Anatomy of Hooves and Claws

Sensory Receptors

• Detect pain, itch, and temperature through encapsulated nerve endings
• Meissner's corpuscles detect light pressure/touch
• Pacinian corpuscle detects deep pressure
• Ruffini corpuscle senses stretch

Epidermal Structures/Derivatives

• Hair follicles
  • Compound hair follicles
  • Sinus (tactile) hairs
• Sebaceous glands
• Apocrine (sweat) glands
• Udders, mammary glands
• Hooves, claws, nails
• Footpads (with merocrine/eccrine sweat glands)
• Anal sac glands and anal sacs
• Circumanal glands (hepatoid glands)

Functions of Hair/Feathers

• Insulation
• Camouflage
• Social display
• Sense/protect
• Sex recognition

Anatomy of Hair

• Produced by a hair follicle
• Hair shaft above the skin surface
• Hair root within the follicle, ending with a bulb
• Hemidesmosomes bind cells to the basal lamina
• Desmosomes bind neighboring cells together
• Both connect to intermediate filaments

Stratum Spinosum

• Polygonal cells, 1-2 cells thick in thin skin, many layers thick in thick skin
• Increased number of tonofilaments (cytokeratin) and desmosomes give a spiny appearance
• Cells are cohesive and resist abrasion
• Cells in this layer retain some capacity for division if needed

Stratum Granulosum

• 3-5 cells thick
• Cells begin to flatten
• Contain basophilic keratohyalin granules
• Lamellar granules (not visible via light microscopy) secreted by cells to form waterproof lipid sheets, “intercellular cement”
• No mitotic activity, last living layer, nucleus and organelles soon to be lost

Stratum Lucidum

• Translucent layer
• Present in thick skin only
• Many keratin filaments, desmosomes present
• Cellular organelles are gone – cells are fully keratinized
• Cytoplasm contains eleidin – protein chemically related to keratin

Stratum Corneum

• Outermost layer
• Many (up to 20) layers thick, thickness varies by location
• Cells consist entirely of keratin, a water-resistant protein
• No nuclei or organelles
• Cells continually shed at the surface

Keratinization

• Process by which keratinocytes differentiate, taking around 21 days in dogs
• Cornification: Production of stratum corneum by terminal epidermal differentiation

Non-Keratinocytes of the Epidermis

• Langerhans cells – immune system
• Merkel’s cells – sensory
• Melanocytes – protection

Langerhans Cells

• Intra-epidermal (within the epidermis) macrophages
• Derived from bone-marrow monocytes
• Role in immunity: antigen presenting to T-cells, mediators of tolerance
• Not often seen with routine H&E, use immunohistochemistry to identify

Merkel’s Cells

• Located in the stratum basale of thick skin
• Can act as sensory mechanoreceptors for cutaneous sensation
• Free nerve endings seen at the base of these cells

Melanocytes

• Present in the stratum basale
• Neural crest origin
• Produce melanin pigment for keratinocytes of stratum basale and spinosum
• Protect cells from effects of UV radiation
• Must have tyrosinase for proper function

Melanin

• Absorbs harmful UV-radiation
• Transforms the energy into harmless amounts of heat
• Must have tyrosinase for proper function

Sensory Nerves of the Dermis

• Nociceptors – free nerve endings that reach the stratum granulosum

Compound Follicles

• Clusters of several hair follicles
• Follicles merge at the level of the sebaceous gland and emerge through one external orifice
• Compound follicles usually have one primary follicle with an apocrine sweat gland and several secondary hair follicles
• Common in carnivores

Arrector Pili

• Smooth muscle attached to follicle of primary hairs
• Contraction causes hair to stand up (goosebumps)
• Role in insulation
• ANS controls this

Sinus Hairs/Whiskers

• Tactile hairs
• Composed of a very large single follicle
• Blood-filled sinus between inner and outer dermal root sheath
• Attached to skeletal muscle to allow for voluntary movement
• Nerve bundles penetrate sheath

Sebaceous Glands

• Located in the dermis
• Produce sebum, a mixture of lipid and cell debris
• Holocrine secretion
• Antibacterial and waterproofing
• Ducts empty into a follicle
• Can be simple, branched, or compound glands

Specialized Sebaceous Glands

• Supracaudal glands – dog
• Circumanal/Hepatoid glands
• Mental organs/glands – cat
• Horn glands – buck goat
• Preputial glands – smegma in horse
• Tarsal (Meibomian) glands – eyelids
• Uropygial gland (the preen gland) in birds

Uropygial/Preen Gland

• Strongly developed in many waterbirds, such as ducks
• The rest of bird skin is aglandular

Apocrine Sweat Glands

• Secrete by apical budding/pinches
• Epithelial cells have apical secretory caps
• Simple coiled tubular glands open into the distal hair follicle
• Contractile myoepithelial cells help express the product
• In domestic animals, located throughout most of the skin
• Function is mainly communication (attraction, marker)

Hooves and Claws

• Modification of skin
• Variation of the stratum corneum
• Supported by highly vascular dermis
• Lack stratum granulosum and stratum lucidum

Hair Growth Cycles

• Hair growth consists of three phases: anagen, catagen, and telogen.
• Anagen phase lasts around 2-3 years, during which hair grows approximately 1 cm per month.
• The majority of hairs (80-90%) are in the anagen phase.
• Catagen phase is a short transitional phase lasting around 2-3 weeks, where hair growth slows down.
• Only 3-4% of hairs are in the catagen phase.
• Telogen phase is a resting phase lasting around 3-4 months, where hair is released from the follicle and a new hair grows from the same follicle.
• 10-15% of hairs are in the telogen phase.

Nociceptors in Hair Follicles

• Nociceptors are sensory neurons that detect painful stimuli.
• In hair follicles, nociceptors are found in the follicular epithelium and the surrounding dermal sheath.
• Nociceptors in hair follicles detect mechanical and chemical stimuli, such as hair pulling or irritation.
• They regulate hair growth and cycling through the release of neurotransmitters.
• Nociceptors may play a role in the development of hair disorders, such as alopecia areata.