The programme for sculpting the architecture of the cortico-cancellous structures of bone, or the regulation of bone regeneration and bone tissue engineering requires three key components: an osteoinductive molecular signal, an insoluble substratum which delivers the signal and acts as a scaffold for the induction of new bone formation, and host recipient cells capable of differentiation into bone cells in response to the osteoinductive soluble signal [2, 4, 6, 9, 10, 12]. All three components are subject to manipulation including the signalling molecules to be delivered and the nature of the carrier biomimetic matrices, which additionally can be loaded with responding cells and tissues [2, 6, 48].
Based on studies performed to determine whether BMPs/OPs present in the concavities are derived from the circulation or are produced after local expression by cells present in the concavity, Northern blot analyses have shown the expression of the mRNA of osteogenic markers of the TGF-β superfamily to be present within the concavities of the biomimetic matrices. We now can propose the following sequence of events culminating in de novo bone formation:
1) Vascular invasion and capillary sprouting within the invading tissue with capillary elongation in intimate contact with the implanted hydroxyapatite biomatrix (Fig. 4A). Vascular invasion is a prerequisite for bone formation. Indeed, in the concavities of the hydroxyapatite substratum bone forms only when a strong vascular invasion is histologically present in proximity to the newly formed bone [2, 12, 40, 47] (Figs. 4A and C).
2) Following the attachment and differentiation of mesenchymal cells at the hydroxyapatite/soft tissue interface of the concavities, expression of BMPs/OPs gene products by differentiating osteoblast-like cells. This has been shown by Northern blot analyses of RNA extracted from tissue harvested from the concavities of the substratum as well as immunolocalization [2, 12].
3) Synthesis of specific TGF-β superfamily gene products as markers of bone formation by induction, as shown by immunolocalisation of OP-1 and BMP-3 within the cell cytoplasm and at the interface of the hydroxyapatite biomatrix with the mesenchymal tissue [2, 12, 40, 47].
4) Intrinsic osteoinduction with further differentiation of osteoblastic cells. This is dependant upon a critical threshold of endogenously produced BMPs/OPs initiating bone formation by induction as a secondary response [2, 9, 10, 12].