Figure 6

Development of columnar longitudinal muscles in mid and late planula. A Mid-planula (~4 d), overview. The first two emerging mesenteries (primary mesenteries) are formed opposite each other. Note that the location of the secondary mesenteries is already set at this stage (inlet). B Detail of A. The base of the mesentery (corresponding to the future site of parietal muscle formation) shows no distinct accumulation of myofilaments. C Detail of A. Single cell (inlet, black arrow: myofilaments) showing first sign of muscle formation. D Late planula (~5 d), overview. Primary mesenteries gradually shift to one side. E Detail of D. The developing retractor muscle in the primary mesentery can already be identified. Myonemes are formed exclusively on one side of the mesoglea. Black arrows: accumulating myofilaments. F Detail of D. Basal part of the primary mesentery. No myofilaments have accumulated yet. Note an amoeboid cell at the branching of the mesoglea, which could be detected occasionally (asterisk). G Detail of D. Secondary mesentery. Future retractor muscle cell highlighted in red. H Detail of G. Retractor muscle cells forming at the tip of the mesentery (in contrast to the primary mesentery), having no bias to one side at this stage. Black arrows: accumulating myofilaments. I Detail of G. Cells at both sides of the base of the mesentery start to accumulate myofilaments (black arrows) adjacent to the mesoglea. All sections are cross-sections of the subpharyngeal region. For easier understanding the mesoglea of all close-ups is highlighted in yellow. Inlets in A and D indicate the location of primary (lines with circles) and secondary mesenteries (lines without circles). ec, ectoderm; en, endoderm; gc, gastric cavity; pm, primary mesentery. Scale: A, D: 50 μm; B, C, G: 5 μm; E, F: 2 μm; H, I: 1 μm.