Fig. 9

Diagram of growth zone in Thamnocephalus. The Thamnocephalus growth zone is divided into anterior and posterior regions based on cell behaviors and gene expression. The posterior domain corresponds to Wnt4 expression (blue gradient); cell cycling in this region is present but low. Although mitosis in the posterior growth zone is not temporally or spatially synchronized, all mitosis in this domain is restricted in anterior–posterior orientation. The anterior growth zone corresponds to WntA expression (red gradient) and lacks cells in S phase. Cells in this region are possibly arrested either in early S phase or at the entry from G1 to S phase, since immediately after the anterior growth zone cells enter S phase again in the newest specified segment (dark green in last added segment). The synchronized S phase and subsequent mitoses in the segments generate the bulk of the visible elongation of the larvae. Wnt6 expression (dark blue bar) is in the telson, posterior to the growth zone while caudal expression (yellow bar) is throughout the growth zone. S phase domains in green, En-expressing cells in red