COLLAGENS AND GROWTH-FACTORS IN HETEROTOP IC OSSIFICATION - EXPERIENCE WITH NONRADIOACTIVE IN-SITU HYBRIDIZATION

Heterotopic Ossification (HO) occurs as a consequence of several disea ses and of various forms of trauma. HO is particularly frequent in par aplegic patients with spinal cord lesions. It is obvious that extraske letal cells are able to differentiate into an osteogenic direction. Ho wever, the mechanisms of the induction process of HO and the stimulati ng agents are not precisely known. A novel tool for studying the ossif ication process at the level of transcription is the technique of non- radioactive in situ hybridisation. Using digoxigenin labeled cDNA prob es we investigated the distribution patterns of types I, II and III co llagen mRNAs and the mRNA of Transforming Growth Factor beta 1 (TGF-be ta 1) in heterotopic ossification of pressure sores of paraplegic pati ents. The three collagen mRNAs as well as the TGF-beta 1 mRNA exhibite d substantially divergent distribution patterns. Type I (alpha 1) coll agen mRNA was predominantly detectable in preosteoblasts, chondroblast s and chondrocytes of the ossification zone. Type II (alpha 1) collage n mRNA was nearly exclusively found in cells of the chondrogenic linea ge. Type III (alpha 1) collagen mRNA was detectable at low levels in s oft tissue, but was strongly expressed by chondroblasts and chondrocyt es of heterotopic cartilage. In contrast expression of TGF-beta 1 mRNA was found in a spatial different distribution pattern in areas of pro liferation of mesenchymal tissue and in different stages of ectopic bo ne formation. As in the case of collagen Typ I (alpha 1) and III (alph a 1) mRNAs the maximum of localization of TGF-beta 1 was dedected in c hondroblastic areas of heterotopic ossification. Taken together our in situ hybridization experiments provide evidence that chondrogenic cel ls play a central role in the process of HO with a phenotypic alterati on in collagen type expression and a strong expression of TGF-beta 1 m RNA. These findings support individual in vivo function for TGF-beta 1 in local cellular regulation of ectopic bone formation.