Integumentary System: Dermis, Epidermal Derivatives, Glands & Hypodermis

Epithelial Tissue & Dermis Introduction

  • Epithelial Tissue Characteristics:

    • Avascular: Lacks its own blood supply.

    • Relies on diffusion from the underlying connective tissue for nutrients and waste exchange.

  • iClickers: Used for participation points, not accuracy. Incorrect answers help the instructor assess teaching effectiveness, so do not switch answers after they are revealed.

Learning Objectives

For today's lecture, you should be able to:

  • Describe the composition of the hypodermis and its relationship to the integument.

  • List the layers of the dermis from superficial to deep.

  • Explain how the integumentary system functions to support general senses.

  • Discuss the structure and function of nails.

  • Identify the functions of hair (note: structure of hair is less emphasized than function).

  • Talk about the glands of the integument.

Dermis: Structure and Composition

  • Location: Deep to the epidermis.

  • Composition: Composed of connective tissue proper and consists of two primary layers.

  • Primary Cell Type: Fibroblasts are the primary cell type of the dermis, distinguishing it from the epidermis where keratinocytes are primary. Fibroblasts will be discussed further with tissue repair.

  • Other Cell Types found in the Dermis:

    • Adipocytes: Found in adipose tissue.

    • Melanocytes: While primary in the epidermis, melanocytes can occur in the dermis. They produce melanin and deposit it into surrounding keratinocytes, determining skin and hair color.

    • Dendritic Cells (Langerhans Cells): These are immune system cells, acting as the first detectors of pathogens that breach the physical barrier of the skin.

  • Structures within the Dermis: The dermis is rich in a variety of structures, including (but not limited to):

    • Dendritic cells

    • Sensory nerve endings

    • Nail roots

    • Arrector pili muscles

    • Blood vessels (nourishing the dermis and epidermis)

    • Sweat glands

    • Sebaceous glands

    • Protein fibers

    • Hair follicles

Note-Taking Strategy: Mind Mapping

  • Consider using mind mapping instead of linear notes. This strategy can help organize information holistically.

  • Videos explaining mind mapping will be provided.

Dermal Extracellular Matrix and Protein Fibers

  • Extracellular Matrix (ECM): The non-cellular component of connective tissue, composed of:

    • Ground Substance: A gel-like liquid that surrounds fibers and cells.

    • Protein Fibers: Provide structural support and other functions.

  • Types of Protein Fibers in the Dermis:

    • Collagen Fibers:

      • Primary Function: Provide tensile strength and structural support.

      • Clinical Relevance: A significant component of scar tissue formation, helping to rebuild strength.

    • Elastic Fibers:

      • Primary Function: Responsible for elasticity and the skin's ability to stretch and recoil.

      • Tissue Type: Predominantly found in elastic connective tissue.

    • Reticular Fibers:

      • Characteristics: Thin, branched fibers.

      • Primary Function: Provide a delicate supportive framework (scaffolding) for organs and aid in overall tissue structure.

  • Textbook Reference: Refer to your textbook for figures illustrating these protein fibers and their functions.

Layers of the Dermis

Papillary Layer (Superficial)

  • Naming: Named for the dermal papillae, which are finger-like projections that extend outward and interlock with the epidermal layer.

  • Composition: Composed of areolar connective tissue.

  • Vascularity: Highly vascularized with many capillary beds located within the papillae.

  • Function: Furnishes blood supply to nourish the avascular epidermis.

Epidermal Layers: A Quick Review

  • Stratum Basale: The deepest layer of the epidermis, directly in contact with the papillary layer of the dermis.

  • Mnemonic for Epidermal Layers (Superficial to Deep in Thick Skin):

    • Corn (Stratum Corneum)

    • Let's (Stratum Lucidum) - Present only in thick skin (palms, soles)

    • Get (Stratum Granulosum)

    • Sun (Stratum Spinosum)

    • Burned (Stratum Basale)

  • Key Learning Points: Be able to identify the layers from superficial to deep and recognize which layer is specific to thick skin.

Reticular Layer (Deep)

  • Location: Deeper and thicker than the papillary layer.

  • Composition: Primarily made up of dense irregular connective tissue.

  • Primary Function of Dense Irregular Connective Tissue: Provides tough, protective support and resists stretching forces applied in multiple directions.

    • Contrast: Dense regular connective tissue resists tension in a single direction (e.g., tendons), and elastic tissue allows stretch and recoil.

  • Exam Focus: Emphasize the functions of tissues rather than merely their structural identification for lecture exams.

Epidermal Derivatives: Nails

  • Definition: Nails and hair are epidermal derivatives, formed from dead keratinocytes.

  • Evolutionary Context: Human flat nails are believed to be modified claws.

    • Animal Examples: Anteaters use claws for digging; Komodo dragons use claws for grasping (and have a toxic bite); horses have hooves (a single modified nail); primates often have flattened nails for aiding precise movements and grasping.

  • Nail Growth:

    • Grown from the nail matrix, which contains living keratinocytes (similar to the stratum basale).

    • These cells continuously divide, pushing the nail outward. The visible nail itself is composed of dead cells.

  • Key Nail Structures:

    • Nail Root: Located beneath the proximal fold (cuticle/eponychium), anchoring the nail to the nail bed.

    • Lunula: The visible half-moon-shaped white area at the base of the nail.

      • Represents the visible portion of the living nail matrix.

      • Its white appearance is due to cells that are not yet fully keratinized and still retain some nuclei.

    • Nail Fold (Cuticle/Eponychium):

      • Cuticle: Composed of dead keratinocytes.

      • Eponychium: Composed of living keratinocytes.

      • Function: Located at the base of the nail, they form a seal to prevent pathogens from entering and protect the delicate nail matrix.

      • They never reach full keratinization, making them softer than the nail plate.

    • Consequence of Damaged Nail Matrix: If the nail matrix is damaged, the nail may not grow back normally.

Epidermal Derivatives: Hair

  • Composition: Consists of keratinized cells that form slender filaments, growing from hair follicles.

  • Hair Types:

    • Lanugo Hair:

      • Characteristics: Super fine, unpigmented hair present on a fetus in the womb.

      • Function: Traps vernix caseosa (a waxy substance) close to the baby's skin, which helps retain water and provides antibacterial properties, supporting the neonate's developing skin.

    • Vellus Hair:

      • Characteristics: Fine, lightly pigmented hair.

      • Distribution: Covers most of the body (e.g., arms, thighs).

    • Terminal Hair:

      • Characteristics: Coarse, densely pigmented hair.

      • Distribution: Found on the scalp, face (especially with high testosterone levels), pubic region, and axillary regions.

  • Hair Color: Derived from melanin produced by melanocytes within the hair bulb and deposited into keratinocytes as they are pushed outward.

    • Melanin's Protective Function: Melanin absorbs UV radiation, thereby reducing damage to both the hair and the underlying skin.

  • Graying/Whitening Hair:

    • Mechanism: A gradual reduction in melanin production.

    • Cause: Melanocytes lose their ability to produce melanin as a person ages.

    • Contributing Factors: Stress can accelerate the process, and genetics (e.g., Steve Martin's early graying) play a significant role.

  • Hair Functions:

    • Traps Debris:

      • Lanugo hair traps vernix caseosa.

      • Hair in the nose and ears helps slow down or trap foreign particles like bugs, pollen, and dust (note: ~90%90\% of dust is dead skin cells from the stratum corneum).

    • Hair Bulb: Swelling at the base of the hair follicle; the only region with living keratinocytes.

      • Surrounded by the hair papilla, which contains connective tissue and a blood supply to nourish the dividing cells.

      • Cells in the hair matrix (within the bulb) divide and push dead, keratinized cells upward to form the hair shaft.

    • Hair Structure (for context):

      • Hair Bulb: The base, containing living cells.

      • Hair Root: Extends from the bulb to the skin's surface within the epidermis.

      • Hair Shaft: The portion of the hair extending beyond the epidermis.

    • Hair Layers (from inner to outer):

      • Medulla: Central core, less compressed cells, provides flexibility and softness.

      • Cortex: More densely packed cells, provides strength and pigment.

      • Cuticle: Outermost layer, composed of highly dense, overlapping cells that protect the hair. Conditioner helps smooth these cells, reducing frizz and damage.

    • Sensory Reception:

      • Root Hair Plexus: A group of nerves surrounding each hair follicle.

      • Function: Detects light touch (e.g., wind blowing through hair, an insect landing).

    • Hair Follicle Structure:

      • An oblique tube surrounding the hair root, extending into the dermis and sometimes the hypodermis.

      • Outer Connective Tissue Root Sheath: Originates from the dermis.

      • Inner Epithelial Root Sheath: Originates from the epidermis.

    • Arrector Pili Muscle:

      • Structure: A small band of smooth muscle attached to the hair bulb and the dermal papillae.

      • Function: When it contracts, it pulls the hair follicle upright, causing the hair to stand on end, resulting in goosebumps.

      • Stimuli:

        • Fear/Rage: In animals, this makes them appear larger and more intimidating (e.g., dog hackles).

        • Cold Exposure: Raising hair traps a layer of warm air close to the body, acting as insulation to reduce heat loss. This effect is most significant in animals with dense fur/feathers (e.g., birds fluffing).

        • Heat Trapping: Terminal hair (e.g., on the scalp) is most effective at trapping heat due to its density, though less effective than animal fur.

    • Visual Identification: Hair color, distribution, and style are significant features for human visual recognition. The presence or absence of hair (e.g., baldness) also contributes to identity.

Exocrine Glands of the Skin

There are two main types of exocrine glands in the skin: sweat glands and sebaceous glands.

Sweat Glands (Sudoriferous Glands)

  • General Characteristics:

    • Both merocrine and apocrine glands have a coiled tubular secretory portion located in the reticular layer of the dermis (sometimes extending into the hypodermis).

    • They have a duct that transports the secreted fluid to the surface of the epidermis.

  • Merocrine (Eccrine) Glands:

    • Distribution: Most widely distributed and numerous type of sweat gland.

    • Secretion: Released directly onto the skin surface.

    • Composition of Sweat: Approximately 99%99\% water, with dissolved metabolites, urea (waste product), and electrolytes.

    • Primary Function: Regulating body temperature through evaporative cooling. As sweat evaporates from the skin, it transfers heat away from the body.

  • Apocrine Glands:

    • Concentration: Primarily found in the axillary (armpit), around the nipple, pubic, and anal regions.

    • Human Function: Have a less significant function in humans compared to other mammals, where they are often involved in scent communication (humans typically don't rely on scent for recognition and often dislike body odor).

    • Secretion: Produce a viscous, protein- and lipid-rich fluid.

      • This fluid is an excellent nutrient source for bacteria.

      • As bacteria metabolize these proteins and lipids, they break them down into malodorous substances, creating body odor.

    • Release Mechanism: Secretions are released directly into hair follicles, not directly onto the skin surface.

  • Hormone Sensitivity: Both apocrine and sebaceous glands are hormone-sensitive.

    • They become more active after puberty. This explains why children typically do not have significant body odor and why acne is more prevalent during adolescence.

Sebaceous Glands

  • Type: These are holocrine glands (meaning the entire cell bursts to release its contents).

  • Association: Primarily associated with hair follicles.

    • Can also be found independently, not connected to hair follicles (e.g., specific white dots around the lips).

  • Secretion: Produce sebum, an oily and lipid-rich substance.

  • Properties: Sebum has some antibacterial properties.

  • Release Mechanism: Released directly into the hair follicle (or onto the skin surface if not associated with a follicle).

  • Functions:

    • Lubricates Hair: Prevents a painful experience as hair grows through the scalp.

    • Lubricates Skin: Keeps the skin supple and prevents dryness.

  • Hormone Sensitivity: Like apocrine glands, sebaceous glands are hormone-sensitive and become more active after puberty.

    • Acne: Often results from blocked sebaceous glands, common during periods of hormonal changes.

Specialized Apocrine Glands

  • Ceruminous Glands:

    • Location: Found in the ear canal.

    • Secretion: Produce cerumen (ear wax), which helps trap foreign particles and lubricate the ear canal.

  • Mammary Glands:

    • Location: In breast tissue.

    • Secretion: Produce breast milk for nourishing offspring.

    • Epithelial Tissue for Secretion: In lactating mammary glands, the tissue responsible for secreting breast milk is typically simple cuboidal epithelial tissue (or low columnar). This single layer of cells is efficient for secretion, unlike stratified layers designed for protection, and is not keratinized.

Thin vs. Thick Skin (Summary)

  • Structures Found in Both Thick and Thin Skin:

    • Sweat glands (merocrine/eccrine)

    • Sensory receptors

  • Structures Typically Absent in Thick Skin (e.g., palms, soles):

    • Hair follicles

    • Sebaceous glands

    • Apocrine sweat glands

General Senses vs. Special Senses

  • General Senses:

    • Distribution: Widely distributed throughout the body.

    • Examples in Integument: Touch, pressure, temperature, pain.

  • Special Senses:

    • Distribution: Localized to specific, dedicated organs.

    • Examples: Vision (eyes), hearing (ears), taste (tongue), smell (nose), balance (ears).

General Sensory Receptors of the Integument

  • Tactile Corpuscle (Meissner's corpuscle): Detects light touch.

  • Lamellar Corpuscle (Pacinian corpuscle): Detects deep pressure and low frequency vibrations.

  • Bulbous Corpuscle (Ruffini corpuscle): Detects stretch receptors (sustained pressure).

  • Tactile Disc (Merkel disc): Detects light touch and pressure.

  • Free Nerve Endings: Detect pain and temperature.

  • End Bulb (Krause corpuscle) (not pictured but mentioned): Detects light touch, pressure, and high frequencies (concentrated in areas like the face, responsible for perceived vibrations at high frequencies).

Hypodermis (Subcutaneous Layer)

  • Relationship to Integument: The hypodermis is associated with the integument but is not considered part of the integument itself (which consists of the epidermis and dermis).

  • Structure:

    • Does not have distinct strata (layers).

    • Composed of adipose tissue and areolar connective tissue.

  • Functions:

    • Protection: Provides cushioning for underlying structures.

    • Energy Storage: Stores triglycerides within adipocytes.

    • Insulation: Adipose tissue acts as an insulator, retaining body heat.

    • Drug Injections: Highly vascularized, making it an ideal site for subcutaneous injections (e.g., flu shots, hormone treatments) for efficient drug absorption into the bloodstream.

  • Fat Distribution and Sex Hormones: The distribution of fat in the hypodermis is significantly influenced by sex hormones.

    • Testosterone Influence: Individuals under the influence of testosterone tend to have less adipose tissue in the hypodermis, with fat tending to accumulate in the neck, upper arms, abdomen, and lower back.

    • Estrogen Influence: Individuals under the influence of estrogen typically have a greater amount of subcutaneous fat, concentrated in the mammaries, hips, buttocks, and thighs. This distribution is believed to be adaptively advantageous, aiding in the storage of specific lipids important for fetal development and promoting more efficient metabolism of fat into energy (beta-oxidation), which could be beneficial during pregnancy or periods of starvation.