title: Sequential changes in vessel formation and micro-vascular function during bone repair.
abstract: BACKGROUND
  Angiogenesis, the process of new vessel formation from a pre-existing vascular network, is essential for bone development and repair. New vessel formation and microvascular functions are crucial during bone repair, not only for sufficient nutrient supply, transport of macromolecules and invading cells, but also because they govern the metabolic microenvironment. Despite its central role, very little is known about the initial processes of vessel formation and microvascular function during bone repair.
  
  
  METHODS
  To visualize and quantify the process of vessel formation and microvascular function during bone repair, we transplanted neonatal femora with a substantial defect into dorsal skin-fold chambers in severe combined immunodeficient (SCID) mice for continuous noninvasive in-vivo evaluation. We employed intravital microscopic techniques to monitor effective microvascular permeability, functional vascular density, blood flow rate and leukocyte flux repeatedly over 16 days. Oxytetracyclin and v. Kossa/v. Giesson staining was performed to quantify the calcification process in vivo and in vitro.
  
  
  RESULTS
  Development of a hematoma surrounding the defect area was the initial event, which was accompanied by a significant increase in microvascular permeability and blood flow rate. With absorption of the hematoma and vessel maturation, permeability decreased continuously, while vascular density and tissue perfusion increased. Histological evaluation revealed that the remodeling of the substantial defect prolonged the in-vivo monitored calcification process.
  
  
  CONCLUSIONS
  The size of the initial substantial defect correlated positively with increased permeability, suggesting improved release of permeability-inducing cytokines. The unchanged permeability in the control group with boiled bones and a substantial defect corroborated these findings. The adaptation to increasing metabolic demands was initially mediated by increased blood flow rate, later with increasing vascular density through increased tissue perfusion rate. These insights into the sequence of microvascular alterations may assist in the development of targeted drug delivery therapies and caution against the use of permeability-altering drugs during bone healing.
authors.affiliation: Department of Zentrum Biomechanik, University Medical Center Hamburg-Eppendorf. Hamburg, Germany. nhansen@uke.uni-hamburg.de
authors.forename: Nils
authors.initials: N
authors.lastname: Hansen-Algenstaedt
authors.forename: Claudia
authors.initials: C
authors.lastname: Joscheck
authors.forename: Lars
authors.initials: L
authors.lastname: Wolfram
authors.forename: Christian
authors.initials: C
authors.lastname: Schaefer
authors.forename: Ingo
authors.initials: I
authors.lastname: Müller
authors.forename: Antje
authors.initials: A
authors.lastname: Böttcher
authors.forename: Georg
authors.initials: G
authors.lastname: Deuretzbacher
authors.forename: Lothar
authors.initials: L
authors.lastname: Wiesner
authors.forename: Michael
authors.initials: M
authors.lastname: Leunig
authors.forename: Petra
authors.initials: P
authors.lastname: Algenstaedt
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Rüther
mesh_terms: D000818:Animals; D001783:Blood Flow Velocity / Q000502:physiology; D001861:Bone Regeneration / Q000502:physiology*; D016025:Bone Transplantation / Q000379:methods; D001842:Bone and Bones / Q000098:blood supply*; D005269:Femur / Q000098:blood supply / Q000473:pathology / Q000637:transplantation; D006801:Humans; D051379:Mice; D016513:Mice, SCID; D008833:Microcirculation / Q000502:physiology*; D018919:Neovascularization, Physiologic / Q000502:physiology*; D010012:Osteogenesis / Q000502:physiology*