[Clinical aspects, differential diagnosis and histogenesis of heterotopic ossification].

Abstract

Investigations regarding proliferation behaviour, histogenesis and bone transformation were carried out on soft tissue samples with 133 heterotopic ossifications (HO), collected from the surgical material of the Institute of Pathology, Berufsgenossenschaftliche Kliniken Bergmannsheil, using methods of conventional histology, histochemistry, immunohistochemistry, electron microscopy and molecular biology, yielding the following results: 1. There is only a limited pathognomonic growth pattern for the variant of non-traumatic heterotopic ossification. Basically, the ossification foci may be divided into central, intermediate and peripheral zones. Expression intensities of the proliferation markers PCNA and MIB-1 show the proliferation behaviour in peripheral areas of osteogenesis to be similar to that found in autonomous osseous neoplasias. The phenotypical picture of zonal architecture found in HO can be seen by the simultaneous demonstration of variable image sequences of multifocally arising ectopic formations of new bone with varying degrees of maturation. 2. Vascular endothelium and pericytes are integrated into the formal histogenesis of heterotopic ossification and belong to osteoprogenitor cells as "stem cells" of heterotopic ossification. 3. The histochemically established expression pattern of alkaline phosphatasis is in good correlation with the degree of activity of the step-by-step development of heterotopic ossification. The intracellular demonstration of alkaline phosphatasis is an indicator for the transformation towards an osteogenic direction. In HO, the enzyme is a major characteristic of osteoprogenitor cells. 4. According to our immunohistochemical results, the proteoglycanes Decorin and PG 100-osteogenic matrix proteins-show a phase-like expression and are involved in osteoneogenesis. They are predominantly found in the area of mineralization. The proteoglycane 100 experiences a "modulation" and can be demonstrated almost selectively in osteoclasts in advanced stages of osteodevelopment. 5. Using in situ hybridization-with digoxigenin labeled cDNA probes- and a propidium iodide counterstaining a co-expression of collagene types I, II and III-mRNA could be demonstrated in samples of ossification. The expression pattern is similar to the collagen expression I. in early phases of embryonal bone development II. with callus proliferation and shows III. also similarities to chondral neoplasias. The results underline the reactive-neoplastic character of heterotopic ossification. 6. TGF-beta 1 mRNA shows a polytopic expression pattern with accumulation in areas of osteogenesis. TGF-beta 1 was found mostly in cartilage cells of heterotopic ossifications, as were collagene types I (alpha 1) and III (alpha 1) mRNAs. In sum, our in situ hybridization results underline the central part of cartilage cells in the ossification process in ectopic osteoneogenesis. This is also indicated by a phenotypical alteration of collagen expression as well as by an accumulation of TGF-beta 1 in chondral ossification areas. In situ hybridization propidium iodide counterstaining offers a reproducable image of morphological structures in the histological slide sample by means of fluorescence microscopy. 7. Phenotyping of osteroclasts in HO revealed their derivation from mature local macrophages. The immunohistochemical characterization with the demonstration of the vitronectin receptor classified the multinucleate giant cells of HO as original osteoclasts. Their origin is in accordance with the histogenetical concept of original bone. 8. Summarizing, the results characterize heterotopic ossification as a reactive proliferative and reversible neoplasia in chronically damaged soft tissue. From the formal pathogenetical point of view, relations could be established to orthotopic osteoneogenesis as well as to impaired proliferation kinetics, similar to osseous autonomous neoplastic lesions. (ABSTRACT TRUNCATED)