogenous fibrin scaffolds are biologically compatible with the host and will not provoke an inflammatory or foreign body response. This study explores the significance of this natural scaffold in determining construct size and vascularization in an in vivo model of tissue engineering.