Detection of Hypoxia Inducible Factors and Angiogenic Growth Factors during Foetal Endochondral and Intramembranous Ossification

During skeletogenesis, the development of a new vascular network, i.e. angiogenesis, is triggered by hypoxia through the activation of the hypoxia inducible factors (HIFs) HIF-1 alpha and HIF-2 alpha. HIFs regulate the expression of several genes, including those coding for angiogenic growth factors such as VEGFA, angiopoietin-1 (ANGPT1) and angiopoietin-2 (ANGPT2). The expression of HIFs and angiogenic growth factors is well documented in endochondral ossification, but few data exist on their expression during intramembranous ossification. In this study, the localization of these factors was examined using immunohistochemistry and RT-PCR in bones of porcine foetuses. Immuno-staining for HIF-1 alpha and HIF-2 alpha was observed during endochondral ossification, whereas only HIF-2 alpha was present at sites of intramembranous ossification. Furthermore, immunostaining for ANGPT2 was present at sites of endochondral and intramembranous ossification. In addition, gene transcripts for ANGPT1, ANGPT2 and VEGFA were detected with RT-PCR in laser capture microdissected isolates from both types of ossification. These results indicate that angiogenesis plays an important role during endochondral and intramembranous ossification. However, the different expression pattern of the HIF-alpha subunits suggests that alternative regulatory pathways trigger angiogenesis in these distinct types of ossification.