Chapter 5 Session 2 - Accessory Structures of the Skin
Overview of Integumentary Accessory Structures
Accessory structures are also referred to as epidermal derivatives.
Embryological origin:
Accessory structures originate from the epidermis during embryological development.
Hair follicles and exocrine glands develop from epithelial columns, which are long cords of epidermal cells growing down into the underlying dermis.
Nails form from thickened epidermal cells nestled deep into the dermis, establishing a specialized area known as the nail field.
Anatomical location:
Though anchored deep within the dermis layer, accessory structures project through the epidermis to reach the skin surface.
Primary categories and broad functions:
Hair Follicles: Produce hairs that provide physical protection to the skull and provide sensitive, delicate touch sensations across the body surface.
Exocrine Glands:
Sweat Glands: Assist in body thermoregulation and excrete organic wastes and electrolytes.
Sebaceous Glands: Produce lipid-rich secretions to lubricate the hair shaft and surrounding skin surface.
Nails: Protect and structurally support the distal tips of fingers and toes under physical stress.
Hair Structure, Types, and Production
Distribution of hair:
Hair is located almost everywhere across the body.
Specific areas completely devoid of hair include:
Palms of hands.
Sides and soles of feet.
Lateral surfaces of fingers and toes.
Lips.
Portions of external genitalia.
Categories of hair:
Terminal Hairs: Large, coarse, heavily pigmented strands. Examples include hairs found on the scalp, eyebrows, eyelashes, and axillary/pubic regions.
Vellus Hairs: Smaller, shorter, fine, and delicate hairs located across the general body surface.
Hair regions:
Hair Shaft: The portion of hair extending from deep within the hair follicle to above the exposed surface of the skin.
Hair Root: The deeper portion anchoring the hair into the skin; extends from the base of the follicle up to the point where the hair shaft loses connection with follicle walls.
Associated follicle structures:
Root Hair Plexus: A network of sensory nerve fibers surrounding the base of each hair follicle, providing sensitivity to hair movement and touch.
Arrector Pili Muscle: A bundle of smooth muscle attached to the connective tissue sheath of the hair follicle. Upon contraction, it pulls the hair shaft erect (producing "goosebumps").
Sebaceous Gland: An exocrine gland discharging an oily coating into the follicle lumen to coat the hair shaft and surrounding skin surface.
Hair formation and internal anatomy:
Hair Bulb: The expanded basal terminal region of the hair follicle surrounding the papilla.
Hair Papilla: A peg of connective tissue containing vital blood vessels and nerve fibers essential for nourishing growing hair.
Hair Matrix: A layer of actively dividing basal keratinocyte cells resting directly on the hair papilla. Mitosis in the matrix produces all cell layers of the hair.
Hair shaft cross-sectional layers (from innermost to outermost):
Medulla: Central core composed of daughter cells formed at the center of the hair matrix; contains flexible, soft keratin.
Cortex: Intermediate muscular layer surrounding the medulla; contains dense, thick layers of hard keratin that give hair its stiffness.
Cuticle: Outermost layer of daughter cells produced at the outer edges of the matrix; composed of thin, extremely tough hard keratin forming the protective exterior coating.
Layers of the hair follicle (from innermost to outermost):
Internal Root Sheath: Directly surrounds the hair root and deeper portion of the shaft; synthesized by cells at the edge of the hair matrix.
External Root Sheath: Continuous extension of the superficial epidermal layers running from the skin surface down to the hair matrix.
Glassy Membrane: A thickened, clear, noncellular basement membrane separating the outer epithelial sheath from surrounding dermal connective tissue.
Connective Tissue Sheath: Dermal connective tissue wrapping around the entire epithelial structure of the hair follicle.
Hair growth dynamics and cycle:
Hairs grow and shed in repetitive cycles alternating between growth and quiescence:
Active Phase:
Basal cells of the hair matrix divide actively, pushing older cells upward and extending hair length.
Lasts between .
Growth occurs at an average rate of approximately .
Regression and Transition Phase: Matrix activity halts and the hair root detaches from the hair bulb.
Resting Phase:
The hair loses its connection to the follicle base, forming a club hair.
The follicle rests until reactivated.
Follicle Reactivation and Replacement:
The follicle regenerates, and new matrix cell division initiates new hair shaft synthesis.
The emerging replacement hair pushes out the old club hair, causing it to shed.
Individual variations in growth rate and cycle duration determine natural maximum hair length.
Exocrine Glands of the Skin
Sebaceous Glands:
Secretion style: Holocrine glands (the entire secretory cell ruptures and breaks down to release stored contents).
Produce an oily lipid secretion called sebum.
Sebum Composition: A complex mixture containing triglycerides, cholesterol, proteins, and electrolytes.
Functions of sebum: Lubricates and conditions the hair shaft, moisturizes the surrounding epidermis, and exerts natural antimicrobial effects.
Mechanism of release: Contractions of the arrector pili muscle compress the gland, forcing stored sebum out into the hair follicle or onto the epidermal surface.
Anatomical subtypes:
Typical Sebaceous Glands: Discharge sebum directly into hair follicles.
Sebaceous Follicles: Large glands that discharge sebum directly onto the skin surface rather than into hair follicles; located on the face, back, chest, nipples, and external genitalia.
Sweat Glands (Sudoriferous Glands):
General secretory method: Release watery secretions by merocrine exocytosis.
Specialized supportive structures: Myoepithelial cells surrounding secretory units contract under autonomic nervous or hormonal stimulation, squeezing the gland to discharge fluids.
Primary function: Flush and cleanse the surface of the epidermis.
Two distinct functional types:
Apocrine Sweat Glands:
Distribution: Restricted to specific regional areas including the axillae (armpits), groin/pubic region, and around the nipples.
Secretion: Sticky, cloudy, viscous fluid with a complex lipid and protein composition that produces noticeable body odor when broken down by surface bacteria.
Regulation: Strongly influenced and regulated by circulating sex hormones.
Specialized Variants:
Ceruminous Glands: Located in the external ear canal, producing cerumen (earwax).
Mammary Glands: Modified apocrine glands specialized for milk production.
Eccrine (Merocrine) Sweat Glands:
Distribution: Found across nearly all areas of the skin; highest concentrations located on the palms of the hands (approx. or ) and soles of the feet.
Secretion: Clear, watery fluid containing dissolved electrolytes (primarily sodium chloride).
Regulation: Controlled primarily by the nervous system.
Functions: Critical for thermoregulation via evaporative cooling, excretion of metabolic wastes, and localized antibacterial defense via antimicrobial peptides and fluid movement.
Structure and Function of Nails
Functional significance:
Dense sheets of keratinized epidermal cells.
Protect the exposed dorsal surfaces of the fingertips and toes.
Limit mechanical distortion of the digits under physical stress when grasping or running.
Anatomical structures of a nail:
Nail Body: The visible, external plate of the nail composed of dead cells packed tightly with hard keratin.
Nail Bed: The underlying layer of living vascularized epidermis covered directly by the nail body.
Lateral Nail Grooves: Longitudinal depressions along each side of the nail body.
Lateral Nail Folds: Folds of normal epithelium bounding the sides of the nail body.
Lunula: Pale, crescent-shaped region located at the proximal base of the nail body where underlying vascularity is obscured by thickened matrix tissue.
Free Edge: Distal projecting portion of the nail body extending past the tip of the digit.
Nail Root: Deep epidermal fold situated proximal to the exposed nail body where active cell division and nail production occur.
Eponychium (Cuticle): Extension of the stratum corneum from the proximal nail fold over the base of the exposed nail.
Hyponychium: Area of thickened stratum corneum underlying the free edge of the nail body.
Diagnostic applications of nail appearance:
Conditions affecting systemic body metabolism alter active nail production and cellular structure.
Clinical indicators:
Pitted and distorted nail plates: Associated with psoriasis.
Concave nails ("spooning"): Associated with specific blood disorders such as chronic iron deficiency.
Effects of Aging on the Integumentary System
Fewer Melanocytes:
Melanocyte activity and density drop; in light-skinned individuals, skin becomes pale.
Increases sensitivity to sun exposure, raising sunburn likelihood.
Drier Epidermis:
Decreased sebaceous gland secretion reduces surface lipid layers.
Thinning Epidermis:
Basal cell division slows in the stratum basale.
Connections between the epidermis and dermis weaken, rendering skin prone to tears, mechanical injury, and infections.
Cutaneous Vitamin synthesis drops, causing muscle weakness and brittle bones.
Diminished Immune Response:
Dendritic cell numbers decline to roughly half of levels present at age 21$.\n - Elevates risk of severe skin damage and opportunistic infections.\n- **Thinning Dermis**:\n - Decreased density of elastic fibers leads to progressive skin sagging and wrinkling.\n- **Decreased Perspiration**:\n - Sweat glands become significantly less active, impairing evaporative cooling and raising the risk of hyperthermia.\n- **Reduced Blood Supply**:\n - Reduced blood flow to the subpapillary plexus cools the skin and stimulates thermoreceptors, making elderly individuals feel cold even in warm rooms.\n - Reduces heat loss capacity during warm environmental conditions.\n- **Slower Skin Repair**:\n - Rate of cellular regeneration drops significantly.\n - Example: Blister repair that takes 3\text{ to }4\,\text{weeks}6\text{ to }8\,\text{weeks}65\text{ to }75\,\text{years}.\n- **Fewer Active Follicles**:\n - Produces thinner, finer hairs; reduced melanocyte output turns hair gray or white.\n- **Altered Hair and Fat Distribution**:\n - Driven by declining circulating sex hormone concentrations.\n\n# Endocrine Interactions and Vitamin D3 Synthesis\n\n- Endocrine impacts on skin function:\n - **Glucocorticoids (Steroid Hormones)**: Loosen intercellular connections between keratinocytes and weaken epidermal effectiveness as a physical barrier to infection.\n - **Thyroid Hormones**: Maintain normal blood flow to the subpapillary plexus in the dermis.\n - **Sex Hormones**: Stimulate epidermal cell divisions, increasing epidermal thickness and accelerating wound repair rate.\n - **Growth Hormone (GH)**: Stimulates fibroblast activity and collagen synthesis; promotes basal cell division to thicken the epidermis and speed up repair.\n - **Growth Factors**: Compounds that stimulate cell growth and cell division:\n - **Epidermal Growth Factor (EGF)**: A peptide hormone synthesized by salivary glands and duodenal glands.\n - Roles of EGF:\n - Promotes basal cell division in the stratum basale and stratum spinosum.\n - Accelerates keratin production in maturing keratinocytes.\n - Stimulates epidermal development and tissue repair after injury.\n - Stimulates synthesis and secretory activity in epithelial exocrine glands.\n\n\n\n- Mechanisms of Vitamin D_3 synthesis and metabolism:\n - When exposed to ultraviolet (UV) radiation, epidermal keratinocytes in the stratum spinosum and stratum basale convert a steroid precursor compound into **cholecalciferol (Vitamin D_3)**.\n - Alternative source: Direct dietary absorption of cholecalciferol from fish, fish oils, shellfish, or fortified foods via the digestive tract.\n - Metabolic processing pathway:\n 1. Cholecalciferol circulates to the **liver**, where it is converted into an intermediary product.\n 2. The intermediary product travels to the **kidneys**, where it is enzymatically converted into the active hormone **calcitriol**.\n 3. Calcitriol acts on the small intestine mucosa to stimulate the essential absorption of calcium (\text{Ca}^{2+}\text{PO}_4^{3-}) ions into blood circulation.\n\n\n\n- Clinical impacts of calcitriol deficiency:\n - **Rickets**: A developmental bone disorder in children resulting from inadequate sunlight exposure or deficient dietary cholecalciferol. Inadequate calcitriol impairs calcium/phosphate deposition in bone tissue, producing soft, flexible, poorly mineralized bones that bow under body weight.\n - **Elderly Bone Loss**: Skin production of cholecalciferol declines by up to 75\% in elderly populations. Combined with lower dietary intake, reduced calcitriol leads to decreased bone mineral density, elevated fracture risk, and prolonged bone healing.\n\n# Wound Healing, Repair, and Scarring\n\n- Four sequential phases of skin regeneration after deep injury:\n - **1. Inflammatory Phase**:\n - Initial trauma damages blood vessels, causing bleeding at the injury site.\n - Localized **mast cells** trigger an inflammatory response.\n - Releases produce localized swelling, redness, heat, and pain.\n - **2. Migratory Phase**:\n - Initiates several hours after injury.\n - A blood clot (**scab**) forms at the surface to temporarily seal the wound.\n - Cells of the stratum basale divide rapidly and migrate along wound edges to replace missing epithelial cells.\n - Patrolling tissue macrophages remove cellular debris and pathogens.\n - When damage extends deep into the dermis, a combination of proliferating fibroblasts, blood clot material, and an expanding capillary network forms **granulation tissue**.\n - **3. Proliferation Phase**:\n - Reached approximately one week following injury.\n - Migrating epidermal cells completely undermine the surface scab.\n - Phagocytic activity is almost complete, and the deep blood clot dissolves.\n - Fibroblasts actively synthesize abundant new collagen fibers and extracellular ground substance.\n - **4. Scarring Phase**:\n - Continues over several weeks post-injury.\n - The superficial scab is shed, and the epidermis becomes fully intact.\n - A shallow depression typically marks the original injury site.\n - Fibroblasts continue creating **scar tissue**—a dense, inflexible, fibrous, poorly vascularized, and noncellular matrix—to permanently bridge the dermal gap.\n\n\n\n- Pathological scarring: **Keloids**:\n - A raised, thickened mass of fibrous scar tissue that begins at an injury site and expands beyond original wound boundaries into surrounding healthy dermis.\n - Covered by a smooth, shiny epidermal surface.\n - Common predilection sites include the upper back, shoulders, anterior chest, and earlobes.\n - Used as a traditional form of body decoration in specific cultures.\n\n\n\n# Questions and Discussion\n\n- **Module 5.6 Review**:\n - *Question*: What are epidermal derivatives?\n - *Answer*: Epidermal derivatives are accessory structures of the integumentary system—including hair follicles, exocrine glands, and nails—that originate from the embryonic epidermis during development.\n - *Question*: Which exocrine glands are in the integument?\n - *Answer*: The integument contains sebaceous glands (typical sebaceous glands and sebaceous follicles) and sweat glands (apocrine sweat glands and eccrine/merocrine sweat glands).\n- **Module 5.7 Review**:\n - *Question*: Describe a typical strand of hair.\n - *Answer*: A typical hair strand is an elongated column of dead, keratinized cells organized into concentric layers: an inner medulla containing soft keratin, a surrounding cortex with hard keratin, and an outer protective cuticle containing hard keratin. It is anchored deep in a hair follicle via its root and extends above the surface as a shaft.\n - *Question*: What happens when an arrector pili muscle contracts?\n - *Answer*: Contraction pulls the hair follicle perpendicular to the skin surface, making the hair strand stand erect and elevating the surrounding skin to form a goosebump, while simultaneously compressing sebaceous glands to release sebum.\n - *Question*: Describe the four phases of the hair growth cycle.\n - *Answer*: The hair growth cycle consists of:\n 1. *Active Phase*: Active cell division in the matrix produces hair growth at approximately 0.33\,\text{mm/day}2\text{ to }5\,\text{years}.\n 2. *Regression and Transition Phase*: Matrix division ceases and the hair root detaches from the base.\n 3. *Resting Phase*: The hair becomes a club hair, losing connection to the follicle base while remaining anchored in place.\n 4. *Follicle Reactivation*: Matrix cells resume mitosis to produce a replacement hair shaft that forces out the old club hair.\n - *Question*: Why is pulling a hair painful, yet cutting a hair is not?\n - *Answer*: Pulling a hair stimulates sensory nerve endings forming the root hair plexus wrapped around the base of the follicle. Cutting hair does not hurt because the visible hair shaft consists entirely of dead, non-innervated keratinized cells.\n- **Module 5.8 Review**:\n - *Question*: Identify and describe the general functions of the exocrine glands found in the skin.\n - *Answer*: Sebaceous glands secrete an oily lipid mixture (sebum) to lubricate hair shafts, condition the epidermis, and inhibit bacterial growth. Sweat glands secrete fluid via merocrine exocytosis; apocrine sweat glands secrete a viscous odorous fluid in response to hormones, while eccrine sweat glands secrete watery electrolyte fluid for thermoregulation, waste excretion, and localized antibacterial protection.\n - *Question*: Describe the secretory method and functions of sebum.\n - *Answer*: Sebum is released via holocrine secretion, where mature glandular cells rupture completely. Sebum functions to lubricate the hair and skin, reduce evaporation, and suppress bacterial growth.\n - *Question*: Deodorants are used to mask the effects of secretions from which type of skin gland?\n - *Answer*: Deodorants mask odorous breakdown products of secretions produced by apocrine sweat glands.\n - *Question*: Name the type of cell that assists the discharge of merocrine secretions, and list the glands in which such cells are present.\n - *Answer*: Myoepithelial cells assist discharge by contracting around secretory units. They are present in both apocrine and eccrine sweat glands, as well as specialized variants like mammary glands.\n- **Module 5.9 Review**:\n - *Question*: Where does nail production occur?\n - *Answer*: Nail production occurs at the nail root, a deep epidermal fold situated proximal to the nail body.\n - *Question*: Define hyponychium.\n - *Answer*: The hyponychium is a thickened area of stratum corneum located directly beneath the free edge of the nail body at the distal tip of the digit.\n - *Question*: Why does the free edge of a nail appear white?\n - *Answer*: The free edge appears white because there are no underlying dermal capillaries directly attached beneath it to impart a pinkish vascular appearance.\n- **Module 5.10 Review**:\n - *Question*: Why does hair turn white or gray with age?\n - *Answer*: Hair turns white or gray because melanocytes within the hair bulb decrease in number and activity, reducing melanin deposition into growing hair shafts.\n - *Question*: Why do people tolerate summer heat less well and become more susceptible to heat-related illness when they become older?\n - *Answer*: Aging reduces the activity and responsiveness of eccrine sweat glands (reducing evaporative cooling) and decreases cutaneous blood flow (reducing heat radiation capacity).\n- **Module 5.11 Review**:\n - *Question*: List some hormones that are necessary for maintaining a healthy integument.\n - *Answer*: Key hormones include thyroid hormones, sex hormones (estrogens and androgens), growth hormone (GH), epidermal growth factor (EGF), and calcitriol (active Vitamin D_3).\n - *Question*: Explain the relationship between sunlight exposure and vitamin D3.\n - *Answer*: Exposure to solar UV radiation converts a steroid precursor in epidermal cells (stratum basale and stratum spinosum) into cholecalciferol (vitamin D_3), which is subsequently processed by the liver and kidneys into calcitriol.\n - *Question*: In some cultures, females must be covered from head to toe when they go outdoors. Explain why these women are at increased risk of developing bone problems later in life.\n - *Answer*: Complete clothing coverage blocks sunlight UV radiation from reaching the epidermis, halting cutaneous synthesis of cholecalciferol. Without sufficient vitamin D_3$$ or dietary supplementation, calcitriol production drops, severely impairing intestinal calcium absorption and causing osteomalacia or reduced bone mineral density.
Module 5.12 Review:
Question: Identify the first step in skin repair.
Answer: The first step is the inflammatory phase, characterized by initial bleeding at the injury site and mast cell activation triggering an inflammatory response.
Question: Describe granulation tissue.
Answer: Granulation tissue is the vascularized connective tissue formed during deep dermal repair, consisting of a blood clot, an invading network of capillary capillaries, and proliferating fibroblasts.
Question: Why can skin regenerate effectively even after considerable damage?
Answer: The skin regenerates efficiently because stem cells (basal cells) in the stratum basale of the epidermis and hair follicles retain high proliferative capacity, and dermal fibroblasts rapidly synthesize new collagen matrix.