Metameric pattern development in the embryonic axis of the mouse. I. Differentiation of the cranial segments.

The overall pattern of the mesoderm in the embryonic axis of the cranial region of mouse embryos was examined with the scanning electron microscope (SEM). A segmental organization was observed first in the paraxial mesodermal wings and midline axis of embryos at the late primitive streak stage. Each segmental unit consists of a somitomere in the paraxial region on each side of an enclosed stretch of midline notochord. Somitomeres appear initially as circular domains of radially arranged cells that swirl about the core center of the unit and are quite similar morphologically to those described recently in chick embryos [12]. Lying in tandem sequence, the segments comprise the chordamesoderm that underlies the neural plate. As additional pairs of somitomeres are added from the primitive streak at the caudal end of the axis, those established in the cranial region remain contiguous and undergo morphogenesis that is coordinate with neurulation. We divide the development of the cranial axis into five phases and associate somitomeres in the mesoderm with neuromeric segmentation in the neural plate. It was found that the first pair of somitomeres comes to underlie the prosencephalon, the second and third pairs underlie the mesencephalon, while the fifth, sixth, and seventh pairs of somitomeres underlie neuromeres of the metencephalon. The eighth pair of somitomeres are the first to separate themselves from the first seven and from the first pair of somites visible at the light microscope level. This study suggests that the cranial axis of the mouse embryo is initially organized into segments like the rest of the body and that subsequent cranial morphology is a consequence of differential development of these segments.