Regulatory mechanisms in the development of bone and cartilage: the use of tissue culture techniques in the study of the development of embryonic bone and cartilage: a perspective.

Tissue culture techniques were used to study a number of factors and mechanisms which are important in the development and metabolism of bone tissue. As an example of an external factor influencing bone development, the importance of the composition of the gasphase which is in equilibrium with the fluid bathing osteoblasts and hypertrophic chondrocytes, was investigated in cultured metatarsal bone rudiments. In vivo, one expects the presence of an O2 gradient in the cartilaginous epiphyses of long bones: low O2 tension between the nonhypertrophic chondrocytes, high O2 tension in periosteum and hypertrophic zone, bordering the marrow cavity. The in vitro findings correlated with these expectations. High CO2 (5%) and high O2 (40%) tensions stimulated calcification; in air, calcification was severely inhibited. On the other hand, maturing chondrocytes were damaged by high O2 tensions. An important cellular mechanism in calcification is the intracellular accumulation of calcium (and phosphate) in osteoblasts and hypertrophic chondrocytes which can be demonstrated with the GBHA stain of Kashiwa [9]. The extracellular role of alkaline phosphatase (AP) present on the cell membranes of these cells was shown to be a less decisive factor in calcification. In the presence of AP inhibitor in a concentration high enough to inhibit AP activity to a large extent, calcification was shown to proceed normally. The effects of a number of hormones known to be important for the development and metabolism of bone tissue was studied using tissue culture (calvaria) as well as culture of different isolated bone cells. The parathyroid hormone (PTH) induced rise of the intracellular cAMP level was found to originate primarily from the osteoblasts not the osteoclasts. Isolated osteoblasts showed a high cAMP response after PTH addition. Cortisol was shown to inhibit PTH induced resorption but to potentiate PTH induced cAMP response in calvaria. Various PTH fragments (desamino 1-34, 2-34, 3-34) were shown to be active as stimulators of bone resorption (although they were less active in this respect than the intact molecule 1-84), but did not stimulate cAMP production in calvaria or isolated osteoblasts. The results obtained strengthened the hypothesis that cAMP is not the (only) mediator in PTH induced bone resorption.