System to identify individual somites and their derivatives in the developing mouse embryo.

The identification of the axial levels of metameric elements along the rostro-caudal axis of vertebrates until now was not possible before late, fetal development, when the vertebral anlagen first appear. We developed a new system for the exact axial identification of somites and their derivatives from early, embryonic stages of mouse development on (Theiler stages (TS) 15 to TS18-19). The initial axial identification of the somites was performed by relating them to the rostral-most two cervical spinal ganglia (SG), that exhibited characteristic morphologies (SG-C1: bar-like, SG-C2: triangular). At all stages of somitic development, the most prominent somite along the rostro-caudal axis correlated with the bar-like SG-C1, and, therefore, we named it the first cervical somite (SO-C1). The next step, the axial identification of the somites independently from the SG, was based on the observation that after in situ hybridization to Myf5, Pax3, Pax1, and Mox1 riboprobes, a distinct and characteristic morphology of the last occipital somite (SO-O5) and the first two cervical somites (SO-C1, SO-C2) can be observed. From TS15 on, these three somites formed a triad of the most prominent somites along the rostro-caudal axis. Also, the dermomyotomal, myotomal, and sclerotomal derivatives of this somite triad were the most prominent in later somitic development. Furthermore, SG-C1 and SG-C2 exhibited a transient bipartite anlagen in their early development, suggesting a "resegmentation" during SG formation. Later, when somites started to dissolve, the caudal moiety of the bar-like SG-C1 anlagen fused to the anlagen of SG-C2.