The Integumentary System

Dr. Barbara T. WizerOur Lady of Holy Cross College

BIO 351

The Skin• The skin is the largest system in the body• It is also an organ (largest in the body)

made of different tissues structurally arranged to function together

• Together with the epidermal derivatives, it forms the integumentary system

• Composed of epidermis and dermis

Functions• The skin provides physical, chemical and biological

protection.– The low pH of skin secretions (acid mantle) retards bacterial growth.– The stratified layers of keratinocytes are a protective barrier.– Glycolipids prevent diffusion of water and dehydration.– Langerhans cells (derived from a type of leukocyte called a

monocyte) which are found in the epidermis and dermis, and macrophages in the dermis play a role in providing immunity.

• Modified cholesterol molecules (7 - dehydrocholesterol) in skin are converted by exposure to UV light to a vitamin D precursor (cholecalciferol= vit. D3) which is important in calcium metabolism. (Cholecalciferol circulates to the liver where it is converted to 25-hydroxy vit. D3, which then circulates to the kidneys, where it is converted to 1,25-dihydroxy vit. D3, which is the active form of vit. D) Vit. D promotes absorption of calcium in the small intestine.

Epidermis• Keratinized stratified squamous epithelium• Cell types:

– Keratinocytes—most common, produce keratin, a fibrous protein that gives the skin its protective properties

– Melanocytes—located in basal layer, synthesize melanin which protects against UV radiation; the pigment accumulates in the superficial portion of keratinocytes and protects their nuclei from UV radiation

– Langerhans cells (= dendritic cells) process cell membrane markers (antigens) from foreign cells

– Merkel cells— serve as light touch receptors, connect to sensory nerve endings

Epidermal Cell Layers• Thick skin covers the palms, fingertips and soles

and has 5 layers or strata– Stratum basale– Stratum spinosum– Stratum granulosum– Stratum lucidum– Stratum corneum

• Thin skin covers the rest of the body and lacks the stratum lucidum

Thick skin: note the thickness of the stratum lucidum

Thin skin: the slide also shows the duct and coils of a sweat gland

Epidermal Strata• Basale—deepest layer, keratinocytes are

actively undergoing mitosis; 10-25% of the cells here are melanocytes; cells divide once every 19 days and take 40-56 days to reach the outermost layer and desquamate

• Spinosum—cells are active in formation of keratin fibers and lamellar bodies which contain glycolipids

Epidermal Strata Continued

• Granulosum—nuclei and organelles begin to disintegrate, cells fill with keratin, and lamellated granules empty a waterproofing glycolipid out of the cell

• Lucidum—clear, flat, dead keratinocytes (this layer is only present in thick skin)

• Corneum--25 or more layers of dead cells which are cornified (= keratinized, i.e. surrounded by a hard protein envelop and filled with keratin); lipids surround the cells and are responsible for many of the permeability characteristics of the skin

Epidermal Derivatives• Sebaceous glands - found everywhere except the palms

and soles, they secrete sebum (oil) and are usually associated with hair follicles. The function of sebum is to lubricate the hair and skin, their secretion is stimulated by hormones, especially androgens.

• Sudoriferous glands – 2 types: eccrine sweat glands are simple tubular glands important in temperature regulation, apocrine – larger glands with more viscous secretions that are located in the axilla

• Ceruminous glands - found in the ear canal, they secrete cerumen which prevents dryness of the skin of the ear canal and repels insects

• Hair follicles - hair is composed of dead, keratinized cells and is produced by hair follicles. Arrector pili muscles are associated with each hair follicle; contraction of the muscles causes the follicles to change position and produces goose bumps on the surface of the skin

Sweat glands – 2 types: eccrine + apocrine

Hair follicle (scalp) with associated sebaceous glands

Note the dense irregular connective tissue of the dermis, the hair follicles, the stratum corneum, and the sweat gland (top center)

Dermis• Dense irregular connective tissue• It is responsible for most of the structural

strength of the skin• Cells found in the dermis include

fibroblasts, macrophages, and mast cells• The dermis contains blood vessels, nerves,

hair follicles, arrector pili muscles, and glands

Dermal Layers• Papillary layer—located just underneath the

epidermis, has more cells and fewer fibers than the deeper reticular layer, fibers are looser, there are many blood vessels in this layer to support the epidermis. The papillae of the dermis of thick skin (on soles and palms) are in parallel, curving ridges that shape the epidermis into fingerprints and footprints which improve grip

• Reticular layer--comprises about 80% of the dermis, blends into the hypodermis

Hypodermis • Also called subcutaneous (subQ) layer• Not technically a part of the skin - it attaches skin

to underlying muscles and is composed of loose connective tissue and fat

• Half the body’s fat is in the subcutaneous layer; variations depend on age, sex, and weight

• The fat in the hypodermis serves as an effective shock absorber.

Rule of Nines

• The “Rule of 9s” is used to estimate the extent of a burn suffered by an adult. It is given as the % TBSA (total body surface area) involved: head = 9% of TBSA, each arm accounts for 9%, each lower extremity accounts for 18%, (i.e. anterior surface = 9% and posterior surface = 9%), anterior and posterior trunk each represent 18%, perineum equals 1%

Skin Cancer• 800,000 new cases annually in the U.S.• Squamous cell carcinoma• Basal cell carcinoma• The above 2 account for 95% of cases, seldom

metastasize• Malignant melanoma – can be deadly• Kaposi’s sarcoma – a cancer of connective tissue

which causes purplish skin lesions, commonly seen in people with AIDS, associated with a type of Herpes virus known as HHV - 8

Skin Infections

• Dermatitis – inflammation of the skin • Impetigo – bacterial infection with Staph or

Strep, mostly seen in kids• Tinea – general term for fungal infection of

skin, Exs. ringworm, athlete’s foot• Wart – caused by infection with

papillomavirus

Scabies• Known as “the 7 year itch”• Caused by infestation with a mite called Sarcoptes

scabiei (related to spiders and ticks)• This parasite burrows into the skin, where it lives and

lays eggs that develop into new mites• Transmitted by skin to skin contact and from

clothing, bedding, and other possessions to next individual

• Nursing home outbreaks are a serious problem

Scabies

Scabies• Causes skin rash and allergic symptoms (intense

itching (pruritis))• There is an especially severe form called Norwegian

or crusted scabies. Elderly are particularly susceptible to this.

• Topical Treatment – Elimite• Topical treatment may be combined with an oral

treatment with ivermectin• Difficult to eradicate, if an outbreak is widespread in

a nursing home, the facility may have to be closed to new admits and no one can be transferred out.

Crusted (Norwegian) Scabies

Skin Disorders

• Scleroderma – autoimmune disease that affects connective tissue. There is a limited (localized) form which causes hardening of the skin, and a systemic form, which also involves internal organs such as the kidneys, heart, and lungs.

• Psoriasis – a chronic inflammatory disorder, has a genetic basis, causes silvery scales

• Eczema – a common inflammatory disorder often seen in association with an allergic reaction called contact dermatitis