Biology 211Anatomy & Physiology I

Skin

The Integumentary System

The Integumentary System

• Skin=Integument• Skin + accessory organs (hair, nails, and cutaneous

glands)= Integumentary system• Dermatology=scientific study and medical treatment of

the integumentary system• Skin-the most vulnerable organ—exposed to radiation,

trauma, infections, injurious chemicals• Lgst. organ—15% body wt

Functions of Integumentary System (skin & its products)

Barrier to keep water and solutes in

Barrier to keep bacterial, dirt, etc. out

Protection against abrasion

Contains sensory receptors for touch, temperature, pressure, pain, etc.

Temperature regulation via hair, sweat, & amount of blood flow

Blood reservoir

Synthesis of vitamin D

Excretion

Epidermis: Stratified squamous epithelium; outer layer is "keratinized" or "cornified"

Dermis:Dense irregular connective tissue

Hypodermis:Adipose connective tissue (technically not part of system

Three Layers of skin:

Epidermis: Avascular. Depends on blood vessels in underlying dermis for its nutrition

Cells formed by mitosis in deepest, or basal, layer, then get pushed into more superficial layers or "stata"

Stratum Basale = Single row of dividing cells

Stratum Spinosum = Three or four layers of cells; Some cell division

Stratum Granulosum = Three or four layers of cells; Actively synthesizing protein keratin

Stratum Lucidum = One or two layers of dying cells

Stratum Corneum = Many layers of flat, dead, scale-like cells full of keratin

(Epidermis)

Primary cell type in epidermis = keratinocytes which produce large amounts of protein keratin

Other cell types:

Melanocytes produce pigment melanin & transfer it to keratinocytes

Langerhans cells (immune cells) protect against pathogens and toxins

Merkel cells detect touch and pressure; transfer this information to sensory receptors in the dermis

Skin Color• Most significant factor is melanin

– 2 forms (1) eumelanin (brownish black) and (2) pheomelanin (reddish yellow)

• Other factors include hemoglobin (red to pink) and carotene (yellow)

• Different races have same # of melanocytes, amount of melanin produced differs

• Melanin accumulates in keratinocytes

Abnormal Color

• Cyanosis—blue, lack of hemoglobin• Erythema—abnormal redness, increased blood flow or

blood pooling• Pallor—pale, ashen, poor blood flow• Albinism—lack of melanin• Jaundice—yellowing, hi levels of bilirubin (hi rate of

erythrocyte destruction• Bronzing—deficiency of glucocorticoids• Hematoma—bruise, blood clot

Keratinocytes move from stratum basale to stratum corneum, dying as they do so. Average = 6 - 8 wks

- Keratinocytes in stratum basale (& stratum spinosum) lie along basement membrane; divide by mitosis

- Older cells die but remain attached to each other

- Older ones pushed toward surface by newer cells

- Cells eventually shed from stratum corneum

- Keratinocytes synthesize large amounts of keratin and flatten out as they move toward surface

- Melanocytes transfer melanin to keratinocytes

(Epidermis)

Dermis:

Dense irregular connective tissue

Separated from epidermis (stratified squamous epithelium) by basement membrane

Highly vascular

Highly innervated

Two Layers:

Papillary layer just below epidermis

Reticular layer forms deep 80%

Dermis:

Contains many types of sensory receptors for touch, pressure, vibration, pain, temperature, etc.

Some = simple nerve endings

Others = complex structures with multiple cell types

Dermis = Dense irregular connective tissue. Thus:

Cells = Fibroblasts / Fibrocytes Macrophages Mast cells Lymphocytes etc.

Fibers = Collagen (therefore strong, flexible) Elastic (therefore stretchable)

Weight gain tears collagen fibers producing striae (stretch marks)

Appendages of the skin

Hair follicles and hairSweat glandsSebaceous (oil) glandsNails on fingers and toes

All begin as epidermis of embryo; grow down into dermis

Hair

- Distribuled over all skin except: palms of hands soles of feet nipples glans of penis & clitoris minor labia

- Formed in follicles located deep in dermis

- Consists of layers of dead, highly keratinized keratinocytes

Function of Hair

• Trunk and limb hair considered vestigial• Scalp only place where it is thick enough to retain heat• Eyebrows enhance facial features• Vibrassae (guard hair) are used to filter particulates from

the ear and nose

Shaft

Bulb

Root

Medulla: Loosely arranged keratinocytesCortex: Densely packed keratinocytesCuticle: Keratinocytes loosely arranged like scales

Black or brown color due to eumelanin produced by melanocytes and transferred to keratinocytes in follicle. Red color produced by pigment called pheomelanin

Blonde color produced by intermediate levels of pheomelanin and low levels of eumelanin

Texture of hair due to The rounder the hair shaft, the straighter it will be The flatter the hair shaft, the more curled it will be

Each hair is associated with:

One or more sebacious (oil) glands

An arrector pili muscle

A plexus of nerves aroundthe root

Sweat Glands

- 2 to 3 million

- Two types: Merocrine: Distributed over all skin except nipples (Eccrine) Simple coiled glands in dermis Duct leads to sweat pore on surface Secreted watery sweat for cooling Apocrine: Located only in axillary, pubic, anal regions Larger than eccrine glands Duct opens into opening of hair follicle Secretes thicker sweat, high content of proteins and fats. Contains specific scent molecules: sexual, fear, etc.

Sweat is usually 99% water with a pH between 4 and 6

Sweat glands produce 500ml of insensible perspiration (no noticable wetness)daily

Diaphoresis—sweating with wetness (up to 1 l per hr when exercising or in heat)

Two specially modified sweat glands:

Ceruminous—found in the external ear canal. Secretion combines with sebum and dead epidermal cells to form earwax (keeps eardrum pliable, canal waterproof and has a bactericidal effect)

Mammary --milk producing glands found in the female breast (modified apocrine glands)

Sebaceous (oil) glands:

- Branched tubular glands

- Duct opens into opening of hair follicle

- Secretes sebum, consisting of lipids, proteins, ions, carbohydrates,

Nails:

- Tips of fingers and toes - Thick layer of densely packed keratinocytes - Produced by nail matrix at proximal end, hidden under eponychium or cuticle

Deeper layers of epidermis = nail bed

Average growth:0.5 mm per week

Nails

• Flat nails allow for fleshy, sensitive fingertips• Serve as tools for digging, grooming and manipulation• Useful in medical diagnosis

– Iron deficiency—flat or concave– Long term hypoxia (oxygen deficiency)—clubbed

Carcinomas (cancers) of the skin:

Basal Cell Carcinoma: Keratinocytes of stratum basale proliferate, invade dermis Relatively low malignancy

Squamous Cell Carcinoma: Keratinocytes of epidermis proliferate May or may not invade dermis Moderate malignancy

Malignant Melanoma: Melanocytes of epidermis proliferate, invade dermis Highly malignant

Burns

• Leading cause of accidental death• Classified by depth of tissue involvement• 1st degree—involves epidermis only• 2nd degree—involves epidermis and some dermis• 3rd degree—involves epidermis, dermis and some

deeper tissue are destroyed• 1st and 2nd degree burns are considered partial thickness

burns, and 3rd degree is a full thickness burn