Histology of tail epidermis and dermis of rat and cotton rat. (A,A′) Longitudinal section of the tail. (B,B′) Vertical section of the tail dermis. (C) Immunofluorescence of Iba1 as a macrophage marker in cotton-rat tail. (D,D′) Connective tissue around the cranial plane of the hair follicles. (E) Immunofluorescence of E-cadherin in cotton-rat tail. (F,F′) Tail epidermis. (G) Thickness of the epidermis and its constituent layers relative to the tail radius. Significant differences between rat and cotton rat are indicated as *P<0.05 (Mann–Whitney U-test). Rat, n=6; cotton rat, n=10. (H) Thickness of the dermis relative to the tail radius; no significant differences (Mann–Whitney U-test). Rat, n=6; cotton rat, n=10. Black arrows: the crack at the cranial plane of the hair follicles; white arrows: the alignment of cell nuclei; white arrowheads: neutrophils; hf, hair follicle.

Histology of tail subcutis of rat and cotton rat. (A,A′) Cross-section of tail. (B,B′) Tail subcutis and median coccygeal artery. (C) Hemorrhage in the subcutis of cotton rat. (D) Infiltration of neutrophils in the subcutis of cotton rat. (E) Thicknesses of lumen, media and adventitia of the median coccygeal artery. (F) Ratio of lumen, media and adventitia to the radius of the median coccygeal artery. Significant differences between rat and cotton rat are indicated as **P<0.01 and ***P<0.001 (Mann–Whitney U-test). Rat, n=6; cotton rat, n=9. Bidirectional arrow: the cleavage plane between skin and subjacent tissue; ar, the median coccygeal artery; d, dermis; t: tail tendon; vr, caudal vertebrae.

Distribution of collagen fibers in the tail subcutis of rat and cotton rat. (A,A′) Tail histology stained by Picrosirius red observed under the light microscope. (B,B′,C,C′) Tail histology stained by Picrosirius red observed under polarized light. Red (D) and green (E) brightness per area in the dermis and subcutis. (F) Brightness ratio of red to green per area in the dermis and subcutis. Significant differences between the dermis and subcutis are indicated as **P<0.01 and ***P<0.001 (Kruskal–Wallis test followed by Scheffé’s method). Significant differences between rat and cotton rat are indicated as #P<0.05 (Kruskal–Wallis test followed by Scheffé’s method). Rat, n=6; cotton rat, n=6. (G,G′) Tail subcutis stained by Elastica van Gieson. Bidirectional arrow: the cleavage plane between skin and subjacent tissue; arrows: bundle elastic fibers; arrowheads: fine elastic fibers; ar: the median coccygeal artery; d, dermis; e, epidermis; sc, subcutis.

Ultrastructure of the tail skin layers in rat and cotton rat by scanning electron microscopy. The images show cross section of the tail skin. The squares in (A,B) are magnified in (A′,B′), respectively. Arrows, fibrous bands. It is noted that the thick fibrous bands connecting between the dermis and subcutis are lacking in the tail of cotton rats.

Ultrastructure of the collagen fibers and fibrils constituting the dermis and subcutis of rat and cotton-rat tails by transmission electron microscopy. Black lines: collagen fibers; dashed black lines: collagen fibrils; arrows: elastic fibers; arrowheads: fragmented collagen fibrils. Although no apparent differences are found in the tail dermis between species (A-A″,B-B″), fragmented collagen fibrils and fewer elastic fibers are noted in the tail subcutis of the cotton rats (C-C″,D-D″).