Nowicki, J.L.*; Takimoto, R.; Foley, A.B.; Burke, A. C.: Global Patterning Information In The Lateral Plate Mesoderm

The somitic mesoderm produces the axial skeleton and the striated muscle of both body and limbs of vertebrates. The limb skeleton and connective tissue develops from cells of the lateral plate mesoderm. To function properly, the elements of the appendicular and axial systems must be coordinated during development; for instance the limbs need to be in register with regional changes in the vertebral column. This patterning is realized during development as cells from the somites migrate into the lateral plate and differentiate into specific muscles. There is a boundary that separates the dorsal compartment (somitic cells only) from the ventral compartment (somitic and LP cells) that we call the somitic frontier. Previous experiments in avian embryos have shown that somitic mesoderm forming structures in the dorsal compartment of the body (axial skeleton, paravertebral muscles, dorsal dermis) is intrinsically patterned, i.e. it maintains its axial, morphological identity despite transplantation to a new anterior-posterior (AP) axial address. Furthermore, Hox gene expression is also maintained in transplanted tissue, strengthening the correlation between Hox gene activity and AP patterning of axial tissues. The somitic cells that cross the somitic frontier and form limb and abdominal muscles, however, appear to come under the influence of the lateral plate. It is uncertain whether the lateral plate mesoderm has an intrinsic axial identity with regard to both morphology and Hox expression. Using the quail-chick chimera system we are performing heterotopic transplants of lateral plate mesoderm to determine the nature of patterning information in the lateral plate, and the role of Hox gene expression in this tissue.