Fig. 2.

The main cell population of the dermis at early regenerative stages shows a gradient of cellular differentiation. Schematics (top) and TEM micrographs (a–f). a Mesenchymal cells at stage 4/5 located at the distal tip near the axial structures look rather undifferentiated. Main features are a secondary boundary layer (always highlighted in red in the schematics), a few vesicles, and a patchy nucleus. b Mesenchymal cells at stage 2/3 in the area next to where the ACC and RNC are adjacent on either side of the RWC. Cells show large RER, many vesicles, and a cytoplasmic pocket containing fibrils likely of collagen. c Mesenchymal cells at stage 4/5 at the very distal tip of the regenerate. Cells show large RER, vesicles, and a more electron-transparent cytoplasmic pocket. d Mesenchymal cells at stage 4/5 at the distal-most tip of the regenerate show a pocket syncytium. e, f Mesenchymal cells at stage 4/5 right under the epidermis show growing spicules. The main cellular features are indicated in the schematic the first time they appear. Red arrows indicate collagen fibrils, capital N indicates nucleus, and asterisks indicate growing spicules. ACC, aboral coelomic cavity; EPI, epidermis; St., stage. Scale bars = 2 μm