Postnatal Msx1 expression pattern in craniofacial, axial, and appendicularskeleton of transgenic mice from the first week until the second year

Phenotypes associated with Msx1 mutations have established the prominent ro le of this divergent homeogene in skeletal patterning. Previous studies hav e been achieved during antenatal development in relation with the early dea th of null mutant mice. Therefore, the present study is devoted to Msx1 hom eogene in the postnatal craniofacial, axial, and appendicular skeleton. A k nock-in transgenic mouse line was studied from the first postnatal week unt il 15 months. Whole-mount beta -galactosidase enzymology identified Msx1 pr otein expression pattern. Maintained expression of Msx1 was observed in gro wing and adult mice, specifically in the sites where Msx1 plays an early mo rphogenetic role during initial skeletal patterning. These included the cra niofacial sutures, autopodium, mandible, and alveolar bone. Furthermore, ac tive membranous and endochondral bone formation involved Msx1 in the entire skeleton. Histologic sections showed that progenitor as well as differenti ating and differentiated cells of all the bone cell lineages could express the Msx1 protein (chondrocytes, osteoblasts, tartrate-resistant acid phosph atase positive osteoclasts and chondroclasts), Recent developments in the g enetic and developmental biology of skeletal morphogenesis demonstrate that genes critical for development are jointly expressed in discrete embryonic signalling and growth centers, the enamel knot in teeth, the cranial sutur e in skull morphogenesis, and the progress zone in the limb buds, The prese nt study suggests that these signalling pathways are jointly important thro ughout the entire lifetime with an exquisite site-specificity spatially rel ated to early patterning. (C) 2001 Wiley-Liss, Inc.