Elemental distributions in the growth plate analyzed with a proton microprobe

Bone formation starts in the embryonal phase from a cartilaginous template. This formation of calcified bone on a cartilage scaffold, called endochondral ossification, also plays a major role during growth. Postnatallongitudinal growth of long bones is limited to a relatively thin region, called the growth plate. In this region, typically a few millimeters thick, cartilage is rapidly converted into bone. Understanding the mechanisms involved in chondrocyte apoptosis is vital for understanding the process of endochondral ossification. A hypothesis of Mansfield is that: Phosphate ions (Pi) induce apoptosis of terminally differentiated hypertrophic chondrocytes and Ca2+ enhances Pi entry into chondrocytes. Furthermore, it is expected that Pi and Ca2+ concentrations are high in the terminal zone of the growth plate, due to turnover of mineralized cartilage into bone and that there is a calcium and phosphorus concentration gradient towards the bone [1]. To assess the latter hypothesis, this study aims at measuring concentration profiles across the growth plate of the femur of fetal pigs. To obtain this information in high detail, nuclear analysis techniques are used. Forward and backward scattering speetrometry is used for the determination of the amount of main matrix elements (H, C, N, and 0) of the cartilage material, while partiele induced X-ray emission (PIXE) is used for determination of the amount of the other elements that play a major role in the development of the bone (P, S, Cl, K and Ca). The results are compared with results found in literature, from scanning transmission electron microscope measurements on rat rib growth plate material [1] and from histochemical measurements on bovine tibial growth plate material [2]. The existence of a phosphate gradient, as found in literature [1, 2], is not proven in this project. But the determined concentration of phosphorus in the resting zone of about 2 mg/ g dry weight, is consistent with the results of the electron microscope measurements [1]. The concentration of calcium determined in this study, is about 2 mg/ g dry weight in the resting zone and increases along the proliferative and hypertrophic zone towards 4 mgjg. Again, these data are consistent with the results of electron microscope measurements [1]. These findings support the theory about the presence of a calcium gradient [1] which might enhance the phosphate induced apoptosis of terminally differentiated epiphyseal chondrocytes [3]. This study also found that sulphur and potassium concentrations increase step-wise between the proliferative zone and the resting zone. This could not be concluded from the spot measurements at different locations in the growth plate with the electron microscope described in literature [1]. Finally, it is concluded that the concentration distributions depend on the state of maturation of the growth plate. The further the growth plate is developed, the clearer the resting zone and proliferative zone become visible as two different zones. Also, an asymmetry between the primary and secondary growth zone shows up. Although the same process takes place in both zones, the secondary growth zone is thinner than the primary. Elemental distributions in the growth plate analyzed with a proton mieroprobe