Integumentary System: Layered Structure, Cells, Glands, and Sensory Function

Layers of the Integument

• Two main layers: Cutaneous Layer (skin) & Subcutaneous Layer (beneath the skin).
• Cutaneous layer (Skin) consists of epidermis and dermis.
• Subcutaneous layer (SubQ): composed of adipose tissue and loose areolar connective tissue; contains blood vessels.
• Witty takeaway: as part of the integumentary system, we have hair, nails, skin glands, sensory receptors; i.e., all structures in the cutaneous and subcutaneous layers.

Epidermis and Dermis (Cutaneous Layer)

• Epidermis:
  • Made of stratified squamous epithelium.
  • Avascular (no blood vessels).
• Dermis:
  • Composed of dense irregular connective tissue.
  • Contains blood vessels.
  • Vascularized connective tissue nourishes the avascular epidermis.

Subcutaneous Layer (Hypodermis)

• Hypodermis contains adipose tissue and loose areolar connective tissue.
• It is vascularized (connected to the dermis).
• Functions: heat insulation, cushioning for internal structures, energy storage.

Functions of the Integument

• Protection: first line of defense against external environment; the skin is the largest organ of the body (organ = two or more tissues organized in a specific way).
• Prevents dehydration (epidermal lipid cement).
• Temperature regulation: sweating cools in heat; hair standing on end traps heat in cold.
• Excretion: sweat contributes to waste removal.
  • Sweat composition: water, NaCl (sodium chloride ions), and other components.
• Absorption: some substances (e.g., nicotine patches, skincare products) can be absorbed through the skin.
• Sensory information: receptors detect temperature, touch, pressure, and pain.
• Vitamin D production: synthesis begins in the skin upon sun exposure; vitamin D is crucial for bone health.
• Witty tips:
  • Epidermal lipid cement prevents dehydration by waterproofing the epidermis.
  • Vitamin D production via skin; modern supplementation added when sun exposure is limited.

Epidermis: Structure and Layers

• The epidermis consists of four or five layers depending on skin thickness.
• Hair presence correlates with skin thickness:
  • Thin skin has 4 layers and generally has hair.
  • Thick skin has 5 layers (additional Stratum lucidum) and is hairless.
• Mnemonic for epidermal layers from superficial to deep: "Come Let's Get Sun Burned"
  • Stratum Corneum
  • Stratum Lucidum (present only in thick skin)
  • Stratum Granulosum
  • Stratum Spinosum
  • Stratum Basale
• Thin skin: 4 layers – Stratum Corneum, Stratum Granulosum, Stratum Spinosum, Stratum Basale.
• Thick skin: 5 layers – Stratum Corneum, Stratum Lucidum, Stratum Granulosum, Stratum Spinosum, Stratum Basale.
• Specialized note: Stratum Lucidum is present only in thick skin (hairless areas like palms and soles).

Specialized Epidermal Cells

• Keratinocytes: predominant cell type throughout all epidermal layers; produce keratin for toughness and friction resistance.
• Keratinocyte stem cells: about 10% of epidermal cells; divide throughout life to replenish keratinocytes.
• Melanocytes: produce melanin; located in Stratum Basale; melanin is transferred to keratinocytes in Stratum Spinosum.
• Merkel cells: located in Stratum Basale; serve as touch receptors and are connected to Merkel discs in the dermis for light touch sensation.
• Langerhans cells (dendritic cells): immune surveillance; reside in Stratum Spinosum; act as intra-epidermal macrophages; present antigens to immune cells.
• Vitamin D and UV interaction: a lipid in the epidermis (7-dehydrocholesterol) is converted to a vitamin D precursor upon UV exposure; this is further processed to calcitriol (active vitamin D) mainly in the kidneys and to a lesser extent in the liver.

Epidermal Layers in Detail

• Stratum Basale (Basal Layer)
  • Deepest epidermal layer; also called the germinative layer.
  • Closest to dermal blood vessels; alive and capable of cell division.
  • Major cell types: keratinocytes (including keratinocyte stem cells), melanocytes, melanocyte stem cells, keratinocyte stem cells, Merkel cells.
  • Melanocytes extend dendrites into the Stratum Spinosum and store melanin in their projections; melanin shields epidermal cells from UV rays.
  • Key concept: the epidermis is avascular; nutrients reach epidermal cells from the dermal blood vessels via diffusion.
  • Witty tips:
  • The epidermis is epithelial tissue and is avascular; nourishment comes from the dermis.
  • As keratinocytes move upward, they become farther from the nutrient supply and progressively die.
• Stratum Spinosum
  • Above Stratum Basale; contains many keratinocytes, Langerhans cells, and dendritic extensions of melanocytes (with melanin).
  • Named Spinosum due to spiny cell connections under the microscope.
  • Langerhans cells surveil for pathogens; located here to remain near nourishing dermis.
• Stratum Granulosum
  • Above Stratum Spinosum; cells undergo apoptosis and contain keratohyalin granules; lipid granules contribute to keratinization and lipid secretion.
  • Desmosomes provide strength and flexibility in the lower live layers; lipid extracellular matrix acts as a waterproof barrier in upper layers.
  • Witty tip: lipid cement between cells causes waterproofing; prevents water loss and entry.
  • Lipid layer also enables vitamin D precursor processing upon UV exposure; calcitriol production is important for calcium/phosphate metabolism and bone health.
  • Vitamin D deficiency in children can cause rickets (bone deformities).
• Stratum Lucidum
  • Only in thick skin; located above Granulosum.
  • Consists of clear, flat, dead keratinocytes; named because cells appear translucent.
• Stratum Corneum
  • Top epidermal layer; in thin skin, it sits above Granulosum; in thick skin, it sits above Lucidum.
  • Composed of flat, dead keratinocytes rich in keratin; constantly shed from the surface.
  • Witty tip: exfoliation removes these cells.

Dermis: Structure and Function

• Lies beneath the epidermis; thicker than the epidermis.
• Major component: dense irregular connective tissue, with fibroblasts producing reticular, collagen, and elastic fibers.
• Contains blood vessels, nerve endings, hair follicles, and sweat glands.
• Provides structural support and nutrients to the epidermis.
• The dermis has two layers:
  • Papillary Layer (superficial): contains dermal papillae that interlock with epidermal ridges to increase surface area for diffusion of nutrients and oxygen; helps prevent separation and blister formation when friction occurs.
  • Reticular Layer (deeper): named for many reticular fibers; provides most of the dermal strength and elasticity.

Accessory Structures in the Dermis

• Hair
  • Hair shaft (visible part) and hair follicle (extends into dermis and hypodermis).
  • Hair bulb is the widened base; hair root is the portion within the skin above the bulb.
  • Melanocytes in the hair follicle produce pigment; aging can cause reduced pigment and gray hair.
  • Dermal papilla of the hair follicle contains hair follicle stem cells and pigment-producing cells.
  • Arrector pili muscle (smooth muscle): contracts to erect hair, producing goosebumps; innervated by motor nerves.
  • Hair is well innervated, increasing skin sensitivity to light touch.
  • Hair functions include protection: nasal hairs trap insects, eyelashes trigger protective eyelid closure, scalp hair protects from sun.
• Sebaceous Glands
  • Always associated with hair; present in thin skin, not in thick skin.
  • Exocrine holocrine glands that secrete sebum (oil).
  • Sebum lubricates hair and skin, provides waterproofing, and has antibacterial properties.
  • Excess sebum can clog ducts, leading to acne.
  • Witty tips:
  • Absence of sebaceous glands in thick skin explains dry skin on hands/feet and the use of external moisturizers.
  • Sebaceous glands are a type of glandular epithelium with ducts that carry secretions toward the hair root.
• Sweat Glands
  • Two types: eccrine (merocrine) sweat glands and apocrine sweat glands.
  • Eccrine (sudoriferous) glands:
  • Found all over the body; located deep in the dermis (reticular layer).
  • Produce sweat to regulate temperature and excrete waste; secretion is controlled by the hypothalamus and sympathetic nervous system; excreted to the surface via a duct.
  • Sweat is clear and odorless; composed of water, NaCl, and other components; can produce substantial volumes (up to ~10 liters/day under extreme conditions).
  • Keratinocytes line the ducts, supporting epidermal regeneration if injured.
  • Apocrine sweat glands (scent glands, modified sweat glands):
  • Located in the dermis, often in axillae, groin, and genital areas; ducts open into hair follicles.
  • Secretions contain lipids and proteins; bacteria on the skin metabolize these components, causing body odor.
  • Begin function at puberty under hormonal influence.
  • Initially thought to secrete via an apocrine mechanism; later findings indicate secretion is via exocytosis like eccrine glands, but the name 'apocrine' remains.
  • Witty tips: toddler days don’t usually require deodorant; the apocrine glands are not fully active until puberty; apocrine secretions are sometimes described as 'thick sweat'.

Sensory Receptors in the Skin

• The skin detects four main senses: touch, pressure, temperature, and pain.
• Mechanoreceptors: detect mechanical deformation and hair position; include Merkel cells, tactile (Meissner) corpuscles, lamellated (Pacinian) corpuscles, and root hair plexus.
• Merkel cells: located in Stratum Basale; respond to light touch; associated with Merkel discs in the dermis.
• Meissner corpuscles (tactile corpuscles): located in dermal papillae; respond to light touch (more superficial than Pacinian); mnemonic: Meissner = Minor touch (Meissner sounds like Me + Minor).
• Pacinian corpuscles (Lamellated corpuscles): located in the hypodermis and deep dermis; detect deep pressure; named for their layered lamellae; mnemonic: P for Pacinian = Pressure, and they’re deep in the skin.
• Hair plexus: nerve fibers wrap around hair follicles; detect movement of hair on the skin and help sense insect contact (e.g., mosquito landing).
• Free nerve endings: distributed throughout the skin; general pain and temperature receptors.
• Nociceptors: free nerve endings in the dermis; detect tissue damage and painful stimuli (extremes of temperature/pressure).
• Thermoreceptors: located in the dermis; detect temperature changes.

Epidermal and Dermal Color, Disease, and Cancer

• Melanocytes: located in Stratum Basale; produce melanin; melanin is transferred to keratinocytes in Stratum Spinosum; melanin distribution and type determine skin color; genetic factors regulate melanin production.
• Albinoism (albinism): melanocytes are present but do not produce melanin.
• Vitiligo: autoimmune destruction of melanocytes leading to white patches.
• Melanoma: the most aggressive form of skin cancer; occurs when melanocytes divide uncontrollably and invade other tissues.

Dermal-Papillary vs Reticular Details (Practical Understanding)

• Papillary Layer:
  • Thin, superficial layer; surface area increased by dermal papillae; acts to exchange nutrients with the avascular epidermis.
  • Interlocks with epidermal ridges to prevent blistering due to shearing forces.
• Reticular Layer:
  • Deep layer with dense irregular connective tissue and abundant reticular and collagen fibers; provides skin strength and elasticity.

Important Concepts and Practical Implications

• The epidermis is avascular and relies on diffusion of nutrients from the dermis.
• The lipid cement in the stratum granulosum is essential for waterproofing; loss of this layer in burns can lead to dehydration and hypovolemia (low blood volume).
• Vitamin D synthesis is linked to UV exposure; deficiency can lead to rickets in children and metabolic bone issues in adults.
• The distribution and activity of melanocytes, not their number, determine skin color variations among individuals; albinism vs vitiligo illustrate different mechanisms of color loss.
• The skin’s sensory apparatus provides rapid detection of environmental changes, enabling protective reflexes (e.g., eyelid reflex, mosquito detection).
• Glands and hair contribute to skin health and protection: sebaceous glands provide waterproofing and antimicrobial benefits (but can contribute to acne when clogged); sweat glands regulate temperature and participate in waste excretion; hair and associated muscles offer protective and regulatory roles.
• Burns severity is determined by tissue involvement:
  • First-degree burns: epidermis only.
  • Second-degree burns (partial thickness): epidermis and part of the dermis; painful due to dermal nociceptors.
  • Third-degree burns (full thickness): epidermis, dermis, and subcutaneous tissue; often less painful initially because nociceptors in the dermis are destroyed.
  • Witty clinical reminder: dehydration risk in burn patients is linked to loss of the lipid cement in the epidermis, even with mild burns.

Quick Reference – Key Numbers and Terms

• Epidermis layers by skin type: Thin skin = 4 layers; Thick skin = 5 layers (adds Stratum lucidum).
  • Thin skin layers (superficial to deep): Stratum Corneum, Stratum Granulosum, Stratum Spinosum, Stratum Basale.
  • Thick skin layers (superficial to deep): Stratum Corneum, Stratum Lucidum, Stratum Granulosum, Stratum Spinosum, Stratum Basale.
• Epidermis renewal time: $t_{ ext{renewal}} \approx 8\text{ weeks}$.
• Sweat production capacity (extreme): $\approx 10\ \text{L/day}$.
• Vitamin D pathway: $\text{7-dehydrocholesterol} \rightarrow \text{vitamin D precursor} \rightarrow \text{calcitriol} (\text{active vitamin D})$, primarily in kidneys; liver also contributes.
• Melanin production is genetically determined in amount/type, not the number of melanocytes.
• Sensory receptors include: Merkel cells, Meissner corpuscles, Pacinian corpuscles, root hair plexus, nociceptors (pain), thermoreceptors (temperature).

Note: The above notes summarize material from the transcript and are organized to mirror the conceptual flow of the lecture content, including tips and mnemonic aids wherever provided.