The reaction of the dura to bone morphogenetic protein (BMP) in repair of skull defects

Efficacy of bone morphogenetic protein (BMP) with osteopromotive membranes--an experimental study in rat mandibular defects

The effect of bone morphogenetic protein (BMP) on healing of standardized bone defects was studied with and without the placement of osteopromotive membranes. Two different bovine BMP (bBMP) preparations were tested. These contained primarily collagen as a carrier. Standardized transosseous bone defects, 5 mm in diameter, were created in mandibles of rats. If left untreated, such "critical size defects" never heal during the lifetime of the animal, whereas covering with an osteopromotive membrane is known to cause complete healing of the defects in 6 weeks. The bBMP was implanted in defects and were either covered with an expanded polytetrafluoroethylene (e-PTFE) membrane (GORE-TEX) or were left uncovered. Control defects did not receive any bBMP and were either covered with membrane or were left uncovered. Histological evaluation was made after 12 d and 24 d of healing, respectively. Implantation of bBMP alone was associated with formation of voluminous amounts of new bone, resulting in essentially complete defect healing at 24 d. However, the combination of membrane and bBMP was clearly less effective in stimulating bone healing, being only about as efficient as when using membranes alone. It was concluded that whereas both bBMP preparations were strongly osteoinductive, no further improvement of bone healing was when the membrane technique was supplemented with bBMP, compared to membrane alone. An explanation may be that the presence of an e-PTFE membrane prevents the degradation of the carrier material in the preparations, thus strongly reducing the availability of bBMP.

Recombinant bone morphogenetic protein-2 enhances bone healing, guided by osteopromotive e-PTFE membranes: an experimental study in rats

It has been shown earlier that it is possible to improve bone healing, to regenerate previously existing bone, and to create new bone by means of an osteopromotive membrane technique. The present study addresses the question of whether it is possible to combine this technique with a locally applied factor, stimulatory to osteogenesis. Circular transosseous 'critical size' defects in mandibles of rats were either implanted with recombinant human bone morphogenetic protein type 2 (rhBMP-2) or were left empty; half the number of implanted and half the number of empty defects were covered with an expanded polytetrafluoroethylene (e-PTFE) membrane (GORE-TEX). Results were evaluated after 12 and 24 days of healing by a histomorphological scoring system. Implantation of rhBMP-2 alone resulted in bony bridging of the defect after only 12 days, but also in voluminous amounts of new bone outside the original defect area. When rhBMP-2 was combined with membrane, newly formed woven bone bridged the defect and the bone contour was maintained by the membrane. The combined treatment with membrane and rhBMP-2 demonstrated a significantly better bone healing than with e-PTFE membrane alone at both 12 days and 24 days of healing. It was concluded that rhBMP-2 has a strong osteoinductive potential and, in contrast to what was found earlier with other types of BMP preparations, this potential was retained when combining the rhBMP-2 with the osteopromotive membrane technique, yielding better bone healing than with the membrane alone, and at the same time maintaining the bone contour.(ABSTRACT TRUNCATED AT 250 WORDS)

Implantation of sepiolite-collagen complexes in surgically created rat calvaria defects

The response of osseous tissue to the implantation of sepiolite-collagen complexes has been studied. Sepiolite, sepiolite-collagen complex and 0.5% glutaraldehyde-treated sepiolite-collagen complex were implanted in created circular defects in rat calvaria. The tissue reactions were analysed using light, transmission and scanning electron microscopies. The patterns of bone growth were radiographically analysed and the bone activity was indirectly quantified by using a point-count method. The reaction against the three implanted materials is characteristic of a foreign body reaction with abundant macrophages and giant cells. Implanted products have been detected in macrophages, which suggest the involvement of phagocytosis in the resorptive process. Bone grew at the implantation sites originating excrescences or sometimes a thin bridge at the defect margins. The studied materials, after implantation in contact with bone tissue, did not produce any toxic effect or necrosis, allowing bone activity.

Temporal dynamics of healing in rabbit cranial defects using guided bone regeneration

PURPOSE

The objective of this study was to histologically evaluate the early stages of bone regeneration using rabbit calvaria defects in conjunction with guided tissue regeneration.

MATERIALS

A semilunar cutaneous-periosteal flap was raised on the forehead of four rabbits exposing the top of the skull. A standardized transosseous skull defect (> or = 15 mm in diameter) was made in the area of the right parietal bone with a rotating round bur. Care was taken not to damage the underlying dura. A flat expanded polytetrafluoroethylene (ePTFE) membrane was placed to cover the defect. The membrane was tightly adapted, extending at least 4 mm onto intact bone, and the flap was sutured. One, 2, 3, and 5 weeks later, the specimens were removed and processed using standard, undecalcified, hard-tissue histologic techniques. Contact radiographs were also taken.

RESULTS

Bone growth increased with time, starting at the borders of the defect. At 1 week, trabeculae of woven bone grew into the highly vascularized loose connective tissue occupying the defect. Two weeks postsurgery, isolated islands of new bone were detected in this connective tissue. Subsequently, neighboring small islands merged to form large islands. In later stages, the primary trabeculae of woven bone were reinforced by layers of regularly deposited lamellar bone.

CONCLUSION

Rabbit calvaria defects treated by guided tissue regeneration heal by ingrowth of woven bone from the defect margins and by formation of bony islands within the defect area. Bone healing showed the histophysiological characteristics of intramembranous bone.

Experimental study on demineralized bone matrix (DBM) and coral as bone graft substitutes in maxillofacial surgery

Three different bone substitutes were implanted in a standardized nasal bone defect in 30 Wistar rats. The results were assessed at 2 months by macroscopic examination, contact radiography, and histologic analysis. Demineralized osseous implants sterilized by ethylene oxide induced bone formation in 90% of the the cases, as no heat-treated graft showed any bone formation. Coralline grafts were osteointegrated in 50% of the cases, but osteoconduction was not sufficient to achieve complete bone repair. This study implies that ethylene oxide sterilization does not impair biologic properties of demineralized grafts, but further studies on more evolved animal species are necessary before human implantation.

Growth factors and wound healing in otolaryngology

An immense amount of knowledge has been gained over the last decade in the realm of polypeptide growth factors. Only recently has this new information made an impact in otolaryngology. This article is a brief overview of peptide growth factors in relation to wound healing and otolaryngology.

Potentiation of transforming growth factor (TGF-beta 1) by natural coral and fibrin in a rabbit cranioplasty model

The association of a biodegradable material and a growth factor could be of clinical value for treating bone defects. We therefore tested the association of transforming growth factor beta (TGF-beta 1) in fibrin glue and coral granules to heal skull defects in rabbits. Adult rabbits underwent a double trepanation symmetrically in both parietal bones. Using histomorphometry, we compared bone repair after 1 month in control animals (n = 5) and in animals treated with either TGF-beta 1 as a single injection of 1 microgram in methylcellulose (n = 5) or in fibrin glue (n = 5), or with coral granules in fibrin glue (n = 4) or with coral granules and TGF-beta 1 1 microgram in fibrin glue (n = 5). We measured the diameter of the remaining defect and the surface of the bone growth. TGF-beta 1 without coral in either methyl cellulose or fibrin induced a partial closure of the defect as assessed by a significant decrease in the defect diameter, compared with the control group. However, the association of TGF-beta 1 in fibrin and coral induced an area of the bone growth higher than in any other groups (P < 0.05). Two months after surgery, this triple association induced a better healing of the defect than coral alone or control group. In each group treated with TGF-beta 1, the mineralization rate was increased not only at the treated side but also in the contralateral defect which was untreated, suggesting a diffusion of the growth factor.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of freeze-dried poly-L-lactic acid discs mixed with bone morphogenetic protein on the healing of rat skull defects

Porous poly-L-lactic-acid discs were prepared by a freeze-drying method (FDPLLA). During preparation of the discs, semipurified bone morphogenetic protein (BMP) was mixed with the poly-L-lactic acid (500 micrograms BMP per disc). The discs were implanted in the skull defects of rats and retrieved after 2 or 4 weeks. With the FDPLLA/BMP discs, new bone formation was observed at 2 weeks after implantation. On the other hand, plain FDPLLA discs failed to produce new bone formation even at 4 weeks after implantation. Calcium content of the retrieved FDPLLA/BMP discs was statistically higher than that of plain FDPLLA discs. These results suggest that FDPLLA may be an effective delivery system for BMP and that the FDPLLA/BMP disc may potentially have usefulness as a bone graft substitute.

Effect of glass bioactivity on new bone development induced by demineralized bone matrix in a rat extraskeletal site

The effects of two kinds of bioactive glass and two kinds of phosphate-free glass on new bone development induced by demineralized bone matrix (DBM) were studied in the rat abdominal muscle pouch model. After 8 weeks' implantation histomorphometric analysis revealed that the amount of new bone in DBM combined with bioactive glass was comparable to DBM without bioactive glass. DBM grafts combined with phosphate-free glass showed significantly less new bone formation. Scanning electron microscopic examination confirmed that new bone bonded to the surface of bioactive glass. The release of ions from the glass seemed to slow down after new bone had bonded to it. Exclusion of phosphate from a bioactive glass resulted in loss of ability to develop the Ca,P-rich surface layer needed for bone bonding.

Immediate reconstruction of massive cranio-orbito-facial defects with allogeneic and alloplastic matrices in baboons

40 cranio-orbito-facial osseous defects were created in 20 adult male baboons (Papio ursinus) to test the effectiveness of an allogeneic and an alloplastic matrix implant for the functional and morphological repair of the disassembled craniofacial complex. In each animal, one defect was reconstructed with a craniofacial bone segment harvested from donor adult baboons, and processed so as to obtain autolysed antigen-extracted allogeneic (AAA) bone matrix, preserving the bone morphogenetic protein (BMP) activity essential for bone induction. The contralateral defect was implanted with spherical macrobeads of polymethylmethacrylate (PMMA) coated with poly-2-hydroxyethylmethacrylate (PHEMA), and sintered into a porous molded implant, replicating the structural anatomy of the avulsed osseous segment. Histological analysis was carried out on undecalcified and decalcified bone sections prepared from specimens harvested at 3, 6 and 12 months after surgery. In AAA bone, the morphogenetic response was characterized by vascular invasion and mesenchymal cell aggregation after partial resorption of the implanted matrix. This was followed by bone deposition at the osteotomy interfaces and within the medullary spaces of the implanted AAA bone. Although bone ingrowth did occur in some PMMA/PHEMA specimens, the majority of implants showed fibrous union at the recipient interfaces. The limited bone ingrowth may be related to narrow interconnections between larger porous spaces after chemical synthesis of the two polymeric components. Osteogenesis in AAA bone appeared consistent with osteoconductive invasion from the viable bone at the recipient interfaces. In addition, the finding of a delicate trabecular-like bone, appositional to the central areas of the implanted matrix, suggests bone formation by induction.

Mineralization of dentinal collagen sheets complexed with alkaline phosphatase and integration with newly formed bone following subperiosteal implantation over osseous defects in rat calvaria

We addressed the question to what extent alkaline phosphatase (ALP) can induce mineralization of a collagenous matrix implanted subperiosteally, and how the graft interacts with the underlying bone. Bovine intestinal ALP was bound to sheets of guanidine-extracted, demineralized bovine dentin by using the crosslinking agent 1-ethyl-3(3-dimethylaminopropyl)carbodiimide.HCl. The complexes (with active enzyme) and control grafts (no enzyme) were implanted over osseous defects in opposite halves of rat calvaria. After time intervals varying from 3-12 weeks, the calvaria were processed for light and electron microscopic examination and histomorphometric analysis. The ALP-containing sheets (but not their controls) rapidly accumulated mineral crystals. As the complexes mineralized, osteoblasts appeared and formed a layer of bone in direct contact with the grafted material. The results indicate that ALP induced the deposition of mineral crystals, and strongly suggest that it is this mineral component which influenced the formation of bone.

Bone induction by composites of bioresorbable carriers and demineralized bone in rats: a comparative study of fibrin-collagen paste, fibrin sealant, and polyorthoester with gentamicin

Host tissue response and heterotopic osteoinduction by composites of demineralized bone matrix and three different substances used as bioresorbable carriers implanted in the abdominal muscles were evaluated by strontium 85 uptake and histology 4 weeks postoperatively in 60 male Wistar rats. Both fibrin-collagen paste and fibrin sealant inhibited bone induction and produced a chronic inflammation; part of the fibrin-collagen paste was still present at 4 weeks. Polyorthoester with gentamicin was almost completely absorbed, induced minimal tissue reaction, and did not inhibit osteoinduction.

Contribution of autogeneic membranous bone chips and bone paste to healing of rabbit skull defects

In skeletal surgery, bone chips and bone paste are often used to facilitate bony repair. However, no comparative investigation between these forms of bone graft implantation has been undertaken. In this study four trephine skull defects were produced in each of 14 adult rabbits and inlays of bone paste and two separate amounts of bone chips were then implanted in each animal. The results were compared relative to a control defect and assessed by gross inspection, light microscopy, and contact radiography after periods of 4 and 15 wk. Bone chips offered only minor advantages over controls in the defects investigated and differences in bony regeneration between the diversified amounts of bone chips were negligible. After bone paste implantation, a cellular and mature bone was rapidly produced. The clinical significance of these findings is discussed.

Bone induction by demineralized bone particles: long-term observations of the implant-connective tissue interface

Allogeneic demineralized bone (DB) powder was applied to subcutaneous pockets and cranial defects of rats and histological, histomorphometrical, and radiological evaluation was performed one to 52 weeks after implantation. In both type of implants cartilage formation was observed after 1 week between DB particles and in former vascular channels and cervices within the particles. Foci of bone formation were observed after 2 weeks in the center of the implant. Remineralization of DB particles only occurred in close contact with the new bone tissue by which they became incorporated, indicating that remineralization of DB is a continuation of a process which has started in live bone. At all times, the interface between implant and connective tissue of the host consisted of a layer of non-remineralized DB particles embedded in fibrous connective tissue. This rim was thinner at the dura-mater-side in the cranial implants, compared with the skin-side of the same implants as well as all sides of the subcutaneous implants. The rim became thinner with time but never disappeared completely. The outer contour of the mineralized implant sometimes ran right through a DB particle. A critical concentration of bone-inducing agent leaking from DB particles seems to be necessary for bone induction. We suggest that at the interface of the implant and loose connective tissue this concentration is never reached, which results in maintenance of the rim.

Regeneration of calvarial defects by a composite of bioerodible polyorthoester and demineralized bone in rats

A study was performed to evaluate regeneration of defects in rat calvaria either unfilled or filled with a bioerodible polyorthoester only, demineralized bone only, or a composite of both. At 4 weeks, histological and radiographic studies showed that defects filled with a composite of bioerodible polyorthoester and demineralized bone or demineralized bone alone were bridged by bone. Unfilled defects or defects filled with polyorthoester only did not heal. The polyorthoester caused slight inflammation that subsided by 3 weeks, and only traces of the filler could be detected at 4 weeks. The polyorthoester provided local hemostasis when used either alone or in composites with demineralized bone. The composite implant was moldable, easily contoured, and technically easier to use than demineralized bone alone.

A glass ionomer for reconstructive surgery. Ionogran--an ionomeric micro implant. A biological evaluation

A porous glass-ionomer (Ionogran) was evaluated and compared to hydroxyapatite Interpore 200. In an in-vivo screening test procedure, microfilter diffusion chambers embedded on the tibial surfaces of adult baboons were used to evaluate cell survival, cell regeneration and cell differentiation. In a bulk testing procedure a standardized bone core was removed from the tibia through the knee joint and the material exposed to the defense mechanisms of the system. Both materials were found to be biocompatible, non-toxic and did not inhibit cell proliferation in the enclosed environment of diffusion chambers. Histological response and cell arrangement within the chambers containing Ionogran were similar to that of the controls with cancellous bone only. Highly specialized haemopoietic tissues were in direct contact with the Ionogran. In the presence of hydroxyapatite new bone was actively formed. Haemopoietic elements were never found in contact with the hydroxyapatite. In the bulk testing procedure, the general histological appearance was that of a reactive tissue response to a low grade persistent stimulation coupled to surface degradation products. The Ionogran was found to be stable, osteo-conductive and promoted osteoid formation when in contact with bone. From a clinical point of view the material could serve as a permanent scaffold, maintaining form, and will be of value in bone reconstructive procedures.

Rapid publication. TGF-beta 1 induces bone closure of skull defects

Transforming growth factor beta 1 (TGF-beta 1) is a multifunctional regulatory protein. It is capable of inducing site-specific healing responses by increasing collagen synthesis and deposition as well as remodeling at sites of soft tissue repair. Large bony defects in the skull heal by fibrous connective tissue and never form bone unless osteoinductive bony fragments or powders are placed in the defect. We have found, however, that the single application of human recombinant TGF-beta 1 in a simple 3% methylcellulose gel to skull defects induced a dose-dependent increase in intramembranous bone formation. Complete bony bridging of defects occurred within 28 days after treatment with 2 micrograms TGF-beta 1. Sites treated with vehicle alone did not heal with bone formation but rather contained dense fibrous connective tissue between the defect margins.

Tissue compatibility of methylmethacrylate in cranial prostheses: a preliminary investigation

An in vivo study using 48 disease-free male Lewis rats was conducted to determine the histologic difference between an alloplastic cranial prosthesis made with a monomer directly from the manufacturer and a triple-distilled monomer. The histologic difference in the tissue reaction between a cranial prosthesis sterilized with ethylene oxide and one sterilized with cobalt-60 irradiation was also evaluated. Histologic tissue biopsies of the cranium and brain tissues were obtained at 1, 3, 6, and 12 weeks. Tissue biopsies after the third week showed minimal inflammation and the microscopic findings were consistent with the reparative stage of wound healing. The findings also suggest that distillation of the monomer in heat-polymerized methyl-methacrylate is unnecessary for cranial prostheses. Cobalt-60 irradiation was found to be an effective alternative method of sterilization for such prostheses.

Bone morphogenetic protein: a review

The osteogenic potential of bone and bone matrix has been characterised only by its biological effects and the parameters influencing it. Recently, the osteoinductive ability of bone matrix had been defined chemically by the description of a bone morphogenetic protein (BMP), and the parameters of osteogenic factors from different species have now been recognised. The current state of isolation, purification and characterisation of these factors is summarised in this review. General aspects of the isolation and testing of BMP preparations, and the results of orthotopic application of BMP implants, including clinical cases, are reported.

Bone-inducing implants in head and neck surgery: an experimental study

The use of autologous bone for head and neck reconstruction requires a separate harvesting procedure which provides limited quantities of bone that may become infected or undergo resorption after being implanted. In this study, a collagen/ceramic carrier containing osteoinductive factor extract (OFE) was used in a rabbit facial augmentation model. Bone-inducing activity of these implants were evaluated in subcutaneous, intramuscular, and subperiosteal sites. Implants with (test) and without OFE (control) were placed on opposite sides of the face in 40 rabbits, and were harvested at 21 days. Bone formation was evaluated by implant alkaline phosphatase determinations and histomorphometry. Osteoblastic activity, bone formation, and preservation of facial augmentation were noted in the OFE implants, showing maximal bone formation when implanted subperiosteally. Control (no OFE) and demineralized bone implants showed no bone formation. Before these implants can be used clinically, novel bone-inducing factors must be manufactured by recombinant deoxyribonucleic acid (DNA) methodology to verify activity of the homogeneous molecule which would be free of other proteins or infectious agents.

Regeneration of cranial suture and bone plate lesions in rabbits. Implications for positioning of osteotomies

The positioning of osteotomies in intramembranous cranial bone was studied by exploring the pattern of bone regeneration in growth areas (the sutural region) as compared to that of the bone plate proper. Trephine defects in the left coronal suture area and the right parietal bone were produced in fifty-nine young rabbits. A pilot study to refine operative and analytical methods comprised 22 animals. The experiments were terminated at one, three, and six weeks after surgery. The bone regenerative response was assessed by x-ray planimetry, plain microscopy, enzyme histochemistry, and fluorescent labelling. Only minor divergences in healing capacity between the two defects were found. No adverse effects on the growth process were indicated. As to clinical management, the findings suggest that osteotomies designed to traverse sutural areas will, under normal circumstances, regenerate in a similar manner and rate to adjoining bone plates.

Investigation of a hydroxyapatite and calcium sulfate composite supplemented with an osteoinductive factor

In the search for a suitable bone graft substitute, a study was conducted using a material that combined a proven osteoconductive composite, hydroxyapatite-calcium sulfate (HA/CS), with an osteoinductive factor, bovine osteogenic factor (OF). The initial study demonstrated the osteoinductive potential of OF in the rabbit muscle model. Once satisfied that the OF was active, it was added to the HA/CS composite and placed in 8-mm trephine defects in the rabbit cranium. This HA/CS/OF was directly compared to HA/CS augmented with a control protein, rabbit serum albumin (RSA). Animals were sacrificed at 4 and 8 weeks. Results demonstrated increased bone formation with the addition of the OF to the composite with bridging of the defects, in most cases, by 4 weeks. No bridging was seen, at this time period, in the other defects left unfilled or filled with HA/CS/RSA. Osteogenic factor, with the appropriate delivery system, can induce bone formation in the rabbit muscle. It may also increase the rate of bone formation at early time periods in a bony defect site when the delivery system is the osteoconductive composite HA/CS.

Pathologic and enzyme histochemical studies on bone formation induced by bone morphogenetic protein in mouse muscle tissue

Bone morphogenetic protein (BMP) irreversibly induced the differentiation of mesenchymal-type cells into osteoprogenitor cells for endochondral ossification. During the process of BMP-induced differentiation in mice, 4 cell type (chondroblasts, osteoblasts, chondroclasts, and osteoclasts) were examined for phosphatase and succinate dehydrogenase using a wide range of buffers (4.0 less than or equal to pH less than or equal to 9.2). During the chondroid tissue-forming stage (1 week), chondroblast-like or osteoblast-like cells expressed phosphatase activity at 6.8 less than or equal to pH less than or equal to 9.2; chondroclast-like or osteoclast-like cells expressed phosphatase activity at 4.0 less than or equal to pH less than or equal to 5.8. However, mature chondrocytes found in hyaline cartilage expressed phosphatase activity between 6.6 less than or equal to pH less than or equal to 7.6 (2 weeks). During the process of endochondral ossification, alkaline phosphatase activity decreased in osteoblast-like cells with traces of acid phosphatase activity still detectable. Chondroclastic and osteoclastic giant cells were characterized by intense succinate dehydrogenase activity.

Reconstituted bovine skin collagen enhances healing of bone wounds in the rat calvaria

The value of reconstituted fibrillar collagen (Zyderm Collagen Implant I, a concentrated solution of pepsin-solubilized, bovin skin collagen) as a bone graft material was tested in 4 mm diameter surgically created defects of rat calvaria. All wounds were allowed to heal for 4 weeks, and were assessed both qualitatively and by computer-assisted morphometry. The fibrillar collagen was found to produce significantly more new bone than no graft or than heat-denatured fibrillar collagen. The fibrillar collagen was generally well tolerated, appeared to act as a hospitable osteoconductor, and became incorporated into the newly formed bone. The effect of collagen concentration was also tested by comparing the fibrillar collagen at 3.5% (Zyderm Collagen Implant I) with 6.5% suspension of collagen (Zyderm Collagen Implant II). There were no significant differences observed, but a definite trend was evident for Zyderm II to encourage more bone formation than Zyderm I. It is concluded that reconstituted fibrillar collagen is a hospitable, osteoconductive substance that enhances bone healing of calvarial defects in the rat.

[Use of allografts in oncology and traumatology]

Fresh allografts of bone are commonly used for repairing small defects, but preserved grafts are required for replacing large sections of long bones. We have collected these grafts in sterile conditions and then preserved the specimens in liquid nitrogen at minus 196 degrees Celsius. Biological, histological and biomechanical studies have confirmed the value of this technique. The method preserves the normal architecture of bone and viable cartilage cells. The medullary cells are gradually destroyed in situ leaving a protein matrix to which mineral components are fixed. Neither of these are antigenic in allografts and immunological problems are rarely encountered. When the recipient site is well vascularised the graft becomes fully integrated within two or three years. Doubt has been expressed as to the functional viability of the cartilage in large allograft joint replacements, and is has been suggested that metal prostheses might be better. Research is in progress in the use of ligament allografts and on the use of bank bone with large prostheses. The functional results are satisfactory in more than 90% of patients treated with these large allografts.

Histopathologic and microdensitometric analysis of craniotomy bone flaps preserved between abdominal fat and muscle

In this study we have investigated the preservation of craniotomy bone flaps within the abdominal wall and the use of such bone flaps for cranioplasty in the early postoperative period. The craniotomy flaps were implanted into a "pocket" between abdominal fat and muscle of adult rabbits, and were later used as autologous grafts in cranioplasty. The flaps were analyzed by histopathologic and microdensitometric methods. In 36 of the flaps, new bone formation near the periosteum was observed. The bone flap-cranium relationship was supplied by a bony tissue. Microdensitometric analysis showed increased optic densities in the experimental group.

Bone cell differentiation and growth factors

Bone morphogenetic protein and bone-derived growth factors are biochemical tools for research on induced cell differentiation and local mechanisms controlling cell proliferation. Bone morphogenetic protein irreversibly induces differentiation of perivascular mesenchymal-type cells into osteoprogenitor cells. Bone-derived growth factors are secreted by and for osteoprogenitor cells and stimulate DNA synthesis. Bone generation and regeneration are attributable to the co-efficiency of bone morphogenetic protein and bone-derived growth factors.