Boon and Bane of Inflammation in Bone Tissue Regeneration and Its Link with Angiogenesis

Abstract

Delayed healing or nonhealing of bone is an important clinical concern. Although bone, one of the two tissues with scar-free healing capacity, heals in most cases, healing is delayed in more than 10% of clinical cases. Treatment of such delayed healing condition is often painful, risky, time consuming, and expensive. Tissue healing is a multistage regenerative process involving complex and well-orchestrated steps, which are initiated in response to injury. At best, these steps lead to scar-free tissue formation. At the onset of healing, during the inflammatory phase, stationary and attracted macrophages and other immune cells at the fracture site release cytokines in response to injury. This initial reaction to injury is followed by the recruitment, proliferation, and differentiation of mesenchymal stromal cells, synthesis of extracellular matrix proteins, angiogenesis, and finally tissue remodeling. Failure to heal is often associated with poor revascularization. Since blood vessels mediate the transport of circulating cells, oxygen, nutrients, and waste products, they appear essential for successful healing. The strategy of endogenous regeneration in a tissue such as bone is interesting to analyze since it may represent a blueprint of successful tissue formation. This review highlights the interdependency of the time cascades of inflammation, angiogenesis, and tissue regeneration. A better understanding of these inter-relations is mandatory to early identify patients at risk as well as to overcome critical clinical conditions that limit healing. Instead of purely tolerating the inflammatory phase, modulations of inflammation (immunomodulation) might represent a valid therapeutic strategy to enhance angiogenesis and foster later phases of tissue regeneration.

FIG. 1.

Bone healing can be divided in phases, which result in regenerated bone. In the upper window, the basic phases are depicted. In the lower window, the three main phases are shown to consist of multiple overlapping/consecutive phases. The further the healing is progressed, the lesser the number of processes that interact to conclude this healing step successfully. Since the quantities of the various elements are often unknown, the stages are depicted as all having a similar magnitude. Color images available online at www.liebertpub.com/teb

FIG. 2.

Inflammation occurs upon injury, activating mediators, which act on the vascular system. Endothelial cells and other cellular components of the vascular wall are activated and eventually increase the proliferation rate of blood vessels.

FIG. 3.

Hematoxylin and eosin staining of hematoma 4 h (A) and 12 h (B) after osteotomy in a sheep model. Blue dots represent nucleated cells, which are markedly less in the 12-h hematoma (marked with arrows). Color images available online at www.liebertpub.com/teb

FIG. 4.

Left: Mean values of live cell counts calculated per gram of hematoma revealed a drop in cell numbers between 4 and 12 h, with a subsequent increase between 12 and 24 h. Right: Cellular composition changes during the first 60 h of healing in a sheep osteotomy model. Proinflammatory cytotoxic T cells (CD8CD5) continually decrease, while the anti-inflammatory regulatory T-helper cell (CD25CD4) percentage increases over time (relative to preinjury blood levels=100%). Color images available online at www.liebertpub.com/teb

FIG. 5.

Movat Pentachrome staining of osteotomy hematoma from a sheep study showing organization processes of the granulation tissue 4 and 7 days after osteotomy. (A) Four days after osteotomy, fibroblasts and fibrocytes are embedded in newly formed connective tissue (blue) in the outer layers of the hematoma; (B) 7 days after osteotomy, the granulation tissue has matured, large areas with connective tissue (blue) replace the hematoma (erythrocytes—red), and in a higher resolution (C), newly formed blood vessels are clearly visible. Color images available online at www.liebertpub.com/teb

FIG. 6.

Material-based delivery can provide (A) localized presentation of therapeutic soluble factors that mainly affect cells at the site of injury as well as (B) sustained presentation of factors; factors delivered by bolus delivery, in contrast, rapidly decrease in concentration with time. Material-based delivery of vascular endothelial growth factor (VEGF) at sites of ischemia shows greater efficacy in promoting angiogenesis (C) and in restoring perfusion to ischemic tissue (D) compared with bolus delivery. In (C), tissue sections from ischemic hind limbs are immunostained for endothelial marker, CD31. In (D), data shown are for mice with no treatment (□), blank material (▵), bolus VEGF delivery (), and material delivery of VEGF (●). Images adopted from Silva and Mooney. Color images available online at www.liebertpub.com/teb