Chapter 5 Integumentary System
The skin is one of the largest and heaviest organ of the body. • In an average adult, • the skin covers about 2 square meters (22 square feet) • weights 4.5-5 kg (10-11 lb) and • varies in thickness (1.5 to 4.0 mm) depending on what part of the body it covers.
The skin covers all of the exposed surface of the body and is continuous with the mucous membranes lining openings into the body surface, such as those of the digestive, respiratory and urogenital systems.
The skin and its appendages(hair, nails, sebaceous and sweat glands) make up a very complex set of organs that accomplishes several functions: • Protection: barrier against the outside • Protection against dehydration • Body Temperature Regulation • Cutaneous Sensation • Metabolic Functions • Blood Reservoir • Excretion
The skin consists of 2 layers, • 1 - the outer epidermis, derived from the embryonic ectoderm and • 2 - the inner dermis which develops from the mesoderm.
The appendages of the skin such as hair, nails, sebaceous glands and sweat glands develop from the embryonic epidermis. • Beneath the dermis is the hypodermis (=subcutaneous tissue = superficial fascia) which anchors the skin loosely to underlying tissues (mainly muscles). The hypodermis is NOT part of the skin
THE EPIDERMIS • The epidermis is a keratinized stratified squamous epithelium consisting of four distinct cell types and five distinct layers. • Its thickness varies: 0.07-0.12 mm over most of the body to 0.8 mm on the palms and 1.4 mm on the soles.
THE EPIDERMIS • Its surface layer, consists of dead cells, rich in keratin, a protein which renders it dry and more or less waterproof, allowing it to resist surface evaporation and preventing excessive water loss. • It also serves as a protective barrier against ultra-violet light, bacteria, many chemicals and abrasion.
The epidermal cells are: • 1 - keratinocytes • 2 - Merkel cells • 3 - melanocytes • 4 - Langerhans cells
1 - the keratinocytes: • compose most of the epidermis. They produce a protein: the keratin that helps waterproof the skin and that protects the skin and the underlying tissues from heat, microbes, abrasion and chemicals.
2 - the Merkel cells: • sensory receptors (touch). They associate with a disc-like sensory nerve ending to form the Merkel disc.
3 - the melanocytes • synthesize the melanin pigments which protect the skin against ultraviolet damage.
4 - the Langerhans cells • macrophages used in the defense against microorganisms.
The epidermis consists of several layers of cells. From the deepest to the most superficial: • 1 - the stratum basale (or stratum germinativum) • 2 - the stratum spinosum • 3 - the stratum granulosum • 4 - the stratum lucidum • 5 - the stratum corneum
1 - the stratum basale (or stratum germinativum): • is the deepest epidermal layer. It consists of one row of cuboidal to columnar shaped cells. Those cells are mostly stem cells that divide rapidly to produce new keratinocytes which push up toward the surface and become part of the more superficial layers
Ten to 25% of cells found here are melanocytes: their long branching processes extend in between epidermal cells and reach into the more superficial Stratum spinosum. • Melanocytes synthesize the pigment melanin.
Melanin passes on from the melanocytes into the keratinocytes and will protect them from the destructive effect of ultraviolet radiation. • Merkel cells are also found in this layer.
2 - the stratum spinosum • is 8-10 layers thick. • The keratinocytes contain thick bundles of intermediate filaments (tonofilaments) made of a tension-resisting protein.
Scattered amongst keratinocytes are the Langerhans'cells that are most abundant in this layer.
Stratum germinativum and stratum spinosum • are immediately adjacent to the dermis and contain the only epidermal cells that receive adequate nourishment (by diffusion of nutrients from the dermis).
As the daughter cells are pushed upward, away from the source of nutrition, they gradually die and their soft protoplasm becomes keratinized (hard).
3 - the stratum granulosum • here the keratinization process begins and the cells begin to die. • This layer is called granulosum because the cells contain granules of the precursor of keratine.
4 - the stratum lucidum • is only found in thickened areas of the epidermis such as the sole of the feet. • The cells appear clear (luci means clear) because of an accumulation of keratin precursor. The cells begin to degenerate.
5 - the stratum corneum • is the outermost layer composed of dead, flat, keratinized cells which are being sloughed off (they are your dandruff and the flakes that slough off dry skin. Corneum means horn: hard dead cells
An easy trick to remember these 5 epidermal layers in order from the most superficial to the deepest: "Can Little Girls Speak German" • Can Corneum • Little Lucidum • GiRl GRanulosum • Speak SPinosum • GERMan GERMinativum
THE DERMIS • The average thickness of the dermis is about 1 to 2 mm and varies from 0.6 mm on the eyelids to 3 mm or more on the soles and palms. It contains lots of nerve fibers, sensory receptors, blood and lymphatic vessels as well as the hair follicles, and the sebaceous and sweat glands. • The dermis consists of two layers:
1 - the Papillary layer • is the outer layer closest to the epidermis. It is composed of areolar loose connective tissue. Its superior region contains fingerlike projections called dermal papillae (also give the layer its name: papillary) that indent the epidermis.
Dermal papillae contain capillaries, nerve endings (pain receptors), Meisner corpuscles (touch receptors: make us feel light touching) and the disc-like sensory nerve endings of the Merkel disks.
2 - the Reticular layer • is the deeper layer and the thickest (4/5th of the dermis). It is made of dense irregular connective tissue containing thick bundles of interlacing collagen fibers and some coarse elastic fibers that run in several directions (but mostly in directions that are parallel to the skin surface).
The collagen fibers in the reticular region provides the skin with strength and extensibility (= ability to stretch) and • The elastic fibers provide its elasticity (= ability to return to the original shape after stretching).
The reticular layer is richly supplied with blood vessels and nerves, and contains sensory endings for touch (Pacinian corpuscle for sensing deep pressure such as bumps), pain, heat, cold, etc.
The epidermis projects down into the dermis to form sweat glands, sebaceous glands and hair follicles.
THE SKIN APPENDAGES • Organs such as hair, sweat glands and sebaceous glands that develop from the embryonic epidermis are labeled appendages of the skin or epidermal derivatives. Nail and the enamel of your teeth also derive from the epidermis.
HAIR • The primary function of hair is protection: in human, hair protect the scalp from injury, from the sun ray and decrease heat loss. Eyebrows and eyelashes protect the eye from foreign particles. Hair in the nostrils filter the air we inhale and hairs in the external ear canal also prevent foreign particles to penetrate there. • Hair are made of dead keratinized cells.
The two regions of a hair are: • 1 - theshaft • 2 - the root
1 - the shaft • It is the portion of the hair that projects from the surface of the skin. • The shape of the shaft determines the curliness of your hair: • round shaft -> straight hair • oval shaft -> wavy/curly hair • flat shaft -> kinkyhair
2 - the root • It is the portion of the hair embedded in the skin. It extends from the epidermal surface into the dermis but in the scalp, it can extend into the hypodermis
The hair (shaft and root) is made of three tubes that fit into each other. • The three tubes are made of keratinized cells.
The outer tube is the cuticle: • it is made of one layer of heavily keratinized cell.
The middle tube is the cortex: • It contains several layers of cells. • The cells contain pigments in dark hair and air bubbles in white hair.
The inner tube is the medulla: • it is composed of 2 or 3 rows of cells containing pigments and air spaces.
The root of the hair is contained inside a bag called the follicle (follicle means bag). • The base of this bag expends forming the hair bulb.
An extension of dermal tissue called the papilla protrudes inside the hair bulb and contains capillaries that supply nutrients to the growing hair. Sensory nerve endings called the root hair plexus surround the bulb.
The wall of this bag is made of two layers: • the external layer is the connective tissue root sheath derived from the dermis and • The internal layer is the epithelial root sheath derived from the epidermis.
Associated with each hair follicle is a tiny band of smooth muscle: the arrector pili. This band of smooth muscle is attached at one end to the papillary layer of the dermis and at the other to the connective tissue root sheath of the follicle.
Contraction of the arrector pili in hairy mammals raises the hair, thus increasing the thickness of its coat. This provide increased protection against heat loss in cold weather or increase in body size to intimidate potential adversaries. In human, contraction of these muscles causes only "goose pimples".
SEBACEOUS GLANDS • or oil glands are simple branched areolar glands. • They secrete the sebum (seb = oil) an oily product. • Sebum is usually secreted into a hair follicle but in a few regions of the body (lips and mammary papilla for example) they are directly secreted onto the skin surface.
Sebum is a natural skin cream: it helps hair from becoming brittle, prevents excessive evaporation of water from the skin, keeps the skin soft and contains a bactericidal agent that inhibits the growth of certain bacteria
Sebaceous glands are scattered all over the surface of the skin except in the palms, soles and the side of the feet.
SWEAT GLANDS • or sudoriferous glands are simple coiled tubular glands. They are divided into two principal types: eccrine and apocrine.