Age-related histological changes in rat tibia

Background: At present, studies on osteochondral morphogenesis only focus on a certain period of time or only provide a pattern diagram, but lack of dynamic tracking observation from the initiation of development to maturity. This study was aimed to dynamically observe the changes of skeleton morphology and structure from embryo to adult, to provide research data for enriching the knowledge of bone and cartilage tissue structure. Materials and methods: In the intrauterine experiment, 5 normal pregnant Wistar rats were sacrificed under anaesthesia at gestational day (GD) 14, 17, 20, respectively. One of their offspring was randomly selected, and a total of 5 offspring were obtained at each time point. In the postnatal experiment, on the 7th and 10th day after birth and at postnatal weeks (PW) 2, 3, 6, 12, 28, 5 offspring rats from 5 different pregnant Wistar rats were randomly selected and sacrificed under anaesthesia at each time point. After obtaining the above offspring, the soft tissue was removed, and the tibia of hind limbs was retained for paraffin-embedded section. After stained with Safranin-O-fast-green and haematoxylin, the morphological development of the tibia was observed under an optical microscope. Results: At GD14, there was no obvious joint space, the whole hind limb was cartilage and bone tissue was not visible. At GD17, visible joint space was seen and the chondrocytes in the centre region appeared to hypertrophy. At GD20, the primary ossification centre was obvious, and a typical epiphysis growth plate structure could be seen. On the 7th day after birth, the chondrocytes in the centre of epiphysis cartilage were hypertrophic and differentiated, the cartilage canal grows from the cartilage surface toward the centre of the epiphysis cartilage, at postnatal day 14, the secondary ossification centre was formed. At this time, the tibia had typical morphological characteristics of the metaphysis, however, there was no obvious layered structure of articular cartilage; the stratified structure of articular cartilage could be seen at PW6, but its mature marker (tidemark) was still not visible; however, at PW12, typical 4 layers of articular cartilage appeared, and the tidemark was visible. The growth plates were clearly visible at PW2, 6 and 12. At PW28, growth plates could still be observed, but its morphology is abnormal. Conclusions: Our results, for the first time, dynamically observed the morphological changes of osteochondral at critical period of development from embryo to adult, especially the process of cartilage canal participating in the formation of secondary ossification centre.