The Epiblast Origin of Avian Somite Cells

Abstract

Our laboratory has been engaged in the study of the acquisition by prospective somite cells of the developmental commitment to form somites. It was shown by the explantation of the so-called unsegmented somite meso-derm (the segmental plate) of chicken and Japanese quail embryos that the entire segmental plate was already committed to somite formation (Sandor and Amels, 1970; Packard and Jacobson, 1976; Sandor and Fazakas-Todea, 1980) and, furthermore, that the future somite pattern was already specified (Packard, 1978; 1980a; 1980b). I suggested at that time that the segmental plate contained about 10 to 12 “prospective somites” that were converted to definitive somites by the process of cleavage (Packard, 1978; 1980a). At about this time Meier (1979) discovered that a segmental pattern could be distinquished on the chick segmental plate through the use of stereo scanning electron microscopy. This pattern consisted of 10 to 12 tandem circular cellular domains that he termed “somitomeres.” Subsequent collaborative efforts between us demonstrated that somitomeres and prospective somites were one and the same and that these somitomeres became somites through a process of compaction (Packard and Meier, 1983; 1984; Cheney and Lash, 1984; Lash, 1985). Recent studies have suggested that the somitomere pattern may be adjusted to some extent following experimental manipulations (Packard, 1986).