Integumentary System :Skin

skin is the largest organ in the body. Also included are the

accessory organs: hair, sweat glands, sebaceous glands, nails

and variations

Skin:The study of skin is dermatology.

The skin has a variety of functions including:*barrier against invasion of micro-organisms*protects underlying structures from injury

*prevents the body from drying*inhibits excess water intake (from aquatic species)

*regulates temperature*receptor for sensations of touch, heat, cold, pressure

and pain.*disposal of waste products

The body produces heat by metabolizing ingested food and the

amount of heat produced is the result of the amount of work done

by the muscles.Most of body heat is lost by:

Radiation: heat energy is transmitted as rays to the

surrounding areas.

Conduction: heat energy passes by direct contact through a non-moving medium.

Eg: when an animal lies on a cold surface it loses heat directly to that surface.

Or when an animal is lying on a surgery table the heat is lost directly to the surface of the

table and through the incision made

Convection: heat transfer occurs in a gas or liquid by the

circulation of currents from one region to another. This is the heat loss responsible for the

wind chill factor

Evaporation; heat is lost as perspiration or other fluids on

the skin. Air currents can increase evaporative heat loss

through convection.

Most of the remainder of heat loss occurs through mucous

membranes and lesser amounts lost through digestive and urinary

tracts

Composition of skinthe skin is composed of two

layers:the epidermis: outer, thinner layerthe dermis: the thicker inner layer

epidermis

dermis

The Epidermis: is made up of stratified squamous

epithilial tissue,the layers from the dermis

outward are

Stratum germinativum: (basal layer)

The cells in this inner most layer multiply continuously to

compensate for the constant loss of cells from the surface of the epidermis. These cells push upward into each layer and

eventually die.

Stratum spinosum:(spinous layer)

The cells begin to shrink and draw apart. This layer is only

one or two cells thick. It is thicker on parts of the body that endure rugged wear (footpads)

Stratum granulosum:(granular layer)

The cells in this layer contain granules visible in the cytoplasm. This layer may not be present in

some areas of thin skin.Keratinization or cornification

packs the cells with fibrous protein called keratin

Stratum lucidum:(clear layer)

This layer is made up of closly packed clear cells. This layer is

found in tissue that endure rugged wear.

Stratum corneum:(horny layer)

This layer is made of flat lifeless cornified cells. They appear as

overlapping dry scales that make up the outer skin layer. If unbroken, they will prevent the entrance of microorganisms.

Keratinization gives the special epidermal parts (hooves, horns, beaks)

their strength.

Cornified

Skin color is determined by the amount of melanin in the

stratum germinativum layer. Heredity, the sun and some

hormones affect the skin color

The absence of normal pigmentation is called albinism.

True albinism prevents pigmentation of the skin, hair and

eyes. This condition may be associated with other lethal traits

as well as a lack of protection from the sun

Skin color can also be affected by the blood supply. Eg. Skin color can take on a blue hue

(cyanosis) when oxygen supply is compromised

The dermis (corium) is made up of dense, fibrous connective

tissue containing blood vessels and nerves. Small involuntary

muscles (arrector pili) are attached to the hair follicles.

When these muscles contract, the hairs stiffen. This increases the animals ability to insulate itself

against cold.

Sebaceous glands, sweat glands, sensation receptors are found in

the dermis layer.

The subcutaneous layer under the dermis is the hypodermis or

subcutis. It consists of loose connective tissue and adipose

tissue.

Skin structure differs throughout the body. It is tough, stretchable

and varies in thickness.

HairMammals have three types of

hair.*primary

*secondary*tactile

Primary hairs are also called guard hairs and these make up

the top coat. These hairs are stiff with a smooth appearance. This arrangement allows rain to run off the coat preventing chilling

of the animal

Secondary hairs are also called wool hairs and make up the

undercoat. These hairs are soft, thin and wavy.

Tactile hair are usually thicker and longer than primary hair and

is most often found about the face. These hairs grow deep

from the hypodermis or superficial muscle layer and act

as a touch receptor.

Hair develops from the hair papilla, located at the base of the

hair follicle. Cells at the base increase, push upward and

keratinize forming the visible hair shaft. Hair follicles are positioned at 30 – 60 degree

angle to the skin.

The two arrangements for hair growth:

simple: primary hairs grow from separate follicular openings

complex: two to five guard hairs erupt from a single follicle. And

are surrounded by clusters of wool hairs

Hair color is important for thermal regulation. In general, lighter color coats are cooler.Animal hair grows in cycles.

Factors affecting the shedding cycle: temperature, nutrition, hormones, genetics, state of

health.

Sweat glands are excretory organs of the skin that cool the

body.Apocrine sweat glands are found throughout the body and secrete a strong smelling substance in the

hair

Eccrine sweat glands are coiled tubular structures embedded in

the dermis and produce a watery sweat. These glands are found in limited areas usually the footpads

and nostrils

The sebaceous glands secrete a substance called sebum which

lubricates the skin and hair, waterproofs the coat, gives the

coat a glossy sheen, increases the spread of sweat, slows bacterial

growth, and serves as a territorial marker.

Examples of these glands are:

Circumoral glands:These glands are found in the

lips of cats. During grooming or when a cat rubs it’s head against

and object, the secretion is deposited to mark territory

Horn glands:These glands produce a musk odor that attracts females to

males during breeding season. They are located caudo-medially

to the horn base.

Anal sacs:These are cutaneous pouches that express a foul smelling fluid that

is expressed during defication and is used as a territorial

marker.

Ceruminous glands:These are classified as modified

sweat glands. They are located in the external ear canal. It excretes a yellowish waxy substance called

cerumen (earwax)

Nails, Claws and Hooves.All three of these structures have the following parts in common:

*wall*sole*pad

In primates the nail is the wall and it grows from the epidermis. The sole is located beneath the nail at the open end. The pad

corresponds with the finger tips

In carnivores the claw is the wall, which has been compressed

laterally. The sole is the underside of the wall and is flaky in texture. The pad corresponds

to the foot pads

Plantigrade animals have well developed footpads.

Digitigrade animals include cats and dogs. Only the digital and metacarpal pads make ground

contactUngulates include hoofed animals

The hoof is the wall, the sole is the underside of the hoof, the footpad is the bulb or heel.

Dewclaws, chestnuts and ergots are modified epidermal

structures. In the dog and pig, the dewclaw has a rudimentary

bone. In ruminants the dewclaws are like miniature hooves and

they serve no practical purpose