Integumentary System: Structure and Function (Chapter 6 Study Notes)

Integumentary System

Overview

The integumentary system consists of the skin and its accessory structures, including hair, nails, sebaceous glands, and sweat glands. This system serves as the body's primary barrier to the external environment and performs several vital functions.

• Protection:

  • Protects against invasion by bacteria and other harmful agents.

  • Shields delicate cells beneath the surface from injury.

  • Inhibits excessive loss of water and electrolytes.

  • Produces protective pigmentation to guard against excessive sun exposure.

• Vitamin D Production:

  • Skin produces the hormonal form of vitamin D when exposed to UV light, which is essential for calcium absorption.

• Temperature Regulation:

  • Blood vessels in the skin dilate when the body is hot and constrict when cold, helping regulate body temperature.

• Excretion:

  • Skin and glands produce waste products.

• Sensation:

  • Millions of nerve endings act as sensory receptors for pain, heat, cold, and pressure.

Skin

Layers of the Skin

The skin is composed of two major tissue layers: epidermis and dermis.

• Epidermis (Stratified Squamous Epithelium):

  • No blood vessels; nutrients received via diffusion.

  • Keratinocytes: Produce keratin protein, making cells durable and resistant to abrasion and water loss. Constantly replaced by deeper layer cells.

  • Keratinization: Process of changing cell shape and composition as keratinocytes move from deep to superficial layers. Malfunction can result in skin diseases.

Strata of the Epidermis

The epidermis is organized into five layers based on keratinization stages:

• Stratum Basale (deepest layer):

  • Single layer of cuboidal or columnar cells anchored by hemidesmosomes and desmosomes.

  • Keratinocyte cell division occurs every 19 days.

• Stratum Spinosum:

  • 8-10 layers of many-sided cells.

  • Formation of lamellar bodies (lipid-filled organelles) and antimicrobial barriers.

• Stratum Granulosum:

  • 2-5 layers of flattened cells.

  • Accumulation of keratohyalin granules in cytoplasm.

  • Lamella bodies release lipids into extracellular space, helping retain water and isolating cells (leading to cell death).

• Stratum Lucidum:

  • 3-5 layers of dead, transparent cells.

  • Present only in thick skin.

• Stratum Corneum:

  • More than 25 layers of dead cells.

  • Cells are shed as desmosomes break apart (dandruff and dander).

  • Cornified cells: Dead keratinocytes with a hard protein envelope of keratin.

Mnemonic for Epidermal Layers

• Brilliant Studying Gives Loads of Confidence (Basale, Spinosum, Granulosum, Lucidum, Corneum)

• "B" in basale stands for "bottom" layer.

Thick vs. Thin Skin

• Thick Skin: All five epithelial strata with many layers in stratum corneum; found on palms, soles, and fingertips.

• Thin Skin: Lacks stratum lucidum and has fewer layers in each strata; covers the rest of the body.

• Callus and Corn: Increased layers of stratum corneum for added protection against friction or pressure.

Skin Color

• Melanin: Pigment for skin, hair, and eye color; protects keratinocyte DNA from UV-induced mutations.

• Produced by melanocytes in stratum basale and spinosum.

• Packaged into melanosomes and transferred to keratinocytes.

• Freckles and pigmented moles are local accumulations of melanin.

Genetics of Skin Color

• All humans have the same number of melanocytes; genetic factors influence:

  • Type and amount of melanin produced

  • Size, number, and distribution of melanosomes

• Melanin is produced from amino acid tyrosine with help of tyrosinase enzyme.

• Albinism: Inability to produce tyrosinase, resulting in lack of pigment.

Tanning and Vitamin D

• Exposure to UV radiation increases melanin synthesis, leading to tanning.

• Immediate response: Oxidation of melanin already present in keratinocytes.

• Delayed response: UV light causes DNA damage in melanocytes, stimulating melanin production (appears within 72 hours).

• Melanin also reduces synthesis of vitamin D in response to UV, affecting calcium ion absorption and homeostasis.

• Populations in high UV regions evolved darker skin to prevent excess vitamin D production; those in low UV regions evolved lighter skin to synthesize enough vitamin D.

Dermis

Structure and Function

The dermis is connective tissue beneath the epidermis, containing fibroblasts, adipocytes, macrophages, and all three protein fibers (mostly collagen). It houses blood vessels, hair follicles, smooth muscles, glands, and lymphatic vessels.

Nerve Endings

• Receptors: Detect pain, itch, tickle, and temperature.

• Hair follicle receptors: Detect light touch.

• Pacinian corpuscles: Detect deep pressure.

• Meissner corpuscles: Detect simultaneous stimulations (light touch).

• Ruffini end organs: Detect continuous touch.

Dermal Layers

• Papillary Layer:

  • Most superficial; loose connective tissue with thin fibers.

  • Contains blood vessels and Meissner corpuscles.

  • Connects dermis to epidermis via collagen fibers.

  • Dermal papillae: Projections containing capillary loops; form dermal ridges, which create epidermal ridges (fingerprints).

• Reticular Layer:

  • Continuous with papillary layer and hypodermis; dense irregular connective tissue.

  • Irregular bundles of collagen and elastic fibers; rich in proteoglycans.

  • Contains blood vessels, glands, hairs, and Pacinian corpuscles.

  • Consists of cleavage lines (oriented directions of fibers); incisions parallel to these lines heal better.

  • Flexure lines: Deep creases in areas where reticular layer is tightly anchored to deeper structures.

Subcutaneous Tissue (Hypodermis)

Structure and Function

• Attaches skin to bone and muscles; supplies skin with blood vessels and nerves.

• Loose connective tissue with collagen and elastic fibers.

• Stores body lipid (adipose tissue) for insulation and energy.

Hair

Structure and Function

• Consists of dead keratinized cells produced by hair follicles.

• Contains hard keratin (tougher and more durable than soft keratin in skin).

• Absent on palms, soles, lips, nipples, and parts of external genitalia.

• Functions:

  • Warns of insects on skin.

  • Guards against physical trauma.

  • Protects from heat loss.

  • Shields skin from sunlight.

Zones of Hair

• Shaft: Region above the skin; contains dead cells that completed keratinization.

• Root: Region below the surface; also contains dead keratinized cells.

• Hair bulb: Base of the root in the dermis; contains live cells undergoing keratinization.

Hair Bulb Structure

• Matrix: Undifferentiated epithelial cells that produce hair and internal root sheath.

• Hair papilla: Dermis projection into hair bulb; contains blood vessels and nerves for nourishment.

• Only stratum basale is present at the hair bulb.

Concentric Layers of Hair

• Medulla: Central axis; 2-3 layers of cells with soft keratin.

• Cortex: Contains hard keratin.

• Cuticle: Single layer of overlapping cells with hard keratin; provides mechanical strength and resists abrasion.

Hair Growth Cycle

• Anagen phase: Hair increases in length; new cells produced and added at base of root.

• Catagen phase: Growth regresses; hair follicle shortens and holds hair in place.

• Telogen phase (resting): No growth; matrix cells stop dividing and die. New cycle begins when a new hair forms.

Hair Color and Muscles

• Melanin produced in hair bulb matrix and distributed to keratinocytes in cortex and medulla determines hair color.

• Arrector pili: Smooth muscle cells associated with hair follicle; contraction causes piloerection (goosebumps) in response to cold or emotional stimuli.

Example: Goosebumps

When arrector pili muscles contract, hair follicles are pulled upright, causing the skin to form "goosebumps" as a response to cold or emotional stress.