The relevance of reversal cells and intermittent parathyroid hormone on the duration of the reversal-resorption phase in intracortical bone remodeling events

Bone loss is a major and growing health problem. When bone gets thinner andmore fragile, it increases the risk of fractures. Fractures increase morbidity andmortality as well as being a great economic burden for society. To improvetreatment of bone loss, it is necessary to improve the understanding of boneremodeling, the process responsible for maintaining bone strength. In boneremodeling, bone-degrading cells called osteoclasts resorb old or damaged bone,which is replaced with new bone mass formed by osteoblasts. Resorption andformation have traditionally been considered separate processes, but recentstudies found that they are coupled by a reversal-resorption phase. This projectfocuses on cortical bone because cortical remodeling occurs in canal structurescalled Haversian systems, giving the opportunity to investigate remodeling inisolated systems with only the involved cells present.

In cortical remodeling, osteoclasts create a canal through the bone matrix. This canal is subsequently partly filled with new bone mass. Osteoclasts open new canals (type 1) and remodel existing canals (type 2). During the reversalresorption phase, reversal cells colonize the canal and eventually differentiate into mature bone-forming osteoblasts. A threshold density of reversal cells is necessary for initiation of bone formation. Deterioration of the coupling prolong or arrest the reversal-resorption phase, leading to extended bone resorption and no initiation of formation. This is thought to be the cause of cortical bone loss and subsequent bone fragility. The hypothesis of this project was that prolonging or arrest of the reversal-resorption phase is caused by lack of reversal cells and/or reduced differentiation. If so, the most suitable treatment would be an anabolic drug such as parathyroid hormone (PTH) rather than the traditional antiresorptives. The overall aim of this thesis was to assess the origin and maturation of reversal cells during the reversal-resorption phase and the effect of iPTH treatment on cortical bone remodeling.

The aim of study 1 was to investigate recruitment, proliferation, and differentiation of reversal cells and the influence of canal type on these factors. Cortical pores (2D cross-sections of canals) in human femoral neck biopsies from adolescents with no diseases known to affect remodeling were classified according to pore type, and eroded, osteoid, and quiescent surfaces were identified. Immunostainings and in situ hybridization made it possible to
investigate the expression of immature osteoprogenitors, mature osteoblasts, and proliferation. The cell densities and cells per circumference were determined on each surface type. As expected, reversal cells were identical to osteoprogenitors and eventually differentiated into osteoblasts. Cell density and cells per circumference increased gradually during the reversal-resorption phase as seen before. Osteoprogenitors proliferated to a bigger extend than osteoblasts. This has previously been observed in trabecular bone.

The aim of study 2 was to summarize the current research on the effect of different types of PTH-treatment on trabecular and cortical bone remodeling on a cellular level. Papers investigating remodeling markers and histomorphometric changes during resorption, reversal-resorption and formation were included in
the review. PTH-treatment increased clinical markers of both resorption and formation. Histomorphometric studies showed signs of increased formation but no to little evidence of increased resorption. This indicates that the effect on formation overcomes that of resorption. The anabolic effects were more prominent in trabecular than cortical bone but may be temporary, although more data on long-term treatment are needed. Different types of iPTH and cyclic PTH had similar effects.

The aim of study 3 was to investigate the effects of short- and long-term treatment with PTH on cortical bone remodeling. Otherwise healthy patients with hypoparathyroidism received daily subcutaneous injections with either PTH (1-84) or placebo. After 6 months, the patients in the PTH group either continued the
treatment or shifted to placebo. The study lasted for another 24 months. Biopsies were taken from the iliac crest at 6 and 30 months. Cortical microstructure was investigated. Pores were classified according to type, remodeling stage, and position, and their number and area were determined. iPTH treatment did not
affect cortical microstructure but did increase resorption and formation equally. The effect lasted for as long as the treatment but stopped when treatment was discontinued.

Overall, this project found that recruitment, proliferation, and differentiation of osteoprogenitors are crucial for initiation of bone formation, and that PTH promotes this transition from resorption to formation. This work provides the basis for future studies on osteoprogenitor behavior in elderly and osteoporotic
people.