Histological development and dynamic expression of Bmp2-6 mRNAs in the embryonic and postnatal mouse cranial base.

The cranial base is formed by endochondral ossification and is characterized by the presence of the synchondrosis growth centers. The aim of this study was to describe the histological development of the mouse midsagittal cranial base area from embryonic day 10 (E10) to the postnatal age of 2 months. The Bmp family of signaling molecules serves important functions in embryo and bone development and may therefore play a significant role in the early formation of the cranial base. To investigate this, we analyzed the mRNA pattern of expression of Bmp2-6 in the mouse cranial base from E10 to 5 days postnatally using radioactive in situ hybridization. We found that the formation of the mouse cranial base corresponds to that of rat and proceeds in a caudorostral sequence. Moreover, all Bmps studied showed distinct and overlapping developmentally regulated expression domains. Bmp2, Bmp5, and Bmp6 were expressed in the early mesenchymal condensations. Later, Bmp2, Bmp3, Bmp4, and Bmp5 were detected in the perichondrium and in the adjacent mesenchyme. Subsequently, Bmp2 and Bmp6 expressions were confined to hypertrophic chondrocytes, while Bmp3, Bmp4, and Bmp5 were expressed in the osteoblasts of the trabecular bone and bone collar. Interestingly, Bmp3 was uniquely expressed postnatally in the resting zone of the synchondrosis growth center, suggesting a role in the regulation of cranial base growth. These results suggest that Bmp signaling may serve specific and synergistic functions at different key stages of cranial base development and growth.