Figure 2. Histological features of the skin in hippos and cetaceans.

Hippo skin (A-D) and cetacean skin (E-G). (A) Whole mount (left) and histological appearance (middle, right) of the upper lip skin in pigmy hippo. In the lip, vibrissa hairs above the skin surface and anagen (active growth) phase vibrissa hair follicles are prominent. Each lip vibrissa follicle has a prominent mesenchymal dermal papilla (black arrowhead, center), a collagen capsule (white arrowheads, center), an epithelial matrix, a mesenchymal ringwulst (black arrowheads, right), and an epithelial bulge (white arrowheads, right). There is no histological evidence of sebaceous glands. (B) Whole mount of tail skin from pygmy hippo (left) shows large hair follicles (white bracket). Histological analysis (right) suggests that tail hair follicles might be of vibrissa type because they are surrounded by a collagen capsule (white arrowheads). There is no histological evidence of sebaceous glands. (C, D) Whole mount and corresponding histological view of upper lip skin (C) and dorsal skin (D) in the pigmy hippo. At both sites, eccrine sweat glands are present. On histology, secretory coils located deep in the dermis are marked with black arrowheads; associated excretory ducts (where obvious) are marked with white arrowheads. In the dorsal skin (D), secretory coils of the glands reside at the very base of the dermis and come in contact with the underlying skeletal muscle layer. There is no histological evidence of dermal adipocytes. (E) Histology of facial skin and a rostral vibrissa hair follicle in an adult gray whale. The epidermis is thick and its basal layer is heavily undulated. The vibrissa follicle has a typical anagen (active growth) phase morphology with a large mesenchymal dermal papilla (green arrowheads, second panel) surrounded by epithelial matrix (yellow arrowheads, second panel). The epithelial outer root sheath compartment located above hair matrix is uncharacteristically thick and has prominent protrusions (yellow arrowheads, third panel). The vibrissa follicle is associated with distinct nerve bundles on either side (yellow arrowheads, fourth panel). (F) Wholemount side view of rostral skin from neonatal common bottlenose dolphin. Vibrissae hairs (black arrowheads) are clearly visible above the skin surface. (G) Histological view of rostral vibrissa hair follicle from neonatal common bottlenose dolphin. The vibrissa follicle has anagen phase morphology. A large dermal papilla (green arrowheads, bottom middle panel) and an uncharacteristically thin epithelial matrix (yellow arrowheads, bottom middle panel) are obvious. Unlike in gray whale, the outer root sheath lacks undulations, and like in gray whale, the vibrissa follicle lacks sebaceous gland. The overlaying epidermis displays prominent rete ridges (yellow arrowheads, right panel). The surrounding dermis contains numerous adipocyte clusters (blue arrowheads, right panel). Scale bar: A-E, G– 50 μm. See also Figure S1 and Figure S2.