The scoliosis (sco) mouse: a new allele of Pax1.

We describe the spontaneous mutant mouse scoliosis (sco) that carries a new allele of Pax1 (un-i, undulated intermediate). The Pax1(un-i) allele is lacking the 5'-flanking region and exon 1 to 4 which is mapped to nt -2636 to -640 and -272 to 4271 of the Pax1 gene. Homozygous mice show a mild form of the known phenotypes of other Pax1 mutants. Adult mice have a lumbar scoliosis and kinky tails. In homozygous embryos the skeleton ossifies early, ossification centers of the vertebral bodies are fused with the ossification centers of the pedicles. Neural arches and spinous processes are underdeveloped but the pedicles and transverse processes are overdeveloped which is in contrast to other Pax1 mutants. In the scapula, the acromion is missing and the deltoid tuberosity of the proximal humerus is shortened and thickened. Among the inner organs the thymus development is affected. In late embryos, the thymus is small and thymocyte numbers are reduced. T-cell development from CD4- and CD8- double negative (DN) to CD4+ and CD8+ double positive (DP) is decelerated. The percentage of CD90+ cells is also reduced but in contrast to other Pax1 mutants no alteration of the expression level of the CD90 (Thy-1) could be found.

N-cadherin is involved in myoblast migration and muscle differentiation in the avian limb bud.

Limb muscle formation involves invasion of the limb bud mesoderm by myogenic precursor cells from the dermomyotomes at limb bud level. Directed cell migration, homing, and differentiation of myogenic cells are controlled by the stationary cells of the limb bud mesoderm. At the level of the extracellular matrix, the molecular basis of migration control has been suggested to be exerted by the distribution of hyaluronan. Here, we demonstrate that N-cadherin-mediated interactions play a role at cell-membrane level in myoblast distribution and differentiation. N-cadherin is strongly expressed by myogenic cells in the chick limb bud and more moderately expressed by stationary mesodermal cells in the myogenic zones and progress zone. After in vivo injection of antibodies and Fab-fragments against the homophilic binding site of N-cadherin into the wing bud mesoderm, aggregates of myoblasts are found predominantly in the dorsal myogenic zone 36 hr after injection apparently due to immobilization. In the same position, areas of myf-5-positive cells are also observed. In injected limb buds, Pax-3-positive cells are less evenly distributed than in uninjected limbs. They are found to spread up to the epidermis and also form loosely arranged aggregates. After prolonged reincubation periods, injected limbs show ectopic myoblasts that are rich in desmin and areas of strongly desmin-expressing myoblasts within muscle blastemas. These effects were not observed after application of antibodies against other parts of the N-cadherin molecule. We conclude that N-cadherin is involved in myoblast migration in the limb buds via homophilic interactions and that it plays a role in signal transduction during myogenesis.

Halves of epithelial somites and segmental plate show distinct muscle differentiation behavior in vitro compared to entire somites and segmental plate.

Medial and lateral halves of the somite are known to differ with respect to their developmental fates: Cells from the medial half of the somite give rise to the epaxial muscle of the back and cells from the lateral half of the somite give rise to the skeletal muscles of the limbs and the ventrolateral body wall. To get a better insight into myogenic determination of somite hemispheres, isolated entire somites as well as medial and lateral parts of somites and of segmental plate from 2 day chick embryos were explanted in vitro. These parts of the paraxial mesoderm were also cocultured in contact with somite surrounding tissues such as neural tube lacking floorplate, neural tube including notochord-floorplate complex, and intermediate mesoderm, which were examined with respect to their muscle promoting or inhibiting influences. Skeletal muscle differentiation was monitored by the use of anti-myosin heavy chain antibody (MF20). It is shown that medial and lateral halves of segmental plate and epithelial somites are capable of undergoing myogenesis in the absence of axial organs. In contrast, cultures of intact segmental plate and epithelial somites from the same levels did not show muscle differentiation. Neural tube lacking floorplate promoted muscle differentiation in the medial halves especially of epithelial somites and also of segmental plate, but not in the lateral halves of the paraxial mesoderm at these levels. Intermediate mesoderm was found to inhibit muscle differentiation in medial and lateral halves of segmental plate and of epithelial somites. We further demonstrate that the arrangement of the myoblasts within tissue cultures is influenced by the presence or absence of axial organs.