Figure 2. Morphology of the hagfish epidermal thread cell.

(A) Cross-section of dorsal epidermis from a Pacific hagfish (Eptatretus stoutii; hematoxylin-eosin-stained; bright-field microscopy). SMC, small mucous cell; LMC, large mucous cell; ETC, epidermal thread cell. (B) The basal layer of epidermis containing epidermal thread cells and large mucous cells, as captured in en face view. ETCs are characterized by granules and threads stained with the fluorescent stain eosin; LMCs appear as circular voids. (C) Longitudinal cross-section of an ETC, showing a cluster of granules, the nucleus located at the basal region of the granules, and a helical thread located mainly along the inner surface of the plasma membrane. (D) Schematic of major cellular components of an ETC. (E) Oblique cross-section of an ETC, showing the relative positions of the granule cluster and threads. Enlarged area shows a region where the thread is intimately associated with the granule cluster. (F) A close-up of the inner plasma membrane, showing the thread packed in a single layer in a switchback pattern. All images were captured with confocal microscopy unless otherwise noted.

Figure 2—figure supplement 1. Abundance of hagfish epidermal cells.

(A) Schematic showing anteroposterior position (P_AP), with 0 representing the snout and 1 the tip of the tail. (Bottom left) Schematic showing dorsoventral positions (P_DV) in a hagfish in cross-section (see Materials and methods). (B) Linear density (λ) of each cell type for dorsal, lateral, and ventral regions along the hagfish (N=1 hagfish). Values are means ± SD. Dots represent data for each cross-section, with color representing P_AP. The linear density of epidermal thread cells (ETCs) was relatively consistent as a function of circumferential position, with the mean value ranging between 17 and 27 mm^–1. The other two cell types, small mucous cells and large mucous cells, had similarly uniform distributions. The small mucous cells exhibited the highest density (126–434 mm^–1), and the large mucous cells had the lowest among the three (2–6 mm^–1). (C) Among all three types of epidermal cells, only the large mucous cells exhibited a change in density with respect to the anteroposterior position. Based on linear regression models, we found no significant effect of anteroposterior position on the densities of ETCs and small mucous cells (p>0.1). However, in large mucous cells, we found a significant effect, with slope = –1.47 ± 0.30, p<0.001. (D) Area density ($\sigma$) of small mucous cells and ETCs sampled from laser confocal images taken in en face view from two hagfishes at three different anteroposterior positions (N=2 samples per location). Values are means ± SD. Colored dots represent data for each sampled area. Data available in Figure 2—source data 1.

Figure 2—figure supplement 2. Morphology of epidermal thread cell (ETC) at different developmental stages.

(A–D) Developmental sequence of ETCs represented by cells of different sizes, with smaller cells at the bottom. Each cell is shown with images stacks at different z-distance, as annotated on the top. Note the variations in thread shape. Scale bars, 5 μm. All images were acquired from hematoxylin-eosin (H&E)-stained slides using confocal microscopy (Zeiss LSM980). The mature ETCs are 54.1±2.8 μm in length (in apical-basal direction) and 22.8±0.6 μm in width (means ± SD; N=5 cells).