Integumentary System Notes (Vet ACBS 400A/500A)

Derived from ectoderm in embryonic tissues.
Cells:
- Keratinocytes: produce keratin
- Melanocytes: produce melanin; melanin helps protect skin from UV light
- Langerhans cells: tissue-resident macrophages that form a network of cells capable of migrating throughout the epidermis
- Merkel cells: senses touch
Layers (deepest to most superficial):
- $\text{Stratum basale}, \, \text{Stratum spinosum}, \, \text{Stratum granulosum}, \, \text{Stratum lucidum}, \, \text{Stratum corneum}$
Important note: the epidermis is the outer protective barrier; it renews continuously through keratinocyte proliferation and maturation.
Clinical relevance: damage to any layer can impair barrier function, sensation, pigmentation, or immune responses.

The germinal (basal) layer that provides the essential germinal cells for regeneration.
Separated from the dermis by the basement membrane.
Mitotic division of germinal cells → progressive maturation (keratinization) → migration toward the surface.
Affected by nutrition, hormones, growth factors, immune responses, and genetics.
Produces keratinocytes; after creation, cells move up through the layers to mature.
Interspersed cells in this layer include:
- Merkel cells
- Melanocytes (melanin production regulated by hormones and genetics)
Functional significance: maintains epidermal turnover and pigmentation; melanocytes influence UV protection.

Known as the "prickly layer" between stratum basale and stratum granulosum.
Keratinocytes are polyhedral with numerous tiny spiny projections in intercellular spaces.
Langerhans' cells are present here; they are damaged by excessive UV exposure and glucocorticoids but are important for immune function.
Part of the phagocytic system; contributes to antigen presentation and skin immunity.

Cells accumulate dense basophilic keratohyalin granules.
Keratohyalin granules play a key role in aggregation of keratin filaments and formation of the cell envelope for cells in the stratum corneum.
Helps form a waterproof barrier to prevent fluid loss from the body.
Lamellar granules contain polysaccharides, glycoproteins, and lipids; these contribute to cohesion of the stratum corneum.
Continues maturation of keratinocytes toward the surface.

Very thin layer consisting of about $2-3$ keratinocytes.
Transition point between granulosum and corneum.
Function: helps to remove noxious materials and prevents unnecessary loss of body fluids.

Keratinocytes have accumulated keratohyalin granules and undergo programmed cell death (lysosomal enzyme release) → cells become dead and filled with mature keratin.
Deeper cells retain desmosomal junctions but are pushed toward the surface by newer cells; desquamation (shedding) occurs.
Primary epidermal permeability barrier; prevents excessive transepidermal water loss.
Structural organization often described as "brick and mortar": bricks = keratinocytes; mortar = lipids deposited in the intracellular interstitium.

Serves as the site for attachment of basal epidermal cells and as a protective barrier between epidermis and dermis.
Important functions:
- Anchors epidermis to dermis
- Maintains a functional and proliferative epidermis
- Maintains tissue architecture
- Participates in wound healing
- Acts as a physical barrier
- Regulates nutritional support between epidermis and underlying connective tissue

Tough but elastic support structure that houses nerves, blood vessels, lymphatics, and glands.
Blood vessels are responsible for thermoregulation.
Cutaneous sensation is mediated by nerve plexuses; both myelinated and unmyelinated nerves are present in the dermis.
Motor nerves are primarily adrenergic and innervate blood vessels and arrector pili muscles.
Composition:
- Ground substance, dermal collagen fibers, and cells (fibroblasts, melanocytes, mast cells, and occasionally eosinophils, neutrophils, lymphocytes, histiocytes, and plasma cells).
- Elastic fibers give skin pliability.
Dermis has fingerlike projections that fit closely into depressions of the epidermis, increasing surface contact for nutrient and signal exchange.

Papillary dermis:
- Contains vascular networks that support the avascular epidermis with nutrients and provide a network for thermoregulation.
- Vasculature can increase or decrease blood flow to conserve or dissipate heat.
- Contains Meissner's corpuscles in highly sensitive areas.
Reticular dermis:
- Dense irregular connective tissue that provides overall strength and elasticity to the skin.
- Houses glands and hair follicles.

Consists of loose connective tissue interspersed with fat; amount varies with body site and physiological state.
Usually more prominent in dogs and cats where skin is easily grasped in folds and moved.
Contains panniculus muscle and blood vessels that feed into the dermis.

Three main types of hair:
- Guard hairs (topcoat)
- Wool hairs (undercoat)
- Tactile hairs (whiskers)
Hair structure:
- Flexible column of closely consolidated and heavily keratinized/dead epithelial cells.
- Distinguishes medulla core, cortex, and outer cuticle.
- If the medulla is thick, hairs are straight and brittle; a thick cortex yields stronger and more pliable hairs.

Proximal end of follicle is joined to the dermis by the arrector pili muscle; the other end attaches to the dermal papillae.
Involuntary contraction causes erection of hair (goosebumps), often in response to cold or sympathetic nervous system activity.
Often associated with sebaceous glands that secrete oils to condition hair and skin.
Inside the follicle, cells condense keratinized dead cells into layers of hair starting at the base of the follicle.
Dermal sheath maintains and regenerates the dermal papilla by progenitor cells.
Dermal papilla regenerates the cycling portion of the hair follicle and generates the hair shaft.

Lie close against the skin and sweep uniformly in broad tracks.
Regular arrangement is important; whorls and partings may disturb uniformity when streams converge/diverge.
Function: promote runoff of rain and help keep the animal warmer by reducing direct skin exposure to cold.

Provide a soft undercoat; thinner, wavy, and often more numerous than guard hairs.
Many wool hairs share a single follicular opening.
Structure: central or primary hairs are the longest and usually guard-type; secondary hairs are shorter and softer.
Secondary hairs provide the undercoat; they may be designated as wool hairs due to a small medulla.

Very thick hairs that protrude beyond the guard hairs.
Commonly found on the face along the upper lip and around the eyes; may also be on the lower lip, chin, and other parts of the head.
Some animals have clusters of tactile hairs at the carpus.
Follicles extend deeper into the subcutis or sometimes into musculature.
Contain a venous sinus between inner and outer layers of the dermal sheath, filled with blood.
Nerve endings within the sheath are responsive to mechanical stimulation; sensitivity is amplified by wave motion of the blood in the sinus.

Review how each layer contributes to protection, sensation, hydration, and thermoregulation.
Be able to explain the functional significance of the epidermal layers, the basement membrane, and the two dermal zones (papillary vs. reticular).
Understand the roles of arrector pili, sebaceous glands, Meissner's corpuscles, and tactile hairs in normal physiology and responses to environmental stimuli.