Figure 5

Organization of the tentacle muscle system in an adult polyp. A Tentacle cross-section. B Detail of A. Typical appearance of a tentacle epithelium. Longitudinally orientated muscle cells are situated side by side at the base of the epidermis and are connected to the mesoglea (highlighted in red). A neural plexus (highlighted in green) is located on top of the muscle layer. Epithelial cells are still connected to the mesoglea by thin processes, which find their way through the discontinuous sheet of muscle cells. In the gastrodermis the epithelial muscle cells are oriented circularly. C Longitudinal section of a longitudinal muscle cell pointing out the basiepithelial organization. Note the close location of the nucleus to the myofilaments. Inlet: parallel arrangement of thick myofilaments (arrow). D Detail of a cross-sectioned longitudinal muscle cell with adjacent nucleus. White arrow: spot-like adherens junction connecting muscle cells to each other. Inlet: Thick myofilaments (black arrow) are distributed irregularly between thin ones. E Ring muscle cell of the gastrodermis (cross-section) with myofilaments located at the base of the cell adjacent to the mesoglea. White arrow: Belt-like apical junctional complex. Inlet: In comparison to the longitudinal muscle the myofilaments of the ring muscles are weakly developed. Black arrow: Thick myofilament. F Epithelial cell (highlighted in yellow) spanning from the external surface to the mesoglea through the sheet of longitudinal muscle cells. Asterisk denotes mesoglea. c, cilium; ed, epidermis; epc, epithelial cell; gc gastric cavity; gd; gastrodermis; lm, longitudinal muscle layer, lmc, longitudinal muscle cell; mv, microvilli; np, neural plexus; nu, nucleus; muscle; rm, ring muscle layer; s, spirocyst; t1, type 1 gland cell with electron-light vesicles; t2, type 2 gland cell with electron-dense vesicles. Scale: A: 50 μm, B: 5 μm, C-F: 2 μm.