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Integumentary System

Integumentary system

The human integumentary system is made up of the skin, hair, nails, and associated glands. Its main function is to protect the body. It prevents excessive water loss, keeps out microorganisms that could cause illness, and shields the underlying tissues from external damage.

The skin helps to regulate body temperature. If heat builds up in the body, sweat glands in the skin produce sweat, which evaporates and cools the skin. When the body overheats, blood vessels in the skin expand and bring more warm blood to the surface, where it cools. When the body gets too cold, the blood vessels in the skin contract, leaving less blood at the body surface, and its heat is conserved.

In addition to temperature regulation, the skin serves as a minor excretory organ. Sweat removes small amounts of wastes produced by the body. These wastes include salts and urea (a chemical compound of carbon, hydrogen, nitrogen, and oxygen). The skin also functions as a sense organ since it contains millions of nerve endings that detect heat, cold, pain, and pressure. Finally, the skin produces vitamin D in the presence of sunlight and renews and repairs damage to itself.

In an adult, the skin covers about 21.5 square feet (2 square meters), and weighs about 11 pounds (5 kilograms). Depending on location, the skin ranges from 0.02 to 0.16 inch (0.5 to 4.0 millimeters) thick. Its two principal parts are the epidermis (the outer layer) and dermis (thicker inner layer). A subcutaneous (under the skin) layer of adipose or fatty tissue is found below the dermis. Fibers from the dermis attach the skin to the subcutaneous layer, and the underlying tissues and organs also connect to the subcutaneous layer.

The epidermis

Ninety percent of the epidermis, including the outer layers, contain cells that produce keratin, a protein that helps waterproof and protect the skin. Keratin is also the major protein found in nails and hair. Pigment cells called melanocytes produce melanin, a brown-black pigment that gives color to the skin and absorbs and reflects the Sun's harmful ultraviolet rays.

Words to Know

Calluses: Abnormal thickenings of the epidermis.

Dermis: Thicker layer of skin lying below the epidermis.

Epidermis: Thinner outermost layer of the skin.

Keratin: Insoluble protein found in hair, nails, and skin.

Melanin: Brown-black pigment found in skin and hair.

Subcutaneous layer: Layer of fatty tissue found beneath the skin.

In most areas of the body, the epidermis consists of four layers. The epidermis on the soles of the feet and palms of the hands has five layers, since these areas receive a lot of friction. Calluses, abnormal thickenings of the epidermis, can occur on any area of the skin where there is irritation or constant pressure. The uppermost layer of the epidermis consists of about 25 rows of flat dead cells that contain keratin. At the skin surface, dead cells are constantly shed.

The dermis

The dermis is thick in the palms and soles, but very thin in other places, such as the eyelids. The dermis is composed of connective tissue that contains protein fibers (called collagen) and elastic fibers. It also contains blood and lymph vessels, sensory nerves, and glands. Sweat glands are embedded in the deep layers of the dermis. Their ducts pass through the epidermis to the outside and open on the skin surface through pores.

Hair and hair roots also originate in the dermis. Hair shafts (containing the bulb of hair) extend from the hair root through the skin layers to the surface. Attached to the hair shaft is a sebaceous gland, which produces an oily substance called sebum. Sebum softens the hair and prevents it from drying. If sebum blocks up a sebaceous gland, a whitehead appears on the skin. A blackhead results if the material oxidizes and dries. Acne is caused by infections of the sebaceous glands. When this occurs, the skin breaks out in pimples and can become scarred.

Nerves in the dermis carry impulses to and from hair muscles, sweat glands, and blood vessels. Bare nerve endings throughout the skin report information to the brain about temperature change (both heat and cold), pressure, and pain.

The Sun and skin

Some skin disorders result from overexposure to the ultraviolet (UV) rays in sunlight. UV rays damage skin cells, blood vessels, and other dermal structures. At first, overexposure to sunlight results in injury known as sunburn. Continual overexposure leads to leathery skin, wrinkles, and discoloration. It can eventually lead to skin cancer, regardless of the amount of melanin in the epidermis. There can be a 10- to 20-year delay between exposure to sunlight and the development of skin cancer.

[See also Cancer ]

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integumentary system

integumentary system: see skin.

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Integumentary System

Integumentary System

Plant integumentary system

Invertebrate integuments

Vertebrate integumentary systems

Human integumentary system

Skin disorders

Resources

The integumentary system includes the skin and the related structures that cover and protect the bodies of plants and animals. The integumentary system of plants includes the epidermis, cuticle, plant hairs, and glands. The integumentary system of invertebrates includes shells and exoskeletons as body covering. The integumentary system of vertebrates comprises skin, scales, feathers, hair and glands. The human integumentary system is made up of the skin that includes glands, hair, and nails. In humans, the skin protects the body, prevents water loss, regulates body temperature, and senses the external environment.

Plant integumentary system

The epidermis is the main surface tissue of young plants and the covering material of all leaves. Usually the epidermis is one cell deep; its cells have thick outer and side walls. On the parts of the plant that are above ground, the epidermal cells secrete an outer waxy cuticle that is water resistant. The thickened cell walls, together with the cuticle, prevent drying out, injury, and fungus infection. The epidermis in aerial parts of the plant gives rise to plant hairs, spines, and glands. In leaves, the epidermis develops guard cells that regulate the size of pores or stomata, which allows the exchange of gases with the atmosphere. Root epidermis lacks the waxy cuticle found in aboveground parts of the plant, allowing it to absorb water from the soil. Root hairs that increase absorbing surface arise from epidermal tissue. When a plant diameter grows, stem and root epidermis is replaced by periderm, which contains cork cells whose walls after cell death provide a protective waterproof outer covering.

Invertebrate integuments

Snails, slugs, oysters, and clams are protected by a hard shell made of calcium carbonate secreted by the mantle, a heavy fold of tissue that surrounds the molluscs internal organs. Spiders, insects, lobsters, and shrimp, have bodies covered by an external skeleton, the exoskeleton, which is strong, impermeable, and allows some arthropods to live on land. The exoskeleton is composed of layers of protein and a tough polysaccharide called chitin, and can be a thick hard armor or a flexible paper-thin covering. Arthropods grow by shedding their exoskeletons and secreting a larger one in a process called molting.

Vertebrate integumentary systems

Keratin, an insoluble protein in the outer layer of the vertebrate skin, helps prevent water loss and dehydration and has contributed to the successful adaptation to life on land. Keratin is also the major protein found in nails, hooves, horns, hair, and wool. Feathers, scales, claws and beaks of birds and reptiles are also composed of keratin.

Human integumentary system

The human integumentary system is made up of the skin, hair, nails, and glands, and serves many protective functions. It prevents excessive water loss, keeps out microorganisms that could cause illness, and protects underlying tissue from mechanical damage. Pigments

called melanin absorb and reflect the suns harmful ultraviolet radiation.

The skin also helps regulate body temperature. If heat builds up, sweat glands produce more sweat, which evaporates and cools the skin. In addition, when the body overheats, blood vessels expand and bring more blood to the skin surface, allowing body heat to be lost. If the body is too cold, on the other hand, blood vessels in the skin contract, putting less blood at the body surface and conserving heat. In addition, the skin serves as a minor excretory organ, since sweat removes small amounts of nitrogenous wastes produced by the body, and functions as a sense organ since it contains millions of nerve endings that detect touch, heat, cold, pain, and pressure. Finally, the skin produces vitamin D in the presence of sunlight, and renews and repairs damage to itself.

In an adult, the skin covers about 21.5 square feet (2 sq m), and weighs about 11 pounds (5 kg). Depending on location, the skin ranges from 0.02-0.16 inch (0.5-4.0 mm) thick. Its two principal parts are the outer layer, or epidermis, and a thicker inner layer, the dermis. A subcutaneous layer of fatty or adipose tissue is found below the dermis. Fibers from the dermis attach the skin to the subcutaneous layer, and the underlying tissues and organs also connect to the subcutaneous layer.

The epidermis

Ninety percent of the epidermis, including the outer layers, contains keratinocytes cells that produce keratin, a protein that helps waterproof and protect the skin. Melanocytes are pigment cells that produce melanin, a brown-black pigment that adds to skin color and absorbs ultraviolet light, thereby shielding the genetic material in skin cells from damage. Touch-sensitive Merkels cells are found in the deepest layer of the epidermis of hairless skin.

In most areas of the body, the epidermis consists of four layers. On the soles of the feet and palms of the hands, where there is a lot of friction, the epidermis has five layers. In addition, calluses (abnormal thickenings of the epidermis), occur on skin subject to constant friction. At the surface, the uppermost layer consists of about 25 rows of flat dead cells that contain keratin.

The dermis

The dermis is made up of connective tissue that contains protein, collagen, and elastic fibers. It also contains blood and lymph vessels, sensory receptors, related nerves, and glands. The outer part of the dermis has fingerlike projections, called dermal papillae, that indent the lower layer of the epidermis. Dermal papillae cause ridges in the epidermis above it, which in the digits give rise to fingerprints. The ridge pattern of fingerprints is inherited, and is unique to each individual.

The dermis is thick in the palms and soles, but very thin in other places, such as the eyelids. The blood vessels in the dermis contain a volume of blood. If a part of the body, such as a working muscle, needs more blood, vessels in the dermis constrict, causing blood to leave the skin and enter the circulation that leads to muscles and other body parts. Sweat gland ducts pass through the epidermis to the outside and open on the skin surface through pores embedded in the deep layers of the dermis.

Hair follicles and roots also originate in the dermis; hair shafts extend from the root through the skin layers to the surface. Also in the dermis are sebaceous glands associated with hair follicles which produce an oily substance called sebum. Sebum softens the hair and prevents it from drying, but if sebum blocks a sebaceous gland, a whitehead appears on the skin. A blackhead results if the material oxidizes and dries. Acne is caused by infections of the sebaceous glands. When this occurs, the skin breaks out in pimples and can become scarred.

The skin is an important sense organ and as such includes a number of nerves that are mainly in the dermis, with a few reaching the epidermis. Nerves carry impulses to and from hair muscles, sweat glands, and blood vessels, and receive messages from touch, temperature, and pain receptors. Some nerve endings are specialized, such as sensory receptors that detect external stimuli. The nerve endings in the dermal papillae are known as Meissners corpuscles, these detect light touch, such as a pat, or the feel of clothing on the skin. Pacinian corpuscles, located in the deeper dermis, are stimulated by stronger pressure on the skin. Receptors near hair roots detect displacement of the skin hairs by stimuli such as touch or wind. Bare nerve endings throughout the skin report information to the brain about temperature change (both heat and cold), texture, pressure, and trauma.

Skin disorders

Some skin disorders result from overexposure to the ultraviolet (UV) rays in sunlight. At first, over-exposure to sunlight results in injury known as sunburn. UV rays damage skin cells, blood vessels, and other dermal structures. Continual overexposure leads to leathery skin, wrinkles, and discoloration and can also lead to skin cancer. Anyone excessively exposed to UV rays runs a risk of skin cancer, regardless of the

KEY TERMS

Chitin Polysaccharide that forms the exoskeleton of insects, crustaceans, and other invertebrates.

Dermis The internal layer of skin lying below the epidermis. It contains the sweat and oil glands, hair follicles, and provides replacement cells for those that are shed from the outer layer.

Epidermis The thinner, outermost layer of the skin. Also the thin outermost covering in plants.

Keratin Insoluble protein found in hair, nails, and skin.

Melanin Brown-black pigment found in skin and hair.

amount of pigmentation normally in the skin. Seventy-five percent of all skin cancers are basal cell carcinomas that arise in the epidermis and rarely spread (metastasize) to other parts of the body. Physicians can surgically remove basal cell cancers. Squamous cell carcinomas also occur in the epidermis, but these tend to metastasize. Malignant melanomas are life-threatening skin cancers that metastasize rapidly. There can be a 10-20 year delay between exposure to sunlight and the development of skin cancers.

Resources

PERIODICALS

Czarnecki, D. 10-Year Prospective Study Of Patients With Skin Cancer. Journal of Cutaneous Medicine and Surgery 6 no. 5 (2002): 427-429.

Fackelmann, Kathy A. Skin Cancers Return: How Big a Threat? Science News (27 June 1992).

Willis, Judith Levine. Acne Agony. FDA Consumer (July-August 1992).

OTHER

Estrella Mountain Community College. The Integumentary System <http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookINTEGUSYS.html> (accessed November 29, 2006).

Skin Deep. video and videodisc. Princeton, NJ: Films for the Humanities and Sciences, 1995.

University of Western Australia, School of Anatomy and Human Biology. Blue HistologyIntegumentary System <http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Integumentary/Integum.htm> (accessed November 29, 2006).

Bernice Essenfeld

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Integumentary System

Integumentary System

Definition

The integumentary system includes the skin and the related structures that cover and protect the body. The human integumentary system is composed of the skin, and includes glands, hair, and nails. The largest organ in the body, the skin protects the body, prevents water loss, regulates body temperature, and senses the external environment.

Description

The integumentary system serves many protective functions for the body. It acts as a mechanical barrier, simultaneously preventing water from entering the body and excessive water loss. It also limits access of microorganisms that could cause illness, and protects underlying tissues from mechanical damage. Pigments in the skin called melanin, give skin its color, and absorb and reflect the sun's harmful ultraviolet radiation.

Function

In addition to serving as a protective barrier, the skin helps to regulate the body temperature by several mechanisms. If heat builds up in the body, sweat glands in the skin produce more sweat that evaporates and cools the skin. When the body overheats, blood vessels in the skin dilate (expand), bringing more blood to the surface, and allowing body heat to dissipate. When the body is too cold, the blood vessels in the skin constrict, shunting blood away from the body surface, thus conserving heat. Along with temperature regulation, the skin serves as a minor excretory organ, since sweat removes small, clinically insignificant amounts of nitrogenous wastes produced by the body. The skin also functions as a sense organ since it contains millions of nerve endings that detect touch, heat, cold, pain and pressure. Finally, the skin produces vitamin D in the presence of sunlight, and renews and repairs damage to itself.

In an adult, the skin covers about 21.5 sq. ft (2 sq. m), and weighs about 11 lb. (5 kg). Depending on location, the skin thickness ranges from 0.02-0.16 in (0.5-4.0 mm). Skin is composed of an outer layer, or epidermis, and a thicker inner layer, the dermis. A subcutaneous layer of fatty or adipose tissue is immediately below the dermis. Fibers from the dermis attach the skin to the subcutaneous layer, and the underlying tissues and organs also connect to the subcutaneous layer.

The epidermis

Ninety percent of the epidermis, including the outer layers, contains keratinocytes, cells that produce keratin, a protein that helps waterproof and protect the skin. Melanocytes are pigment cells that produce melanin, a brown-black pigment that adds to skin color and absorbs ultraviolet light, thereby shielding the genetic material in skin cells from damage. Merkel's cells disks are touch-sensitive cells found in the deepest layer of the epidermis of hairless skin.

In most areas of the body, the epidermis consists of four layers. On the soles of the feet and palms of the hands where there is considerable friction, the epidermis has five layers. Calluses, abnormal thickenings of the epidermis, occur on skin subject to constant friction. At the skin surface, the outer layer of the epidermis constantly sheds the dead cells containing keratin. The uppermost layer consists of about 25 rows of flat dead cells that contain keratin.

The dermis

The dermis is made up of connective tissue that contains protein, collagen, and elastic fibers. It also contains blood and lymph vessels, sensory receptors, related nerves, and glands. The outer part of the dermis has fingerlike projections, called dermal papillae, that indent the lower layer of the epidermis. Dermal papillae cause ridges in the epidermis above it, which in the digits give rise to fingerprints. The ridge pattern of fingerprints is inherited, and is unique to each individual. The dermis is thick in the palms and soles, but very thin in other places, such as the eyelids.

The blood vessels in the dermis contain a volume of blood. When a part of the body, such as a working muscle, needs more blood, blood vessels in the dermis constrict, shifting blood from the skin to supply muscles and other body parts. Sweat glands with ducts that pass through the epidermis and open on the skin surface through pores are embedded in the deep layers of the dermis. Hair follicles and hair roots also originate in the dermis, and the hair shafts extend from the hair root through the skin layers to the surface. The dermis also contains sebaceous glands associated with hair follicles. Sebaceous glands produce an oily substance called sebum. Sebum softens the hair and prevents it from drying, but if sebum blocks a sebaceous gland, a whitehead appears on the skin. A blackhead results if the material oxidizes and dries. Acne pimples are caused by infections of the sebaceous glands.

The skin is an important sense organ and as such includes a number of types of nerves, which are mainly in the dermis, with a few reaching the epidermis. Nerves carry impulses to and from hair muscles, sweat glands, and blood vessels, and receive messages from touch, temperature, and pain receptors. Some nerve endings are specialized such as sensory receptors that detect external stimuli. The nerve endings in the dermal papillae, known as Meissner's corpuscles, detect light touch, or the feel of clothing on the skin. Pacinian corpuscles, located in the deeper dermis, are stimulated by stronger pressure on the skin. Receptors near hair roots detect displacement of the skin hairs by stimuli such as touch or wind. Bare nerve endings throughout the skin supply information to the brain about temperature change (both heat and cold), texture, pressure, and trauma.

Role in human health

Along with its vital roles as shield against micro-organisms and regulating body temperature, skin often provides information about overall health and a variety of medical conditions. The color, texture, temperature, and elasticity of skin can aid in diagnosing a variety of disorders. For example, patients with hepatitis may have a characteristic yellow tinge to their skin. Similarly, cold sores and fever blisters are indications of infection with herpes simplex virus, and warts (intraepidermal skin tumors) result from infection with human papilloma virus (HPV).

Skin testing is an important diagnostic tool in the evaluation of allergies. Skin testing involves a series of superficial injections of one or more suspected allergens. A positive response, such as redness, or inflammation, at the site of the skin test, helps to pinpoint the culprit.

Common diseases and disorders

Acne, caused by clogged pores and bacterial infection, is commonly diagnosed in teenagers and young adults. Acne may be mild, moderate, or severe and is characterized by blackheads, whiteheads, papules, pustules, and cysts on the face, shoulders, chest, and back. Mild acne may be treated with topical antimicrobial agents to kill the bacteria on the skin and topical retinoids to open the pores. Moderate and severe acne often respond to treatment with systemic antibiotics such as tetracycline or doxycycline.

Common bacterial skin infections are impetigo, folliculitis, and cellulitis. Impetigo is a contagious skin infection caused by streptococci or staphylococcus. It produces crusty patches on the skin. Local outbreaks may be treated with antibacterial ointment, and patients with widespread infections are given oral antibiotics. Infectious folliculitis produces erythema (redness) and pustules. It is caused by staphylococcus and treated with oral antibiotics. Cellulitis is swelling, erythema, warmth, and pain caused by infection of the dermis and subcutaneous tissue often near a wound site. Cellulitis is usually caused by Group A streptococci or staph aureus and is treated with a course of anti-strep or anti-staph antibiotics.

Common viral skin disorders include infection with herpes simplex or herpes zoster. Herpes simplex is responsible for cold sores, fever blisters, and lesions on the genitals and buttocks. Herpes zoster produces a painful rash characterized by vesicles. Both conditions are treated with acyclovir, or other orally administered anti-viral agent.

Skin reactions include eczema, allergic contact dermatitis (rashes), such as those resulting from contact with poison ivy, sumac, or oak, and hives. Contact dermatitis, an eruption of itchy skin vesicles, is an allergic skin reaction. Patients are advised to avoid contact with the suspected allergen, and mild cases may be treated with warm soaks and topical ointments to reduce inflammation and soothe inflamed skin.

Cosmetic damage as well as potentially fatal skin disorders may result from overexposure to the ultraviolet (UV) rays in sunlight. At first, overexposure to sunlight results in injury known as sunburn. UV rays damage skin cells, blood vessels, and other dermal structures. Continual overexposure produces leathery skin, wrinkles, and discoloration and may also lead to skin cancer. Anyone excessively exposed to UV rays runs a risk of skin cancer, regardless of the amount of pigmentation normally in the skin. Seventy-five percent of all skin cancers are basal cell carcinomas that arise in the epidermis and rarely metastasize (spread) to other parts of the body. Physicians can surgically remove basal cell cancers. Squamous cell carcinomas also occur in the epidermis, and these may metastasize. Malignant melanomas are life-threatening skin cancers that metastasize rapidly. There can be a 10 to 20 year delay between exposure to sunlight and the development of skin cancers.

Dermatology is the medical specialty devoted to the diagnosis and treatment of skin disorders. In addition to the disorders previously described, primary care physicians and dermatologists are frequently called upon to diagnose and treat the following conditions:

  • alopecia (hair loss)
  • athlete's foot (fungus infection)
  • moles
  • psoriasis (scaly skin on the scalp, trunk, arms and legs)
  • rosacea (symmetrically distributed papules and pustules on the nose and cheeks)
  • scabies (skin infestation with mites that produces inflammatory papules in the wrists, web spaces, and sides of feet)
  • seborrheic dermatitis (facial redness and scaling)
  • spider veins, varicose veins
  • vitiligo (loss of skin color on patches of skin, usually affects the face and extremities)

Today, many dermatologists also provide a range of cosmetic services to reduce the signs of aging, such as wrinkles, sagging skin and discoloration, and reverse some of the effects of sun damage to skin. Microdermabrasion, laser skin resurfacing, and injections of collagen are among the techniques used to improve the appearance of skin.

KEY TERMS

Chitin— Polysaccharide that forms the exoskeleton of insects, crustaceans, and other invertebrates.

Dermis— Thicker layer of skin lying below the epidermis.

Epidermis— Thinner outermost layer of the skin.

Keratin— Insoluble protein found in hair, nails, and skin.

Melanin— Brown-black pigment found in skin and hair.

Resources

BOOKS

Freedberg, Irwin, et al. Fitzpatrick's Dermatology In General Medicine, Fifth Edition. Vols. I, II, and III. New York, NY: McGraw-Hill, 1999.

Odom, Richard B., et al. Andrew's Diseases of the Skin. Philadelphia, PA: W.B. Saunders Company, 2000.

OTHER

Skin Deep. Video and videodisc. Princeton, NJ: Films for the Humanities and Sciences, 1995.

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Integumentary System

Integumentary system

The integumentary system includes the skin and the related structures that cover and protect the bodies of plants and animals. The integumentary system of plants includes the epidermis, cuticle, plant hairs, and glands . The integumentary system of invertebrates includes shells and exoskeletons as body covering. The integumentary system of vertebrates comprises skin, scales, feathers, hair and glands. The human integumentary system is made up of the skin which includes glands, hair, and nails. In humans, the skin protects the body, prevents water loss, regulates body temperature , and senses the external environment.


Plant integumentary system

The epidermis is the main surface tissue of young plants and the covering material of all leaves. Usually the epidermis is one cell deep; its cells have thick outer and side walls. On the parts of the plant that are above
ground, the epidermal cells secrete an outer waxy cuticle that is water resistant. The thickened cell walls, together with the cuticle, prevent drying out, injury, and fungus infection . The epidermis in aerial parts of the plant gives rise to plant hairs, spines, and glands. In leaves, the epidermis develops guard cells that regulate the size of pores or stomata, which allows the exchange of gases with the atmosphere. The epidermis of roots lacks the waxy cuticle found in the parts of the plant above ground, allowing the root epidermis to absorb water from the soil . Root hairs that increase the absorbing surface of the root arise from epidermal tissue. When a plant grows in diameter, the epidermis is replaced by the periderm, in the stem and the roots. The periderm contains cork cells whose walls after cell death provide a protective waterproof outer covering for plants making up the bark of older trees.


Invertebrate integuments

Snails , slugs , oysters, and clams are protected by a hard shell made of calcium carbonate secreted by the mantle, a heavy fold of tissue that surrounds the mollusc's internal organs. Spiders, insects , lobsters , and shrimp , have bodies covered by an external skeleton, the exoskeleton, which is strong, impermeable, and allows some arthropods to live on land. The exoskeleton is composed of layers of protein and a tough polysaccharide called chitin, and can be a thick hard armor or a flexible paper-thin covering. Arthropods grow by shedding their exoskeletons and secreting a larger one in a process called molting.

Vertebrate integumentary systems

Keratin, an insoluble protein in the outer layer of the skin of vertebrates, helps prevent water loss and dehydration and has contributed to the successful adaptation to life on land. Keratin is also the major protein found in nails, hooves, horns, hair, and wool. Feathers, scales, claws and beaks of birds and reptiles are also composed of keratin.


Human integumentary system

The human integumentary system is made up of the skin, hair, nails, and glands, and serves many protective functions for the body. It prevents excessive water loss, keeps out microorganisms that could cause illness, and protects the underlying tissues from mechanical damage. Pigments in the skin called melanin absorb and reflect the sun's harmful ultraviolet radiation . The skin helps to regulate the body temperature. If heat builds up in the body, sweat glands in the skin produce more sweat which evaporates and cools the skin. In addition, when the body overheats, blood vessels in the skin expand and bring more blood to the surface, which allows body heat to be lost. If the body is too cold, on the other hand, the blood vessels in the skin contract, resulting in less blood is at the body surface, and heat is conserved. In addition to temperature regulation , the skin serves as a minor excretory organ , since sweat removes small amounts of nitrogenous wastes produced by the body. The skin also functions as a sense organ since it contains millions of nerve endings that detect touch , heat, cold, pain, and pressure . Finally, the skin produces vitamin D in the presence of sunlight, and renews and repairs damage to itself.

In an adult, the skin covers about 21.5 sq ft (2 sq m), and weighs about 11 lb (5 kg). Depending on location, the skin ranges from 0.02-0.16 in (0.5-4.0 mm) thick. Its two principal parts are the outer layer, or epidermis, and a thicker inner layer, the dermis. A subcutaneous layer of fatty or adipose tissue is found below the dermis. Fibers from the dermis attach the skin to the subcutaneous layer, and the underlying tissues and organs also connect to the subcutaneous layer.


The epidermis

Ninety percent of the epidermis, including the outer layers, contains keratinocytes cells that produce keratin, a protein that helps waterproof and protect the skin. Melanocytes are pigment cells that produce melanin, a brown-black pigment that adds to skin color and absorbs ultraviolet light thereby shielding the genetic material in skin cells from damage. Merkel's cells disks are touch-sensitive cells found in the deepest layer of the epidermis of hairless skin.

In most areas of the body, the epidermis consists of four layers. On the soles of the feet and palms of the hands where there is a lot of friction , the epidermis has five layers. In addition, calluses, abnormal thickenings of the epidermis, occur on skin subject to constant friction. At the skin surface, the outer layer of the epidermis constantly sheds the dead cells containing keratin. The uppermost layer consists of about 25 rows of flat dead cells that contain keratin.


The dermis

The dermis is made up of connective tissue that contains protein, collagen , and elastic fibers. It also contains blood and lymph vessels, sensory receptors, related nerves, and glands. The outer part of the dermis has fingerlike projections, called dermal papillae, that indent the lower layer of the epidermis. Dermal papillae cause ridges in the epidermis above it, which in the digits give rise to fingerprints. The ridge pattern of fingerprints is inherited, and is unique to each individual . The dermis is thick in the palms and soles, but very thin in other places, such as the eyelids. The blood vessels in the dermis contain a volume of blood. If a part of the body, such as a working muscle, needs more blood, blood vessels in the dermis constrict, causing blood to leave the skin and enter the circulation that leads to muscles and other body parts. Sweat glands whose ducts pass through the epidermis to the outside and open on the skin surface through pores are embedded in the deep layers of the dermis. Hair follicles and hair roots also originate in the dermis and the hair shafts extend from the hair root through the skin layers to the surface. Also in the dermis are sebaceous glands associated with hair follicles which produce an oily substance called sebum. Sebum softens the hair and prevents it from drying, but if sebum blocks up a sebaceous gland, a whitehead appears on the skin. A blackhead results if the material oxidizes and dries. Acne is caused by infections of the sebaceous glands. When this occurs, the skin breaks out in pimples and can become scarred.

The skin is an important sense organ and as such includes a number of nerves which are mainly in the dermis, with a few reaching the epidermis. Nerves carry impulses to and from hair muscles, sweat glands, and blood vessels, and receive messages from touch, temperature, and pain receptors. Some nerve endings are specialized such as sensory receptors that detect external stimuli. The nerve endings in the dermal papillae are known as Meissner's corpuscles, which detect light touch, such as a pat, or the feel of clothing on the skin. Pacinian corpuscles, located in the deeper dermis, are stimulated by stronger pressure on the skin. Receptors near hair roots detect displacement of the skin hairs by stimuli such as touch or wind . Bare nerve endings throughout the skin report information to the brain about temperature change (both heat and cold), texture, pressure, and trauma.


Skin disorders

Some skin disorders result from overexposure to the ultraviolet (UV) rays in sunlight. At first, overexposure to sunlight results in injury known as sunburn. UV rays damage skin cells, blood vessels, and other dermal structures. Continual overexposure leads to leathery skin, wrinkles, and discoloration and can also lead to skin cancer . Anyone excessively exposed to UV rays runs a risk of skin cancer, regardless of the amount of pigmentation normally in the skin. Seventy-five percent of all skin cancers are basal cell carcinomas that arise in the epidermis and rarely spread (metastasize) to other parts of the body. Physicians can surgically remove basal cell cancers. Squamous cell carcinomas also occur in the epidermis, but these tend to metastasize. Malignant melanomas are life-threatening skin cancers that metastasize rapidly. There can be a 10-20 year delay between exposure to sunlight and the development of skin cancers.


Resources

periodicals

Czarnecki, D. "10-Year Prospective Study Of Patients With Skin Cancer." Journal of Cutaneous Medicine and Surgery 6, no. 5 (2002): 427-429.

Fackelmann, Kathy A. "Skin Cancer's Return: How Big a Threat?" Science News (June 27, 1992).

Willis, Judith Levine. "Acne Agony." FDA Consumer (July-August 1992).

other

Skin Deep. Video and videodisc. Princeton, NJ: Films for the Humanities and Sciences, 1995.


Bernice Essenfeld

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chitin

—Polysaccharide that forms the exoskeleton of insects, crustaceans, and other invertebrates.

Dermis

—The internal layer of skin lying below the epidermis. It contains the sweat and oil glands, hair follicles, and provides replacement cells for those that are shed from the outer layer.

Epidermis

—The thinner, outermost layer of the skin. Also the thin outermost covering in plants.

Keratin

—Insoluble protein found in hair, nails, and skin.

Melanin

—Brown-black pigment found in skin and hair.

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Integumentary System

Integumentary System


The integumentary system of an organism is its protective outer covering. All organisms have an integument or covering that separates the organism from its environment and serves several other important functions. The integument of vertebrates (animals with a backbone) is called skin. Skin can vary widely, from the impenetrable shell of an armadillo to the amazingly smooth skin of a porpoise.

Every organism has some sort of covering that holds together its body organs and fluids and makes it separate from its environment. This outer covering protects it from foreign bodies and matter and sometimes allows it to communicate with the world outside itself. In both one-celled organisms and plants, the integument is the same as its cell membrane and any secretion or coating that it produces. More complex invertebrates (animals without a backbone) have an integument that consists of a single layer of cells called an ectoderm. Only in vertebrates is the integument a many-layered, complex organ system that serves many functions.

THE INTEGUMENTARY SYSTEM OF PLANTS

The integument, or outer covering, of plants does the same thing that skin does for animals—it protects plants from injury and prevents underlying tissue from drying out. Higher plants, or those that have seeds and a vascular system (an internal system of tubing that carries fluid), have a living epidermis usually one cell thick. It may be thin, like the covering on lettuce leaves, or thick and tough like that on pine needles. The epidermis has openings called stomates that regulate temperature and water loss. It can have coatings like the wax on an apple or sensitive hairs like those on a Venus's flytrap. Overall, most plant integuments are fairly fragile compared to animal coverings.

THE INTEGUMENTARY SYSTEM OF INVERTEBRATES

The integument of more complex invertebrates is usually a single layer of cells that secrete some type of cuticle. The cuticle oozes out of the epidermis and hardens, affording it some type of protection. In crustaceans like crabs and lobsters, this functions as an external skeleton and is very hard and tough. Insects have an outer covering made up of chitin fibers that is secreted by the epidermis. Chitin forms a type of flexible, natural plastic and acts as an outer skeleton to which muscles are attached. In insects, the cuticle is a living structure and can produce sensitive hairs as well as bristles, scales, claws, or wings, depending on the species. Insects grow by shedding the cuticle and growing a new, larger one. Mollusks, like clams and snails, also form a hard, external shell.

INTEGUMENTARY SYSTEMS IN VERTEBRATES

Only in vertebrates, however, is the integumentary system considered to be a vital organ. That is, it not only provides protection for the delicate tissue underneath, but it gathers and conveys information to the organism itself about the outside environment. The skin of all vertebrates consists of two layers: the relatively thin (outer) epidermis, and the tough, inner dermis. The epidermis is several cells thick, and its outermost layer of cells is made up of dead cells composed of keratin, the protein found in hair, nails, claws, beaks, feathers, scales, and quills, among other things. Vertebrates replace this outer layer of dead cells every twenty-eight days, with the new layer identical to the old. In mammals, the inner dermis is highly developed, and is richly supplied with blood vessels, glands, and nerve endings. Besides acting as a barrier against infection and retaining the body's fluids, the dermis also has a regulatory function of letting the organism know whether to raise or lower body temperature or to move to where it is cooler or warmer. In endothermic (warm-blooded) animals, the skin plays an important role in regulating the body's temperature. In humans, the epidermis also contains a dark pigment called melanin that protects the skin from the Sun's ultraviolet radiation. It is the amount of melanin an individual's body produces that accounts for what we describe as the many different colorings of human "races."

Mammals. In mammals, the presence of hair is a distinguishing characteristic. As an outgrowth from a mammal's skin, hair grows from a pit in the dermis called a hair follicle. This pit also has a small gland that secretes an oily substance that keeps the hair oiled. For most mammals, the hair's main function is to act as insulation against the cold. Hair also serves as a sensory organ (like long whiskers) for certain night-prowling animals. Eyelashes in humans serve to make the eyes reflexively shut if they are hit by a speck of dust. There are also other glands in the dermis that keep the skin oiled and waterproof. Humans also have sweat glands in the dermis that act as a temperature control by means of evaporation, and most mammals also have dermal glands that produce odors that are thought to be a form of sexual communication.

In humans, the skin is considered the largest organ of the body. It changes considerably over time, and as it ages it becomes less elastic and more wrinkled. As with all vertebrates, human skin provides both protection from and communication with its environment. The dermis is rich with nerve fibers that can respond rapidly to changing environmental conditions, reporting its findings to the brain, which makes the necessary adjustments.

Besides its practical functions as barrier and regulator, the human skin also possesses the exquisite and indescribable sense of touch. Its surface is usually home to many bacteria, mostly harmless, and the skin can be subject to many diseases or injuries.

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