Lecture 7: Integumentary System

The integument is the outer most covering of the body. Although commonly referred to as skin, it also includes blood vessels, nerves and sensory receptors, sweat and oil glands, other derivatives such as hair and nails. It is a complex organ that serves primarily a protective function.

I. Regions

 

A. Epidermis—outermost layer; mostly epithelial cells; non-vascular

B. Dermis—fibrous connective tissue; vascular

C. Hypodermis (superficial fascia)—not skin; protective; adipose and loose connective tissue

 

A. Epidermis—thick keratinized stratified squamous epithelium consisteing of four cell types and five layer

1. Cell types

            a. Keratinocytes—produce keratin; primary cell of epidermis

                        i. Arise in stratum basale

                        ii. Pushed to surface as continuously mitotic cells reproduce

                        iii. Keratin is produced as cells migrate

                        iv. Cells connected by desmosomes

                        v. At surface, cells are dead keratin filled plasma membrane

            b. Melanocytes—produce pigment, melanin

 

                        i. In stratum basale

                        ii. Have processes that contact all keratinocytes

                        iii. Transfer melanin to keratinocytes

iv. Melanin accumulates near cell’s apical surfaces to shield nucleus from UV

            c. Merkel cells—associated with sensory nerve endings; form Merkel disc

            d. Langerhan’s cells (epidermal dendritic cells)

                        i. Made in bone marrow and migrate to epidermis

                        ii. Form a continuous network

                        iii. Function as macrophages

2. Layers of epidermis

 

 

            a. Stratum basale (stratum germinativum)—deepest layer

                        i. Single layer of mitotically active cells

                        ii. Give rise to keratinocytes (youngest)

                        iii. Includes melanocytes and some Merkel cells

b. Stratum spinosum (Prickly layer)—weblike network of cells formed by intermediate filaments attached to desmosomes

            i. Comprised of keratinocytes

            ii. Includes melanin granules and Langerhans cells

            c. Stratum granulosum—thick; 3-5 cell layers; keratinocytes are modified

                        i. Flattened; nuclei and organelles lost

                        ii. Keratohylaline and lamellated granules accumulate

iii. Lamellated granules are glycoproteins, released into extracellular space, that reduce water loss

iv. Cells more resistant to destruction

d. Stratum lucidum—a few rows of clear, flattened, dead keratinocytes; layer occurs only in thick skin

i. Keratohyalin granules—gummy substance associated with keratin filaments

ii. Cells aggregate in parallel arrays

            e. Stratum corneum (Horny layer)—outer most layer; most of epidermis thickness

                        i. 20-30 cell layers thick

ii. Keratin, thickened plasma membranes and glycoproteins protect against abrasion and loss of water

iii. Cornified or horny cells—remnants of cells from this layer

 

B. Dermis—strong, flexible tissue layer

1. Cells

            a. Fibroblasts

            b. Macrophages

            c. Mast cells

            d. WBC’s

2. Nervous fibers with sensory receptors

3. Blood vessels

4. Hair follicles

5. Swaet glands

6. Layers

            a. Papilary layer—thin; superficial

                        i. Connective tissue with blood vessels

ii. Dermal papillae—superficial layer; project into epidermal layer; include capillary beds, Meissner’s corpuscles

            b. Reticular layer—most of dermis; dense irregular connective tissue

                        i. Collagen fibers—strength and resiliency; hydration

                        ii. Elastin fibers—stretch recoil

 

C. Skin color

1. Pigments

            a. Melanin—ranges in color from yellow to redish-brown to black

                        i. Tyrosine derivative

                        ii. Tyrosinase

iii. Racial differences—amount and persistence of melanin in keratinocytes

iv. Melanocytes are stimulated by sunlight; protects DNA from UV

            b. Carotene—yellow to orange

                        i. Accumulates in stratum corneum and fatty tissue of hypodermis

                        ii. Obvious in palms and soles

            c. Hemoglobin—in RBC’s

i. Redish hue through transparent skin of caucasians lacking melanin

                        ii. Cyanosis—deoxygenated blood gives skin a blue appearance

2. Emotional and disease states

a. Redness (erythema)—embarrassment, fever, hypertension, inflammation, allergy

b. Pallor—paleness; emotional stress, anemia, low blood pressure

c. Jaundice—liver disorder (bile pigments in blood and deposited in body tissue)

d. Bruises—blood escapes circulatory system and forms a hematoma

 

D. Skin appendages

 

1. Sweat glands (sudoriferous)

            a. Eccrine

                        i. Coiled, tubular

                        ii. Palms, soles of feet

                        iii. Duct opens to pore on surface

                        iv. Hypotonic secretion

                        v. Regulated by sympathetic NS

                                    a. Involuntary

            b. Apocrine

                        i. Large; ducts empty into hair follicle

                        ii. Axillary and anogenital areas

                        iii. Interaction with skin bacteria results in odor

            c. Ceruminous—modified apocrine

                        I. Ear canal; cerumen (wax)

            d. Mammary—produce milk

2. Sebaceous glands

            a. Secrete sebum

                        i. Holocrine—accumulate lipids until they burst

                        ii. Slow water loss when humidity is low

                        iii. Softens hair

                        iv. Bactericidal

                        v. Stimulated by androgens

3. Hair and hair follicles

 

            a. Warmth (not humans)

            b. Sensory functions

            c. Protection

            d. Filtration

4. Nails—modification of epidermis

 

            a. Free edge, body, proximal root

           

II. Function of the Integumentary System

A. Protection

1. Chemical barriers

            a. Acid mantle—low pH selects against bacteria

            b. Bactericidal substances in sebum

2. Physical and mechanical barriers

            a. Skin is continuous and keratinized

            b. Blocks movement of water soluble substances

            c. Permits lipid soluble, organic solvents

3. Biological barriers

1. Immune responses initiated in skin

            a. Antigen presentation to lymphocytes

                        i. Langerhan’s cells

            b. Macrophage activity

B. Body temperature regulation

 

1. Endothermic evaporation cools body

            a. Sweat glands secrete sweat continuously

            b. Body temperature increase

                        i. Blood vessels dilate

                        ii. Sweat glands stimulated

C. Cutaneous sensation

1. Sensory receptors (externoceptors)

            a. Meissner’s corpuscles--vibration

            b. Pacinian corpuscles—deep pressure

            c. Root hair plexuses

            d. Nociceptors

D. Metabolic functions

1. Conversion of cholesterol into vitamin D precursor

E. Blood reservoir

1. Shunted by nervous function between skin reservoir (5% of volume) to general circulation

F. Excretion

1. Limited amount of nitrogenous wastes

2. Water and salts in sweat