Bone Formation Zones in Heterotopic Ossifications: Histologic Findings and Increased Expression of Bone Morphogenetic Protein 2 and Transforming Growth Factors ß2 and ß3

Abstract

Heterotopic ossifications (HOs) formed after total endoprosthetic replacement of the hip joint were collected during revision surgery (n = 7). Tissues collected during regular hip arthroplasty (n = 12) were used as reference. Histomorphometric analysis was performed for assessment of bone formation activity in HOs and reference bone. HOs were dissected with histological guidance into three zones: formed bone, zone of active bone formation, and zone with fibrous connective and fibrocartilagineous tissue. Relative expression of the mRNA for bone morphogenetic protein 2 (BMP-2), transforming growth factor beta2 (TGF-beta2), and TGF-beta3 was determined by reverse-transcription polymerase chain reaction relative to beta-actin. Expression of all three growth factors was higher than in orthotopic bone. Similarly, the osteoid surface density was increased in HOs. The levels of all growth factors were higher in the zone of active bone formation or remodeling than in the zone of formed bone. In matured HOs, the osteoid surface density as well as mRNA levels were lower, although still significantly raised, indicating that bone formation slows down after 2 years. Immunohistochemical analysis demonstrated the presence of TGF-beta1, TGF-beta2, TGF-beta3, and BMP-2 proteins in the zone of bone formation. We conclude that bone formation after heterotopic bone induction is initially intense, slows down within 2 years, and thereupon continues as active remodeling mainly on the border of HO. Our data indicate that BMP-2, TGF-beta2, and TGF-beta3 are involved in bone formation in HO.

Bone tissue content of TGF-β2changes with time in human heterotopic ossification after total hip arthroplasty

“…It should, however, be noted that overall bone content of TGF-b 2 was lower compared to the other TGF beta isoforms (see Table I). We also showed that the TGF-b 3 expression was increased (Toom et al 2007) while no difference on the protein level was seen in this study. It should be emphasized that in the tissues the protein level do not always correspond to the mRNA profile.…”

“…This was confirmed by the current study, since relatively little cartilage formation was found. We have earlier also shown the increased expression on TGF-b 2 and TGF-b 3 in the immature HOs as compared to the mature HOs, with the method, where beta-actin was used as a control for expression (Toom et al 2007). This study shows that the HO content of TGF-b 2 is clearly higher in the immature HOs, compared to the mature HOs and the increased expression thus leads to increased formation of TGF-b 2 .…”

“…They demonstrated the expression of TGF-b 1 in the areas of endochondral ossification by using digoxigenin-labeled cDNA probes. Our earlier results indicate that TGF-b 2 and TGF-b 3 isoforms are related to the intramembranous ossification, since it seems to be the main way of the HO formation after THA (Toom et al 2007). This was confirmed by the current study, since relatively little cartilage formation was found.…”

“…HO formation is relatively rapid in initial phases but slows down later during maturation of the ectopic bone, which usually takes 12 -18 months (Toom et al 2007). At this later stage the osteoblastic activity is lower but remodeling still takes place.…”

ANA Deficiency Enhances Bone Morphogenetic Protein-induced Ectopic Bone Formation via Transcriptional Events

“…Recently, ectopic and heterotopic ossifications have been reported to be in association with high levels of expression of BMP2 (22). Here, we examined whether ANA regulates BMP activities with respect to ectopic bone formation.…”