Histopathological and immunohistochemical assessment of fabricated coral shell hydroxyapatite (CSHA) and plasma rich fibrin (PRF) in remodeling of the induced critical size defect of the mandibular bone in the dogs: Twenty-seven adult dogs of both sexes were included and equally divided into three equal groups: control, plasma-rich fibrin (PRF) and hydroxyapatite group (CSHA). The experimental mandibular bone defect was induced in a circular shape, and the dimensions of the defect were 14×5mm. Evaluation of the healing progress of the defect and associated macroscopical, histopathological, and Immunohistological findings was recorded in all studied groups at 7, 15, and 30 days post-operatively. Macroscopically, the healing was evaluated by the presence of new bone tissue filling the bone gap defect in all groups during different follow-up periods. In the plasma-rich fibrin (PRF) group, the gap was highly filled with hard, firm tissues that filled all borders and the centre of the induced gap in comparison with the coral shell hydroxyl apatite group (CSHA), which is partially filled with hard tissue. Histopathologically, the progress of healing in the PRF group was represented by the presence of highly mature connective tissue and new woven bone formation at seven days and well-developed mature bone inside defective bone at 15 and 30 days post-operatively, whereas in the CSHA group, the results were represented by the occlusion of highly mature connective tissue and new woven bone formation inside the induced hole at 15 and 30 days post-operatively. At 30 days post-surgery, in the control group, there was the presence of newly formed woven bone surrounded by the edge of the mandible bone. The immunohistochemical expression of the alkaline phosphatase (ALP) in the mandible bone at 30 days PS in the control group was represented by weak positive expression, while mild positive expression was indicated in the CSHA group and moderate positive expression in the PRF group. In conclusion, this research exhibited the role of both CSHA and PRF in improving the healing process of defective mandible bones, with a clear superiority of the beneficial value of using PRF. The histopathological and immunohistochemistry assessments emphasize these results.