Chapter 6
Integument
Introduction
• The integument, or skin, is a composite organ.
– It is composed of the epidermis, dermis, and
basement membrane.
• The integument is one of the largest organs of the
body, making up about 15% of the body weight.
– The epidermis produces hair, baleen, feathers, claws,
nails, horns, beaks, and some scales.
– The dermis gives rise to dermal bones and
osteoderms of reptiles
– Collectively they form the teeth, scales, and denticles
of fishes.
• As the critical border between the organism
and its environment, the integument has a
variety of specialized functions.
– It forms part of the exoskeleton and thickens to
resist mechanical injury and establishes a barrier
to prevent the entry of pathogens.
– The integument helps hold the shape of an
organism and regulates osmotic movements.
– It holds feathers for locomotion, hair for
insulation, and horns for defense.
– Skin pigments also aid in protection from UV
radiation and display bright color for courtship.
Dermis
• The most conspicuous part of the dermis is the fibrous
connective tissue composed mainly of collagen.
– Collagen fibers may be woven into distinct layers called
piles.
– Alternating layer of piles give shape to the skin and
prevent it from sagging.
• In aquatic vertebrates allow the skin, aligned in orderly
piles forming a stratum compactum to stretch when
pulled at oblique angles.
– This flexibility allows it to accommodate to lateral bending,
but resists distortion in body shape.
– Because it does not wrinkle it allows water to flow
smoothly without turbulence over the body
• In terrestrial vertebrates, the stratum
compactum is less obvious because
movement depends more on the limbs than
on body movements.
– Consequently, collagen fibers are present in
terrestrial vertebrates, but are much less regularly
ordered and do not form piles.
Epidermis
• The epidermis, outer layer of the skin, in many vertebrates,
produces a thick layer of mucus to moisten and protect the
skin.
– In fishes it provides some protection from bacteria and lessens
resistance to laminar flow
– In amphibians it aids in similar functions and helps prevent
desiccation.
• In terrestrial vertebrates it often forms a keratinized, or
cornified layer, called the stratum corneum.
– All cells in this layer are dead cells.
– Their formation various proteins accumulate and form keratin in
a process called keratinization.
– This layer helps to reduce water loss in dry terrestrial
environments.
• In areas of the body where friction is
common, such as the soles of feet or palms of
hands, a thick protective layer, or callus, forms
to protect from mechanical damage.
• Finally, scales form within the integument of
many aquatic and terrestrial vertebrates.
– Scales are basically folds in the integument.
– If dermal contributions predominate, as in dermal
bone, the fold is termed a dermal scale.
– An epidermal fold is termed an epidermal scale.
Fishes
• With few exceptions the skin of fishes is nonkeratinized and covered instead with mucus.
• Unlike terrestrial animals, the epidermis of fishes is
alive and active on the body surface.
– The mucus layer forms from individual cells in the
epidermis with contributions from multicellular glands.
– The mucus coat, or cuticle, resists penetration by
infectious bacteria and contributes to laminar flow of
water.
– It also makes fish slippery to predators and includes
chemicals that are repugnant, alarming, or toxic to
predators.
• Fish dermis often gives rise to dermal bone and dermal
scales.
Chondrichthyes
• In cartilaginous fishes, dermal bone is absent, but
the surface denticles, termed placoid scales
persist.
– These scales are what give the skin of sharks and rays
their rough feel.
• The dermis is composed of fibrous connective
tissue, especially elastic collagen fibers, whose
regular arrangement forms a fabric-like wrap in
the dermis.
– This gives the skin strength and prevents wrinkling
during swimming.
• Placoid scales themselves form in the dermis,
but project through the dermis to reach the
surface.
– A cap of enamel forms the tip, dentin lies
beneath, and a pulp cavity resides within.
• Chromatophores occur in the lower part of
the epidermis and upper part of the dermis.
Bony Fishes
• The epidermis includes a basal layer of cells
above which are the stratified epidermal cells.
– Within these layered cells are secretory club cells that
produce the mucus cuticle, or slime
• The dermis of bony fishes is subdivided into a
superficial layer of lose connective tissue and a
deeper layer of dense fibrous connective tissue.
– Chromatophores are found in the dermis.
– The most important structural component of the
dermis is the scale.
• Based on their appearance several types of
scales are recognized among bony fishes.
• The cosmoid scales, seen in primitive fishes,
sit upon a double layer of bone.
• In cosmoid scales, there is a thick, welldeveloped layer of dentin (cosmine) beneath a
thin layer of enamel.
• The ganoid scale is distinguished by a surface
of thick enamel (ganoin) without an
underlying layer of dentin.
• The teleost scale lacks enamel, dentin and a
vascularized bony layer.
• Two kinds of teleost scale are recognized.
– The cycloid scale, composed of concentric rings,
and the,
– Ctenoid scale with of projections along its
posterior margin
Tetrapods
• Although keratinization occurs in fishes, among
terrestrial vertebrates it becomes a major feature of
the integument.
• Extensive keratinization produces a prominent outer
cornified layer, the stratum corneum, that resists
mechanical abrasion.
– The stratum is also mixed with lipids to increase the
resistance to water loss in the dry terrestrial environment.
• Multicellular glands, which are more prevalent than in
fishes, reside within the dermis and reach the surface
through common ducts that pierce the cornified layer.
Amphibians
• Amphibians are unique, in that during the
course of their lives they metamorphose from
aquatic to a terrestrial form.
• In most modern amphibians, the skin is also
specialized as a respiratory surface.
– Some species rely entirely on cutaneous
respiration
• Frogs and salamanders lack all traces of dermal
scales.
• In these vertebrates the dermis is composed of
fibrous connective tissues and a stratum that
affords some protection from abrasion as well as
protecting from loss of moisture.
• Generally the skin of frogs and salamanders
includes two types of multicellular glands, mucus
and poison glands that are located in the dermis.
• Chromatophores may occasionally be found in
the epidermis, but most reside in the dermis.
• Capillaries, which reside in the dermis of most
vertebrates, extend into the epidermis of
amphibians, and air in cutaneous respiration.
Reptiles
• The skin of reptiles reflects their grater commitment to
terrestrial life.
– Keratinization is much more extensive, and skin glands are
fewer than in amphibians.
• Scales are present, but are fundamentally different that
those of fishes.
– The reptilian scale usually lacks the bony under support or
any significant structural support of the dermis.
– Instead, it is a fold of the epidermal surface, forming an
epidermal scale.
– Large, plate-like scales are referred to a scutes and may be
modified into crests, spines, or hornlike processes.
• The dermis of reptiles is composed of fibrous
connective tissues.
• The epidermis is divided into three layers, the
stratum basale, stratum granulosam, and stratum
corneum.
– This changes in reptiles that shed large portions of
cornified skin.
– Shedding of the cornified layer, termed molting or
ecdysis, results in removal of extensive sections of the
superficial epidermis.
• Integumentary glands of reptiles are restricted to
certain areas of the body
– Most are thought to play a role in courtship or to
discourage predation.
Birds
• In birds the epidermis comprises the stratum
basale and stratum corneum.
– Between which is a transitional layer of cells that
transform into the keratinized layer of the corneum.
• Bird skin has few glands.
– The major gland, or uropygial gland, located at the
base of the tail secretes lipids that is smeared onto
the feather to repel water.
– The other gland, in marine birds, is a salt gland which
excretes excess salt ingested with food.
• Feathers distinguish birds from all other
vertebrates.
– Feathers are structurally elaborate and come in a
variety of forms.
– All feathers are nonvascular and nonnervous
outgrowths of the skin.
• Generally the modern bird feather is built from a
tubular central shaft, the rachis, which carries on
either side a vane, or series of barbs with
interlocking connections called barbules.
– The rachis continues proximally as the barbless
calamus, or quill, which anchors the feather to the
body and is moved by dermal muscles.
• In modern birds, feathers are of many types,
serving many functions.
• Flight feathers are long and the vanes
asymmetrical around the stiffened rachis.
• Contour feathers cover the body and usually
have symmetrical vanes.
• Down feathers lack a distinctive rachis and
non-interlocking barbs extend from the
calamus as a fluffy feather important in
insulation.
• Old feathers are shed, or molted, and the beginning of a
new feather soon grows, the feather filament soon grows
out of the follicle as a consequence of cellular proliferation
at the follicular base.
– The feather filament continues to grow out from the follicle
accompanies by the highly vascularized dermal core, which
extends through the follicle above the surrounding integument.
• Dermal muscles, connected in a network of muscles, act to
erect the feather.
• The patterning zone determines the spacing between
feather parts.
– This spacing allows adjacent barbs and barbules to separate as
they unfurl.
– Preening of the unfurling feather encourages the overlap and
interlocking of barbules a the mature feather takes final shape.
Function of Feathers
• There are several types of feather.
1. Contour feathers aerodynamically shape the
surface of the bird.
2. Down feathers lie close to the skin as thermal
insulation.
3. Filoplumes are often specialized for display
4. Flight feathers of the wings are a type of contour
feather.
•
These feathers have some value as insulation,
however their ,main function is stabilization in flight.
• Chromatophores occur within the epidermis,
and their pigments are carried into the
feathers to give them color.
• Light refraction off the feather barbs also play
a role in pigmentation of feathers and adds
iridescence.
Evolution of Feathers
• In modern birds it is easy to see that feathers assist in
flight.
• But, they likely had a different function in the past.
• One theory is that early feathers played a role in
surface insulation.
– Thus, protecting ectothermic birds from excessive heat or
endothermic birds in trapping heat generated from within.
• Another theory is that birds were initially used to aid in
gliding and that they later functioned to assist in flight.
• Regardless of their original purpose, feathers were
modified reptilian scales (or at least derived from a
similar process)
Mammals
• As in other vertebrates, the two main layers of
mammalian skin are the epidermis and
dermis, which join and interface through the
basement membrane.
• Beneath this lies the hypodermis that is made
of connective tissues and fat.
Epidermis
• In mammals the epidermis may be locally
specialized as hair, nails, or glands.
• Epithelial cells of the epidermis are keratinocytes
and belong to keratinizing systems that form the
dead, superficial cornified layer of the skin.
– Surface keratinized cells are continually exfoliated and
replaced by cells arising from the deepest layer of the
epidermis.
• The pattern of keratinization is most obvious in
areas that are most often in contact with their
surroundings (soles of feet, pads of paws).
• Chromatophores can be found all over the
body surface.
– They secrete granules of pigment called melanin.
• Skin color results from the combination of the
yellow stratum corneum, the red underlying
blood vessels, and the dark pigmented
granules secreted by the chromatophores.
Dermis
• The mammalian dermis is double layered.
– The outer papillary layer and the inner reticular layer.
• Blood vessels, nerves, and smooth muscle occupy
the dermis but do not reach into the epidermis.
• The mammalian dermis produces dermal bones,
but these contribute to the skull and pectoral
girdle.
– They rarely form scales, except in certain situations
where there is secondary development of dermal
bones in the integument. (Armadillos)
• Blood vessels and nerves enter the dermis;
hair follicles and glands project inward from
the epidermis.
• The dermis is usually composed of irregularly
arranged fibrous connective tissue that is mixd
with elastic fibers.
– This gives it the ability to stretch and retun to its
original shape.
– As the skin ages it loses this elasticity and begins
to sag.
Hair
• Hairs are slender, keratinous filaments.
– The base of the hair is the living root.
– Its remaining length makes up the nonliving shaft.
• The outer surface of the shaft often forms a
scaly cuticle, beneath it is cortex and at the
core is the medulla.
• Hair shafts grow from a living follicle, which goes
through a cycle of activity in three stages;
1. Growth: active proliferation of cells in the hair
papilla.
2. Degeneration: hair producing cells become inactive
and die.
3. Resting: lasts weeks to months
•
•
Eventually stem cells in the follicle resume the growth
phase and the old hair shaft falls out.
The cycle is intrinsic and does not seem to change with
cuing of the hair shaft.
• Chromatophores in the hair follicle contribute to
hair color.
– As we age hairs become gray, because special stem
cells within the follicle begin to die.
• A thick covering of hair, fur or pelage, is
generally composed of guard hairs and
underfur.
– Guard hairs are large coarse hairs that are the
most abundant type of hair on the outer surface.
– The underfur is usually much finer and shorter.
• Both types of fur function largely as insulators.
• Some hairs are specialized.
– Sensitive nerves are associated with the roots of
whiskers, vibrissae, located around the snout of
many mammals.
• Such sensory functions may have been the firs use of
fur, prior to evolving into a thermal insulator.
Glands
• Principally there are 3 types of integumentary glands;
sebaceous, eccrine, and apocrine.
– Scent, sweat, and mammary glands are derived from them.
1. The sebaceous glands produce an oily secretion, sebum,
that is released onto the hair follicles in order to condition
and waterproof fur.
2. Eccrine glands produce thin, watery fluids, are not
associated with hair follicles, and begin to function before
puberty.
– In most mammals they are associated with the soles of the feet
and hands, tails, and any other surface in contact with abrasive
surfaces.
3. Apocrine glands produce a viscous, lipid-containing fluid,
are associated with hair follicles, begin to function at
puberty and are used primarily for chemical signaling.
• Mammary glands produce milk, a watery
mixture of fats, carbohydrates, and proteins
that nourish the young
• Ectodermal mammary ridges, within which
the glands form, are located along the
ventrolateral surface of the embryo
– The number of mammary glands varies among
species.
• With few exceptions mammary glands
become functional only in females.
– Lactation, release of milk, can occur with or
without the development of breasts or nipples.
Nails, Claws, and Hooves
• Nails are plates of tightly compacted, cornified
epithelial cells on the surface of fingers and toes; thus,
they are products of the keratinizing system of the skin.
• Nails protect the tis of digits from inadvertent
mechanical injury and help stabilize the skin so that a
secure grip can be maintained.
– Only primates have nails, all other vertebrates have either
claws or hooves.
• Claws are laterally compressed, keratinized projections
from the tips of digits
• Hooves are enlarged, keratinized plates on the tips of
the ungulate digit.
Horns and Antlers
• Mammals, dinosaurs, and some extinct turtles
are the only vertebrates to possess true horns or
antlers.
• Both antler and horn are composed of a mixture
of skin and underlying bone.
– In horns, the integument produces a tough, cornified
sheath that fits over the bony core that is never
branched.
– In antlers, the overlying skin, velvet, apparently
shapes and provides a vascular supply to the growing
bone.
• True antlers are only found in members of the
Cervidae family.
– Typically only males possess antlers, which are
branched and shed annually.
– Under hormonal control and tend to appear for
mating season.
• True horns are found among members fo the
family Bovidae.
– Commonly horns occur in both genders, are
retained year round, and continue to grow
throughout life.
– The horn is unbranched and formed by a bony
core and a keratinized sheath.
• The horns of giraffe and rhino are different
still;
• In giraffe horns form from separate
cartilaginous processes that ossify, fuse to the
top of the skull, and remain covered with
living, non-cornified skin.
• The rhinoceros horn does not contain a bony
core, and is a exclusive product of the
epidermis.
– It forms from compacted keratinous fibers.
Baleen
• The integument within the mouths of
mysticete whales forms plates of baleen that
act as strainers to extract krill from the water
gulped into the mouth.
Fin