The Metameric Organization of the Presomitic Mesoderm and Somite Specification in the Mouse Embryo

Abstract

The earliest expression of a metameric pattern in the mammalian embryo at the light microscopic level is the appearance of neuromeres in the neural tube (Neal, 1918; Adelman, 1925; Bartelmez, 1923) and the formation of somites in the paraxial mesoderm (Butcher, 1929; Rugh, 1968; Theiler, 1972). Somites, which are tandem blocks of mesodermal cells, are arranged in a craniocaudal series and they are formed by the successive segmentation of the paraxial mesoderm. Situated caudal to the most recently formed somite, there is a portion of the paraxial mesoderm which always remains overtly unsegmented. This tissue, which is known as the presomitic mesoderm in the mouse embryo, is contiguous with the tissue at the caudal end of the embryonic axis (Fig. 1). It is believed that an active recruitment of cells to the presomitic mesoderm occurs within the caudal region of the embryo through the activity of the primitive streak, and at a later stage, of the tail bud (Flint et. al., 1978; Tam, 1981). The presomitic mesoderm in the mouse embryo is developmentally homologous to the caudal paraxial mesoderm of amphibian embryos (Woo Youn et al., 1980) and to the segmental plate of avian and reptilian embryos (Bellairs, 1979; Packard & Meier, 1983, 1984).