Epidermis

The epidermis covering the embryo initially consists of a single layer of cuboidal cells resting on a basal lamina (Fig 25.1A). Shortly after neurulation, these ectodermally‐derived cells divide and give rise to a superficial layer of flattened cells, the periderm and an underlying layer of cuboidal cells, the basal layer (Fig 25.1 B). Further proliferation of the cells of the basal layer gives rise to an intermediate layer resulting in a stratified covering, the epidermis (Fig 25.1 C, D and E). After gastrulation, ectodermal cells differentiate along either epidermal or neural pathways, a process determined by the relative expression of Wnt, Fgf and Bmp signals. It has been proposed that the process of stratification of the epidermis, which occurs in association with the appearance of the first suprabasal cells, is determined in part by a dramatic shift in spindle orientation, as the majority of basal cells now divide perpendicular to the basal lamina. The exchange of water, sodium and glucose between amniotic fluid and the epidermis probably involves peridermal cells. Close to mid‐pregnancy, the basal epidermal cells deep to the periderm undergo differentiation, forming the typical epithelial layers characteristic of postnatal stratified squamous epithelium, consisting of a stratum basale (stratum germinativum), stratum spinosum, stratum granulosum and stratum corneum. Cells in these epithelial layers which synthesise the scleroprotein keratin, are termed keratinocytes.

The signalling pathways determining stratification and differentiation of the skin have been identified through murine knockout studies. These pathways include Notch and the transcriptional regulator p63. Keratinocyte growth factor, otherwise known as Fgf‐7, which is produced by fibroblasts of the underlying mesenchymally‐derived dermis, also regulates the growth of the basal cells of the epidermis. As the epithelium differentiates into its characteristic layers, the peridermal cells, which undergo apoptosis, are shed into the amniotic fluid. Loss of the peridermal layer and formation of the stratum corneum of the stratified squamous epithelium coincide with cessation of exchange of water and electrolytes between amniotic fluid and epidermis. This loss of exchange may also be related to the commencement of kidney function and the passing of urine into the amniotic cavity with its accumulation within the amniotic sac. In some areas of the body, proliferation of cells of the basal layer gives rise to epidermal pegs which extend into the underlying developing dermis. During the period of epidermal proliferation, cells of neural crest and mesodermal origin also contribute to the population of cells found in the skin. Melanoblasts, derived from the neural crest, migrate to the underlying mesenchyme and later move to the basal layer of the epithelium where they differentiate into melanocytes, cells which synthesise melanin pigment. Melanin is stored intracellularly as granules referred to as melanosomes. These pigment granules, which are moved to the tips of dendritic processes of melanocytes, are transferred to adjacent keratinocytes by a process referred to as cytocrine secretion. Within keratinocytes, melanosomes act as a barrier to solar radiation. Melanosomes also impart pigmentation to the skin, hair, hooves, horns and a number of ocular structures.

Langerhans cells, derived from the bone marrow, are of the monocyte–macrophage lineage. These cells, which are more numerous in the stratum spinosum than in other layers of the epithelium, are present in the epidermis from an early stage in embryonic development. Langerhans cells, which act as antigen‐presenting cells for T lymphocytes, are a peripheral component of the immune system.

A third cell type, the Merkel cell, which migrates to the basal layer of the epidermis, functions as a sensory cell through its interaction with free nerve endings. Merkel cells originate from epidermal progenitors during development and can detect tactile stimuli and changes in contact pressure.

Dermis

The dermis, which develops during the late embryonic period, arises from mesenchymal cells derived in part from dermatomal cells and also from somatopleural mesoderm. The mesenchyme differentiates into the connective tissue cells which give rise to collagenous and elastic fibres. The dermis, which is located immediately deep to the epidermis, has areas known as dermal papillae, that project into the overlying epidermis. The superficial papillary layer of the dermis is composed of loose connective tissue, while the thicker underlying reticular layer contains dense irregular connective tissue. Afferent nerve fibres, which grow into the dermis, innervate both the dermis and epidermis.

Hypodermis

Beneath the dermis in most regions of the body, mesenchymal cells form a layer of loose connective tissue, the hypodermis, consisting of irregular bundles of collagen fibres interspersed with elastic fibres and adipocytes. This layer of subcutaneous connective tissue anchors the skin to underlying structures. Hypodermis is not present in particular regions such as the lips, cheeks, eyelids, auricles of the ears and anus. Bundles of skeletal muscle, the subcutaneous muscle, develop in the hypodermis in specific regions of the body, such as the thoracic and cervical regions. The nature and depth of the hypodermis vary considerably with species. Because the hypodermis is less dense in carnivores and sheep than in other domestic species and contains a high proportion of elastic fibres, the skin in these animals can be easily raised when grasped. In pigs, the hypodermis, which is a comparatively thick layer, attaches the skin firmly to the underlying structures. Porcine fat in the hypodermis forms a well‐defined layer, the panniculus adiposus, which may be up to 5 cm thick. In horses, cattle and goats, which have a thin layer of hypodermis, the skin closely follows the outline of the underlying structures. The presence of fat in the hypodermis contributes to insulation against heat loss.

The skin contains a variety of nerve endings which are more numerous in hairless areas than in hair‐covered areas. While sensory fibres are prominent in the dermis and hypodermis, they also extend to the external root sheaths of hair follicles and between the cells of the deeper layers of the epidermis. Nerve endings in the skin can be divided morphologically into free nerve endings and encapsulated nerve endings. Free nerve endings, which are found principally in the epidermis, detect stimuli associated with pain, heat and cold. Structures with diverse morphology, referred to as encapsulated nerve endings, are located in the dermis or hypodermis and serve as mechanoreceptors. Innervation of blood vessels and sweat glands is supplied principally by the sympathetic division of the autonomic nervous system.

Three vascular networks parallel to the skin surface, the subcutaneous, cutaneous and superficial plexuses, provide the arterial blood supply to the skin. The subcutaneous plexus is derived from arterial branches to superficial cutaneous structures. The cutaneous plexus, which supplies the hair follicles and sweat glands, arises from branches of the subcutaneous plexus. The superficial plexus, which develops from branches of the cutaneous plexus, supplies the papillary processes. The epidermis derives its nutrients and oxygen by diffusion from the capillary loops in the papillary processes. A network of veins corresponding to the arterial plexuses provides venous drainage. In superficial regions of the dermis, a lymphatic network drains into cutaneous lymphatic vessels.

Hair

One of the features which distinguishes mammals from other vertebrates is the presence of hair. Slightly raised elevations on the smooth bare skin in areas around the lips, periorbita, cheeks and lower jaw of the foetus are the first macroscopic evidence of hair development. With the exception of notable anatomical regions, the entire body surface of domestic animals is covered by closely spaced hairs. Areas devoid of hair include the muzzle, muco‐cutaneous junctions, hooves and digital pads. Marked variation in hair density, type, distribution pattern and colour is evident among species and, within species, hair characteristics are breed related.

The primordial structures from which hairs develop arise during the early foetal period when the epidermis is composed of three layers. Solid proliferations from the basal layer of the epidermis project into the underlying mesenchyme, forming hair buds or pegs (Fig 25.2). As the hair peg extends into the dermis at an oblique angle, an aggregation of mesenchymal cells, known as the hair papilla, projects into the tip of the peg. The epidermal cells of the peg grow around the hair papilla like an inverted cup, forming the hair bulb. The structure formed from the epidermal ingrowth, together with the hair papilla, is referred to as a hair follicle. The inner layer of epidermal cells of the hair bulb which gives rise to the hair shaft and epithelial root sheaths is known as the germinal matrix.

The formation of hair follicles requires interactions between cells in the basal layers of the epidermis and the underlying mesoderm. Prior to the formation of hair placodes, a range of marker molecules are expressed in both the dermis and epidermis, including Wnt‐10b, Edar, Dkk‐4 and Keratin 17. Wnt signals are generally considered the first dermal signals which subsequently direct the formation of a hair follicle. These signals activate ectodysplasin (Eda), a Tnf family ligand confined to ectoderm, which binds to its receptor Edar and has the dual effect of inhibiting Bmp signals and inducing expression of Shh. Eda, together with Wnt signals from the hair pegs, contribute to the initiation of hair bud formation. These signals regulate expression of Shh and Bmp genes. The signalling molecule Shh induces the aggregation of mesenchymal cells in the dermis and promotes the development of individual hair follicles. The Bmp signals, in conjunction with Wnt inhibitor, Dkk‐4, suppress hair follicle development in regions of the dermis immediately adjacent to an existing hair follicle primordium, thus regulating the spacing of hair follicle formation. Bmp inhibitors, including Noggin and Follistatin, are expressed in the placode region, thereby ensuring that a follicular fate is followed.

The developing hair follicle which connects the germinal matrix with the surface becomes canalised and the layer of epidermal cells surrounding the newly‐formed space gives rise to the external root sheath of the follicle. Cells in the centre of the germinal matrix adjoining the hair bulb proliferate and are displaced into the lumen of the external root sheath, forming the hair shaft. Continued proliferation of the basal cells of the matrix force the hair shaft towards the surface of the skin from which it subsequently projects. As cells of the hair are pushed towards the surface and move further away from the papilla, their source of nutrients, they undergo keratinisation. Cells at the periphery of the germinal matrix proliferate and grow between the hair shaft and the external root sheath, forming the internal root sheath. This internal root sheath, which extends halfway along the follicle, produces soft keratin. Melanocytes present in the hair bulb impart pigmentation to the developing hair.

Hair keratin expression has a distinct pattern along the length of hair shafts. The keratins Ha‐2 and Hb‐2 are expressed specifically in the hair cuticle, the layer of cells on the surface of the hair shaft, while Ha‐1 expression begins at the transitional region between the matrix and cortex and continues throughout the lower and middle portions of the cortex. Differential expression of these keratin proteins may influence hair texture.

Mesenchymal cells surrounding the developing hair follicle differentiate into a connective tissue sheath. A small band of smooth muscle, also derived from mesenchymal cells in the dermis, attaches this connective tissue sheath to the superficial layer of the dermis on the side of the hair follicle which forms the greater angle beneath the epidermal layer (Fig 25.2 E). On contraction, these muscle bands, known as arrector pili muscles, decrease the greater angle between the hair follicle and the skin surface, thereby moving the hair shaft into an erect position. Arrector pili muscles are especially well developed along the dorsal midline of dogs where they cause the hair to become erect in response to a threat of aggression.

Primordia of sebaceous glands form as cellular outgrowths from the basal epithelial layer of the walls of developing hair follicles at levels closer to the surface than the points of attachment of arrector pili muscles (Fig 25.2 D). Smaller epidermal outgrowths, superficial to sebaceous gland primordia, may develop from the follicular wall forming the primordia of sweat glands (Fig 25.2 D).

Hair follicles are classified as either primary or secondary. Primary hair follicles have a large diameter and the bulbs are located deep in the dermis. Arrector pili muscles and both sebaceous and sweat glands are normally associated with primary follicles. A single hair which emerges from these follicles is referred to as a guard hair. Initially, primary hair buds tend to develop at closely spaced time intervals and at even distances from each other. Subsequently, new primary follicles develop among those already established, resulting in groups of two, three or four follicles in close proximity to each other. Hair follicles which have a relatively small diameter and are located more superficially in the dermis than primary follicles are referred to as secondary follicles. Hairs which emerge from secondary follicles are referred to as secondary or under hairs. While secondary follicles have associated sebaceous glands, unlike primary follicles they lack sweat glands and arrector pili muscles.

Hair follicles may be described as simple, when a single hair is present, or compound (complex), when two or more hairs project through a common pore (Fig 25.3). In dogs and cats, compound hair follicles develop postnatally. In canine skin, up to 15 secondary buds develop from the primary follicles in a manner analogous to the development of primary buds, giving rise to hair shafts which project from the skin surface through a common pore.

There is wide variation not only in hair follicle types but also in their surface distribution among domestic animals. Only primary hair follicles, which are distributed evenly in rows over the body surface, are present in horses and cattle. In pigs, primary hair follicles occur in clusters with three or four primary hair follicles per cluster. From the compound follicles in canine skin, which occur in clusters of three, a single large primary hair surrounded by a group of smaller secondary hairs emerges. In cats, single large primary follicles are surrounded by two to five compound follicles. Each compound follicle has three coarse primary hairs and from 6 to 12 secondary hairs.

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