Stages of tentacle muscle differentiation observed using TEM. (A) DAPI staining of a transverse section of the tentacle root. The light blue line delineates the tentacle root. (B-E) TEM images of myogenesis stages in the median expansion of the tentacle root. (B) TEM view of the region boxed in yellow in (A), showing the muscle stem cells / progenitors. (C) TEM view of the region boxed in orange in (A) showing spindle-shaped muscle cell progenitors. (C’) Higher magnification of the region boxed in (C). White arrowheads point to cytoplasmic connections and plasma membrane interdigitations. (D) TEM view of the region boxed in red in (A), showing a muscle cell in differentiation (delineated by the white dotted line), and resulting from the fusion of several muscle cell progenitors, as indicated by the multinucleate organisation of the cell. These cells exhibit a developed endoplasmic reticulum, many mitochondria and are devoid of myofilaments. (E) TEM view of the region boxed in purple in (A), showing a mature muscle cell, with aligned nuclei (Nu) and distinct myofilaments (Myo). White holes in (B-D) are fixation artefacts. LR: Lateral Ridge; Mi: Mitochondria; MR: Median Ridge; Myo: Myofilaments; Nu: Nucleus. Scale bars: A: 100 μm; B: 10 μm; C, C’: 2 μm; D: 5 μm; E: 2 μm.