Endochondral ossification involves a well ordered sequence of cellular events. Chondrocytes change their morphology and functions and are ultimately removed by the process of apoptosis. A variety of apoptotic-related signals have been characterised. These include Fas receptor (FasR)/Fas ligand (FasL), p53 and Bcl family. However, there is little known regarding the activity of these signals in the process of fracture healing. The purpose of this study was to investigate mRNA expression of apoptotic signals using RNase protection assay (RPA) and immunohistochemistry in endochondral bone formation.

BALB/C mice aged 8 to 10 weeks were used for this study. First, a transverse fracture was made in the right tibia. Mice were euthanised at 1, 2 and 3 weeks postfracture. The calluses were harvested and studied for the expression of caspase-8, a key enzyme of apoptosis, and apoptosis inducers: tumour necrosis factor-alpha (TNF-α) and its receptor p55, FasL and Fas receptor (FasR), and TNF-related apoptosis-inducing ligand (TRAIL). Four mice at each timepoint were used for immunostaining of fracture callus. Sections were incubated with primary antibody then labelled by avi-din-biotin complex method. Another four to ten tibiae were used for RPA. Fracture callus were harvested and snap frozen in liquid nitrogen. RNA was isolated by TRI reagent and BCP, and mRNAs expression of apoptotic signals were detected.

At each timepoint, mRNA of caspase-8, TNF-α, p55, FasL,FasR and TRAIL were detected by RPA. Immunostainings clearly showed that those apoptotic-related proteins were expressed by callus chondrocytes. Cartilaginous callus is replaced by woven bone in endochondral ossification. In this process, chondrocytes should be removed by the process of apoptosis in which death factors are elaborated directly in both an autocrine and paracrine manner.