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. Dev. Dyn. 1997;210:216–226. © 1997 Wiley‐Liss, Inc.