The Integumentary System

Overview

The integumentary system is composed of the skin and its associated structures, serving as the body's primary barrier against the external environment. It plays crucial roles in protection, sensation, thermoregulation, and metabolic functions.

• Skin consists of two main layers: the epidermis and dermis.

• Subcutaneous tissue (hypodermis) lies beneath the skin and shares some functions.

Structure of the Skin

Main Layers

• Epidermis: Outermost protective shield, composed of epithelial tissue and is avascular.

• Dermis: Underlies the epidermis, made up of dense connective tissue, and is vascular.

• Subcutaneous tissue (Hypodermis): Not part of the skin proper, but anchors skin to underlying structures, provides shock absorption and insulation. Composed mainly of adipose tissue and some areolar connective tissue.

Functions of the Skin

• Protection from mechanical, chemical, and biological threats

• Prevention of water loss

• Sensation (touch, pain, temperature)

• Thermoregulation

• Metabolic functions (e.g., vitamin D synthesis)

The Epidermis

Cell Types

The epidermis is a keratinized stratified squamous epithelium containing four main cell types:

• Keratinocytes: Produce keratin, the protein that gives skin its strength and waterproofing properties. Major cell type; millions slough off daily, renewing the epidermis every 25–45 days.

• Melanocytes: Spider-shaped cells in the deepest epidermis. Produce melanin pigment, packaged into melanosomes, which protect keratinocyte DNA from UV damage.

• Dendritic cells (Langerhans cells): Star-shaped macrophages that patrol the epidermis, ingesting foreign substances and activating the immune system.

• Tactile epithelial cells (Merkel cells): Sensory receptors for touch, located at the epidermal-dermal junction.

Layers of the Epidermis

The epidermis is organized into four or five distinct layers (strata):

• Stratum basale: Deepest layer, single row of actively dividing stem cells. Also called stratum germinativum. Contains 10–25% melanocytes.

• Stratum spinosum: Several cell layers thick, cells have a spiky appearance due to desmosomes. Contains melanosomes and dendritic cells.

• Stratum granulosum: Four to six layers of flattened cells. Keratinization begins here; cells accumulate keratohyaline and lamellar granules. Cells above this layer die due to lack of nutrients.

• Stratum lucidum: Present only in thick skin (palms, fingertips, soles). Thin, translucent band of dead keratinocytes.

• Stratum corneum: Outermost layer, 20–30 rows of dead, keratinized cells. Protects deeper layers, prevents water loss, and acts as a barrier.

Thick skin contains all five layers; thin skin lacks the stratum lucidum.

The Dermis

Structure and Cell Types

The dermis is a strong, flexible connective tissue layer containing fibroblasts, macrophages, and occasional mast and white blood cells. It houses nerves, blood vessels, lymphatics, hair follicles, and glands.

Layers of the Dermis

• Papillary dermis: Thin, superficial layer of areolar connective tissue. Contains dermal papillae, which project into the epidermis and contain capillary loops, free nerve endings, and tactile corpuscles (Meissner's corpuscles).

• Reticular dermis: Makes up ~80% of dermal thickness. Dense irregular connective tissue with elastic and collagen fibers for strength, stretch, and hydration. Contains cleavage (tension) lines and flexure lines.

Dermal Modifications

• Friction ridges: Formed by dermal papillae and ridges, enhance grip and touch sensitivity, and create unique fingerprints.

• Cleavage lines: Parallel collagen fibers; important for surgical incisions.

• Flexure lines: Dermal folds at joints, visible on hands and feet.

Skin Color

Pigments Determining Skin Color

• Melanin: Produced by melanocytes from tyrosine via tyrosinase. Packaged in melanosomes, protects DNA from UV. Two forms: reddish-yellow and brownish-black. Skin color differences are due to amount and type of melanin.

• Carotene: Yellow to orange pigment, most visible in palms and soles. Accumulates in stratum corneum and hypodermis; can be converted to vitamin A.

• Hemoglobin: Pinkish hue in fair skin due to transparency and underlying blood.

Hair

Structure and Function

• Composed of dead, keratinized cells (hard keratin).

• Functions: Sensory detection, protection from trauma, heat loss, sunlight, and filtering particles (eyelashes, nose hairs).

Parts of Hair

• Root: Portion within scalp, where keratinization is ongoing.

• Shaft: Portion above scalp, where keratinization is complete.

• Medulla: Central core of large cells and air spaces.

• Cortex: Several layers of flattened cells surrounding the medulla.

• Cuticle: Outermost layer of overlapping single cells.

Hair Color

• Determined by melanocytes in the hair follicle. Combinations of melanins produce different colors; red hair has pheomelanin. Gray/white hair results from decreased melanin and air bubbles in the shaft.

Hair Follicle Structure

• Extends from epidermal surface into dermis.

• Bulb: Expanded area at deep end; contains follicle receptor (root hair plexus) for touch.

• Peripheral connective tissue sheath: Derived from dermis.

• Glassy membrane: Basement membrane.

• Epithelial root sheath: Derived from epidermis.

• Hair papilla: Dermal tissue with capillaries supplying nutrients.

• Matrix: Actively dividing area producing hair cells.

• Arrector pili: Smooth muscle causing "goose bumps."

Nails

Structure and Function

• Scale-like modifications of epidermis containing hard keratin.

• Protect distal, dorsal surfaces of fingers and toes.

• Parts: Root (embedded), nail plate/body (visible), free edge.

• Nail bed: Epidermis under nail plate.

• Nail matrix: Responsible for nail growth.

• Nail folds: Skin overlapping nail border; cuticle (eponychium) projects onto nail body.

• Hyponychium: Area under free edge, accumulates dirt.

• Lunule: White, thickened matrix region.

• Color and shape can indicate disease.

Cutaneous Glands

Sweat Glands (Sudoriferous Glands)

• Distributed over most skin surfaces; about 3 million per person.

• Eccrine (merocrine) sweat glands: Most numerous, abundant on palms, soles, forehead. Ducts open to skin surface; function in thermoregulation. Sweat is 99% water, salts, metabolic wastes, and dermcidin. pH 4–6.

• Apocrine sweat glands: Located in axillary and anogenital areas. Secrete viscous sweat containing fatty substances and proteins; bacteria cause body odor. Ducts empty into hair follicles; begin functioning at puberty.

• Modified apocrine glands: Ceruminous glands (earwax) and mammary glands (milk).

Sebaceous (Oil) Glands

• Widely distributed except in thick skin (palms, soles).

• Develop from hair follicles; secrete into follicles.

• Inactive until puberty; stimulated by androgens.

• Secrete sebum: oily, bactericidal, softens hair and skin.

Summary Table: Cutaneous Glands

Cutaneous Sensations

• Exteroceptors: Respond to external stimuli.

• Touch: Meissner's corpuscles (dermal papillae), Merkel cells.

• Deep pressure: Pacinian corpuscles (deeper dermis/subcutaneous tissue).

• Hair movement: Hair follicle receptors.

• Pain: Free nerve endings.

Metabolic Functions

• Synthesizes vitamin D for calcium absorption.

• Keratinocytes disarm carcinogens and activate hormones (e.g., convert cortisone to hydrocortisone).

• Makes collagenase for collagen turnover, preventing wrinkles.

Blood Reservoir

• Skin can hold up to 5% of total blood volume.

• Blood vessels can constrict to redirect blood to other organs during exercise.

Excretion

• Excretes limited amounts of nitrogenous wastes (ammonia, urea, uric acid).

• Sweating causes salt and water loss.