Invasion of Blood Vessels into Cartilage Implanted into Skull Defects

In previous studies, cartilage nodules grown in a rotating bioreactor were used to heal defects in the skulls of mice.  Histological samples of skulls with or without implants were taken at 2 and 3 weeks. These sections showed that most of the cartilage was gone by 2 weeks after implantation, and by 3 weeks, none of the implanted cartilage remained. Cartilage in the implanted nodules is organized into “growth plate-like regions,” with the typical growth plate zones, and which undergo mineralization and secrete Indian hedgehog in prehypertrophic regions and VeGF in the hypertrophic zone. In this study, cartilage remaining after two weeks of implantation was examined to determine if vascularization proceeded as in vivo. Objective: To determine whether growth plates of implanted cartilage undergo vascularization in the same way as growth plates in situ. Methods: Histological serial sections from 3 implant studies, stained to distinguish bone, cartilage and vessels, were used. Cartilage was located and any zones present noted as was whether or not the implant was surrounded by a layer of bone, and where the vessels penetrated the nodule. Results:   Most remaining cartilage consisted of hypertrophic cells, although in some sections, a proliferative zone was seen.  The cartilage remnant appeared to be encircled by a layer of bone, except where the proliferative zone remained. Vessels entered the lacunae of hypertrophic cells via breakdown of the horizontal septa. Conclusion: Vascularization of the cartilage implants occurs in the same manner as vascularization in growth plates in situ, making the formation of ectopic bone a good model to study endochondral ossification.