Skin (Integument)

•Consists of three major regions

–Epidermis  – outermost superficial region

–Dermis – middle region

–Hypodermis (superficial fascia) – deepest region

Skin (Integument)


•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

Layers of the Epidermis: Stratum Basale
(Basal Layer)

Layers of the Epidermis: 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

Layers of the Epidermis: 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

Layers of the Epidermis: Stratum Lucidum
(Clear Layer)

•Thin, transparent band superficial to the stratum granulosum

•Consists of a few rows of flat, dead keratinocytes

•Present only in thick skin

Layers of the Epidermis: Stratum Corneum
(Horny Layer)

•Outermost layer of keratinized cells

•Accounts for three quarters of the epidermal thickness

•Functions include:


–Protection from abrasion and penetration

–Rendering the body relatively insensitive to biological, chemical, and physical assaults


•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

Layers of the 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

Layers of the 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


•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; secrete 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 that secrete cerumen

–Mammary glands – specialized sweat glands that secrete milk

Sebaceous Glands

•Simple alveolar glands found all over the body

•Soften skin when stimulated by hormones

•Secrete an oily secretion called sebum

Structure of a Nail

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


•Filamentous strands of dead keratinized cells produced by hair follicles

•Contains hard keratin which is tougher and more durable than 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

•Pigmented by melanocytes at the base of the hair

Hair Function and Distribution

•Functions of hair include:

–Helping to maintain warmth

–Alerting the body to presence of insects on the skin

–Guarding the scalp against physical trauma, heat loss, and sunlight

•Hair is distributed over the entire skin surface except

–Palms, soles, and lips

–Nipples and portions of the external genitalia

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

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

–Male pattern baldness – caused by follicular response to DHT

Functions of the Integumentary System

•Protection – chemical, physical, and mechanical barrier

•Body temperature regulation is accomplished by:

–Dilation (cooling) and constriction (warming) of dermal vessels

–Increasing sweat gland secretions to cool the body

•Cutaneous sensation – exoreceptors sense touch and pain

Functions of the Integumentary System

•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

•Most skin tumors are benign and do not metastasize

•A crucial risk factor for nonmelanoma skin cancers is the disabling of the p53 gene

•Newly developed skin lotions can fix damaged DNA

Skin Cancer

•The three major types of skin cancer are:

–Basal cell carcinoma

–Squamous cell carcinoma  


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


•Cancer of melanocytes is the most dangerous type of skin cancer because it is:

–Highly metastatic

–Resistant to chemotherapy


•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)


•Treated by wide surgical excision accompanied by immunotherapy

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


•First-degree – only the epidermis is damaged

–Symptoms include localized redness, swelling, and pain

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

–Symptoms mimic first degree burns, but blisters also appear

•Third-degree – entire thickness of the skin is damaged

–Burned area appears gray-white, cherry red, or black; there is no initial edema or 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




Skeletal Cartilage

•Contains no blood vessels or nerves

•Surrounded by the perichondrium (dense irregular connective tissue) that resists outward expansion

•Three types – hyaline, elastic, and fibrocartilage

Hyaline Cartilage

•Provides support, flexibility, and resilience

•Is the most abundant skeletal cartilage

•Is present in these cartilages:

–Articular – covers the ends of long bones

–Costal – connects the ribs to the sternum

–Respiratory – makes up the larynx and reinforces air passages

–Nasal – supports the nose

Elastic Cartilage

•Similar to hyaline cartilage but contains elastic fibers

•Found in the external ear and the epiglottis


•Highly compressed with great tensile strength

•Contains collagen fibers

•Found in menisci of the knee and in intervertebral discs

Growth of Cartilage

•Appositional – cells in the perichondrium secrete matrix against the external face of existing cartilage

•Interstitial – lacunae-bound chondrocytes inside the cartilage divide and secrete new matrix, expanding the cartilage from within

•Calcification of cartilage occurs

–During normal bone growth

–During old age

Bones and Cartilages of the Human Body

Classification of Bones

•Axial skeleton – bones of the skull, vertebral column, and rib cage

•Appendicular skeleton – bones of the upper and lower limbs, shoulder, and hip

Classification of Bones: By Shape

•Long bones – longer than they are wide
(e.g., humerus)


•Short bones

–Cube-shaped bones of the wrist and ankle

–Bones that form within tendons (e.g., patella)

•Flat bones – thin, flattened, and a bit curved (e.g., sternum, and most skull bones)

•Irregular bones – bones with complicated shapes (e.g., vertebrae and hip bones)


Function of Bones

•Support – form the framework that supports the body and cradles soft organs

•Protection – provide a protective case for the brain, spinal cord, and vital organs

•Movement – provide levers for muscles

•Mineral storage – reservoir for minerals, especially calcium and phosphorus

•Blood cell formation – hematopoiesis occurs within the marrow cavities of bones

Bone Markings

•Bulges, depressions, and holes that serve as:

–Sites of attachment for muscles, ligaments, and tendons

–Joint surfaces

–Conduits for blood vessels and nerves

Bone Markings: Projections –
Sites of Muscle and Ligament Attachment

•Tuberosity – rounded projection

•Crest – narrow, prominent ridge of bone

•Trochanter – large, blunt, irregular surface

•Line – narrow ridge of bone


•Tubercle – small rounded projection

•Epicondyle – raised area above a condyle

•Spine – sharp, slender projection

•Process – any bony prominence

Bone Markings: Projections –
Projections That Help to Form Joints

•Head – bony expansion carried on a narrow neck

•Facet – smooth, nearly flat articular surface

•Condyle – rounded articular projection

•Ramus – armlike bar of bone

Bone Markings: Depressions and Openings

•Meatus – canal-like passageway

•Sinus – cavity within a bone

•Fossa – shallow, basinlike depression

•Groove – furrow

•Fissure – narrow, slitlike opening

•Foramen – round or oval opening through a bone

Gross Anatomy of Bones: Bone Textures

•Compact bone – dense outer layer

•Spongy bone – honeycomb of trabeculae filled with yellow bone marrow

Structure of Long Bone

•Long bones consist of a diaphysis and an epiphysis


–Tubular shaft that forms the axis of long bones

–Composed of compact bone that surrounds the medullary cavity

–Yellow bone marrow (fat) is contained in the medullary cavity


–Expanded ends of long bones

–Exterior is compact bone, and the interior is spongy bone

–Joint surface is covered with articular (hyaline) cartilage

–Epiphyseal line separates the diaphysis from the epiphyses


Bone Membranes

•Periosteum – double-layered protective membrane

–Outer fibrous layer is dense regular connective tissue

–Inner osteogenic layer is composed of osteoblasts and osteoclasts

–Richly supplied with nerve fibers, blood, and lymphatic vessels, which enter the bone via nutrient foramina

–Secured to underlying bone by Sharpey’s fibers

•Endosteum – delicate membrane covering internal surfaces of bone

Structure of Short, Irregular, and Flat Bones

•Thin plates of periosteum-covered compact bone on the outside with endosteum-covered spongy bone (diploλ) on the inside

•Have no diaphysis or epiphyses

•Contain bone marrow between the trabeculae


Location of Hematopoietic Tissue (Red Marrow)

•In infants

–Found in the medullary cavity and all areas of spongy bone

•In adults

–Found in the diploλ of flat bones, and the head of the femur and humerus

Microscopic Structure of Bone: Compact Bone

•Haversian system, or osteon – the structural unit of compact bone

–Lamella – weight-bearing, column-like matrix tubes composed mainly of collagen

–Haversian, or central canal – central channel containing blood vessels and nerves

–Volkmann’s canals – channels lying at right angles to the central canal, connecting blood and nerve supply of the periosteum to that of the Haversian canal


Microscopic Structure of Bone: Compact Bone

•Osteocytes – mature bone cells

•Lacunae – small cavities in bone that contain osteocytes

•Canaliculi – hairlike canals that connect lacunae to each other and the central canal


Microscopic Structure of Bone: Compact Bone

•Osteoblasts – bone-forming cells

•Osteocytes – mature bone cells

•Osteoclasts – large cells that resorb or break down bone matrix

•Osteoid – unmineralized bone matrix composed of proteoglycans, glycoproteins, and collagen

Chemical Composition of Bone: Inorganic

•Hydroxyapatites, or mineral salts

–Sixty-five percent of bone by mass

–Mainly calcium phosphates

–Responsible for bone hardness and its resistance to compression

Bone Development

•Osteogenesis and ossification – the process of bone tissue formation, which leads to:

–The formation of the bony skeleton in embryos

–Bone growth until early adulthood

–Bone thickness, remodeling, and repair

Formation of the Bony Skeleton

•Begins at week 8 of embryo development

•Intramembranous ossification – bone develops from a fibrous membrane

•Endochondral ossification – bone forms by replacing hyaline cartilage

Intramembranous Ossification

•Formation of most of the flat bones of the skull and the clavicles

•Fibrous connective tissue membranes are formed by mesenchymal cells