Fracture Healing via Periosteal Callus Formation Requires Macrophages for Both Initiation and Progression of Early Endochondral Ossification

Abstract

The distribution, phenotype, and requirement of macrophages for fracture-associated inflammation and/or early anabolic progression during endochondral callus formation were investigated. A murine femoral fracture model [internally fixed using a flexible plate (MouseFix)] was used to facilitate reproducible fracture reduction. IHC demonstrated that inflammatory macrophages (F4/80(+)Mac-2(+)) were localized with initiating chondrification centers and persisted within granulation tissue at the expanding soft callus front. They were also associated with key events during soft-to-hard callus transition. Resident macrophages (F4/80(+)Mac-2(neg)), including osteal macrophages, predominated in the maturing hard callus. Macrophage Fas-induced apoptosis transgenic mice were used to induce macrophage depletion in vivo in the femoral fracture model. Callus formation was completely abolished when macrophage depletion was initiated at the time of surgery and was significantly reduced when depletion was delayed to coincide with initiation of early anabolic phase. Treatment initiating 5 days after fracture with the pro-macrophage cytokine colony stimulating factor-1 significantly enhanced soft callus formation. The data support that inflammatory macrophages were required for initiation of fracture repair, whereas both inflammatory and resident macrophages promoted anabolic mechanisms during endochondral callus formation. Overall, macrophages make substantive and prolonged contributions to fracture healing and can be targeted as a therapeutic approach for enhancing repair mechanisms. Thus, macrophages represent a viable target for the development of pro-anabolic fracture treatments with a potentially broad therapeutic window.

Drug delivery strategies to control macrophages for tissue repair and regeneration

“…Monocytes differentiate into macrophages, which appear to control each subsequent phase of tissue repair to ensure successful progression through the healing process. 4 Although the mechanisms by which macrophages modulate healing are poorly understood, the critical role of macrophages in tissue repair has been repeatedly demonstrated by pharmacologic or genetic depletion of macrophages, which reduces or delays healing in many tissues, including skin, 5 muscle, 6 liver, 7 bone, 8,9 and cardiac tissue. 10 A landmark study by Godwin et al 11 showed that macrophage depletion reduced limb regeneration in the salamander, which otherwise has a massive regenerative capacity.…”

Snapshot: Targeting Macrophages as a Candidate for Tissue Regeneration

“…This is a clear message that endochondral ossification was severely disturbed in mice without sufficient macrophages. A similar failure of bone reconstruction was observed by Raggatt's research group (Raggatt et al, 2014). They created transgenic mice, in which mature myeloid cells carried a Fas-based suicide gene.…”

Similitudes entre Ontogenia y Regeneración Ósea Post-fractura

“…Es un proceso complejo que involucra una interacción coordinada entre varias líneas celulares, factores de crecimiento y componentes de la matriz extracelular (Claes et al, 2012;Raggatt et al, 2014). Por otro lado en la reparación, el tejido que se forma es un tejido cicatricial, con características diferentes al original (Córdova, 2010).…”

“…Además se ha demostrado la presencia de macrofagocitos durante esta fase tanto en humanos (Andrew et al, 1994;Oni, 1997), como en animales (Hankemeier et al, 2001;Alexander et al, 2011;Raggatt et al). Fig.…”