Modeling Principle

The Beagle Skull Defect Model is a standardized calvarial critical-sized bone defect pathological model recognized by SCI, which is constructed by removing full-layer cranial cortical bone and diploic tissue via surgical grinding on the skull vertex. It accurately reproduces the pathological characteristics after debridement of craniocerebral trauma, resection of skull tumors and craniotomy in clinic, including osseous skull defect, destruction of local bone marrow structure under intact dura mater, massive loss of endogenous osteoblasts, limited creeping osteogenesis ability of peripheral bone tissue in defect area, preferential filling of defect cavity by fibrous connective tissue, and failure of spontaneous complete osseous healing. It overcomes the experimental deficiencies of rodents, which have huge differences from human calvarial repair behavior in skull thickness, diploic structure, bone cell distribution, skull osteogenic repair rate and response sensitivity to osteoinductive materials.

The calvaria of adult Beagle dogs consists of three layers: outer table, diploë and inner table. The anatomical stratification, bone mineral density, distribution of bone marrow stromal stem cells, intramembranous ossification repair mechanism, recruitment rule of inflammatory cells and activation mode of TGF-β/BMP osteogenic signaling pathway are highly homologous to human skull. Animals are fixed in prone position under general anesthesia, scalp and galea aponeurotica are stripped layer by layer through midline incision on the top of the head to fully expose the bilateral parietal bone area of the skull vertex. A medical dental burr is used for uniform grinding to penetrate the outer table and diploë until the superficial layer of inner table cortex, so as to create circular full-thickness skull defects with fixed diameter, while the dura mater is kept intact to maintain intracranial physiological homeostasis. Immediately after defect formation, local bone marrow tissue is damaged and massive in-situ osteoblasts undergo apoptosis and disappear. The body only relies on a small number of osteoblasts in the periosteum around the defect for slow creeping proliferation. After organization of blood clot inside the defect cavity, it is rapidly filled by fibroblasts and collagen fibers to form dense fibrous scar tissue that blocks the creeping growth of bone callus. Long-term observation shows only a small amount of punctate bone callus in the defect area without continuous osseous bridging, stably forming a permanent nonunion phenotype of calvarial critical defects. This model fully recapitulates the classic repair cascade of human skull defects: full-thickness skull tissue defect – loss of in-situ osteogenic components – limited osteogenesis of periosteum – fibrous tissue occupation – spontaneous healing disorder of critical defects.

The skull of Beagle dogs is flat and open, multiple symmetrical defects can be constructed on both sides, the defect diameter and grinding depth can be accurately standardized and controlled, the bone repair cycle is mild with low individual data dispersion, and there are no spontaneous skull lesions or basic intracranial diseases. Three pathological stages can be clearly distinguished: acute inflammatory blood clot stage of defects, fibrous tissue proliferation and filling stage, and terminal nonunion stage with slow bone callus growth. It serves as a standardized large-animal gold-standard model for translational researches on various porous bone scaffolds, composite stem cell bone grafts, osteoinductive active factors, sustained-release osteogenic drugs and skull repair materials.

Modeling Success Criteria

In-vivo Imaging and General Skull Phenotype

Micro-CT of the sham control group showed continuous and intact skull cortex with uniform bone density, and the general skull was smooth without defects. Micro-CT of the model group at Week 12 showed extremely low bone volume fraction in the defect area, only a small amount of punctate bone callus at the defect edge, and the defect closure rate was less than 10%. Gross anatomical observation showed depression in the skull defect area, and gray-white dense fibrous tissue filled the defect cavity on the section without continuous osseous bridging structure. The macroscopic phenotype of critical skull defect nonunion was typical with extremely significant statistical difference between groups, which could preliminarily confirm the successful construction of the Beagle skull defect model.

Quantitative Biochemical Gold-Standard Serum Indexes

The serum levels of TNF-α and IL-1β in the acute phase of the model group were significantly higher than those in the sham group; the expression of bone formation markers OC and ALP was down-regulated in the middle and late observation period, while the bone resorption marker CTX-Ⅰ increased compensatorily; the contents of osteogenic proteins TGF-β1 and BMP-2 in local defects decreased obviously, which fully matches the biochemical diagnostic characteristics of clinical skull defects: local inflammatory response, decreased endogenous osteogenic activity, excessive proliferation of fibrous tissue and blocked osseous healing.

Histopathological Characteristics of Skull Defect Tissues

Sequential pathological changes of critical skull defects can be observed via combined multiple staining of defect tissues with HE, Masson, Goldner and ALP/TRAP:

1. Acute stage at Week 4 after modeling: blood clot formed in defect cavity with a small amount of inflammatory cell infiltration, and a small number of ALP-positive osteoblasts could be seen in bone tissue at the defect edge;
2. Middle fibrous proliferation stage at Week 8 after modeling: massive disorderly proliferation of blue-stained collagen fibers filled the defect cavity, the number of osteoblasts was further reduced, and almost no new bone mineralization signal was detected by Goldner staining;
3. Terminal stage at Week 12 after modeling: thick collagen fibers formed dense scar tissue occupying the whole defect space, no trabecular connection existed between bone tissues at both ends of the defect, only scattered trace mineralized bone matrix at the edge. It perfectly recapitulates the progressive pathological evolution law of permanent bone nonunion caused by inflammation and fibrous proliferation in human critical skull defects.

Gold-Standard Indexes for Osteogenic and Inflammatory Pathways

Trauma-activated NF-κB inflammatory pathway persisted in the defect area of the model group, inhibiting the expression of BMP-2/TGF-β1 osteogenic signal axis. Fibroblasts in the defect cavity were massively activated to synthesize collagen fibers, physically blocking the creeping migration of bone cells. The activity of limited osteoblasts around the defect was suppressed by the inflammatory microenvironment, failing to synthesize sufficient bone matrix to complete defect bridging repair. It accurately conforms to the complete pathological mechanism of skull defects prepared by surgical grinding: bone tissue defect – persistent activation of local inflammation – inhibition of osteogenic signals – occupation of fibrous scars – permanent nonunion of critical defects, serving as the core academic basis for verifying effective model formation.

Model Advantages

This model is a well-recognized large-animal gold-standard model for critical skull defects constructed by burr in bone tissue engineering and craniomaxillofacial surgery SCI fields. The 12-week standardized observation cycle can stably reproduce pathological phenotypes highly consistent with humans, including local skull inflammation, fibrous scar hyperplasia, insufficient endogenous osteogenesis and permanent defect nonunion. The layered skull structure, intramembranous ossification mode, bone cell response and defect repair kinetics of Beagle dogs are highly matched with human beings. The defect size is standardized and controllable with mild lesion progression and low intra-group individual differences, and the experimental repeatability is far superior to small animals such as mice, rats and rabbits. The modeling adopts physical grinding to remove osseous tissues without systemic toxic and side effects caused by chemical induction, which highly restores the real pathological state of skull defects caused by clinical craniotomy and trauma. Multiple experimental defects can be set on both sides of the same animal to save the amount of experimental animals, and the skull specimens are abundant for simultaneous multi-dimensional detection of imaging, biochemistry, pathology and molecules. It is suitable for preclinical pharmacodynamic and safety evaluation of various artificial bone scaffolds, composite bioactive factors, stem cell bone repair systems and skull repair implants. Experimental data are widely recognized by high-impact SCI journals of bone materials, orthopedics and craniomaxillofacial surgery, which is applicable for National Natural Science Foundation projects, master & doctoral proposal writing, orthopedic graduation theses and translational medical researches on skull defect repair.

Research Applications

The Beagle Skull Defect Model is mainly used to analyze basic pathological mechanisms including local traumatic inflammation induced by surgical grinding removal of full-thickness skull tissue, massive loss of osteoblasts, inhibition of BMP osteogenic pathway, fibroblast proliferation & collagen deposition, fibrous scar occupation blocking bone callus creeping, and spontaneous osseous healing disorder of critical skull defects. It is specially adopted to screen and verify porous bone scaffold materials, recombinant bone active factors, stem cell composite bone grafts, natural plant osteogenic extracts and skull implant repair consumables with functions of inhibiting local inflammatory response in defects, improving osteogenic differentiation ability of bone marrow stromal cells, up-regulating BMP/TGF-β osteogenic signals, reducing abnormal proliferation of collagen scars, inducing nascent trabecular bone formation in defect areas and realizing osseous closure repair of skull defects. It is widely used for excavation of regenerative targets for craniofacial bone defects, elucidation of regulatory network between inflammation and osteogenic differentiation, as well as preclinical in-vivo efficacy verification of various bone repair products in large animals. It is an indispensable essential standardized large-animal gold-standard model in the fields of bone tissue engineering, craniomaxillofacial surgery and regenerative medicine.

beagle skull critical defect model, calvarial bone defect repair, in-vivo evaluation of bone scaffold biomaterials, bone regeneration, bone graft & osteoinductive material preclinical study