SKIN

Skin is the largest organ of our bodyforming the outermost protective layer,covering the whole body. It accounts15% of total body. It is an impermeablebarrier.

The skin along with its derivativescollectively constitutes integumentarysystem of the body.

FUNCTIONS OF THE SKIN

• Protection

• Sensation

• Temperature regulation

• Storage of food

• Excretion

• Synthesis of vitamin D

• Grip

STRUCTURE OF SKIN

HUMAN SKIN

EPIDERMIS• Outermost thin layer.

• Originates from ectoderm of embryo.

• Formed from stratified epithelium piled up layer after layer.

• Shows three region-

1. Stratum corneum(outermost cornified layer)

2. Granular layermiddle part)

3. Malphigian layer(inner germinative layer.)

STRATUM CORNEUM

• Outermost layer, consisting of piled up flattened dead cells.

• Made up of fibrous structural protein called keratin.

• Acts as barrier to pathogen, mechanical damage, and reduce water loss through evaporation.

• Continuously replaced by newly formed cells of Malpighian layer.

GRANULAR LAYER

• Very thin middle layer.

• Consists of two to three sub layers of flattened living cells.

• Keeps on giving new cells on the outer side towards the cornifiedlayer.

MALPIGHIAN LAYER• Innermost layer, formed of columnar

living cells.

• Cells divide constantly to form new cells, pushed upwards continuously.

• Pigment melanin is present in these cells, which acts as a protective screen against ultraviolet light.

• Content of these skin soon gets converted into tough keratin.

Difference between keratin and melanin

• keratin is (protein) a protein which hair and nails are comprised of while melanin is any of a group of naturally occurring dark pigments, especially the pigment found in skin, hair, fur, and feathers.

• Both keratin and melanin are important in the skin, but have different roles. The keratin is basically tough, and helps keep the skin nice and tough so it can cope with all the bumps it gets all the time. Melanin on the other hand helps protect your skin from radiation damage (like sunlight). Melanin is pretty cool, as it gets made by special cells in the skin and forms a kind of hat over the nucleus to shield the DNA inside it from the sun. The more sunlight you get, the more melanin the skin forms to protect itself, which is why you get a sun tan.

LEUCODERMA• Leucoderma is particularly known by the

presence of white patches which could be localized to smaller areas in the beginning. . These white patches are the result of the partial or complete loss of the skin pigmentation. However, with the passage of time, the skin patches might get enlarged. Most of the major skin changes might become noticeable around the age of 10 to 30 years. Such skin color changes are more noticeable in people with darker skin complexions.

LEUCODERMA

ALBINISM

• Albinism is a congenital disorder characterized in humans by the complete or partial absence of pigment in the skin, hair and eyes. Albinism is associated with a number of vision defects, such as photophobia, nystagmus, and amblyopia. Lack of skin pigmentation makes for more susceptibility to sunburn and skin cancers.

ALBINISM

DERMIS

• This layer lies just below the epidermis.

• It originates from the mesoderm of the embryo.

• This layer is 2-3 times thicker than the epidermis. It is made up of thick elastic connective tissue.

• The dermis consists of the following structures-blood vessels, nerve fibres, sensory organs, sweat glands, touch receptors, hair follicles, etc.

FUNCTION OF DERMIS

• Collagen and fibres make the dermis tough and flexible.

• Blood capillaries provide nourishment and oxygen and removes excretory products.

• Blood capillaries help in regulating body temperature.

• Sensory corpuscles and nerve endings make it sensitive to sensations.

• Subcutaneous fat store food reserves and insulates the body against heat loss

DERIVATIVES OF SKIN

• The derivatives of skin are;

• Hair

• Glands

• Nails

HAIR• Hairs are the cornified epidermal outgrowths.

They develop from the cells of the Malpighianlayer but remain embedded deep in the dermis.Each hair is divisible into three parts-

• Hair shaft

• Hair root

• Hair bulb

STRUCTURE OF HAIR

GOOSE FLESH

• Due to extreme cold or fright, thearrector muscle present in thedermis pull the hair into a verticalposition. It is called goose flesh

NAILS• Nails are the hard, keratinized plate like

structure, present at tip of the fingers and toe.They are derived from epidermis and form a protective cover over the finger tips

• .A nail is divided into three parts-

1. Nail plate,

2. Nail bed and

3. Nail matrix.

NAIL

SEBACEOUS GLANDS

• These are glandular outpushings of the follicle wall.These are branched alveolar glands, associated with the hair follicles,open into them or even directly to the outside.They give out oily secretion called sebum,containingfatty acids, waxes and steroids. It spreads over the hair and skin and make them oily and waterproof

MODIFIED FORMS OF SEBACEOUS GLANDS-

• ACNE

• PIMPLES

• WHITE HEADS

• BLACK HEADS

• FRECKLES.

ACNE

PIMPLES

BLACK HEADS

FRECKLES

SWEAT GLANDS

• Simple coiled tube situated deep in the dermis.

• Derived from the Malpighian layer.

• It opens out by a long spiral duct.

• Its opening is called sweat pore, surrounded by a network of blood capillaries.

• The sweat glands bring about excretion and also cause cooling of the skin through evaporation of sweat.

TYPES OF SWEAT GLANDS

• ECCRINE SWEAT GLAND: These are distributed over the entire body surface. They produce sweat and open on the surface of skin.

• APROCRINE SWEAT GLAND: They are located in the axillary (ex-armpits) and genital areas. They open in hair follicles and release the sweat in the cavity of follicles. Therefore sweat produced by these glands mixes with the sebum.

SWEAT• Sweat is watery secretion from the sweat

glands.It contains about 99% water, 0.2-0.5% salt(Nacl) and traces of urea. Because of Naclsweat is salty.

• Sweating normally occurs during summer to cool off body heat but sweating also occurs due to nervousness, fright and weakness. This is called cold sweating or cold sweat. This sweat is often accompanied with nausea and pain.

MAMMARY GLANDS• These mammary glands are modified sebaceous glands

• Rudimentary in male and well developed and functional infemales.

• They produce milk after the birth of a child to child to nourishthe new born.

• Each mammary gland is contained in a breast and is formedof a cluster of 15 to 20 lobes of glandular tissue.

• Each breast has a conical projection called nipple, whichreceives 15 to 20 milk ducts.

• The functioning of mammary glands is production of milkwhich is related to the reproductive hormones-prolactin.

SKIN AND HEAT REGULATION OF BODY

• Some of the living animals are able to keep their body temperature constant between 35-40 degree centigrade. They are called warm blooded or homeothermals or endothermal.

• On the other hand, some animals are unable to keep the body temperature constant. They are called cold-blooded or poikilothermal or ectothermal animals.Cold blooded animals undergo hibernation during winter season and aestivation during summer season.

SOURCES OF HEAT PRODUCTION-

• Chemical reactions occurring in body cells.

• Vigorous activities.

• Ingestion of hot foods and beverages.

SOURCES OF HEAT LOSS-

• Skin-By convection, conduction, radiation and through evaporation of sweat.

• Lungs- During exhalation. Some heat is lost during vaporization of water through lungs.

• Urine and sweat.

• Food-when cold food, water, or cold beverages are taken into the body.

In cold weather

• Surface blood vessels contract. Blood capillaries lie in the skin far from the surface.

• Sweat glands stop producing sweat.

• Hair pulled up the contraction of arrectormuscles so layer of air trapped against the skin surface is thicker. This provides insulation.

• Shivering occurs by rhythmic contraction of skin muscles which produces heat.

In hot weather• Surface blood vessels relax. Blood capillaries in

the skin lie near to the surface.

• Sweat glands produce more sweat, which cools the skin as it evaporates from the surface.

• Hair drop down against skin surface as arrector muscles relax so less air is trapped for insulation.

• No shivering.

• Vasodilation- The blood vessels (arterioles) in the skin dilate. This ensures an increased blood flow through them. Thus, excess heat is given up by the blood to the environment by radiation.

• Vasoconstriction- The blood capillaries near the surface of the skin become narrow so that less blood flows through them and less heat is lost from the blood to the surroundings.