BASIC STRUCTURE AND PROMINENT FUNCTIONS OF
VERTEBRATE INTEGUMENT
INTRODUCTION
The integument or the outer cover of the body is commonly referred to as the
skin. Together with its derivatives it makes up the integumentary system.
It is continuous with the mucous membrane lining the mouth, eyelids, nostrils,
rectum and the openings of the urino-genital ducts.
The skin functions primarily to cover and protect the tissues lying beneath it. In
other words, it forms the external protective covering of an animal.
Forms interface between organism and external environment.
Part that the predator sees first, and which offers the first line of defense.
Abundantly supplied with sensory nerve endings, which are affected by
environmental stimuli and play an important role in communication.
General metabolism of the body, temperature regulation and water loss.
Character of the skin and its derivatives shows variation in different regions of
the body, in different individuals, in the same individual as age advances and in
different groups of vertebrates.
The type of environment whether aquatic or terrestrial is of importance in
connection with these variations.
The evolution of vertebrate integument is correlated with the transition of
vertebrates from an aquatic to a terrestrial environment.
Nevertheless, basic similarities exist in the integument of all vertebrates.
INTEGUMENT PROPER
In Annelids, Arthropods, integument consists of single layer of cells, the
EPIDERMIS, together with an outer non-cellular CUTICLE, secreted by the
cells.
Annelids have a body covered with an external thin collagenous cuticle (never
shed or molted).
In Arthropods, the chitinous and rigid cuticle makes up the exoskeleton. Periodic
shedding of this cuticle is termed Ecdysis.
THE VERTEBRATE SKIN DIFFERS FROM INVERTEBRATE SKIN
TWO LAYERS – Outer epidermis derived from ectoderm
Inner dermis or corium of mesodermal origin.
The relative amount of the two layers varies with the environment.
EPIDERMIS – the epidermis is made of stratified epithelium (several layers of
columnar epithelium cells).
These cells are held together tightly by minute intercellular bridges found on the
surface of cells.
The innermost layer is stratum Malpighii or stratum germinativum placed
over a thin basement membrane. These cells divide constantly to produce new
cells.
Move upwards, tend to become flattened, protoplasm becomes horny
(keratinisation).
In fishes and amphibians, this keratinised layer forms a cuticle, but in amniotes,
it forms stratum corneum, of hard, horny, flat, cornified cells made largely of
keratin, which is tough, waterproof and insoluble protein.
It affords protection against mechanical injuries, fungal and bacterial attacks and
prevents desiccation.
In many Tetrapoda, this layer is shed periodically in pieces or all at once. No
stratum corneum in cyclostomes and fishes (since they are fully aquatic) here the
epidermis has mucous glands, secreting mucus to keep the skin slimy and protects
it from bacteria.
The epidermis has no blood vessels and is nourished by capillaries in the dermis.
The epidermis rests on a thin basement membrane which separates it from the
dermis
Dermis has an outer loose layer and inner dense layer
Made up of dense connective tissue having cells, muscles, blood vessels, lymph
vessels, collagen and elastic fibres, and nerves.
Amphibians and reptiles -collagen fibres at right angles in three planes
Birds and mammals, they have an irregular arrangement.
Substances pass by diffusion from the dermis to the epidermis.
Skin contains pigment, if present in epidermis, it occurs as a diffuse substance or
as granules.
If in dermis, then in the form of granules in special branching cells called
chromatophores. The pigment can either collect as a central ball making the skin
lighter or spread out into all the branches making the skin darker, thus,
chromatophores bring about colour variations. Chromatophores are of many
kinds,
Melanophores that contain brown to black pigment
Lipophores or xanthophores which contain yellow red fatty pigments
Iridocytes or guanophores contain crystals of guanine which reflect light.
Under dermis, the skin has subcutaneous loose areolar tissue which separates the
skin from the underlying muscles, it may contain fat and muscles, especially in
mammals.
Integument of Anamnia shows a decrease in thickness and also a decrease in the
degree of ossification. These are of advantage in allowing greater mobility and in
amphibians, they permit respiration by the skin.
But in Amniota, the skin becomes progressively thicker to prevent loss of water
and to retain body heat.
STRUCTURE OF INTEGUMENT IN CYCLOSTOMATA
Epidermis is multi-layered (stratified) but has no keratin.
It has three types of unicellular gland cells: mucus glands (secrete mucus), club
cells (scab-forming cells) and granular cells (unknown function).
Below epidermis is the cutis formed of collagen and elastin fibres.
Star- shaped pigment cells are also present in the cutis.
STRUCTURE OF INTEGUMENT IN PISCES
The epidermis has several layers of simple and thin cells, but there is no dead
stratum corneum. The outermost cells are nucleated and living. The stratum
Malpighii replenishes the outer layers of cells which have some keratin.
Unicellular goblet or mucous gland cells are found in the epidermis, as in all
aquatic animals. The mucous makes the skin slimy reducing friction between the
body surface and water, protects the skin from bacteria and fungi and assists in
the control of osmosis.
Multicellular epidermal glands like poison glands and light producing
organs may also be found. The epidermis rests on a delicate basement membrane.
The dermis contains connective tissue, smooth muscles, blood vessels, nerves,
lymph vessels and collagen fibres. The connective tissue fibres are generally not
arranged at right angles but run parallel to the surface. Scales are embedded in
the dermis and projected above the epidermal surface. The colours of fishes are
due to chromatophores and iridocytes.
STRUCTURE OF INTEGUMENT IN AMPHIBIA:
The epidermis has several layers of cells, six to eight cells in thickness and is
divisible into three layers: stratum corneum, stratum germinativum and a
basal portion in contact with the basement membrane.
The outermost layer is a stratum corneum, made of flattened, highly keratinised
cells.
Such a dead layer appears first in amphibians and is best formed in those which
spend a considerable time on land.
The stratum corneum is an adaptation to terrestrial life (protects body and
prevents excessive loss of moisture).
In ecdysis, stratum corneum is cast off in fragments or as a whole in some.
(moulting / desquamation i.e., removal of unicellular sheet of stratum corneum).
The dermis is relatively thin in amphibians, it is made of two layers -
upper loose stratum spongiosum and a lower dense and compact stratum
compactum. Connective tissue fibres run both vertically and horizontally. Blood
vessels, lymph spaces, glands and nerves are abundant in the stratum spongiosum.
There are two kinds of glands, multicellular mucous glands and poison glands
in the dermis, but they are derivatives of the epidermis.
Mucous gland produces mucus (slimy protective covering, helps in respiration).
Amphibian skin is an important organ of respiration.
Poison glands produce a mild but unpleasant poison which is protective.
In the upper part of the dermis are chromatophores. (melanophores and
lipophores)
Ability of the skin for changing colour to blend with the environment is well
developed.
INTEGUMENT IN REPTILIA.
The integument is thick and dry, it prevents any loss of water, it has almost no
glands. The only glands present are scent glands for sexual activity.
The epidermis has a well-developed stratum corneum well adapted to terrestrial
life. The horny scales of reptiles are derived from this layer.
Ecdysis is necessary to remove dead outer layers, hence scales are shed
periodically in fragments or cast in a single slough as in snakes and some lizards
Scales often form spines or crests.
Below the epidermal scales are dermal bony plates or osteoderms in tortoises,
crocodiles and some lizards (Heloderma).
The dermis is thick and has an upper layer and a lower layer, upper layer
has abundance of chromatophores in snakes and lizards.
Lower layer has bundles of connective tissue in which collagen fibres lie at right
angles.
Leather of high commercial value can be prepared from the skin of many reptiles
like lizards, snakes and crocodiles.
Many lizards and snakes have elaborate colour patterns, they may be for
concealment or as warning colours. There is marked colour change in certain
lizards such as chameleon, the colour may change with the environment for
concealment or it may change in courtship or threat.
The ability of chameleons and some other animals to change colour is known as
metachrosis. (metachromatism)
In Calotes, chromatophores are controlled by the posterior lobe of pituitary
whereas in chameleons they are controlled by the Autonomic Nervous System.
INTEGUMENT IN BIRDS
Thin, loose, dry and devoid of glands.
There is only a uropygial gland at the base of the tail, its oil is used for preening
(to clean and tidy its feathers with its beak) and waterproofing the feathers
(aquatic birds)
Epidermis is delicate except on shanks and feet where it is thick and forms
epidermal scales. The rest of the body has a protective covering of epidermal
feathers.
The keratin producing powers of the epidermis are devoted to producing feathers
and scales.
The dermis is thin and has interlacing connective tissue fibres, abundant muscle
fibres for moving feathers, blood vessels and nerves.
The dermis has an upper and lower compact layer, between which is a vascular
layer, the dermis also contains fat cells.
The skin has no chromatophores.
Pigment is found only in feathers and scales.
Colour patterns in birds are vivid (concealment, recognition and sexual
stimulation)
Colours are produced partly by pigments and partly by reflection and refraction
from the surface of the feathers.
INTEGUMENT IN MAMMALS
Skin is elastic and waterproof, much thicker than in other animals, especially the
dermis is very thick and is used in making leather.
Epidermis is thickest in mammals.
Outer stratum corneum containing keratin, cells not dead as believed before.
Below this is stratum lucidum (barrier layer), chemical called eleidin
Below this stratum granulosum, darkly staining granules of keratohyalin
Below this is stratum spinosum whose cells are held together by spiny
intercellular bridges.
Lastly stratum germinativum which rests on a basement membrane
Dermis is best developed in mammals.
Upper layer is papillary layer made up of elastic and collagen fibres with
capillaries in-between, thrown into folds called dermal papillae, especially in
areas of friction
Greater lower part of dermis is reticular layer, having elastic and collagen fibres.
In both layers there are blood vessels, nerves smooth muscles, certain glands
tactile corpuscles and connective tissue fibres in all directions.
Below dermis the subcutaneous tissue contains a layer of fat cells forming
adipose tissue
In the lowest layer of epidermis there are pigment granules, no pigment bearing
chromatophores in mammaIs (in man, branching dendritic cells or melanoblasts)
FUNCTIONS OF THE INTEGUMENT
▪ PROTECTION
▪ TEMPERATURE CONTROL
▪ FOOD STORAGE
▪ SECRETION
▪ EXCRETION
▪ SENSATION
▪ RESPIRATION
▪ LOCOMOTION
▪ DERMAL ENDOSKELETON
▪ SEXUAL SELECTION
1. Protection: The integument forms a covering of the body and is protective. It
protects the body against entry of foreign bodies and against mechanical injuries.
It protects the tissues against excessive loss of moisture, this is very important
because both aquatic and terrestrial animals are dependent upon water in their
bodies for various metabolic activities. The integument forms protective
derivatives, such as scales, bony plates, layer of fat, feathers and hair which
reduce the effect of injurious contacts. In some animals the skin shows protective
colouration which makes the animals resemble their environment, thus, making
them almost invisible to their enemies. Poison glands of toads, slippery skin of
aquatic animals and an armour of spines of some mammals are protective devices
of the integument. The skin forms a covering which prevents the passage of water
and solutes in one of the following ways: (a) By formation of cuticle in
Protochordata and embryos of fishes and amphibians, (b) By secreting a coat of
mucus in fishes and aquatic amphibians, and (c) By formation of keratin layers
in the epidermis of tetrapoda. Keratin is formed from the cytoplasm of
degenerating cells of the epidermis which finally form a layer of horny stratum
corneum.
2. Temperature Control: Heat is produced constantly by oxidation of food stuffs
in tissues. This heat is distributed evenly by the circulating blood. The body heat
is lost constantly with expired breath, with faeces and urine, and from the surface
of the skin. The integument regulates heat and maintains a constant temperature
in endothermal animals. In birds the heat is regulated by adjustment of feathers
which retain a warm blanket of air, when feathers are held close to the body, they
remove warm air and body cooled, when feathers are fluffed out, they keep the
warm air enclosed. In mammals, constant evaporation of sweat regulates the body
heat. In cold weather contraction of skin’s blood capillaries reduces the loss of
body heat. In some animals, fat in the skin prevents loss of heat because it is a
non-conductor of heat.
3. Food Storage: The skin stores fat in its layers as reserve food material which is
used for nourishment in times of need. In whales and seals the fat of the skin
forms a thick layer, called blubber which is not only reserve food but also
maintains the body temperature.
4. Secretion: The skin acts as an organ of secretion. Glands of the skin are
secretory. In aquatic forms there are secretory mucous glands whose secretions
keep the skin moist and slippery. In mammals, sebaceous glands secrete oil which
lubricates the skin and hairs. Mammary glands produce milk for nourishment of
the young. In birds uropygial glands secrete oil for preening the feathers. Odours
of scent glands attract the opposite sex. Lacrymal glands’ secretion wash the
conjunctiva of eyeball in mammals. Ear wax (cerumen) secreted by the glands of
auditory meatus greases the eardrums and avoids insects to enter the canal.
5. Excretion: The integument acts as an organ of excretion. Shedding of the
corneal layer during ecdysis removes some waste substances. In mammals
metabolic waste (salts, urea and water) is removed from the blood by means of
sweat. Chloride secreting cells are found in gills of marine fishes.
6. Sensation: The skin is an important sense organ because it has various kinds
of tactile cells and corpuscles which are sensory to touch, temperature changes,
heat, cold, pressure and pain.
7. Respiration: In amphibians, the moist skin acts as an organ of respiration, in
frogs the respiratory function of the skin is greater than that of the lungs.
8. Locomotion: Derivatives of the integument bring about locomotion in some
animals, such as the fins of fishes aid in locomotion in water, the web of skin in
the feet of frogs and aquatic birds aid in swimming, feathers of the wings and tail
of birds are used for flying, and extensions of the integument forming “wings” of
flying lizards, extinct pterodactyls, flying squirrels and bats.
9. Dermal Endoskeleton: The skin contributes to the endoskeleton. It forms the
dermal bones of vertebrates and also forms parts of the teeth. Endoskeleton of
head protects the brain and sense organs. In the body it protects the soft, tender
viscera.
10. Sexual Selection: The skin acts as an organ of sexual selection. It provides the
feathers of birds which often have brilliant colours which are for sexual attraction.
Some integumentary glands of mammals produce odours far attracting the
opposite sex. Antlers of male deer distinguish it from female.
Besides the above functions, mammalian skin synthesizes the vitamin D with the
help of Sebum of sebaceous glands. Brood pouches beneath skin in some fishes
and amphibians protect unhatched eggs. Nasal glands of tetrapods, keep the
nostrils free of dirt and water. Skin also has the power of absorption of oils,
ointments, etc
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