Skin - Anatomy & Physiology

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Development

Origin of the epidermis

Following neurulation, the precursor of the epidermis is formed from the cells covering the embryo. This then becomes 2 layers, the outer forming the periderm and the inner forming the basal layer or stratum basale. A further layer is formed from the basal layer called the spinous layer or stratum spinosum. The basal and spinous layers are termed the Malphigian layer, the cells of which then divide to produce the granular layer or stratum granulosum. As the cells of the granular layer differentiate and keratinise, they form the outer cornified layer or stratum corneum.

Structure and Function

Epidermis

Monkey epidermis showing the layers present. Copyright RVC2008

The epidermis is a stratified squamous epithelium and is composed of 4 cell layers anchored to a basal lamina of connective tissue. Keratinocytes migrate through the epidermis from the basal layer. This migration begins in the stratum basale, then moves up through the stratum spinosum, stratum granulosum and the stratum corneum.

• Stratum basale

The deepest layer of the epidermis is the stratum basale. It consists of a single layer of columnar epithelium which is adhered to the basal lamina. Nutrition is supplied from the underlying capillaries of the dermis. Migration of cells towards the surface begins here.

• Stratum spinosum

Above the stratum basale is the stratum spinosum. Spinous cells are large polygonal cells with prominent desmosomal intercellular filaments. When cells move into this layer, they shrink and move apart. Keratinisation begins in the stratum spinosum and continues in the stratum granulosum.

• Stratum Granulosum

External to the stratum spinosum, lies the stratum granulosum. Granular cells are thin, flattened keratinocytes with variably prominent keratohyaline granules. At its outer surface, the stratum granulosum secretes oragnelles called lamellar bodies, which are composed of lipid and enzymes which fuse with the plasma membrane and help to form the water concentration gradient that leads to transepidermal water loss.

• Stratum Corneum

The outer layer, stratum corneum, consists of anucleate, thin cells filled with keratin filaments and proteins that form a cell envelope resistant to solvents and enzymes; there is also a permeability barrier to water and ions. The cells of the stratum corneum are embedded in a lipid-rich matrix which helps to hold the cells together, while the desmosomes that were present in lower layers have disappeared. For the stratum corneum to maintain a constant thickness, there is a constant turnover of exfoliated corneocytes, being replaced by new corneocytes. Enzymes in the lamellar bodies from the stratum granulosum help to break down the intercellular lipid 'glue' that holds the cells in place and there is also some degradation of desmosomes.

Cells of the Epidermis

• Langerhans' cells

These are dentritic cells derived from bone marrow. They act as antigen presenting cells in the epidermis as part of the mononuclear phagocyte immune system. Langerhans' cells express MHC I and II molecules and immunoglubulin Fc receptors and are a key feature of delayed hypersensitivity responses.

• Melanocytes

These are also dentritic cells and are derived from the neural crest. They are immersed in the epidermis where they form close contact with a defined number of keratinocytes - an epidermal-melanin unit. Melanin is composed of 2 types: eumelanin which is black-brown and phaeomelanin which is reddish-brown. Melanocytes produce pigment in melanosome structures which are membrane-bound structures involved in the transfer of pigment to keratinocytes. An important function of melanin is UV protection and absence of pigment confers increased sensitivity to UV damage.

• Merkel cells

These are part of the sensory apparatus of the skin and are located within the stratum basale of the epidermis where they act as slow-adapting maechanoreceptors. They are usually closely associated with the afferent myelinated nerve fibres, arranged in a plate-like structure with the base of the Merkel cell, together forming a Merkel's corpuscle.

Dermis

The epidermis interfaces with the underlying dermis at the basement membrane zone (BMZ). The binding includes anchoring junctions called hemidesmosomes and focal adhesions. The former comprise ECM proteins such as laminin-5 and collagen type IV molecules, which form the lamina densa part of the BMZ. Collagen type VII is important in forming anchoring fibrils between the dermis and the basal lamina.

Rete ridges are epidermal projections into the dermis in areas of mechanical loading such as the nasal planum and footpads.

The dermis consists of dense fibrous tissue composed of collagen type I and III, with some elastin fibres and provides a supporting mattress for the carriage of blood vessels, lymphatic vessels, nerves and sensory receptors, hair follicles, sebaceous and sweat glands, with circulating white cells.

Glands

• Sweat glands

Sweat is considered to be the product of apoptosis releasing cell contents into the gland lumen, together with secretion where exocytosis, microapocrine blebbing, transcellular and perhaps paracellular processes involved in varying degrees. Sweat is composed of secretions and cellular debris and includes inorganic ions, water, immunoglobulins, amino acids and waste products including urea and lactic acid. Equine sweat is rich in proteins including glycoproteins, albumin and gammaglobulins.

Sweat is degraded by bacteria leading to odour. Dogs and cats produce very little sweat and use panting to lower body temperature.

• Mammary glands

These are greatly modified, enlarged sweat glands and are present in both male and female animals. Depending on species, they extend from the axillae to the groin, associated with teats and a complex array of sinuses and ducts. Age, pregnancy and endocrine factors have an important role in determining glandular activity.

• Sebaceous glands

One or more sebaceous glands surround each hair follicle at a point about 1/3 of its length from the surface. Each gland is composed of numerous acini attached to a common excretory duct through which they discharge their secretions onto the hair shaft and epidermis. In the skin and eyelids such glands release the cell contents together with secretory products with holocrine secretion. This secretion involves apoptosis in which the secretory products and cell debris are released into the gland lumen. The secretory products vary from species to species and include various fatty acids, diglycerides, cholesterol, diesters, wax esters and giant-ring lactones.

The function of sebum is that of an emolient of the hair and skin, preventing excessive dryness. It may combine with sweat to form an emulsion within the stratum corneum and a form of cement around the hair pores preventing bacterial invasion of the epidermis.

• Circumanal and supracaudal tail glands

Hepatoid sebaceous glands restricted to the perineum, dorsal tail and prepuce, they may have an endocrine and/or pheromone function. They empty into special sweat glands.

• Submental and circumoral glands

Large glands in the lip region of cats; presumed to have a territory marking role. Circumoral glands are enlarged integumentary glands bordering on the mucocutaneous jumctions of the mouth or oral margins associated with greeting and related behaviours, especially in scuiriomorph rodents.

• Inguinal glands

Sexually-related scent glands prominent in ungulates and lagomorphs.

• Anal sacs

Sebaceous (and modified apocrine in the dog) glands lie within the wall of the anal sacs, with a duct carrying material to the anal opening. The foul smelling secretion may act as a territorial marker. Secretions are expressed on defecation. The secretions are composed of fatty and serous materials with cellular debris.

Cutaneous Appendages

Footpads

Thickened skin of the canine footpad. Copyright RVC2008

Cats and Dogs have a digitigrade stance with digital, metacarpal and metatarsal pads in contact with the ground. A carpal pad is also present, but seems to be superfluous.

Pads are covered with a very thickened epidermis with no hair follicles. They are hyperpigmented and have atrichial sweat glands opening to the surface under the influence of epinephrine.

There is little dermis and the hypodermis is composed of collagenous and elastic fibres mixed with adipose tissue to form a digital cushion that provides protection from mechanical impact.

The Nose

The covering of the nose is very thickened with rete ridges providing anchoring. The surface has a cobblestone appearance in dogs and cats. Hair and sebaceous glands are absent, atrichial glands are present.

Claws

Claws have a variety of functions including locomotion, climbing, fighting and hunting. They are composed of a coronary band, lateral compressed walls and a flaky ventral sole. The hard keratinised epidermis is formed from the nail matrix that is an extension of the hirsute skin, with an underlying vascular dermis.

Cats can unsheath their claws by muscle action and retract them with an elastic ligament on the dorsal aspect of the middle and distal phalanges.

Depending on the pigmentation of the claw, the dermis may or may not be visible. Care must be taken when clipping claws to avoid laceration of the 'quick' and subsequent haemorrhage.

The claws of the accessory first digits on the medial aspect of the foot of dogs is often termed the dew claw.

Eyelids

The palpebral margin is thicker than the remainder of the eyelid and cilia grow from the margin. There are various glands in the region, including Moll's glands that are modified sweat glands, associated with the cilia. Meibomian glands, Zeis glands associated with the cilia and the lacrimal glands together produce the tear film.

Beak

The beak of birds is formed from the bones of the maxilla and mandible with a horny, keratinised covering, the rhamphotheca.

Microscopically, it is similar to the skin, with a modified epidermis. The stratum corneum is very thick, containing cell bound calcium phosphate and layered crystals of hydroxyapatite.

The beak is very sensitive to heat, cold, pressure and pain due to a high number of mechanoreceptors (Herbst corpuscles) being presence.

The corpuscles are recognisable histologically as papillae, originating from the dermis ending in crater-like structures at the distal tip of the beak.

The cere is situated at the base of the upper beak and is composed of keratinised skin. The nostrils are located here in many avian species. The colour of the cere is influenced by diet and hormones.

Immunology

The immune system is involved in several diseases of the skin, with different types of hypersensitivity reactions occuring.

See also Skin Immunology

Species Differences

Avian Skin

The general make-up of the avian skin is similar to that of mammals, having an epidermis a dermis and a subcutaneous layer. In comparison, however, it is much thinner, effectively glandless and contains [Feather - Anatomy & Physiology|feathers]]. Generally, the skin is thin enough to be transparent, aiding examination of superficial internal organs including the liver.

The epidermis consists of 3 layers:

• The basal (germinative) layer
• Intermediate layer
• Outer (cornified) layer

Striated muscles in the epidermis move the skin.

The dermis is divided into:

• A superficial layer which varies in thickness depending on position on the body and age of the bird. This layer contains loosely arranged layers of collagen in interwoven bundles.
• A deep layer containing fat, [Feather - Anatomy & Physiology|feather follicles]], smooth muscles that control the movement of the feathers, blood vessels and nerves that supply the dermis and epidermis.

The subcutaneous layer is formed mainly by loose connective tissue. It also contains fat, both as a layer, and in discrete fat bodies. These are readily observed as yellow deposits beneath the skin.

Areas of fat deposition vary from species to species (high in aquatic birds) and the time of year (pre-migration deposition).

Common areas of fat deposition are lateral to the pectoral muscles, in the cloacal region and on the dorsum.

Skin of the legs and feet

• Podotheca - the non-feathered areas of the legs and feet. Scales are formed from raised, heavily keratinised epidermis separated by folds of less keratinised tissue overlying a proliferative germinal layer, giving it a 'pimpled' architecture.

Glandular tissue

The avian skin is effectively glandless, lacking sebaceous and sweat glands and most of the skin is thin, dry and inelastic. The exceptions are:

• Uropygial gland
• Glands of the ear canal
• Pericloacal glands
• Keratinocytes

The Uropygial gland is also known as the preen gland. It is a bilobed gland located dorsal to the cloaca at the end of the pygostyle. It opens through a caudally directed nipple.

This holocrine gland is NOT present in all species of bird. It is well developed in some parrots (African Greys) but absent in others (Amazons). It is also present in most finches but only some columbiformes.

The uropygial gland is involved in maintaining feather condition and secretions spread by preening. It serves a waterproofing function. The secretions contain a pro-vitamin D, converted by UV light to active vitamin D. The secretions are also believed to suppress the growth of micro-organisms, serving an anti-bacterial function.

Keratinocytes are important in birds without a uropygial gland. Developing dermal cells (keratinocytes) undergo metamorphosis from cuboidal or squamous nature, lose organelles, produce lipids and fibrous proteins (keratin) and dehydrate and lyse.

This function is unique to birds and it is suggested that the lipid production by the keratinocytes makes the entire skin an 'oil-producing' gland.

Given that birds do not have sweat or odour producing glands, it is noted that when stressed (e.g. during handling), some parrot species emit a musty odour. This appears to arise from volatile fats emitted directly onto the skin by rapidly lysing keratinocytes.

Links

http://en.wikipedia.org/wiki/Feathers