The ventral epithelium of Trichoplax adhaerens (Placozoa): Cytoskeletal structures, cell contacts and endocytosis

Summary

All cilia emerge from ciliary pits supported along their circumference by 22–24 dense rodlets that are connected by filaments to a surrounding sheath of endoplasmic reticulum. The proximal part of the basal body is provided with two short lateral rootlets and one long terminal rootlet, all associated with microtubules. The lateral rootlets are in turn connected by fine fibrous material to the dense supporting rodlets which follow the contour of the ciliary pit and extend along the ciliary membrane beyond the level of the basal plate where the central pair of microtubules originates. The proximal part of the basal body has fine fibrous connections to the endoplasmic reticulum while its distal portion is surrounded by nine curved sheets. The terminal cell contactions are by belt desmosomes that are accompanied by a bundle of microfilaments which encircle the apical region of the cell and insert at the cell membrane. Tight junctions are lacking. Endocytosis was demonstrated by the uptake of cationized ferritin. The structures associated with the ciliary pits are probably associated with the firm anchorage of the ciliary base since Trichoplax adheres to the substrate as it moves propelled by its ventral cilia. The marginal bundle of microfilaments may be involved in folding of the organism during feeding.