Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation.

[1] Skin (including cutaneous and subcutaneous tissues) plays crucial roles in formation, structure, and function of extraskeletal apparatus such as horns of bovids (e.g., cattle) and rhinos, cervids' antlers, giraffids' ossicones, armadillos' osteoderm, and os penis/os clitoris.

For example, the skin plays a key role in protecting the body against pathogens[3] and excessive water loss.

[8] Primarily, fur augments the insulation the skin provides but can also serve as a secondary sexual characteristic or as camouflage.

Amphibian skin plays key roles in everyday survival and their ability to exploit a wide range of habitats and ecological conditions.

The epidermis can be further subdivided into the following strata or layers (beginning with the outermost layer):[14] Keratinocytes in the stratum basale proliferate through mitosis and the daughter cells move up the strata changing shape and composition as they undergo multiple stages of cell differentiation to eventually become anucleated.

During that process, keratinocytes will become highly organized, forming cellular junctions (desmosomes) between each other and secreting keratin proteins and lipids which contribute to the formation of an extracellular matrix and provide mechanical strength to the skin.

The epidermis and dermis are separated by a thin sheet of fibers called the basement membrane, which is made through the action of both tissues.

[16] The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain.

Dermis and subcutaneous tissues are thought to contain germinative cells involved in formation of horns, osteoderm, and other extra-skeletal apparatus in mammals.

The papillae provide the dermis with a "bumpy" surface that interdigitates with the epidermis, strengthening the connection between the two layers of skin.

The disinfected skin surface gets recolonized from bacteria residing in the deeper areas of the hair follicle, gut and urogenital openings.

Instead, the color of the skin is largely due to chromatophores in the dermis, which, in addition to melanin, may contain guanine or carotenoid pigments.

Structurally, the duct is derived via keratinocytes and passes through to the surface of the epidermal or outer skin layer thus allowing external secretions of the body.

[24] Granular glands can be identified as venomous and often differ in the type of toxin as well as the concentrations of secretions across various orders and species within the amphibians.

These glands are alveolar meaning they structurally have little sacs in which venom is produced and held before it is secreted upon defensive behaviors.

This causes the epidermal layer to form a pit like opening on the surface of the duct in which the inner fluid will be secreted in an upwards fashion.

The cells are found radially around the duct and provide a distinct attachment site for muscle fibers around the gland's body.

[24] The ducts of the mucous gland appear as cylindrical vertical tubes that break through the epidermal layer to the surface of the skin.

Among the amphibians, there are taxa which contain a modified intercalary region (depending on the function of the glands), yet the majority share the same structure.

[25] The epidermis of birds and reptiles is closer to that of mammals, with a layer of dead keratin-filled cells at the surface, to help reduce water loss.

In these animals, there is no clear differentiation of the epidermis into distinct layers, as occurs in humans, with the change in cell type being relatively gradual.

Hair is a distinctive feature of mammalian skin, while feathers are (at least among living species) similarly unique to birds.

[23] Cutaneous structures arise from the epidermis and include a variety of features such as hair, feathers, claws and nails.

This reaction-diffusion system combines an activator, Sonic hedgehog, with an inhibitor, BMP4 or BMP2, to form clusters of cells in a regular pattern.

Skin aging is caused in part by TGF-β by blocking the conversion of dermal fibroblasts into fat cells which provide support.