Temporal and spatial changes in element distribution in bone and cartilage

Background: Bone and cartilage act as reservoirs for a number of elements, and the perturbation of these elements is suggested to contribute to various diseases. Even so, little is known about their respective temporal and spatial distribution. Methods: Three knee joints of three mice on the first day after birth, three knee joints of 3-week-old mice, and three knee joints of 20-week-old mice were prepared. We performed element mapping in the bone and cartilage of normal mouse knee joints. We measured element distribution in articular cartilage, trabecular bone, cortical bone, joint cartilage, growth plates, and joint cavities using energy dispersive X-ray spectrometry. Results: The analysis revealed the following: (1) The main elements in articular cartilage of 1-day-old mice were Na, O, P, S, and Ca; and those in bone were Na, O, P, S, K, Ca, and Mg. (2) The main elements in the growth plate of 3-week-old mice were Na, N, O, P, S, Cl, K, and Ca; those in joint cartilage were Na, O, P, S, Cl, and K; and those in bone were Na, O, P, Ca, and Mg. (3) The main elements in the growth plate of 20-week-old mice were Na, O, P, S, Cl, K, and Ca; and those in bone were Na, O, P, S, K, Ca, and Mg. After corrections were made for the Na ratios of these elements, we investigated the temporal and spatial changes in the distribution of each element. On the first day after birth, a spatial change was seen in the growth plate cartilage: the more the cartilage matured toward hypertrophy, the more S and Ca it contained. Temporal changes in element distribution in the growth plate cartilage, articular cartilage, and bone were observed. Growth plate cartilage of the older mice contained more S and Ca than that of the younger mice. Bone of the older mice contained more Ca and Mg than that of the younger mice. Spatial changes in element distribution in the cortical bone were also seen; that is, the more the cortical bone matured toward diaphysis, the more Mg and the less S it contained. In contrast, no temporal or spatial changes in element distribution were observed in the joint space. No significant temporal or spatial changes in the distribution of P, Cl, or K were seen. Conclusions: These results suggest that element mapping may be useful for identifying the age and maturity of different skeletal tissues. In particular, it may help distinguish between immature cartilage and mature cartilage based on the Ca and S contents.