MusFIG 11: Structure and components of C. elegans fibrous organelles (FOs).

A. Graphic rendition of a section through the body with the cuticle (gray) and the hypodermis (beige) peeled off from the dorsal left quadrant to expose fibrous organelles (small dots), attachment plaques (large oval dots), and the underlying muscle (green). FOs are oriented circumferentially over the muscle cells, orthogonal to the anterior–posterior orientation of the myofilaments. (Orange layer) Basal lamina.
B. Components of DB and M lines. Many of the proteins found in DBs, M lines, and attachment plaques are similar to those seen in vertebrate focal adhesions.
C. Schematic illustration of the muscle–hypodermis–cuticle anchorage structures that allow mechanical force to be transmitted across the muscle cell membrane to the cuticle (Labouesse, 2006). FOs correspond to the structures formed by hemidesmosome-like electron-dense plaques found on either side of the hypodermal plasma membranes, with interconnecting intermediate filaments (IFs). On the muscle side, the transmembrane extracellular matrix receptor PAT-2/PAT-3 integrin and the basement membrane protein UNC-52/perlecan are concentrated at the sites around the DB and M lines. On the hypodermal side, homologous transmembrane proteins MUA-3 and MUP-4 may be localized at the apical hypodermal surface and bind to intermediate filaments within the hypodermis and collagens in the outside cuticle.
D. FOs are localized in circumferentially oriented bands over the muscle quadrants. FOs are shown by GFP labeling of intermediate filaments in the hypodermis of a transgenic animal expressing the mua6::GFP reporter gene. Epifluorescent image, left lateral view. (Strain source: V. Hapiak and J. Plenefisch. Image source: R. Lints.).
E. Longitudinal TEM thin section showing hemidesmosomes on both sides of the hypodermis. (Image source: [Hall] fat-3 T411.)

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