Figure 4. Somite derivatives of E. burgeri.
(a–f) Early pharyngula. (a) Epithelial somite in the caudalmost region of the trunk. (b) Twist is strongly expressed in the ventral somite. (c) Pax1/9 is weakly expressed in the median ventral somite. (d) Sagittal section showing rostrocaudally migrating putative sclerotomal cells at the anterior level of the trunk. (e, f) Transverse sections at the anterior level of the trunk showing migrating putative sclerotomal cells expressing Twist (e) and Pax1/9 (f) (arrowheads). (g–j) Middle pharyngula. (g) Mesenchymal cells located at the lateral side of the notochord. (h) Pax1/9 expression by mesenchymal cells. (i) Pax3/7 expression on the lateral side of a somite. (j) MyoD is strongly expressed at the medial side of a somite. (k, l) Late pharyngula. (k) Mesenchymal cells located at the dorsal aorta and the lateral side of the notochord. (l) Clear expression of Pax1/9 is evident in the late pharyngula. (m, n) Schematic summary of the development of putative vertebral elements. (m) Medial ventral somites differentiate into mesenchymal cells (blue) in the early pharyngula. (n) The mesenchymal cells differentiate into the cartilaginous nodules in the adult. ao, dorsal aorta; mu, mucus gland; n, notochord; nt, neural tube; sc, sclerotome; som, somite. Arrows indicate the migrating sclerotomes. Scale bars, 100 μm.