Chapter 6

Learning Targets:By the end of this lecture, you should be able to:1.List functions of skin.2.Describe the various layers of the skin.3.Explain the physiology behind skin and hair color.4.List and describe the various accessory organs of the skin.

Organs are body structures composed of two or more different tissues.

The skin and its accessory organs make up the integumentary system.

Skin is also known as the cutaneous membrane

Functions of the skin include:

Sensation (housing sensory receptors)

Heat regulation (maintains homeostasis)

Absorption (certain chemicals/ medications)

Protecting underlying tissues

Excreting wastes

Synthesizing certain chemicals & vitamins

The skin consists of: Epidermis

Epithelial tissue

Dermis Connective tissue Epithelial tissue Smooth muscle tissue Nervous tissue Blood

Subcutaneous layer (hypodermis) Loose connective

tissue Adipose Tissue

Lacks blood vesselsAs cells divide and

grow, older cells are pushed away from the dermis

Farther the cells move, poorer their nutrient supply becomes, and in time, they die and are shed

In humans, it is thinnest on the eyelids at 0.05 mm (0.0020 in) and thickest on the palms and soles at 1.5 mm (0.059 in).

Keratinization - cytoplasm of older cells fills with keratin protein (tough and waterproof) as they move to the outer layers

Normally, production of epidermal cells is balanced with the loss of dead cells (so skin doesn’t wear away)

Where skin is rubbed or pressed regularly, cell division increases… causes calluses

Life span of a typical epidermal cell is about 2-4 weeks

Melanocytes - specialized cells in epidermis produce melanin

Melanin - a dark pigment that absorbs UV radiation & prevents mutations in DNA of skin cells

Melanin is produced because of UV light exposure

Melanocytes found in deepest portion of epidermis, cellular extensions pass between epidermal cells

Largely due to melanin

All people have the same number of melanocytes

Differences result from amount of melanin produced and size of pigment granules

More pigment located near equator to protect from UV radiation

As your body is exposed to UV light, your melanocytes produce more melanin to block the UV light.

The melanin moves outside of the melanocytes to protect you, tanning your skin or making it freckle in the process.

Sunburn results when your skin cannot produce melanin quickly enough to prevent UV rays from injuring blood vessels near kin’s surface

Most deadly type of skin cancer

Uncontrolled production of melanocytes

Usually starts on or near a mole

For women, most common on legs; for males, on the back

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Boundary with epidermis is uneven because epidermal ridges project internally

Dermal papillae then extend into spaces

Fingerprints - determined by genes and changes slightly as fetus moves during pregnancy (so identical twins usually don’t have exact same fingerprints)

Composed of dense connective tissue with fibers that give the skin toughness and elasticity

Dermal blood vessels supply nutrients to all skin cells and help regulate body temperature

Nerve cells and sensory receptors also present

Hair follicles, sebaceous (oil-producing), and sweat glands also present

Ink is injected into dermis

Tattoo visible through epidermis

Tattoo wouldn’t work on epidermis because it would shed quickly

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Dermis doesn’t shed like epidermis

It can regenerate when injured

Starts to produce new tissue (has different texture and quality than surrounding tissue)

Will become a scarStretch marks are

caused by rapid growth and tearing of elastic tissue; causes scarring

Binds skin to underlying organs

No sharp boundary between dermis and hypodermis

Adipose tissue for insulation

Blood vessels supply nutrients

NailsProtective coverings on

ends of digitsThe whitish, thickened,

half-moon shaped region (lunula) covers the most actively growing region

Cells here divide and become keratinized; pushes nail plate forward

Thumbnail grows slowest; middle nail fastest

HairPresent on all skin surfaces

except palms, soles, lips, nipples and parts of external reproductive organs

Each hair develops from a group of epidermal cells called hair follicles

As the epidermal cells divide and grow, older cells are pushed up

These cells become keratinized and die (remains are hair)

Hair= dead epidermal cells

A bundle of smooth muscle cells attaches to each hair follicle

called the arrector pili muscle

The muscle is positioned so that a short hair w/in the follicle stands on end when the muscle contracts

If person is emotionally upset or cold, nerve impulses may stimulate the arrector pili muscles to contract, causing goose bumps

Humans have as many hair follicles per square inch of skin as a chimpanzee

All of the 5 million hair follicles form during the 5th month of prenatal development

Perhaps humans evolved to have less hair for hunting(could run for longer times); perhaps to avoid parasites.

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Genes determine hair color by directing amount of pigment melanocytes produce

A lot of melanin= darker hair

No melanin= white

Sebaceous Glands

Associated with hair follicles

Secrete sebum to keep hair and skin soft, pliable and waterproof

Sweat GlandsLined with sweat-

secreting epithelial cells

Eccrine GlandsMost numerousForehead, neck, backProduce sweat

Water, Salt and Waste (Urea and Uric Acid)

Apocrine GlandsResponsible for natural body

odorBecome active at pubertyAxillary regions and groinUsually connect to hair follicles

Modified sweat glandsMammary glands secrete milkCeruminous glands secrete ear wax

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