Correlation of fluorescence and electron microscopy of F-actin-containing sensory cells in the epidermis of Convoluta pulchra (Platyhelminthes: Acoela)
Phalloidin-stained whole mounts of acoel turbellarians show brightly fluorescing club-shaped structures distributed over the epidermis and concentrated especially at the anterior and posterior tips of the body. By correlating electron micrographic images and fluorescence images of Convoluta pulchra, these structures can be seen to be sensory receptors with a central cilium surrounded by a collar of microvilli. The other candidate for showing fluorescence in the epidermis, namely gland necks, can be ruled out since their distribution is too dense to resemble the distribution of the fluorescent structures seen here. The collared sensory receptors were inserted between epidermal cells, and each bore a central cilium surrounded by a collar of 6–18 microvilli and an additional centrally positioned 2–7 microvilli of which 2 or 3 were associated with a modified rootlet called the swallow’s nest. Confocal scanning laser microscopy resolved the core of actin filaments within the microvilli of the collar and their rootlet-like connections to the base of the sensory cell. Such receptors could also be identified by fluorescence microscopy in several other species of acoel turbellarians.