Integument

Stratum basale

• innermost layer of the epidermis, interfaces with dermis

• responsible for proliferation, replication, & differentiation normally or following injury

• differentiate into higher layers of epidermis

Arrector pili

• smooth muscle associated with hair follicles that cause piloerection

• functions in thermoregulation & communication (hackles)

• more common/dense on the dorsum

Stratum spinosum

• prominent desmosomes (prickle cells)

• thickness/number of layers depends on anatomic location

Stratum granulosum

• thin layer of keratinocytes w/dense basophilic keratohyalin granules (contain components responsible for keratinization)

• produces hydrophobic phospholipid secretory product

• not always present in every layer of the skin, seen less in thinner skin

Stratum cornea

• variably thick outer layer of epidermis

• flattened anuclear eosinophilic keratinocytes (aka corneocytes) w/thickened, insoluble, hydrophobic cell membrane

• considered dead but have some metabolic/signal function

• surrounded by hydrophobic phospholipid secretory product

• most differentiated cell layer

Papillary dermis

• the thin superficial layer of the dermis

• plexus supplies blood to the epidermis

Reticular dermis

• majority of dermal collagen thickness

• imparts structural properties, supports adnexa and vasculature

Dermis

• composed primarily of collagen w/some elastin present, interspersed with ground substance

• provides structural/tensile strength, thermoregulation, adnexal support, immune response, & fluid/electrolyte/nutrient support

• vascularized, majority supply is subcutaneous/deep dermal that branches to form cutaneous plexus that supplies blood to adnexa

• aka quick in carnivores, corium in horses

Hair follicle

• simple (one hair per shaft) vs compound (multiple hairs from same shaft, contributes to soft hair coat)

• primary (larger) vs secondary (smaller/undercoat)

• grow via cyclical proliferation (anagen/growth, catagen/transition, telogen/resting), cycle based on hormones, daylight, temperature, nutrition, & growth factors

• zones: bulb (deep), isthmus, infundibulum (superficial)

Sebaceous glands

• lobular glands adjacent to hair follicles that secrete sebum (moisturizing & antimicrobial) into the isthmus of the follicle via holocrine secretion

Epitricheal glands

• coiled tubular glands lined by cuboidal epithelium that secrete sweat into the hair follicle via apocrine secretion

Eccrine glands

• coiled tubular glands that secrete sweat onto the surface of the skin via merocrine secretion

• primarily associated with non-haired skin

Subcutis

• lowest layer of the skin (beneath dermis)

• primarily lipid/adipose tissue

• thickness/amount depends on area of the body & physiologic status

• stores energy, provides insulation, & acts as protection

Hoof wall

• a thick, compacted layer of stratum corneum

• stratum externum & medium made by coronary epithelium

• stratum interum layer made by laminae

Laminae

• specialized finger-like projections of epidermal epithelium

• holds the hoof wall onto P3 bone

• produce stratum internum

• primary & secondary (projections from the primary that increase surface area)

Melanocytes

• produce melanin in the basal layer via melanosome organelle & transfer pigment to keratinocytes

• uniform distribution

Melanin

• functions to scatter/reflect UV rays & scavenge free radicals