Chapter 5Chapter 5

Integumentary Integumentary SystemSystemLecture 5a - Skin Structure

Integumentary system

• Includes the skin, sweat and oil glands, hairs, and nails

• Major Function – protection

• Accounts for about 7% of total body weight in the average adult – (9-11 pounds)

Skin Structure

Figure 5.1

Skin Structure

• Two layers

•1. Epidermis - outermost layer–Epi = upon–Stratified squamous epithelium–Often keratinized (hardened by keratin)

Skin Structure• Two layers

•2. Dermis–Dense connective tissue

• Only Dermis is vascularized• Nutrients reach the epidermis by

diffusion

Hypodermis

•Subcutaneous tissue–Just deep to skin

•Not considered part of skin–Functions as a shock absorber and insulates - Prevents heat loss from body

•Mostly adipose tissue (so it stores fat)

Hypodermis

•The hypodermis thickens during weight gain–In females, the hypodermis first thickens in thighs and breasts

Hypodermis

–In males, the hypodermis first thickens in the anterior abdomen - Sometimes described as a ‘beer belly’

Hypodermis•Loosely anchors skin to

underlying structures (mostly muscles)–Skin is loose enough to slide somewhat over structures – many blows glance off our bodies.

Epidermis

• Composed of keratinized stratified squamous epithelium

• Consists of four distinct cell types and four or five layers

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

Cells of the Epidermis

• Most are Keratinocytes – produce the fibrous protein keratin

• Keratin gives the epidermis protective properties–Kera = horn in Greek

• Connected by desmosomes• Keratinocytes arise from the deepest

layer of epidermis whose cells undergo continual mitosis

• Cells are pushed upward by the production of new cells underneath

• When keratinocytes reach surface they are dead, scalelike structures filled with keratin

• Millions of these dead cells rub off daily–We have a totally new epidermis every 25 to 45 days

• Persistent friction causes accelerated cell and keratin production and a thickening of the epidermis called a callus

•Melanocytes – spider-shaped cells that produce the brown pigment melanin (Melan = black)

• As melanin is made it accumulates in the processes (the arms) of the melanocytes cells.

• Then they are taken up by the keratinocytes and accumulate on the superficial side of the keratinocyte nucleus–Forms a pigment shield to protect nucleus from ultraviolet radiation in sunlight

Other Cells of the Epidermis

• Langerhans’ cells – star-shaped cells that arise from bone marrow and migrate to the epidermis as epidermal macrophages that help activate the immune system

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

Layers of the Epidermis

Figure 5.2b

Layers of the Epidermis

• Thick skin–5 layers or strata (strata = sheets)

–Covers palms, fingertips, soles of feet

• Thin skin–4 layers

• Deepest epidermal layer firmly attached to the dermis

• It is a single row consisting of mostly young keratinocytes, with 10-25% melanocytes, and an occasional Merkel cell

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

Layers of the Epidermis: Stratum Basale (Basal

Layer)

Figure 5.2b

• Several cell layers thick• Cells contain a weblike system of

intermediate filaments attached to desmosomes

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

Layers of the Epidermis: Stratum Spinosum (Prickly

Layer)

Figure 5.2b

• Thin; three to five cell layers in which drastic changes in keratinocyte appearance occurs –Cells flatten, nuclei & organelles disintegrate, and keratohyaline and lamellated granules accumulate

Layers of the Epidermis: Stratum Granulosum (Granular

Layer)

Layers of the Epidermis: Stratum Basale

(Basal Layer)

Figure 5.2b

•Present only in thick skin• Thin, transparent band

superficial to the stratum granulosum

• Consists of a few rows of flat, dead keratinocytes

Layers of the Epidermis: Stratum Lucidum (Clear

Layer)

• Outermost layer of keratinized cells• Accounts for three quarters of the

epidermal thickness• Functions include:

– Waterproofing– Protection from abrasion and penetration– Rendering the body relatively insensitive

to biological, chemical, and physical assaults

Layers of the Epidermis: Stratum Corneum

• Average person sheds 40 pounds of skin flakes from the stratum corneum layer in a lifetime–Provides food for dust mites!

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• Richly supplied with nerve fibers,

blood vessels, and lymphatic vessels.

• Most hair follicles, oil and sweat glands are derived from epidermal tissue but reside in the dermis

• In animals, this is the hide that makes leather products

Layers of the Dermis: Papillary Layer

• Areolar connective tissue with collagen and elastic fibers and blood vessels

Layers of the Dermis: Papillary Layer

• Its superior surface contains peglike projections called dermal papillae

Layers of the Dermis: Papillary Layer

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

• On palms of the hands and soles of the feet, dermal papillae lie on top larger mounds called dermal ridges

• The epidermis that lies on the dermal ridges is called the epidermal ridges

• These epidermal ridges are genetically determined and unique to each of us

• These are our fingerprints!

Layers of the Dermis: Reticular Layer

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

• Is dense irregular connective tissue• Collagen fibers in this layer add

strength and resiliency to the skin• Elastin fibers provide stretch-recoil

properties

• Most bundles of collagen fibers run parallel to the skin surface.

• Less dense regions of collagen fibers form cleavage or tension lines in the skin – invisible from surface

• These cleavage lines are important to surgeons – Incisions made parallel to these

lines, the skin gapes less and heals more readily than when the incision is made across cleavage lines

• Epidermal ridges (fingerprints) and cleavage lines are called skin markings

• Another type of skin marking is Flexure lines

• These are externally visible• Dermal folds that occur at or

near joints• See them as creases in your palms,

wrists, finges, soles, and toes

Homeostatic imbalances• Stretching of the skin can tear

the dermis– Indicated by silvery white scars called striae (streaks) commonly called stretch marks

Homeostatic imbalances• Stretching orTrauma like a burn or a

poor fitting shoe, or digging a hole with a shovel, can cause a blister–Separation of the epidermal and dermal layers by a fluid filled pocket

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

Melanin

• Made in melanocytes and passed to keratinocytes

• All humans have about the same number of melanocytes– Individual differences in skin coloring

reflect the amount of melanin made and retained

– Darker skinned people produce more melanin and their keratinocytes retain it longer

Melanin• Melanocytes are more active

when exposed to sunlight –Protects DNA of skin cells from UV radiation by absorbing the light and dissipating the energy as heat

–Causes a ‘tan’

• Even with Melanin’s protection, excess sun exposure eventually damages the skin

•Leathery skin, depressed immune system, skin cancer

Carotene and hemoglobin

• Carotene – yellow to orange pigment (also found in carrots)– Accumulates in the stratum corneum and

the hypodermis– Most intense when carotene-rich foods are

eaten

• Hemoglobin – pinkish hue reflecting the crimson color of oxygenated hemoglobin in red blood cells in the dermal capillaries

Homeostatic imbalances• When hemoglobin is poorly

oxygenated, both the blood and the skin appear blue–Called cyanosis (cyan = dark blue)–Lots of melanin may mask cyanotic appearance so dark-skinned individuals will only show blue tint in their mucous membranes and nail beds

Homeostatic imbalances• Skin often becomes cyanotic

during heart failure and severe respiratory disorders

Homeostatic imbalances

• Alterations in skin color signal certain diseases and emotional stimuli

• Redness – may indicate embarrassment, fever, hypertension, inflammation, or allergy

Homeostatic imbalances

• Jaundice or yellow cast – usually signifies a liver disorder

Homeostatic imbalances

• Bronzing – a bronze or almost metallic appearance of the skin is a sign of Addison’s disease

• Black and Blue marks or bruises – reveal where blood escaped from the circulatory system and clotted beneath the skin – called hematomas

We will quiz over this next time!

Do study Guide pages 101-105(they will be checked)