The Fracture Callus Is Formed by Progenitors of Different Skeletal Origins in a Site‐Specific Manner

Abstract

We evaluated repair following a mid‐diaphyseal fracture of the tibia in 3‐month‐old mice. We observed differences in the repair process at three different sites of the callus. Site 1: bone developing from the outer layer of the periosteum of the cortex; site 2: bone developing within the bridge/central region of the fracture; and site 3: bone developing within the marrow of the ends of broken bones. We characterized these sites by correlating datasets from X‐ray CT and histology. Correlated data demonstrated the involvement of different cells and different rates of mineralization. The origin of the progenitors and mechanism of progenitor differentiation involved at these sites was then evaluated using lineage tracing of cells expressing Prx1 and Col.2. The Prx1 progeny contributed to intramembranous bone formation (IBF) at site 1 and endochondral bone formation (EndoBF) at site 2 but not to intramedullary bone formation (IMBF) at site 3. IBF at site 1 was confirmed without a chondrocyte intermediate unlike EndoBF at site 2. Additionally, the presence of Col.2 progeny contributed to EndoBF in site 2 and IMBF in site 3 but not to IBF in site 1. However, the Col.2 progeny in IMBF in site 3 appeared to come from Col.2‐expressing osteocytes originating in the cortices of the ends of the fractured bone. In conclusion we have identified three sites of bone fracture repair that differ in their origin of cells and their mechanisms of bone formation. © 2019 The Authors JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Insights Into Pulp Biomineralization in Human Teeth

“…As mentioned, a multiscale correlative microspectroscopic approach (17)(18)(19)(20)(21) was taken to gather insights into pulp biomineralization. Methods include a length-scale mapping of physical, chemical, and biochemical characteristics of pulp, dentin, and cementum.…”

“…Specimens were infiltrated with LR-white resin (Electron Microscopy Sciences, Hatfield, PA) in a gelatin capsule and polymerized for 2 days at 60 • C and cut into 90 nm thin ultrasections with an ultramicrotome (Reichert Ultracut E, Leica Microsystems, Inc., Buffalo Grove, IL). The ultrasections were collected on formvar/carbon-coated Ni grids (Electron Microscopy Sciences) and were imaged for ultrastructure using scanning transmission electron microscopy (STEM, SIGMA VP500, Carl Zeiss Microscopy, Pleasanton, CA) mode at 15 keV (17,18).…”

“…Methods will include correlative spatial mapping of physicochemical and biochemical characteristics of pulp, and contextualize these properties within the DPC to reveal the inherent vunerabilities of pulp. This multiscale correlative approach will provide insights into the plausible predisposed nature of tissues (17)(18)(19)(20)(21), and in this case pulp, that can form biominerals as a response to significant shifts in the IFP. From a practical standpoint, it will reveal the need to question the effect of clinical interventions such as local anesthesia (22), dental restorations (23), periodontal treatment (24), and orthodontics (16), all of which can significantly affect functional homeostasis of the pulp.…”

Periosteal Skeletal Stem Cells and Their Response to Bone Injury

“…By tracing the Lac-Z-positive cells, it was concluded that repairing cells came from adjacent tissues; cells derived from the periosteum were always found on the periosteal surface; cells derived from the BM were always found within the marrow cavity; and cortical bone involvement was very limited. In the callus, cartilage cells are almost always derived from the periosteum, while osteoblasts are derived from both the periosteum and BM ( Colnot, 2009 ; Wang et al, 2019 ). In a similar study, the authors stated that approximately 70% of the total regenerated cells came from the periosteum ( Zhang et al, 2005 ).…”