SKIN→ The skin, the body’s largest organ, is the outer body covering ofan animal.→ It forms a barrier that helps prevent harmful microorganismsand chemicals from entering the body, and it also prevents theloss of life-sustaining body fluids.→ It protects the vital structures inside the body from injury andfrom the potentially damaging ultraviolet rays of the sun.→ The skin also helps regulate body temperature, excretes somewaste products, and is an important sensory organ.→ Both delicate and resilient, the skin constantly renews itself andhas a remarkable ability to repair itself after injury.
Structure of the SkinThe skin consists of an outer, protective layer (epidermis) and an inner, livinglayer (dermis). The top layer of the epidermis is composed of dead cellscontaining keratin, the horny protein that also makes up hair and nails
MAJOR LAYERS of the SKIN●EPIDERMIS →The outer layer of skin which is only a few cells thick;it contains pigments and pores, and its surface is madeof dead cells that it sheds from the body.●DERMIS→ the thick sensitive layer of skin or connective tissuebeneath the epidermis
KERATINOCYTESAbout 90% of the cells in the epidermis arekeratinocytes, produces a tough, fibrous proteincalled keratin. This protein is the main structuralprotein of the epidermis, and it provides many ofthe skin’s protective properties.
The old keratinocytes at the upper surface of theskin constantly slough off. Meanwhile, cells in thelower layers of the epidermis divide continually,producing new keratinocytes to replace thosethat have sloughed off. By the time the cellsreach the uppermost layer of the epidermis, theyare dead and completely filled with the toughprotein.
MELANOCYTESScattered among the keratinocytes in theepidermis are melanocytes, cells that produce adark pigment called melanin. This pigment givescolor to the skin and protects it from the sun’sultraviolet rays.
Differences in skin color result from differences inthe amount of melanin produced and howmelanosomes, packets of melanin, are arrangedin the keratinocytes.
Particularly in people with light skin, melaninsometimes accumulates in patches, formingfreckles, age spots, or liver spots.
A type of melanincalled pheomelaninmakes redheadedpeople more sensitiveto the sun.
A total lack ofmelanin, a geneticcondition calledalbinism, makespeople extremelysensitive to the sun.Peoplewith albinismhave very light skin,hair, and eyes.
LANGERHANS CELLSThe epidermis also contains a type of immunecell known as a Langerhans cell. Produced in thebone marrow, Langerhans cells take up sentrylikepositions in the epidermis, where they help cellsof the immune system recognize potentiallydangerous microorganisms and chemicals.
MERKEL CELLS Another cell in the epidermis is the Merkel cell,found in sensitive, hairless areas such as thefingertips and lips. Located in the deepest layer ofthe epidermis, Merkel cells contact nerve endingsin the dermis below and function as a type oftouch receptor.
COLLAGENThe main structural component of the dermis is aprotein called collagen. Bundles of collagenmolecules pack together throughout the dermis,accounting for three-fourths of the dry weight ofskin. Collagen is also responsible for the skin’sstrength.
ELASTINAnother protein in the dermis, elastin, is the maincomponent of elastic fibers. These proteinbundles give skin its elasticity. Collagen andelastin are produced by cells calledfibroblasts, which are found scattered throughoutthe dermis.
Papillary LayerThe upper part of thedermis is known as thepapillary layer. It ischaracterized by dermalpapillae, tiny, fingerlikeprojections of tissue thatindent into theepidermis above.
In the thick skin on the palms and soles, theepidermis conforms to the shape of theunderlying dermal papillae, forming ridges andvalleys that we know as fingerprints. These ridgesprovide traction that helps people grasp objectsand surfaces.
Some dermal papillae contain touch receptorscalled Meissner’s corpuscles, and many containloops of tiny blood vessels. The extensive networkof blood vessels in the dermal papillae plays animportant role in the regulation of bodytemperature.
The blood vessels dilatein hot environments tohelp dissipate heat, andthey constrict toconserve heat in coldenvironments.
The lower layer of thedermis is called thereticular layer. It is madeprimarily of coarsecollagen and elasticfibers.
Skin appendages such asglands and hair folliclesare often anchored inthe reticular layer of thedermis.
The reticular layer also contains several differenttypes of sensory receptors, nerve cells specializedto detect various stimuli.
HAIRHair is composed primarily of keratin. The deadkeratinocytes fuse together to form the hair. Atthe base of the follicle is the bulb, which containscells that give rise to the keratinocytes that makeup the hair, as well as blood vessels that nourishthe growing hair.
Lengthening fibers of keratin-filled dead cells, grouped around the semi hollowmedulla, make up the cortex. A living structure called the bulb (visible as a whitelump at the end of a plucked hair) surrounds and feeds the root, which lies in apocket of the epidermis called the follicle.
Each hair follicle also contains the arrector pili, amuscle that contracts in response tocold, fright, and other emotions. When themuscle contracts, it pulls the hair in the follicleinto a vertical position.
This response may help some mammals keepwarm or look bigger to frighten or intimidate theirenemies.
But in humans, because of our sparse coat ofbody hair, it merely produces “goose bumps.”
The color of hair is due to melanin. Dark haircontains true melanin like that found in the skin.
Blond and red hair result from types of melaninthat contain sulfur and iron.
Hair goes gray when melanocytes age and losethe enzyme necessary to produce melanin.
White hair occurs when air bubbles becomeincorporated into the growing hair.
NAILSNails on the fingers andtoes are made of hard,keratin-filled epidermalcells. They protect theends of the digits frominjury, help us graspsmall objects, andenable us to scratch.
The part of the nail that is visible is called the nailbody or nail plate, and the portion of the nailbody that extends past the end of the digit iscalled the free edge. The nail groove is the fold onthe side and the eponychium is the skin that holdsthe nail root.
Most of the nail bodyappears pink because ofblood flowing in thetissue underneath. Thepale, semicircular areacalled the lunula appearswhite due to anunderlying thick layer ofepidermis that does notcontain blood vessels.
The part of the nail that is buried under the skin iscalled the root. Nails grow as epidermal cellsbelow the nail root and transform into hard nailcells that accumulate at the base of the nail,pushing the rest of the nail forward.
GLANDS●Sweat GlandsAn adult human has between 1.6 to 4 millionsudoriferous glands, or sweat glands. Most are of a typeknown as eccrine sweat glands, which are found almostall over the surface of the body and are most numerouson the palms and soles. Eccrine sweat glands begin deepin the dermis and connect to the surface of the skin by acoiled duct.
In addition, nerve fibers that encircle the sweatglands stimulate the glands in response to fear,excitement, or anxiety.
Other sweat glands, known as apocrine sweatglands, are much less numerous than eccrinesweat glands. They are also anchored deep in thedermis, but open into hair follicles rather thanonto the surface of the skin.
Apocrine sweat glands are located mainly in thearmpit, genital area, and around the nipples ofthe breasts.
● Oil GlandsOil, or sebaceous, glands are found all over the bodyexcept on the palms, the soles, and the top of the feet.They are most numerous on the face and scalp. Mostsebaceous glands open into hair follicles, but the glandsalso occur in some hairless areas, such as the lips andinside the mouth.
Glands of this typeproduce an oilysubstance called sebum,which keeps the skin andhair from drying out andinhibits the growth ofcertain harmful bacteria.
● Wax GlandsWax, or ceruminous, glandsare located in the ear canal.They secrete a waxysubstance that helpsprevent foreign particlesfrom entering the ear.Ceruminous glands aremodified sweat glands.
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