Chapter 5

Integumentary System

Skin (Integument)

• Consists of three major regions

• Epidermis – outermost superficial region

• Dermis – middle region

• Hypodermis – deepest region

Epidermis

• Composed of keratinized stratified squamous epithelium, consisting of four distinct cell types and four or five layers

• Cell types include keratinocytes, melanocytes, Merkel cells, and Langerhans’ cells

• Outer portion of the skin is exposed to the external environment and functions in protection

Cells of the Epidermis

• Keratinocytes – produce the fibrous protein keratin

• Melanocytes – produce the brown pigment melanin

• Langerhans’ cells – epidermal macrophages that help activate the immune system

• Merkel cells – function as touch receptors in association with sensory nerve endings

Layers of the Epidermis:

Stratum Basale (Basal Layer)

• Deepest epidermal layer firmly attached to the dermis

• Consists of a single row of the youngest keratinocytes

• Cells undergo rapid division, hence its alternate name, stratum germinativum

Stratum Spinosum (Prickly Layer)

• Cells contain a weblike system of intermediate filaments attached to desmosomes

• Melanin granules and Langerhans’ cells are abundant in this layer

Stratum Granulosum (Granular Layer)

• Thin; three to five cell layers in which drastic changes in keratinocyte appearance occurs

• Keratohyaline and lamellated granules accumulate in the cells of this layer

Stratum Lucidium (Clear Layer)

• Thin, transparent band superficial to the stratum granulosum

• Consists of a few rows of flat, dead keratinocytes

• Present only in thick skin

Stratum Corneum (Horny Layer)

• Outermost layer of keratinized cells

• Accounts for three quarters of the epidermal thickness

• Functions

• Waterproofing

• Protection from abrasion and penetration

• Renders the body relatively insensitive to biological, chemical, and physical assaults

Dermis

• Second major skin region containing strong, flexible connective tissue

• Cell types include fibroblasts, macrophages, and occasionally mast cells and white blood cells

• Composed of two layers – papillary and reticular

Dermis: Papillary Layer

• Papillary layer

• Areolar connective tissue with collagen and elastic fibers

• Its superior surface contains peglike projections called dermal papillae

• Dermal papillae contain capillary loops, Meissner’s corpuscles, and free nerve endings

Dermis: Reticular Layer

• Reticular layer

• Accounts for approximately 80% of the thickness of the skin

• Collagen fibers in this layer add strength and resiliency to the skin

• Elastin fibers provide stretch-recoil properties

Hypodermis

• Subcutaneous layer deep to the skin

• Composed of adipose and areolar connective tissue

Skin Color

• Three pigments contribute to skin color

• Melanin – yellow to reddish-brown to black pigment, responsible for dark skin colors

• Freckles and pigmented moles – result from local accumulations of melanin

• Carotene – yellow to orange pigment, most obvious in the palms and soles of the feet

• Hemoglobin – reddish pigment responsible for the pinkish hue of the skin

Sweat Glands

• Different types prevent overheating of the body; secret cerumen and milk

• Eccrine sweat glands – found in palms, soles of the feet, and forehead

• Apocrine sweat glands – found in axillary and anogenital areas

• Ceruminous glands – modified apocrine glands in external ear canal and secrete cerumen

• Mammary glands – specialized sweat glands that secret milk

Sebaceous Glands

• Soften skin when stimulated by hormones

• Simple alveolar glands found all over the body

• Secrete an oily secretion called sebum

Hair

• Helps maintain warmth, alerts the body to presence of insects on the skin, and guards the scalp against physical trauma, heat loss, and sunlight

• Filamentous strands of dead keratinized cells produced by hair follicles

• Contains hard keratin, which is tougher and more durable than the soft keratin of the skin

• Made up of the shaft projecting from the skin and the root embedded in the skin

• Consists of a core called the medulla, a cortex, and an outermost cuticle

Hair

• Pigmented by melanocytes at the base of the hair

Hair Follicle

• Root sheath extending from the epidermal surface into the dermis

• Deep end is expanded forming a hair bulb

• A knot of sensory nerve endings (a root hair plexus) wraps around each hair bulb

• Bending a hair stimulates these endings, hence our hairs act as sensitive touch receptors

Hair Follicle

Types of Hair

• Vellus – pale, fine body hair found in children and the adult female

• Terminal – coarse, long hair of eyebrows, scalp, axillary, and pubic regions

Hair Thinning and Baldness

• Alopecia – hair thinning in both sexes

• True, or frank, baldness – genetically determined and sex influenced condition (i.e., male pattern baldness)

Structure of a Nail

• Scalelike modification
of the epidermis
on the distal,
dorsal surface
of fingers
and toes

Functions of the Integumentary System

• Protection – chemical, physical, and mechanical barrier

• Body temperature

• Regulated by dilation (cooling) and constriction (warming) of dermal vessels

• Sweat glands increase secretions to cool the body

• Cutaneous sensation – exoreceptors sense touch and pain

• Metabolic functions – synthesis of vitamin D in dermal blood vessels

• Blood reservoir – skin blood vessels store up to 5% of the body’s blood volume

• Excretion – limited amounts of nitrogenous wastes are eliminated from the body in sweat

Skin Cancer

• Basal cell carcinoma

• Squamous cell carcinoma

• Melanoma

Basal Cell Carcinoma

• Least malignant and most common skin cancer

• Stratum basale cells proliferate and invade the dermis and hypodermis

• Slow growing and do not often metastasize

• Can be cured by surgical excision in 99% of the cases

Squamous Cell Carcinoma

• Arises from keratinocytes of stratum spinosum

• Arise most often on scalp, ears, and lower lip

• Grows rapidly and metastasizes if not removed

• Prognosis is good if treated by radiation therapy or removed surgically

Melanoma

• Cancer of melanocytes is the most dangerous type of skin cancer

• Melanomas have the following characteristics (ABCD rule):

• A: Asymmetry; the two sides of the pigmented area do not match

• B: Border is irregular and exhibits indentations

• C: Color (pigmented area) is black, brown, tan, and sometimes red or blue

• D: Diameter is larger than 6 mm (size of a pencil eraser)

Melanoma

• Treated by wide surgical excision accompanied by immunotherapy

• Chance of survival is poor if the lesion is over 4 mm thick

Burns

• First-degree – only the epidermis is damaged

• Symptoms include localized redness, swelling, and pain

• Second-degree – the epidermis and upper regions of dermis damaged

• Symptoms mimic first degree burns, but blisters also appear

• Third-degree – involve entire thickness of the skin

• Burned area appears gray-white, cherry red, or black, and there is no initial edema nor pain (since nerve endings are destroyed)

Rule of Nines

• Estimates the severity of burns

• Burns considered critical if:

• Over 25% of the body has second-degree burns

• Over 10% of the body has third-degree burns

• There are third-degree burns on face, hands, or feet

Developmental Aspects of the Integument: Fetal

• Epidermis develops from ectoderm

• Dermis and hypodermis develop from mesoderm

• Lanugo – downy coat of delicate hairs covering the fetus

• Vernix caseosa – substance produced by sebaceous glands that protects fetus’s skin in the amnion

Developmental Aspects of the Integument: Adolescent to Adult

• Skin and hair become oilier and acne may appear

• After one reaches the 20s and 30s, skin shows the effects of cumulative environmental assaults

• Scaling and dermatitis become more common

Developmental Aspects of the Integument:
Old Age

• Epidermal replacement of cells slows and skin becomes thinner

• Skin becomes dry and itchy

• Subcutaneous fat layer diminishes, leading to intolerance of cold

• Decreased elasticity and loss of subcutaneous tissue leads to wrinkles

• Decreased numbers of melanocytes and Langerhans’cells increase the risk of skin cancer