Integumentary System Notes - BIO 205

Epidermis

  • The epidermis consists of 55 strata: Stratum Corneum, Stratum Lucidum, Stratum Granulosum, Stratum Spinosum, Stratum Basale.

  • General role: protective outer layer; keratinization and continual turnover of keratinocytes.

  • Stratum Basale (basal layer): deepest epidermal layer; site of mitosis; new keratinocytes originate here and migrate upward.

  • Stratum Spinosum (prickly layer): several cell layers; keratinocytes connected by desmosomes, providing strength.

  • Stratum Granulosum (granular layer): keratinocytes begin keratinization; contains keratohyalin granules; forms a waterproof barrier.

  • Stratum Lucidum (clear layer): thin, seen only in thick skin (e.g., palms, soles); provides an additional barrier.

  • Stratum Corneum (horny layer): outermost layer of dead, flattened keratinocytes; major protective barrier.

  • Note on sensation: tactile corpuscles are sensory structures for touch but are located in the dermis, not within the epidermal layers.

Dermis

  • The dermis lies beneath the epidermis and makes up most of the skin's thickness.

  • Two main subregions: Papillary Layer (superficial) and Reticular Layer (deep).

  • Papillary Layer:

    • Loose connective tissue with dermal papillae that interlock with the epidermis.

    • Contains capillaries and Meissner's (tactile) corpuscles for light touch.

  • Reticular Layer:

    • Dense irregular connective tissue with thick collagen and elastic fibers.

    • Houses hair follicles, sebaceous glands, eccrine and apocrine sweat glands, nerves, and various sensory receptors.

  • Sensory structures in the dermis include:

    • Tactile corpuscles (Meissner's corpuscles) — light touch.

    • Lamellated corpuscles (Pacinian corpuscles) — deep pressure and vibration; also described as shock absorbers.

  • Dermis provides structural support and nutrient/waste exchange for the epidermis; it makes up the majority of the skin's thickness.

Hypodermis

  • Also called the subcutaneous layer; lies beneath the dermis.

  • Not a true part of the integument but anchors the skin to underlying tissues.

  • Contains adipose tissue for insulation and energy storage (peripheral, practical description).

Hair Follicles and Glands

  • Hair Follicle:

    • Derived from the epidermal layer.

    • Contains the hair shaft and root; hair papilla provides nutrients to the growing hair.

  • Hair Shaft and Root:

    • Shaft: portion of the hair extending above the skin surface.

    • Root: portion within the follicle.

  • Arrector Pili Muscle:

    • Small smooth muscle attached to each hair follicle.

    • When stimulated (e.g., cold, fear), hair stands up; causes goosebumps.

  • Sebaceous Gland:

    • Produces sebum, which coats the epidermis and hair shaft; provides lubrication and has antibacterial properties.

  • Eccrine Sweat Gland:

    • Produces sweat; helps regulate body temperature through evaporative cooling.

  • Apocrine Sweat Gland:

    • Associated with hair follicles; secretes into hair pores (restricted distribution in typical locations); contributes to odor development with bacterial breakdown in certain areas.

Sensory Receptors and Other Sensory Features

  • Tactile Corpuscles (Meissner's Corpuscles):

    • Located in the dermal papillae; highly sensitive to light, fluttering touch; involved in fine touch discrimination.

  • Lamellated Corpuscles (Pacinian Corpuscles):

    • Located deeper in the dermis (and sometimes in the hypodermis); respond to deep pressure and high-frequency vibration; function as shock absorbers.

Quick Connections and Practical Relevance

  • The integumentary system integrates barrier protection, temperature regulation, sensation, and secretion.

  • Practical implications:

    • Wound healing involves re-epithelialization primarily from cells in the Stratum Basale.

    • Sebaceous and sweat glands contribute to skin lubrication and thermoregulation, with clinical relevance to acne, dehydration, and thermoregulatory disorders.

    • Sensory structures in the dermis enable tactile perception and protective reflexes.

  • Real-world relevance: understanding these layers helps in diagnosing skin conditions, wound care, dermatological treatments, and appreciating how everyday factors (friction, temperature, moisture) affect skin health.

Key Terms

  • Stratum Corneum, Stratum Lucidum, Stratum Granulosum, Stratum Spinosum, Stratum Basale

  • Papillary Layer, Reticular Layer

  • Meissner's Corpuscles (Tactile Corpuscles), Pacinian Corpuscles (Lamellated Corpuscles)

  • Dermal Papillae

  • Hair Follicle, Hair Shaft, Hair Root, Hair Papilla

  • Arrector Pili Muscle

  • Sebaceous Gland, Eccrine Sweat Gland, Apocrine Sweat Gland

  • Goosebumps, Sebum, Sweat

In the integumentary system, keratin is a fibrous protein that is the primary structural component of the epidermis, hair, and nails. Its main role is to provide strength, toughness, and flexibility to these structures. Within the epidermis, keratinocytes produce keratin, undergoing a process called keratinization, particularly in the Stratum Granulosum. As these cells migrate to the Stratum Corneum, they flatten and fill with keratin, forming a highly durable and waterproof barrier that protects the body from environmental damage, pathogens, and water loss.

Melanin determines skin color based on the amount and type of melanin produced by melanocytes, which are located in the Stratum Basale of the epidermis. The variations in skin tone are not due to the number of melanocytes, but rather how much of the two main types of melanin
—Eumelanin (producing brown/black pigments) and Pheomelanin (producing red/yellow pigments)
—is generated and distributed in the skin.

Fingernail Anatomy

Fingernails are specialized structures derived from the epidermis, primarily composed of a tough protein called keratin. They serve several functions, including protecting the fingertips, enhancing tactile sensitivity, and aiding in grasping small objects.

Main Components:

  1. Nail Plate (Corpus Unguis):

    • The visible, hard, translucent part of the nail, consisting of flattened, keratinized cells.

    • It slides over the nail bed as it grows.

  2. Nail Bed (Stratum Onguium):

    • The skin underneath the nail plate.

    • It is rich in blood vessels, giving the nail its pinkish color.

    • Contains longitudinal ridges that interlock with corresponding grooves on the underside of the nail plate, enhancing adhesion.

  3. Nail Matrix (Matrix Unguis):

    • The active growing part of the nail, located at the proximal end of the nail bed, beneath the lunula and hidden by the eponychium.

    • Consists of specialized epidermal cells that continuously divide and produce new nail cells.

    • The health and activity of the matrix determine the shape, thickness, and growth rate of the nail.

  4. Nail Root (Radix Unguis):

    • The portion of the nail plate that extends under the skin, proximally to the nail bed.

    • It is embedded in a groove in the epidermis and originates from the nail matrix.

  5. Lunula:

    • The visible, whitish, half-moon-shaped area at the base of the nail, just above the eponychium.

    • It is the distal end of the nail matrix, where the newly formed, less opaque nail cells are still partially keratinized.

  6. Eponychium (Cuticle):

    • A fold of skin at the proximal end of the nail, covering the nail matrix.

    • It acts as a protective barrier, sealing the area between the nail plate and the skin, preventing bacteria and fungi from entering.

  7. Paronychium (Nail Folds):

    • The skin folds that frame the nail plate on its sides.

  8. Hyponychium:

    • The thickened layer of epidermis beneath the free edge of the nail.

    • It forms a protective barrier, preventing external substances from entering the nail bed.

  9. Free Edge:

    • The part of the nail plate that extends beyond the finger or toe tip.

Fingernail Physiology
Nail Growth:

  • Nails grow from the specialized epidermal cells in the nail matrix.

  • Cells in the matrix divide and then differentiate, producing keratinocytes that become highly keratinized and flattened.

  • These hardened cells are pushed forward, forming the nail plate.

  • Fingernails grow at an average rate of approximately 33 mm per month, though this can vary due to age, nutrition, and overall health.

  • It takes about 44 to 66 months for a fingernail to grow from the matrix to the free edge.

Function of Nails:

  • Protection: Guards the sensitive fingertips and toes from trauma and injury.

  • Support: Provides counter-pressure to the pulp of the finger, enhancing tactile sensation for fine manipulation.

  • Tool: Aids in scraping, scratching, and picking up small objects.

  • Indicator of Health: Changes in nail color, texture, or shape can sometimes indicate underlying systemic diseases or nutritional deficiencies.