SkinSkin SkinSkin It completely covers the body & It completely covers the body &

is continuous with the is continuous with the membranes lining body orificesmembranes lining body orifices

It is the largest organ of the It is the largest organ of the body, of 1.5 to 2 m body, of 1.5 to 2 m ²²surface areasurface area

Structure of skinMain Layers: • Epidermis • dermis

Epidermis Composition: st. keratinised sq. ep.Thickness : varies, thickest on the palms &

solesColor: varies from white to black, No blood vessels , but deeper layers are

bathed in interstitial fluid of dermisIt is regenerated continuously every 2-4

weeksLayers ( strata)

Differentiation

- Cells move from basal layer to stratum corneum

1) Synthesis & modification of proteins, especially keratins

2) Appearance of new organelles, reorganization of organelles, loss of organelles

3) Change in cell size & shape

4) Changes in properties of membranes

5) Dehydration

6) Desquamation

• Maintenance of healthy epidermis:1. Desquametion of keratinized cells2. Effective keratinizatrion of cells

approaching surface3. Continual cell division of germinal

layer

Fingerprints • Projection of cells in the dermis

called papillae. They cause ridges of different pattern in every individual, their impression makes Fingerprints

Skin colour1. Melanin 2. Hemoglobin3. Bile pigments & carotenes

Dermis• Composition: mainly of collagen, but

also contains elastic,reticular fibers&muscular tissue

• NB:(facial expressions are due to the skeletal muscle connection to the dermal collagen fibers to produce: smile, frown, eyebrow movement…)

• Striae?

Structures in it:1. Blood & lymph vessels2. Sensory nerve endings3. Sweat gland & their

ducts4. Hairs, arrector pili m &

sebaceous glands

Functions Pressure detection;

metabolism (duplication ofcells)…

Dermis

Sweat glandsMerocrineApocrine

•Found in thick skin•Simple coiled tubular•Ducts are long & run zigzag to open on the surface of epidermis•cool the body

•Found in thin skin of axilla, pubic region, found only near hair folliclesand respond to stress andsexual arousal•Large sweet glands•Start function at puberty•Produce milky secretion fatty acids→ support bacteria →body odor

Sebaceous Glands

• Oily secretion called sebum that contains broken-down cells

• anti-bacterial properties• flexibility of cutaneous

membrane

Ceruminous Glands

• secretion combines with sebum to produce earwax

• waterproof• keeps eardrum flexible• bitterness repel mites &• other pests

Mammary Glands

• modified apocrine sweat gland• thicker secretions containing

proteins and fats• released by ducts that open at

the nipple

Hypodermis• A sub dermal layer of adipose tissue or otherwise calledsubcutaneous fat; which is made up of loose, fibroustissue, rich in blood vessels, lymphatic vessels andnerves;• The base of hair follicles and the coiled tubes of sweatglands may also project down into the hypodermis;• This is the layer that pads the body, serves as an energyreservoir and provides thermal insulation, (it isdifferently distributed in females vs males)

Functions of Hairs • Sensation• Communication• Facial, pubic and axillary hair• Eyebrows• Protection• Barrier• Guard hairs in nose• Eyebrows• Thermoregulation

Different body hair types

• lanugo -- fine, unpigmented fetal hair

• vellus -- fine, unpigmented hair of children and women

• terminal hair -- coarse, long, pigmented hair of scalp,

• pubic and axillary regions

Structure of Hair and Follicle

• Hair is filament of keratinized cells• shaft is visible above skin; root is below

within follicle• Follicle is oblique tube within the skin• bulb is swelling in base where hair

originates• vascular tissue (papilla) in bulb

provides nutrients

• Epithelial root sheath is an extension of the epidermis (lies next to hair root)

• Connective tissue root sheath is derived from the dermis (surrounds it)

• Hair receptors entwine each follicle

• Piloerector muscles : smooth muscles that cause goose bumps

Nails

• Clear, hard derivative of stratum corneum

densely packed cells filled with hard keratin

• Flat nails allow for fleshy, sensitive fingertips

Functions of skin1. Protection 2. Regulation of body temp3. Formation of vit. D4. Cutaneous sensation5. Absorption6. Excretion 7. Psychological and social functions• appearance and social acceptance• facial expression and nonverbal communication

Vitamin D Production

Sensory functions•. It is equipped with a variety of nerve endings that react to heat, cold, touch, texture, pressure, vibration, and tissueinjury (pain);

• The sensory receptors are specially abundant on theface, palms, fingers, soles, nipples and genitals. thereare relatively few on the back, and joint areas;

• Some receptors are naked dendrites that penetrate intothe epidermis, and most others are limited to the dermisand hypodermis, where specialized connective tissuesgive the nerve cells more selective sensitivity toparticular stimuli.

Types of sensory nerve endings

Free nerveendings

Merkel’s(tactile) disks

hair folliclereceptors

Meissner’s(tactile)corpuscles

Ruffiniendings

Pacinian(Lamellated)corpuscles

Dermis,around hairfollicle

Dermis(papillae offingertips,palms, lips,eyelids)

Dermis,hypodermis

Dermis,hypodermis

Light touch,texture,vibration (20-40 Hz range

Deep touch,Pressure,vibration (150-300 Hz )

Thermoregulation

• thermoreceptors and sweat glands

• hypothalamus controls cutaneous arteries and sweat glands to

• retain or dissipate heat

Mechanisms of heat loss

• Evaporation: insensible & sweat• Radiation• Conduction• Convection

Control of body temp• Nervous control1. Temperature regulation center

(hypothalamus)2. Vasomotor center

Effect of vasodilatation & vasoconstriction

Activity of sweat glands• Evaporation of sweat from body

surface takes heat from the body core.

Fever • Caused by pyrogens• They act on hypothalamus by

releasing prostaglandins which reset the thermostat to a higher temperature

• Activating heat promoting mechanism