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. 2021 Dec 23;10:e74336. doi: 10.7554/eLife.74336

Figure 1. Morphology of sea anemone hair cells.

(A–F) Confocal sections of oral tentacles of Nematostella vectensis at the primary polyp stage. Filamentous actin is labeled with phalloidin (Pha), and nuclei are labeled with DAPI (dapi). DiI is used to label cell membrane of a subset of hair cells. In A, the distal end of the tentacle is to the top, and in B–I, the apical surface of the ectodermal epithelium is to the top. A: sections through the tentacle. Numerous hair bundles (arrowheads) are evident on the tentacle surface. (B) Sections through the hair bundles at the tentacle tip, showing stereovilli (st) and their prominent rootlets (rl) of central hair cells. (C–D) Sections through a DiI-labeled hair cell (hc) at the tentacle tip. Note that the hair cell has an apical cilium (ci) surrounded at its base by stereovilli (st), and basally extended thin neurites (ne). An empty space within the cell body shows the location of a nucleus (nu), as evidenced by DAPI staining (D). (E–H) Sections through a DiI-labeled hair cell (hc) located near the tip of a tentacle. Arrowheads in F point to microvilli of the mechanoreceptor hair bundle contributed by peripheral support cells, which are DiI-negative. (I) Electron microscopic section of an apical region of the tentacular ectodermal epithelium of N. vectensis polyp, showing a hair cell (hc) and a support cell (su). The hair cell has stereovilli that extend dense filaments into the cytoplasm, forming 2–3 µm-long rootlets (rlst), as well as numerous clear vesicles (ve), while the support cell has apical microvilli (mv) and electron-dense vacuoles (va). Abbreviations: ms muscle fibers; rlci ciliary rootlet; ec ectoderm. Scale bar: 10 µm (A–H); 2 µm (inset in B); 500 nm (I).

Figure 1.

Figure 1—figure supplement 1. Diagram of the hair bundle mechanoreceptor of sea anemones.

Figure 1—figure supplement 1.

A cilium (ci) extends from a central hair cell (hc). It is encircled by stereovilli (sv), also produced by the hair cell, as well as smaller-diameter microvilli (mv) contributed by surrounding support cells (su), forming a cone-shaped structure. Abbreviations: ne neurites; nu nucleus.
Figure 1—figure supplement 2. Life cycle transition in the sea anemone cnidarian Nematostella vectensis.

Figure 1—figure supplement 2.

Confocal sections of N. vectensis at metamorphosis from a free-swimming planula (A; 3–5 days post-fertilization [dpf]), through the tentacle-bud stage (B; 5–7 dpf), into a primary polyp (C; 7–10 dpf). Filamentous actin is labeled with phalloidin (Pha). All panels show side views of the animal with the oral pole facing up. (A) The planula consists of ectoderm (ec) and endoderm (en) separated by an extracellular matrix, and develops muscle fibers (ms) in the endoderm. (B) At the tentacle-bud stage, four tentacle primordia known as the tentacle buds (tb) emerge in the circumoral ectoderm. (C) Four primary tentacles (te) and the body column then elongate along the oral-aboral axis, forming a primary polyp. Scale bar: 50 µm.
Figure 1—figure supplement 3. Morphology of nematocytes in the tentacles of Nematostella vectensis polyps.

Figure 1—figure supplement 3.

(A, B) Confocal sections of N. vectensis primary polyp, labeled with an antibody against tyrosinated ∂-tubulin (‘tyrTub’). Filamentous actin is labeled with phalloidin (Pha), and mature cnidocysts (cnidae) are labeled with a high concentration of DAPI in the presence of EDTA (Szczepanek et al., 2002). The section shows the ectoderm at the tentacle tip, with the epithelial surface facing up. Note that nematocytes (nm) have an apical cilium (ci) surrounded by microvilli (mv) without rootlets. Stereovilli (st) and their prominent rootlets (rl) of hair cells are also shown. (C) An electron micrograph of a section of N. vectensis primary polyp, showing an apical structure of a cnida(cn)-containing nematocyte in the tentacular ectoderm. Microvilli (mv) without rootlets occur on the apical cell surface. Scale bar: 10 µm (A, B); 500 nm (C).
Figure 1—figure supplement 4. Hair cell development begins in the ectoderm of tentacle primordia at metamorphosis in sea anemones.

Figure 1—figure supplement 4.

Confocal sections of Nematostella vectensis at the late planula (A, B) and tentacle-bud (C, D) stages. Filamentous actin is labeled with phalloidin (Pha), and nuclei are labeled with DAPI (dapi). All panels show side views of animals with the blastopore/mouth facing up. A and C show longitudinal sections through the center, and B and D show surface ectoderm of tentacular primordia boxed in A and C, respectively. Note that 3–5 µm-long stereovilli characteristic of hair cells become evident at the tentacle-bud stage (arrowheads in D), indicative of hair cell differentiation. Scale bar: 50 µm (A, C); 10 µm (B, D).