We found that when guinea pig spontaneous knee osteoarthritis occurs,the superficial chondrocytes in the tibial plateau degenerate first, and the chondrocytes undergo hypertrophic changes, which lead to the synthesis and secretion of MMP-13 proteins. Then,the chondrocytes undergo apoptosis. Eventually, empty chondrocyte lacunae remain in the superficial cartilage tissue. Then,the number of chondrocytes in the superficial zone decreases, resulting in their inability to continue to synthesize and secrete extracellular matrix. Due to stimulation by OA-initiation factors, such as mechanical factors, collagen II fibers in the superficial zone degrade. The superficial cartilage tissue is damaged, and longitudinal cracks appear on the surface.
Knee osteoarthritis is a common degenerative disease in middle-aged and elderly people. When OA occurs, the cartilage tissue of the knee joint degenerates, chondrocyte hypertrophic changes occur, apoptosis occurs, and empty chondrocyte lacunae remain in cartilage tissue. The cartilage matrix degrades, and the cartilage tissue cannot withstand stress, resulting in injury. However, when OA-initiation factors appear, whether the chondrocytes or extracellular matrix degenerate first remains unclear [5,6,7,8,9].Studies suggest that the extracellular matrix in the cartilage tissue degrades firstly [14, 15]. Cartilage tissue is similar to a concrete structure, and the collagen II fibers in the cartilage matrix are similar to supporting “steel bars” that protect the chondrocytes scattered therein. Some scholars have claimed that chondrocyte degeneration may be the initiating factor in the occurrence of OA, but there is no definite evidence to prove this idea. It is generally accepted that when OA occurs, both chondrocytes and the extracellular matrix undergo degeneration. However, when the cartilage matrix degrades, it loses its protective effect on chondrocytes, leading to chondrocyte degeneration and apoptosis. According to histology, the articular cartilage of the tibial plateau can be divided into the superficial zone, middle zone and deep zone. The superficial zone is the most superficial area of articular cartilage. The collagen II fibers in the superficial cartilage matrix are mainly transverse, and the long axis of chondrocytes is parallel to the collagen II fibers, exhibit a “flat length” shape. In addition, the superficial cartilage tissue is the first area affected when OA occurs, as has been basically concluded by academic circles. Some studies have suggested that the physiological status of superficial chondrocytes can directly or indirectly affect the status of middle zone and deep zone chondrocytes, so superficial cartilage tissues are very important in the occurrence and development of knee OA [16].According to the OARSI knee cartilage degeneration score system in guinea pigs, grade 0 is normal cartilage tissue;the superficial cartilage tissue is intact, without damage, and the chondrocytes exhibit no hypertrophic changes. The grade 1 cartilage tissue is uneven and damaged, and the density of chondrocytes in the damaged area, most of which have disappeared, is extremely low. However, this system does not indicate whether chondrocytes or extracellular matrix degenerate firstly when OA changes from OARSI grade 0 to grade 1.
At present, there are many experimental methods to simulate human knee OA, such as the application of traumatic OA caused by surgery, the injection of drugs (such as papain) into the joint cavity to produce knee OA, and the use of an external fixator to brake the knee joint and cause OA. However, these methods use exogenous intervention factors, so identifying an animal model that can simulate the natural degenerative process of the human knee joint is the ideal way to study the occurrence and development of knee OA. Hartley guinea pigs are a typical animal model in OA-related studies [17]. Degeneration and loss of knee cartilage tissue spontaneously appear with the increase in age; in other words,spontaneous knee osteoarthritis appears. In our experiments, we found that at the age of 8 months, the cartilage tissue of knee joint was basically normal, the superficial chondrocytes did not change hypertrophy, and the cartilage matrix did not degrade. Therefore, the cartilage tissue of 8-month-old Guinea pig could be regarded as the normal knee cartilage. We also found that when guinea pigs were 10 months old, the surface of the tibial plateau cartilage tissue was intact, but the superficial chondrocytes showed significant hypertrophic changes. Some chondrocytes expressed more Caspase-3 protein, which indicated apoptosis, and there were many empty chondrocyte lacunain the superficial cartilage tissue. However, there was still no significant degradation of the collagen II fibers in the cartilage, which was confirmed by the serum concentration of CTX-II, the collagen II degradation product, in the serum. When guinea pigs were 12 months old, the tibial plateau cartilage tissue was uneven and damaged, and the superficial chondrocytes exhibited hypertrophic changes, followed by apoptosis. There was much collagen II degradation in the cartilage tissue, which was confirmed by the serum concentration of CTX-II, the collagen II degradation product, in the serum. According to the OARSI rating system, we considered the tibial plateau cartilage tissue of 12-month-old guinea pigs to be grade 1 and that of 10-month-old guinea pigs to be grade 0 ~ 1.In other words, we captured the stage when superficial chondrocytes preceded extracellular matrix degeneration during spontaneous knee osteoarthritis in guinea pigs.
Therefore, when knee OA occurs, the degeneration of chondrocytes in the superficial cartilage tissue is the initial factor. Therefore, in follow-up studies, we should focus on the changes in the physiological status of chondrocytes in cartilage tissue and consider this to be one of the key points for the early diagnosis and preventative treatment of degenerative knee OA.