THE segmental organization of the vertebrate embryo is first apparent
when somites form in a rostrocaudal progression from the paraxial meso
derm adjacent to the neural tube. Newly formed somites appear as paire
d epithelial spheres that become patterned to form vertebrae, ribs, sk
eletal muscle and derms(1-3). Paraxis is a basic helix-loop-helix tran
scription factor expressed in paraxial mesoderm and somites(4). Here w
e show that in mice homozygous for a paraxis null mutation, cells from
the paraxial mesoderm are unable to form epithelia and so somite form
ation is disrupted. In the absence of normal somites, the axial skelet
on and skeletal muscle form but are improperly patterned. Unexpectedly
, however, we found that formation of epithelial somites was not requi
red for segmentation of the embryo or for the establishment of somitic
cell lineages. These results demonstrate that paraxis regulates somit
e morphogenesis, and that the function of somites is to pattern the ax
ial skeleton and skeletal muscles.