In tissue engineering, the generation of tissue constructs comprising preformed microvessels is a promising strategy to guarantee their adequate vascularisation after implantation. Herein, we analysed whether this may be achieved by seeding porous scaffolds with adipose tissue-derived microvascular fragments. Green fluorescent protein (GFP)-positive microvascular fragments were isolated by enzymatic digestion from epididymal fat pads of male C57BL/6-TgN(ACTB-EGFP)1Osb/J mice. Nano-size hydroxyapatite particles/poly(ester-urethane) scaffolds were seeded with these fragments and implanted into the dorsal skinfold chamber of C57BL/6 wild-type mice to study inosculation and vascularisation of the implants by means of intravital fluorescence microscopy, histology and immunohistochemistry over 2 weeks. Empty scaffolds served as controls. Vital microvascular fragments could be isolated from adipose tissue and seeded onto the scaffolds under dynamic pressure conditions. In the dorsal skinfold chamber, the fragments survived and exhibited a high angiogenic activity, resulting in the formation of GFP-positive microvascular networks within the implants. These networks developed interconnections to the host microvasculature, resulting in a significantly increased functional microvessel density at day 10 and 14 after implantation when compared to controls. Immunohistochemical analyses of vessel-seeded scaffolds revealed that >90 % of the microvessels in the implants' centre and ~60 % of microvessels in the surrounding host tissue were GFP-positive. This indicates that the scaffolds primarily vascularised by external inosculation. These novel findings demonstrate that the vascularisation of implanted porous scaffolds can be improved by incorporation of microvascular fragments. Accordingly, this approach may markedly contribute to the success of future tissue engineering applications in clinical practice.

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Inosculation of preformed microvessels with the host microvasculature represents a promising approach to accelerate vascularization of tissue constructs. Herein, we analyzed whether cultivation of prevascularized tissue constructs promotes inosculation by reducing the perivascular cell coverage of the preformed microvessels. Poly(ester-urethane) scaffolds were implanted into FVB/N-TgN (Tie2/green fluorescent protein [GFP]) 287 Sato mice to generate prevascularized tissue constructs with GFP-positive microvessels. These constructs were then cultivated for 3 or 10 days before implantation into dorsal skinfold chambers of FVB/N mice to analyze inosculation and vascularization by intravital fluorescence microscopy and immunohistochemistry. Noncultivated tissue constructs served as controls. Cultivation reduced the number of α-smooth muscle actin-positive preformed microvessels within the constructs and increased the production of vascular endothelial growth factor. After 3 days of cultivation, tissue constructs still exhibited good cell viability, whereas apoptotic cell death was massively increased in the 10-day-cultivated group. After implantation, inosculation of preformed microvessels was accelerated in the 3-day-cultivated constructs. This resulted in an improved vascularization, as indicated by an increased functional microvessel density and blood perfusion. Immunohistochemical detection of GFP-positive microvessels revealed that internal and external inosculation occurs in parallel. In conclusion, this study demonstrates that inosculation of in situ prevascularized tissue constructs can be easily accelerated by destabilization of preformed microvessels and angiogenic activation during short-term cultivation.

The engineering of preformed microvessels offers the promising opportunity to rapidly vascularise implanted tissue constructs by the process of inosculation. Herein, we analyzed whether this process may further be accelerated by cultivation of prevascularised tissue constructs in Matrigel before implantation. Nano-size hydroxyapatite particles/poly(ester-urethane) scaffolds were implanted into the flank of FVB/N-TgN (Tie2/GFP) 287 Sato mice to allow the ingrowth of a granulation tissue with green fluorescent protein (GFP)-positive blood vessels. After harvesting, these prevascularised constructs were then transferred into dorsal skinfold chambers of FVB/N recipient mice to study the process of inosculation. The constructs were implanted directly after embedding in Matrigel or after 3 days of cultivation in the extracellular matrix. Matrigel-free constructs served as control. Cultivation in Matrigel resulted in the outgrowth of CD31/GFP-positive vascular sprouts. Vascularisation of these constructs was markedly improved when compared to the other two groups, as indicated by a significantly elevated functional microvessel density between days 6 to 14 after implantation into the dorsal skinfold chamber. This was associated with an increased number of GFP-positive blood vessels growing into the surrounding host tissue. Thus, the blood supply to prevascularised tissue constructs can be accelerated by their pre-cultivation in an angiogenic extracellular matrix, promoting external inosculation of the preformed microvascular networks with the host microvasculature.