Equine Integumentary System - Horse Anatomy

Skin

Epidermis

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, which act like bridges between the cells. 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 mechanoreceptors. 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 extra cellular matrix (ECM) proteins such as laminin-5 and collagen type IV molecules, which form the lamina densa part of the BMZ, while the lamina lucida is composed of Collagen type V. Collagen type VII is important in forming anchoring fibrils between the dermis and the basal lamina.

The dermis consists of dense fibrous tissue composed of collagen type I, III and V, 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.

Hair

All hair has a similar structure: an outer cuticle, a cortex containing pigmentation, an inner medulla and a root or bulb from which the hair grows out of the follicle to form the shaft.

Each hair follicle has a sebaceous gland and an arrector pili muscle associated with it.

Hair follicles are divided into three parts:

The infundibulum corresponds to the area from the opening of the sebaceous duct to the surface of the skin.

The isthmus is the area between the opening of the sebaceous duct and the attachment of the arrector pili muscle.

The inferior segment extends from the attachment of the arrector pili muscle to the dermal hair papilla.

The hair follicle originates from a peg of epidermal cells that grows down into the underlying dermis, where it forms a hair cone over a piece of dermis called the dermal papilla. The papilla provides the blood and nerve supply for the growing hair. From the hair cone, the cells keratinise and form a hair. As the hair grows up through the epidermis to the skin's surface, the cells at the point of the cone die, forming a channel, the hair follicle.

Hair Types

Guard Hairs

These are generally stiff and straight and form the outer haircoat of an animal. They are uniformly distributed across the skin surface and give the haircoat a smooth appearance. The smoothness of the coat is important in allowing rain to fall from the surface without penetrating deeper to the epidermis and causing loss of body temperature. The oily coating of the haircoat comes from the secretions of the sebaceous glands in the epidermis, associated with the hair follicle. This also contributes to the ‘waterproofing’ of the haircoat. Each hair consists of an outer cuticle, with a cortex and innermost medulla. It is composed of highly keratinised, dead epithelial cells, with the arrangement into the 3 layers conferring flexibility. The arrector pili muscle is attached to the proximal end of the follicle and the dermis close to the point at which the hair is externalised. Involuntary contraction of this muscle in cold ambient temperatures causes erection of the hairs to trap warm air against the skin, thus providing insulation. This can also be induced by the ‘fight or flight’ mechanism of the sympathetic nervous system. Modifications of the guard hairs in horses include the coarse hair of the mane, tail and the tufts of hair ('feathers') around the fetlock region.

Wool Hairs

These are thin, wavy, soft hairs that form the undercoat of animals. They are often much more numerous than the tough guard hairs and usually shorter. Their number increases in winter when they serve to keep the body warm.

Tactile Hairs

Tactile hairs are associated with a sensory function and are much thicker and stiffer than other hairs. They are mainly found on the face and can also be referred to as whiskers or vibrissae. Their follicles are much deeper into the hypodermis than guard or wool hairs and possess a venous sinus and nerve endings with mechanoreceptors. During embryonic development, they appear before other hair types. These hairs are mostly found on the face, near the eyes and on the upper lip.

Tylotrich Hairs

These large hair follicles are rapidly adapting mechanoreceptors. They may be present anywhere on the body. These structures consist of a single large hair, surrounded by neurovascular tissue at the level of the sebaceous gland.