Experimental studies of growth response to hormonal treatment

Abstract

Statural growth occurs as a result of the collective growth of long bones and the axial skeleton. Linear bone growth occurs at both ends of the bones in a thin cartilaginous structure termed the growth plate, located between the epiphysis and metaphysis. In the growth plate, resting chondrocytes at the epiphyseal border give rise to rapidly dividing, proliferative chondrocytes which, successively, generate terminally differentiated hypertrophic chondrocytes whose fate is to die from apoptosis, or possibly trans-differentiate into osteoblasts. Longitudinal bone growth is regulated by a diversity of factors, both within the growth plate and externally e.g., by hormones.

The overall aim of this thesis was to analyze the effect of different hormonal treatments with regards to linear bone growth and the growth plate. To achieve this, animal models of ex vivo and in vivo bone growth were employed.

In the first study, we developed and characterized an experimental model of long-term postnatal rat metatarsal bone cultures. By optimizing the culture conditions, we managed to culture the bones for up to 160 days. When treating the bones with dexamethasone, a well-known inhibitor of bone growth, for varying duration of time we could observe that catch-up growth did occur ex vivo, but also that the potential for this was limited to postnatal bones and only after short-term exposure. The mechanism behind the observed catch-up growth was increased proliferation whilst hypertrophic differentiation remained unaltered. We conclude that the potential for catch-up growth is intrinsic to the growth plate and that the model is suitable for in vitro studies of this phenomenon.

In study two, we investigated the growth promoting potential of combined treatment with growth hormone (GH) and insulin-like growth factor-I (IGF-I) to that of GH alone in normal juvenile rats. We found an additive effect of combination treatment on cortical bone mass, but no extra stimulation of linear bone growth compared with GH monotherapy.

Finally, in study three we continued the characterization of the postnatal rat metatarsal bone cultures by adding IGF-I from different timepoints to the serum-free conditions. We found that IGF-I stimulated linear bone growth at all timepoints, with increasing magnitude, through augmented proliferation and hypertrophic differentiation but also that growth, when followed longitudinally, merged with the trajectory of the control bones. The findings are discussed with an inclusive perspective on possible explanations to the results. We conclude that the model is suitable also for the study of postnatal catch-up growth after hormonal replacement treatment.