• Skin and related structures:
• Hair, hooves, horns, claws, skin-related glands
• Functions: prevents desiccation; reduces threat of injury; assists in maintaining normal body temperature; excretes water, salt, and organic wastes; receives and conveys sensory information; synthesizes vitamin D; stores nutrients

• Epidermis
• Dermis
• Hypodermis

Cell types:

Keratinocytes – produce keratin, the tough, fibrous, waterproof protein that gives skin its resiliency and strength 

Melanocytes – produce melanin pigment

Merkel cells – phagocytize microinvaders; macrophage specific to epidermis 

Langerhans cells – found in stratum spinosum; may be involved in allergic and cell-mediated immune response in skin 

• Stratum germinativum (basal layer): Deepest layer
• Stratum spinosum (spiny layer)
• Stratum granulosum (granular layer): Middle layer
• Stratum lucidum (clear layer)

• Stratum corneum (horny layer): Outermost layer

• Deepest layer
• Consists of a single row of keratinocytes attached to epithelial basement membrane 
• Merkel cells, melanocytes, keratinocytes, found in this layer

• Contains several layers of cells held together by desmosomes 
• Langerhans cells found in this layer

• Middle layer
• Composed of two to four layers of flattened, diamond-shaped keratinocytes that contain lamellated granules of glycolipids 
• Glycolipids play a role in helping waterproof the skin and slowing water loss across the epidermis

• Found in very thick skin 
• Composed of a few rows of flattened dead cells 
• Contents of the keratogranules combine with intracellular tonofilaments to form keratin fibrils

• Composed of 20 to 30 rows of keratinocyte “remnants” 
• Sometimes called horny or cornified cells

• Hairy skin usually consists of three epidermal layers rather than five (stratum basale, stratum spinosum, and stratum corneum)
• The surface of hairy skin is covered in scalelike folds. 
• A knoblike elevation can be seen periodically
• Tactile elevation or epidermal papillae
• Usually associated with a tactile hair (tylotrich hairs)

• Composed of dense irregular connective tissue 
• Collagen, elastic, and reticular fibers 
• Also includes hair follicles, nerve endings, glands, smooth muscle, blood vessels, and lymphatics 
• Fibroblasts, adipocytes, and macrophages also present
• Two layers: 

1. Papillary layer 
2. Reticular layer

Papillary layer 

• Underneath the epithelial layer of the epidermis 
• Composed of loose connective tissue with loosely woven fibers and ground substance 
• Dermal papillae help cement the epidermis and the dermis together
• Blood vessels, pain, temperature,  and touch receptors also present

Reticular layer 

• Consists of dense irregular connective tissue
• Bundles of collagen fibers from papillary layer blend into those of reticular layer 
• Most fibrous bundles tend to run parallel to each another.
• Separations between bundles represent tension lines in skin
• In areas where a great deal of bending occurs, dermal folds or flexure lines are present.

• Composed of areolar tissue containing adipose, blood and lymphatic vessels, and nerves 
• Contains special touch receptor  – the pacinian corpuscle (sensitive to heavier pressure than Meissner's corpuscle) 
• Fibers of hypodermis are continuous with those of dermis 
• Hypodermal layer permits skin to move freely over underlying bone and muscle without putting tension on skin

• Pigmentation
• Paw Pads
• Planum Nasale
• Ergots and Chestnuts
• Cutaneous Pouches in Sheep

• Result of presence or absence of melanin granules in the extensions of melanocytes 
• No pigmentation if granules are concentrated around nucleus of the melanocyte 
• As granules move into the cellular extensions and into surrounding tissue, pigmentation becomes macroscopically apparent 
• The more granules present, the darker the pigmentation
• Melanocyte-stimulating hormone controls dispersion of granules
• Keratinocytes arrange melanin on the side of the cell with greatest amount of sun exposure
• Acts to protect keratinocytes from exposure to damaging ultraviolet rays

• Thick layers of fat and connective tissue with exocrine sweat glands and lamellar corpuscles
• Outer surface is the toughest and thickest skin in the body 
• Often pigmented; composed of all five epidermal layers
• Stratum corneum is thicker than all other layers combined 

Conical papillae can be seen covering entire pad

• Top of the nose in cats, pigs, sheep, and dogs
• Planum nasolabiale: the muzzle of cows and horses
• Usually pigmented; aglandular except in sheep, pigs, and cows
• Composed of only three epidermal layers:
• Stratum germinativum, stratum spinosum, stratum corneum  
• Not present: stratum lucidum, stratum granulosum

• Dark horny structures found on the legs of horses, ponies, and other members of the equine family 
• Thought to be vestiges of carpal and tarsal pads of second and fourth digits ("splint bones")

• Infoldings of skin 
• Infraorbital, interdigital, and inguinal pouches 
• Contain fine hairs and numerous sebaceous and oil glands 
• Secrete a fatty yellow substance which covers and sticks to the skin when dry

• Hair
• Hair strands and follicles
• Types of hair
• Glands of the skin
• Sebaceous and sweat glands
• Tail glands
• Anal sacs
• Claws and dewclaws
• Hoof
• Horns

• Functions in maintaining body temperature; camouflage
• Hair shaft: visible above the skin 
• Hair root: buried within the skin 
• Hair follicle: anchors the hair 
• Deepest part of hair follicle expands to form a hair bulb
• At the base of the hair bulb is a mound of dermal cells called the papilla.
• Hair strands are formed as epithelial cells mature, fill with keratin, and move away from the papilla.

• Root sheath layers: connective tissue root sheath, external root sheath, and internal root sheath 
• Each hair strand is organized into three layers: cuticle, cortex, and medulla
• Root hair plexus: web of sensory nerve endings 
• Touch receptor

• Anagen phase: cells are added at the base of the root, hair lengthens
• Catagen phase: period of transition between anagen and telogen phases
• Telogen phase: maximum length of hair is achieved, hair stops growing, hair follicle shortens, and hair is held in a resting phase

• Melanocytes transfer melanin to the cortical and medullary cells that form the hair strand. 
• Different colors result from the quantity and type of melanin incorporated into the hair. 
• Horses produce only one type of melanin; dogs produce two.
• As animals age, melanin production decreases and hair begins to turn gray. 
• White hair is formed when the cortex loses its pigment entirely and the medulla becomes completely filled with air.

1.Primary or guard hairs

• Straight or arched; thicker and longer than secondary hairs

2.Secondary or wool-type hairs

• Softer and shorter than primary hairs; wavy or bristled in the dog; predominant hair type in species with wool-type coats

3.Tactile (or “sinus”) hairs 

• Contain numerous sensory endings 
• Commonly known as whiskers; also mixed intermittently throughout the hair coat 
• Also called sinus hair because of the large blood sinus located in the connective tissue portion of the follicle

• Small, smooth muscle 
• Attached to each hair follicle 
• Innervated by sympathetic nervous system
• Contraction of the muscle pulls the hair to an erect position

• Sebaceous Glands
• Sweat glands (sudoriferous glands)
• Eccrine sweat glands 
• Apocrine sweat glands
• Tail Glands
• Anal Sacs

• Located in the dermis; may be simple or complex alveolar structures
• Most have a single duct that empties into hair follicle; others have ducts that empty directly onto surface of skin
• Epithelial cells lining sebaceous gland manufacture and store sebum
• Because the epithelial cell is lost in the process of secretion, the sebaceous gland is classified as a holocrine structure.

• Composed primarily of glycerides and free fatty acids
• Arrector pili muscle contracts and compresses sebaceous gland, forcing sebum through the duct into the hair follicle
• Coats the base of the hair and surrounding skin 
• Helps trap moisture, keeps hair soft, pliant, and somewhat waterproof 
• Sebum also helps reduce the skin's risk of infection.

• Also called sudoriferous glands 
• Found over the entire body of most domestic species 
• Sweat helps cool the body through evaporation.
• Two types of sweat glands: 
• Eccrine 
• Excretory portion consists of a simple coiled tube located in the dermis or hypodermis 
• Empty onto surface of skin through a long duct
• Apocrine
• Coiled excretory portion buried in the dermis or hypodermis; single excretory duct 
• Empty into hair follicles

• Oval region at the dorsal base of the tails of most dogs and cats
• Contains coarse, oily hairs 
• Very large apocrine and sebaceous glands present
• Thought to assist with recognition and identification of individual animals

• Cats and dogs have anal sacs similar to musk glands of skunks.
• Located at the 5 and 7 o'clock positions relative to the anus
• Connected to the lateral margin of the anus by a small single duct 
• Lined with sebaceous and apocrine glands 
• When an animal defecates or becomes frightened, some or all of the anal sac contents are expressed. 

Claws

• Hard outer coverings of the distal digits
• Usually pigmented
• Function in maintaining traction and serve as tools for defense and catching prey 
• Claws are nonretractable except in most cat species

Dewclaws 

• Evolutionary remnants of digits 
• In the dog, the dewclaw is the first digit.
• In the cow, pig, and sheep, the medial and lateral dewclaws are the second and fifth digits, respectively.

• Horny outer covering of digits of some animals
• Another name for “hoof” is ungula. 
• Hoofed animals are called ungulates. 
• Hooves rest on tissue called the corium. 
• The corium is attached to the periosteum of the distal phalanx. 
• The outer hoof is a modified epithelial layer, and the corium is modified dermis. 
• The skeletal foot of the horse includes the distal part of the second phalanx, the distal sesamoid bone (navicular bone), and the entire third phalanx (coffin bone).
• The coffin bone has a layer of corium, which in turn is covered by the cornified hoof. 
• The hoof and the corium form interdigitations called laminae. 
• The equine hoof is generally divided into three parts: the wall, the sole, and the frog. 

• Laminar: primary and secondary lamina. Located between hoof wall and third phalanx. Provides nutrients to stratum internum
• Perioplic corium: located in the coronary sulcus and supplies nutrients to periople
• Coronary corium: found in the coronary sulcus and provides nutrients to the sole
• Sole corium: superior to the sole and provides nutrients to the sole
• Frog corium: superior to the frog and provides nutrients to the frog

Lamellae – interlocking of dermal and epidermal lamellae 

The wall:

• External portion of the hoof visible from the anterior, lateral, and medial views; divided into the toe, the quarters, and the heels 

The sole:

• Plantar, or palmar, surface of the hoof; outer layers are avascular and lack innervation

The frog:

• Triangular horny structure located between the heels on the underside of the hoof 
• Divided by a central depression known as the central sulcus 
• Digital cushion: a thick pad of fat and fibrous tissue lies beneath the sensitive frog
• Lateral cartilages extend proximally from the distal phalanx 

• Epidermal in origin 
• Structurally similar to hair 
• Composed of keratin 
• In adults the horn is hollow and communicates directly with the frontal sinus. 
• The corium lies at the root of the horn and is bound to the horn process by periosteum. 
• The body of the horn is composed of tightly packed tubules.
• The wall of the horn is thinner at the base than at the apex.