Skin

The skin forms a protective barrier against the external environment and contributes to temperature regulation, sensory perception and immune defence. Although its basic organization is conserved across mammals, species differ in epidermal thickness, hair follicle density, skin gland distribution and mechanisms of thermoregulation, reflecting adaptations to different environments and physiological demands.

Skin samples from the groin and back are available for macaque, pig, rat and mouse, providing representation of different anatomical regions. Additional eyelid samples are available for macaque, rat and mouse, but not for pig or human. The human RNA dataset includes skin samples from the leg and suprapubic region and providing a broadly comparable reference for cross-species analysis. For tissue specificity analysis, samples from the different anatomical regions are combined into one main tissue type, skin, although regional differences in cellular composition and gene expression can be explored at the individual sample level.

As skin contains stratified squamous epithelium, connective tissue and other shared cellular components, some overlap in gene expression is expected with tissues such as the lip, oral mucosa and other epithelial or connective tissue-rich organs.

Figure 1. Tissue sample overview and the hierarchy of grouped tissues representing skin

Comparative transcriptomics of the mammalian endocrine tissues

The transcriptomic data is used to identify genes with elevated expression in individual tissues or groups of tissues, classified into three categories:

Tissue enriched: Genes with elevated expression in a single tissue compared with all other tissues.
Group enriched: Genes with elevated expression in a limited group of tissues compared with the remaining tissues.
Tissue enhanced: Genes with elevated expression in one or more tissues compared with the average expression across tissues (gap ratio ≥2)

Tissue specificity is evaluated at two levels: first, within each species using all available tissue types, and second, using only tissues that can be directly compared between the species and human. This distinction makes it possible to explore both tissue-specific expression within each species and the conservation of expression patterns across evolution.

The results can be explored for individual genes on the gene summary pages or searched to identify genes with specific expression patterns. To assess evolutionary conservation, pairwise overlap analysis compares each species individually with human, identifying genes that share the same tissue specificity classification in corresponding tissues and quantifying the extent of overlap between species.

Tissue	Species	Elevated	Human elevated overlap
Tissue	Species	Elevated	Complete overlap	Partial overlap
Skin	Macaque	192	52	69
	Mouse	137	19	46
	Rat	186	34	71
	Pig	167	47	61

Skin – back and groin

Skin provides a protective barrier and contains specialized structures involved in sensation, temperature regulation and immune defence. The density of hair follicles, sweat glands and sebaceous glands differs considerably across mammals, with pigs having relatively sparse hair and humans possessing extensive eccrine sweat glands, whereas rodents rely more heavily on other mechanisms of thermoregulation.

Eyelid

The eyelid protects the eye and distributes the tear film across the ocular surface during blinking. Its specialized structures, including eyelashes, sebaceous glands and the meibomian glands, are broadly conserved, although their size, arrangement and secretory composition differ between species.

Skin

Comparative transcriptomics of the mammalian endocrine tissues

Skin – back and groin

Eyelid

Species coverage