Strategies for promoting neurovascularization in bone regeneration

Abstract

Bone tissue relies on the intricate interplay between blood vessels and nerve fibers, both are essential for many physiological and pathological processes of the skeletal system. Blood vessels provide the necessary oxygen and nutrients to nerve and bone tissues, and remove metabolic waste. Concomitantly, nerve fibers precede blood vessels during growth, promote vascularization, and influence bone cells by secreting neurotransmitters to stimulate osteogenesis. Despite the critical roles of both components, current biomaterials generally focus on enhancing intraosseous blood vessel repair, while often neglecting the contribution of nerves. Understanding the distribution and main functions of blood vessels and nerve fibers in bone is crucial for developing effective biomaterials for bone tissue engineering. This review first explores the anatomy of intraosseous blood vessels and nerve fibers, highlighting their vital roles in bone embryonic development, metabolism, and repair. It covers innovative bone regeneration strategies directed at accelerating the intrabony neurovascular system over the past 10 years. The issues covered included material properties (stiffness, surface topography, pore structures, conductivity, and piezoelectricity) and acellular biological factors [neurotrophins, peptides, ribonucleic acids (RNAs), inorganic ions, and exosomes]. Major challenges encountered by neurovascularized materials during their clinical translation have also been highlighted. Furthermore, the review discusses future research directions and potential developments aimed at producing bone repair materials that more accurately mimic the natural healing processes of bone tissue. This review will serve as a valuable reference for researchers and clinicians in developing novel neurovascularized biomaterials and accelerating their translation into clinical practice. By bridging the gap between experimental research and practical application, these advancements have the potential to transform the treatment of bone defects and significantly improve the quality of life for patients with bone-related conditions.

Keywords: Biomaterials; Blood vessels; Bone; Nerve; Neurovascularization.

Fig. 1

Distribution of blood vessels and nerves in different parts of the body, including the teeth, jaws, and femurs. a The inferior alveolar artery and vein within the mandible. b The inferior alveolar artery and vein branching out to the pulp chamber of the mandibular molar. c The inferior alveolar nerve within the mandible. d The inferior alveolar nerve branching out to the pulp chamber of the mandibular molar. e Distribution of blood vessels within the femur, which mainly consists of periosteal artery, nutrient artery and emissary vein. f Distribution of nerves within the femurs, including sensory nerves and sympathetic nerves

Fig. 2

Distribution of the neurovascular system in bone. a The blood supply of a long bone. The marrow cavity contains a large central venous sinus, a dense network of medullary sinusoids, and longitudinal medullary arteries and their circumferential rami [33]. b A simplified schematic of the neuronal distribution in the mouse femur [34]. c A schematic illustrating the general pattern and course of the sensory nerve fibers and blood vessels in the periosteum and mineralized bone [11]. d A schematic of the morphology and distribution of type H and type L blood vessels. Arrowhead marks the entry of the arteriole through the cortical bone [35]. CGRP calcitonin gene-related peptide

Fig. 3

Expression of neuro-associated molecules and related events during bone healing. a The line chart displays the temporal sequence of events following a bone fracture. b Distinctive distribution of neuropeptides across the 4 phases of bone healing [17]. BDNF brain-derived neurotrophic factor, CGRP calcitonin gene-related peptide, NGF nerve growth nerve, NPY neuropeptide Y, Sema3A semaphorin 3A, SP substance P

Fig. 4

Conductive-based biomaterials promote innervated and vascularized bone regeneration. a Schematic of the (core)-polycaprolacton/(shell)–DNM biomimetic periosteum (PD)@black phosphorus promoting neurogenic bone regeneration [203]. b Schematic of the process used for the fabrication of the gelatin methacryloyl (GelMA)/GeP@Cu electroactive hydrogel and its multiple therapeutic actions supporting bone regeneration [127]. c Fabrication of Mo₂Ti₂C₃ MXene hydrogel and its application in bone defects. d Immunofluorescence results showed MXene hydrogel increased the relative intensity of 5-hydroxytryptamine (5-HT) and bone morphogenetic protein 2 (BMP-2) expression at 8 weeks [130]. Scar bar = 100 μm. bFGF basic fibroblast growth factor, BMSCs bone marrow mesenchymal stem cells, CGRP calcitonin gene-related peptide, HF hydrofluoric acid, ROS reactive oxygen species, TPAOH tetrapropyl ammonium hydroxide, VEGF vascular endothelial growth factor

Fig. 5

Strategies of delivering nerve growth factor (NGF) using biomaterials address burst release of NGF for innervated and vascularized bone regeneration. a Immunofluorescence results showed laminin (LM)332/polyethyleneglycol (PEG) effectively delivers bone morphogenetic protein 2 (BMP-2) and promotes the expression of late osteogenic markers osteopontin (OPN) and osteocalcin (OCN). b Immunofluorescence represented images of DRG cells in diverse culture conditions, which showed DRG cultured in LM411/PEG + GCRDVPMSMRGGDRCG peptide (VPM) hydrogels with 1 µg/ml of β-NGF showed the longest neurite outgrowth [165]. Scale bar = 500 µm. c The adsorption capacity of the acellular scaffold was leveraged to construct a sustained release system of NGF, which promoted sensory nerves reinnervation and bone repair [110]. d The schematic diagram showed the preparation of bioprinted constructs, which promote bone regeneration through sensory nerves and blood vessels regeneration [221]. AlgMA alginate methacrylate, BMP-2 bone morphogenetic protein 2, BMSCs bone marrow mesenchymal stem cells, CGRP calcitonin gene-related peptide, DAPI 4',6-Diamidino-2-phenylindole, DRG dorsal root ganglia, FN fibronectin, GelMA gelatin methacryloyl, MSCs mesenchymal stem cells, OM osteogenic media, TrkA tyrosine kinase receptor A, UV ultraviolet

Fig. 6

Inorganic ions-based biomaterials promote innervated and vascularized bone regeneration. a Schematic showing diffusion of implant-derived Mg²⁺ promotes osteogenic differentiation toward the periosteum that is innervated by sensory neurons [292]. b Immunofluorescence staining of overexpressing semaphorin 3A (Sema3A) in sensory nerves showed a large number of Leptin receptor (LepR)⁺ cell, Calcitonin gene-related peptide (CGRP)⁺ nerve fibers, and CD31⁺ vessels. Scale bar = 100 μm. c 3D-reconstructed superficial and interior images of femoral condyle defects showed overexpressing Sema3A in sensory nerves could accelerate bone regeneration [304]. Scale bar = 100 μm. d Ce-eggshell membrane (ESM) enhanced gene expressions of vascular endothelial growth factor (VEGF), platelet-derived growth factor-BB (PDGF-BB) and immunofluorescence images demonstrated a significant upregulation of SLIT3 in macrophages after Ce-ESM simulation [136]. *P < 0.05; **P < 0.01; ***P < 0.001, ns non-significant. Scale bar = 50 μm. cAMP cyclic adenosine monophosphate, CALCRL calcitonin receptor-like receptor, CGRP calcitonin gene-related peptide, CREB1 cAMP-responsive element binding protein 1, DRG dorsal root ganglion, MAGT1 magnesium induces magnesium transporter 1, PDSC periosteum-derived stem cell, RAMP1 receptor activity-modifying protein 1, SLIT slit guidance ligand, TRPM7 transient receptor potential cation channel subfamily M member 7, VEGF vascular endothelial growth factor

Fig. 7

Natural and engineered exosome-based biomaterials promote innervated and vascularized bone regeneration. a At different stages of bone regeneration, Schwann cell-exosome (SC-exos)/hydrogel improves the osteogenic microenvironment and promotes neurovascularized bone regeneration [133]. b Schematic showing electrospun biomimetic periosteum loaded with aptamers engineered exosomes. These entities can target injured axons and regenerate blood vessels and bone. c Compared with the control group, aptamers engineered exosomes promoted dorsal root ganglion (DRG) axons growth and showed clear guidance [137]. d The N-exos-functionalized LDM-printed hierarchical porous scaffolds could promote the axonal growth and calcitonin gene-related peptide (CGRP) expression of sensory neurons and synergistically stimulate the osteogenic differentiation capacity of bone marrow mesenchymal stem cells (BMSCs) [142]. ARG arginase, BDNF brain-derived neurotrophic factor, Exos exosomes, FFT Fast Fourier Transform, GDNF glial-derived neurotrophic factor, iNOS inducible nitric oxide synthase, JUK3 c-Jun N-terminal kinase 3, LDM low temperature deposition modelling, MAPK mitogen-activated protein kinase, NF200 neurofilament 200, NGF nerve growth factor, NT3 neurotrophin-3, PPE PCL@PEI@exosome, PPEA PCL@PEI@exosome@aptamer, SMAD small mothers against decapentaplegic homolog, TGF-β transforming growth factor-β