ABSTRACT. We examined time-dependent histological changes of the calcified fibrocartilage area in a tibial cranial cruciate ligament (CCL) insertion after ligament resection in rabbits. The animals were divided into two groups: those undergoing CCL substance resection in the right stifle (resected group) and those receiving the same operation without CCL resection in the left stifle (sham operated group). Five animals were euthanized with deep anaesthesia at four time periods (1, 2, 4 and 6 weeks), and Haematoxylin-eosin and Safranin-O stainings and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end labeling (TUNEL) staining were performed. The average percentage of TUNEL-positive chondrocytes and the average thickness of the glycosaminoglycan (GAG)-stained area in the calcified fibrocartilage area were measured. Two and 4 weeks after the surgery, the average percentages of TUNELpositive chondrocytes in the resected group (23.8 ± 10.3% and 15.9 ± 6.7%, respectively) were significantly higher than those in the sham operated group (8.9 ± 3.8% and 7.4 ± 1.6%, P<0.05, respectively). Six weeks after the surgery, the average thickness of the GAGstained area in the resected group (7.7 ± 13. 5µm) was significantly smaller than that in the sham operated group (69.4 ± 39.9 µm, P<0.05). Our results suggest that the average percentage of TUNEL-positive chondrocytes became a peak in 2 weeks and that histological changes occurred in 6 weeks. The chondrocyte apoptosis can induce decrease of GAG-stained area after resection of CCL. Therefore, chondrocyte apoptosis in the calcified cartilage area in the CCL tibial insertion might lead to histological changes. KEY WORDS: apoptosis, cranial cruciate ligament, insertion.J. Vet. Med. Sci. 69(3): 253-258, 2007 The cranial cruciate ligament (CCL) is one of the intraarticular ligaments of the stifle joint that connects the femur and tibia (equivalent of anterior cruciate ligament, ACL in human). The CCL insertion site consists of 4 distinguishable tissue zones of transition, that is, ligaments, uncalcified fibrocartilage, calcified fibrocartilage, and bone [6,8,28]. Gradual hardness changes of different tissues reduce stress concentration at the insertion site [6,28]. Containing glycosaminoglycan (GAG) in the cartilage zone provides water absorbability and flexibility to the ligament [5]. GAG and fibrocartilage layers are presumed to resist shear stress at the insertion site [28]. A mechanical load is necessary to produce cartilage layers and GAG at the insertion site of ligaments and tendons [5,14,22].In humans, traffic accidents or sports activities sometimes cause knee ligament injuries such as an injury to ACL. Many dogs also rupture or tear CCL due to their physical action [27]. In humans after rupturing ACL, the early reformation of synovial tissue over the ends of the ligament combined with the formation of myofibroblast-like cells with contractible properties in this synovial tissue might be responsible in part for the retraction of the ...