Objective To investigate the role of estrogen in spontaneous repair of osteochondral defect,so as to provide guidance for the application of estrogen in tissue engineering.Methods Seventytwo healthy adult female SD rats (4-month-old) were assigned to negative control group (group A,n =24),ovariectomized group (group B,n =24) and ovariectomized + estrogen treatment group (group C,n =24) according to the random number table.Rats in group C were intraperitoneally injected 0.1 mg/kg estradiol benzoate once per week after operation.An osteochondral defect model (diameter:2 mm,height:1.5 mm) was established in femoral trochlea of rats 4 weeks after operation.At 2,10 and 20 weeks after the modeling,8 rats from each group were sacrificed.Femoral condyles were collected to measure bone volume fraction (BVF),number of bone trabeculae (Tb.N),trabecular thickness (Tb.Th) and trabecular spacing (Tb.Sp) in the osteochondral defect area by micro-CT scanning.Morphological changes were observed by HE staining,histomorphological changes and cartilage regeneration by safranine O-fast green staining,osteoclast count by tartrate-resistant acid phosphatase (TRAP) staining,distribution of matrix metalloproteinase-9 (MMP-9) by immunohistochemistry and expressions of osteoprotegerin (OPG),receptor activator of nuclear factor-κB ligand (RANKL) and MMP-9 by real-time-PCR.Results Compared to group B,increased BVF and Tb.N and decreased Tb.Sp were observed in groups A and C at each time point.More bone matrix formations were observed in groups A and C than in group B at 2 weeks,cysts (also known as cystic cavities) in regenerated subchondral bone and cracks in subchondral bone plate were observed in group B at 10 and 20 weeks,and proliferation of osteoclasts was active within lesions in group B at 10 and 20 weeks.Compared to group B,lowered levels of RANKL and MMP-9 (P ＜ 0.05) and significantly increased level of OPG (P ＜ 0.01) were observed in groups A and C at each time point,and osteoclast count in groups A and C was lowered at 10 and 20 weeks (P＜0.05).No regeneration of cartilage layer was observed in all groups.Conclusions Excessive proliferation of osteoclasts results in formation of cystic cavities and cracks.Estrogen improves the speed and quality of bone repair by regulating activities of osteoblasts and osteoclasts,so it can be used to treat osteochondral defect in tissue engineering.