Microanatomy of the Integument

Definition and Function of the Integument

• The integument is defined as the combination of the skin and its various epidermal derivatives.
• Epidermal derivatives include a wide range of specialized structures:
  • Foot pads and claws.
  • Hooves and horns.
  • Glands, which include specialized sets like the anal sacs.
  • Uropygial glands, specifically found in birds.
  • Wattles, combs, and spurs.
  • Feathers.
• The skin is referred to using the Latin prefix/suffix cutis or the Greek dérma.
• It represents the largest organ in the body, accounting for approximately $8-16\,\%$ of total body weight.
• The integument serves several critical biological functions:
  • Protection: Acts as a physical, chemical, and biological barrier against environmental agents.
  • Fluid Balance: Prevents the excessive loss of water and electrolytes.
  • Thermoregulation: Facilitates the regulation of body temperature.
  • Sensation: Provides sensory feedback from the environment.
  • Metabolism: Involved in calcium homeostasis and energy storage.
  • Immune function: Serves as an active part of the body's immune defense system.

Embryological Origins and Structural Components

• The skin is composed of three primary layers with distinct embryological origins:
  • Epidermis: Derived from the ectoderm. It consists of stratified squamous keratinized epithelium. This layer is avascular and serves as a thick protective barrier.
  • Dermis: Derived from the mesoderm. It is composed of connective tissue that supports the epidermis and contains blood vessels and nervous tissue.
  • Hypodermis (Subcutis): Also known as the subcutaneous layer. It consists of loose connective tissue and is often rich in adipocytes, forming what is known as the panniculus adiposus.

Variations in Skin Architecture: Thick vs. Thin Skin

• Thick Skin:
  • Characterized by a significantly thick epidermis.
  • It is hairless.
  • Contains merocrine or eccrine sweat glands.
  • Common examples include paw pads and the muzzles of certain species (e.g., bovine muzzle).
• Thin Skin:
  • Characterized by a relatively thin epidermis.
  • Contains hair follicles, which are often associated with the arrector pili muscle.
  • Contains sebaceous glands and apocrine sweat glands.

The Dermo-Epidermal Junction and Dermal Zones

• The interface between the dermis and epidermis is characterized by specific structural interdigitations:
  • Epidermal Pegs (or Ridges): These are downward projections of the deep epidermis that extend into the dermis.
  • Dermal Papillae: These are upward projections of the superficial dermis that extend into the epidermis.
  • The purpose of this junction is to increase the surface area between layers, strengthening the bond and bringing the blood supply of the dermis closer to the avascular epithelium.
  • These structures are most prominent in areas subjected to traction, such as footpads, the nasal planum, and the scrotum. They are not normally visible in typical haired skin.
• There are two distinct zones within the dermis:
  • Papillary Layer: The superficial zone composed of loose connective tissue.
  • Reticular Layer: The deeper zone composed of dense irregular connective tissue. This layer contains collagen fibers, elastic fibers, resident and transient cells, blood vessels, nerves, and lymphatics.

Cellular Composition of the Epidermis

• Keratinocytes: The most common cell type, making up approximately $95\,\%$ of the total number of cells in the epidermis.
• Non-Keratinocytes:
  • Langerhans cells: Function as tissue macrophages for immunity.
  • Merkel’s cells: Function in mechanoreception (sensory Detection).
  • Melanocytes: Provide protection against Ultraviolet (UV) radiation.
• The basic structural plan of the epidermis involves:
  • The generation of a constant supply of cells.
  • Facilitating cell adherence via desmosomes (between cells) and hemidesmosomes (between basal cells and the basal lamina).
  • Replacing cytoplasm with keratin.
  • Developing a waterproof barrier through the flattening of cells and the creation of multiple layers.

Hierarchical Layers of the Epidermis

• As keratinocytes mature, they progress from the deepest layer toward the surface, eventually dying and sloughing off.
• From deep to superficial, the layers are:
  1. Stratum Basale: The deepest layer located at the dermo-epidermal junction. It consists of a single layer of cuboidal to columnar epithelium attached to the basal lamina via hemidesmosomes. This layer is mitotically active and contains stem cells and melanocytes.
  2. Stratum Spinosum: Composed of polygonal cells. In thin skin, it is $1-2$ cells thick, while in thick skin, it is many layers thick. Cells are held together by an increased number of tonofilaments (cytokeratin) and desmosomes, giving them a spiny appearance. This layer retains some mitotic capacity.
  3. Stratum Granulosum: A layer $3-5$ cells thick where cells begin to flatten. It is mitotically inactive. Cells contain basophilic keratohyalin granules and lamellar granules. Lamellar granules (not visible via light microscopy) secrete waterproof lipid sheets termed "intercellular cement."
  4. Stratum Lucidum: A translucent layer present only in thick skin. It contains many keratin filaments and desmosomes. Cellular organelles are absent, and the cytoplasm contains eleidin, a protein related to keratin.
  5. Stratum Corneum: The outermost layer, which can be up to $20$ layers thick. Cells are entirely dead, lack nuclei and organelles, and consist primarily of the water-resistant protein keratin.

Keratinization and Specialized Non-Keratinocytes

• Keratinization: The process by which keratinocytes differentiate. In dogs, this process takes approximately $21$ days.
• Cornification: The production of the stratum corneum through terminal epidermal differentiation.
• Detailed Non-Keratinocyte Functions:
  • Langerhans Cells: Bone-marrow-derived monocytes located within the epidermis. They act as intra-epidermal macrophages that present antigens to T-cells and mediate tolerance. They are not visible with routine H&E staining.
  • Merkel’s Cells: Located in the stratum basale of thick skin. They function as sensory mechanoreceptors and are associated with free nerve endings at their base.
  • Melanocytes: Neural crest-derived cells located in the stratum basale. They produce melanin pigment, which is transferred to keratinocytes in the stratum basale and spinosum.
  • Melanin: Absorbs harmful UV radiation and converts the energy into harmless heat. Proper function requires the enzyme tyrosinase.

Sensory Receptors and Innervation of the Dermis

• Nociceptors: Free nerve endings that reach as far as the stratum granulosum to detect pain, itch, and temperature.
• Encapsulated Nerve Endings:
  • Meissner’s Corpuscles: Detect light pressure and touch.
  • Pacinian Corpuscles: Detect deep pressure and vibration.
  • Ruffini Corpuscles: Detect stretch.

Anatomy and Physiology of Hair and Feathers

• Functions: Insulation, camouflage, social display, sensation/protection, and sexual dimorphism.
• Hair Anatomy:
  • Hair Shaft: The portion located above the skin surface.
  • Hair Root: The portion within the follicle, ending in the hair bulb.
  • Hair Bulb: The expanded terminal portion of the root that fits over the dermal papilla.
• Hair Follicle Layers:
  1. External/Outer Root Sheath: Includes the glassy membrane (thickened basement membrane) and is continuous with the epidermis.
  2. Internal Root Sheath: Consists of a few layers of squamous cells and a cuticle of overlapping keratinized cells that interlock with the hair cuticle.
  3. Hair Structure: Divided into the medulla (innermost layer of loose cuboidal cells and air), cortex (dense compact keratinized cells), and hair cuticle (single layer of flat keratinized cells).
• Hair Matrix Cells: Located in the hair bulb covering the dermal papilla. These cells are equivalent to the stratum basale and give rise to the hair.

Hair Growth Cycles and Follicle Types

• Hair Growth Phases:
  • Anagen: The active growth phase where hair bulb cells are mitotically active.
  • Catagen: The regressive stage where metabolic activity slows and the follicle base migrates toward the surface.
  • Telogen: The resting or quiescent phase where growth stops. New hair eventually grows beneath the telogen follicle, causing the old hair to shed.
  • The cycle is regulated by daylight, ambient temperature, nutrition, and hormones (including estrogen, testosterone, adrenal steroids, and thyroid hormone).
• Follicle Classification:
  • Primary Hair Follicle: Large diameter, rooted deep in the dermis. Associated with sebaceous glands, arrector pili muscles, and sweat glands. Produces guard hairs.
  • Secondary Hair Follicle: Smaller diameter, rooted nearer the surface. May have sebaceous glands but lack apocrine glands and arrector pili muscles. Produces underhairs.
  • Compound Follicles: Clusters where several follicles (usually one primary and several secondary) merge at the level of the sebaceous gland and emerge through a single external orifice. These are most common in carnivores.
• Sinus Hairs (Whiskers): Tactile hairs featuring a very large single follicle with a blood-filled sinus between the inner and outer dermal root sheaths. They are attached to skeletal muscle for voluntary movement and have large nerve bundles penetrating the sheath.

Cutaneous Glands: Sebaceous and Sweat Glands

• Sebaceous Glands:
  • Located in the dermis; usually empty into a hair follicle.
  • Exhibit holocrine secretion (the cell itself becomes part of the secretion).
  • Produce sebum (lipids and cell debris) for waterproofing and antibacterial protection.
  • Types include simple, branched, or compound.
  • Specialized versions: Supracaudal (dog), Circumanal/Hepatoid, Submental (cat), Horn glands (goat), Preputial (horse/smegma), Tarsal/Meibomian (eyelids), and Uropygial/Preen gland (birds).
• Apocrine Sweat Glands:
  • Secrete via apical budding/pinching (apical secretory caps).
  • Simple coiled tubular glands that typically open into the distal hair follicle.
  • Possess contractile myoepithelial cells to help express the product.
  • Mainly used for communication (attraction, markers) in domestic animals.
  • Specialized versions: Mammary glands, Ciliary glands (eyelid), Ceruminous glands (ear wax), and glands of the anal sac.
• Merocrine (Eccrine) Sweat Glands:
  • Secrete via exocytosis.
  • Open directly onto the skin surface.
  • In domestic animals, these are minor except in the footpads of dogs. They function in thermoregulation and electrolyte balance.

Mammary Gland Microanatomy

• Structure: Tubuloalveolar glands connected by ducts and separated into lobules by connective tissue septae.
• Cellular Composition: Epithelial (milk-secreting) cells and myoepithelial cells.
• Ductal System:
  • Clusters of alveoli form lobules.
  • Lactiferous ducts (cuboidal epithelium) drain into the lactiferous sinus (cuboidal).
  • Teat sinus (cuboidal) leads to the teat canal, which is lined with stratified squamous epithelium.
  • The teat contains a smooth muscle sphincter.

Specialized Integumentary Structures: Hooves and Claws

• These are modifications of the skin and variations of the stratum corneum. They belong to a vascular dermis and typically lack the stratum granulosum and stratum lucidum.
• Equine Hoof Components:
  • Corium (Dermis): Highly vascular and innervated connective tissue. Includes the Coronary corium, Sole corium, and Laminar corium.
  • Laminar Corium: Consists of $500-600$ primary dermal laminae (sensitive laminae) that interdigitate with the epidermal laminae.
  • Hoof Wall (Epidermis) Layers:
    1. Stratum Externum (Tectorium): Continuing from the perioplic epidermis; provides the "glaze."
    2. Stratum Medium: The majority of the wall, produced from coronary epidermis with keratin arranged in parallel tubules and intertubular horn.
    3. Stratum Internum (Lamellatum): Consists of approximately $600$ primary epidermal laminae (insensitive laminae) that interdigitate with the sensitive dermal laminae.
  • White Line: The junction where the sole joins with the hoof wall.