Table 1. Anatomical features of the skin.

Anatomical features of the skin in human, cow, pig, hippo, and two cetaceans.

Cellular/anatomical feature	Human	Pig	Cow	Hippo	Grey whale (facial skin only)	Common bottlenose dolphin (facial skin only)
Epidermal thickness	29.6 to 96.5 μma	45.8 to 91.7 μmb	34.9 to 55.6 μmc	River hippo: 216 to 1,134 μmd (adult trunk); 42 +/− 7 μm (neonatal, facial skin) Pygmy hippo: 44 +/− 12 μm (neonatal, facial skin	6,334 +/− 872 μm	945 +/− 267 μm
Epidermal rete ridges	Medium-size rete ridges that differ in size across body	Medium-size rete ridges that differ in size across body	Absence of definitive rete ridges	Shallow rete ridges that differ in size across body	Prominent, branched rete ridges	Prominent, branched rete ridges
Cytoplasmic lipid vacuoles in epidermal living layers	Absent	N/A	N/A	Extensive cytoplasmic lipid vacuoles in all epidermal layers	Extensive cytoplasmic lipid vacuoles in all epidermal layers	Extensive cytoplasmic lipid vacuoles in all suprabasal epidermal layers
Lipid distribution in stratum corneum	Inter-cellular	N/A	N/A	Mostly intercellular	Mostly intracellular	Mostly intracellular
Stratum basale	Present	Present	Present	Present	Present	Present
Stratum spinosum	Present, thin	Present, medium	Present	Present, medium	Present, very thick	Present, very thick
Stratum granulosum	Present	Present	Present	Poorly defined	Absent	Absent
Stratum corneum	Present, variable thickness	Present, variable thickness	Present, thin	Present, medium	Present, very thick	Present, very thick
Epidermal barrier	Intact	Intact	Intact	Intact	Disrupted	Disrupted
Pelage hair follicles	Present, variable hair density across the body	Present, variable hair density across the body	Present, high hair density	Present, variable hair density; sparse over most of the body	Absent	Absent
Vibrissa hair follicles	Absent	Present	Present	Present	Present, limited to cranial skin	Present, limited to cranial skin
Ability of hair follicles to cycle	Able to cycle	Able to cycle	Able to cycle	Able to cycle	Able to cycle	Hair follicles cycle only once in neonates and degenerate in adults to become pits
Hair follicle-associated sebaceous gland	Present, variable in size and often prominent	Present, rudimentary	Present	Absent	Absent	Absent
Meibomian glands	Present	Present	Present	Absent	Absent	Absent
Sweat glands	Apocrine and eccrine glands	Apocrine glands only	Apocrine glands only	Eccrine glandsd; possibly apocrine glands	Absent	Absent
Dermal adipose tissue	Prominent	Prominent	Poorly defined	Absent	Abundant adipocyte clusters throughout the dermis	Abundant adipocyte clusters throughout the dermis

^aData from references [130] and [131]

^bData from reference [77].

^cData from reference [132].

^dGlands are specialized and produce “colored” sweat with proposed sunblock and/or antimicrobial properties [41].