Lesson 7
◊
Lesson Outline:
♦ General Features of the Integument
♦ Embryonic Origins of the Epidermis
♦ Specializations of the Epidermis
o Glands
o Keratin and Stratum Corneum
◊ Objectives:
At the end of this lesson you should be able to:
♦ Describe the basic structure and function of the epidermis
♦ Describe the embryonic origin of the different layers of the integument
♦ Describe the phylogenetic trends seen in the development of the
epidermis.
◊
References:
Chapter 6: 105-120
◊
Reading for Next Lesson:
Chapter 6: 120-127
Integument
General Features
Skin is a composite organ - composed of and outer layer or layers of epithelium (the
epidermis), and a much thicker inner layer (the dermis) composed of closely packed
fibrous connective tissue.
The epidermis arises from the single cell layer of the ectoderm while the dermis arises
firm the mesoderm of the dermatome.
Variations are found in the integument of craniates involving:
- the relative number and complexity of skin glands
- the extent of differentiation and specialization of the
epidermis
- the extent to which bone develops in the dermis
Basic Functions
Protection
-mechanical barrier
- prevents entrance of pathogens
- cryptic/warning or camouflage colouration
- protection from UV light
Exchange Surface
-water
-wastes
- ions
- gases
- heat
Sensory Reception
Epidermis
Embryonic Origin
The single layer of ectoderm proliferates to become a multilayered epidermis in most
vertebrates (but not all). It produces an inner layer called the basal lamina.
Cells in the basal lamina are constantly undergoing mitosis, pushing older cells outward
and replacing those lost at the surface.
Epidermal cells may also undergo the process of keratinization to form a keratinized or
cornified layer. Keratin is a class of proteins which are produced in specific epidermal
cells just before they die to form a nonliving tough, water proof layer the stratum
corneum. This layer can be of varying degrees of thickness and can be used to protect the
soles of the feet to the roof of the mouth. It may also differentiate into hair, hooves, horn
and other specialized cornified structures.
Keratinizing systems are systems where an elaborate interaction between the epidermis
and dermis gives rise to cornified structures such as teeth, denticles and scales in most
fish.
Invagination of the surface epidermis forms a variety of skin glands.
These are called exocrine glands if the retain a duct to the surface and endocrine glands if
they separate from the surface and secrete their products into the vascular system. One of
the most common types of glands is the mucus secreting glands.
The dermis contains connective tissue, blood vessels, lymphatic vessels, nerves and
pigment cells. It sometimes adheres closely to the underlying muscle but is usually
separated from it by an intervening layer of loose connective tissue and adipose tissue the hypodermis.
Specializations of the Epidermis
Two kinds of nonliving coverings overlie the living epidermis of vertebrates. These
coverings are produced by the epidermis. In fishes and aquatic amphibians it is a thin coat
of mucus that is continually being replaced. In terrestrial vertebrates it is the stratum
corneum (dead keratinized cells).
The epidermis is glandular to some extent in all vertebrates - more so in fish and
amphibians.
Multicellular epidermal glands invade the dermis where they are surrounded by blood
capillaries that provide them with nutrients, and in the case of endocrine glands, transport
their products.
Epidermal Glands
Fish
- most are single cells
- goblet cells secrete only mucus
- granule cells secrete mucus and other substances
- mucus is used for: reducing friction
avoiding predation
cocoons for estivation and sleeping
nutritious mucus for feeding young in
some species
- slime glands in hagfish and lamprey are used for protection
and for suffocating prey
- granule cells also produce some toxins and alkaloids
- some deep sea teleosts have multicellular glands that have
been adapted to serve as light emitting organs or photophores. The basal
layer of cells in the gland emit light as a product of an enzymatic reaction.
The mucus cells above them act as a magnifying lens.
Amphibians - most epidermal glands are multicellular
- leydig cells (antiviral and antibacterial substances)
- mucus glands for moist skin,
- poisons glands for defense (poison dart frog, bufotenin in
toads etc.)
Tetrapods - epidermal glands are multicellular
- they can be tubular (rare except in mammals) or saccular
(alveolar)
- They vary in they way in which they liberate their
products.
- merocrine glands secrete their products across the cell
membrane and the cell remains intact. (eg. sweat glands)
- in holocrine glands, the cells themselves are secreted
(eg. sebaceous and oil glands)
- in apocrine glands the secretion accumulates in the
apical portion of the cell which is then pinched off along with
some cytoplasm. The cell then repairs itself. (eg. mammary glands)
Mucous Glands
Mucus secreting epidermal glands have all but disappeared in terrestrial tetrapods
except in mammals where they are confined to areas where lubrication is essential.
Granular Glands
Found only in amphibians and reptiles.
They are defensive and secrete toxins or alkaloids to ward of
predators.
They also secrete pheromones which are chemical cues
important in reproductive behaviour.
Femoral glands in lizards secrete a substance on the inner
thighs of males that hardens to form temporary spines to restrain females
during copulation.
Avian Oil Glands
Uropygial glands near the rump secrete a water repellent oil that is transfered to
the feathers during preening. It is largest in aquatic birds.
Sebaceous Glands
These are the mammalian equivalent of the avian oil gland. They are present
wherever there is hair and secrete oil to water proof hair and fur. They are found
independent of hair on the lips, glans penis, labia minora and skin surrounding the nipples
where they serve a lubricating function.
Sudoriferous Glands
These are sweat glands found primarily in mammals which serve a
thermoregulatory role. In furry mammals they are confined to the least furry regions(feet,
lips, etc.). They are most abundant in hairless mammals (such as humans).
Scent Glands
Sebaceous and sudoriferous glands also produce a variety of scents, most of
which are pheromones. These can be found on any part of the body and can be used to
drive away enemies or attract mates.
Mammary Glands
Found only in mammals, they are used for nutrition.
The glands themselves consist of many lobules that are each composed of many
alveoli (same term as in the lung) that secrete the milk.
The glands may open directly to the surface (monotremes) or through a raised
epidermal papilla (nipple - multiple openings) or into a common chamber in an enlarged
epidermal papilla (teat).
The glands usually proliferate and become enlarged under hormonal stimulation
just before females give birth to their young. (no need to produce milk when it is not
required)
Adipose tissue can build up beneath the mammary gland (in the hypodermis) to
form a breast. Breasts and mammary glands are not synonymous. The adipose tissue does
not secrete milk. Breasts are primarily for sexual display and courtship. In most mammals
they only develop during mating season. Humans are one of the few mammals where
they are permanently fully developed after puberty.
Keratin and Stratum Corneum
Fish and Amphibians - keratin is not found very often in the
epidermis of fish or amphibians (it is found in the dermis).
- Some salamanders have a thin stratum corneum
- Lampreys and hagfish have cornified epidermal spines
that act as teeth for grasping food or rasping prey.
Tetrapods - Keratinization becomes highly developed in the epidermis of tetrapods.
It provides protection against desiccation on land. Keratinization and formation of
a stratum corneum also occur
where friction or direct mechanical abrasion insult the epithelium. For example, the
epidermis in the oral cavity of aquatic and terrestrial vertebrates often exhibits a
keratinized layer, especially if the food eaten is unusually sharp or abrasive. In areas of
the body where friction is common, such as the soles of the feet or palms of the hands,
the cornified layer may form a thick protective layer, or callus, to prevent mechanical
damage (figure 6.6). The stratum corneum may be differentiated into hair, hooves, horn
sheathes, or other specialized cornified structures.
With the acquisition of an amnion, which liberated the amniotes from water, the
stratum corneum became increasingly specialized in various regions of the body for
protection against abrasion, for offense and defense (scales, claws and horny
protuberances), and as an adjunct for thermoregulation (hair and feathers).
Epidermal Scales:
Scales are present as thickenings of the stratum corneum in amniotes. In snakes
and lizards they form as overlapping folds of the epidermis. Despite appearances, the
epidermis is continuous. Many reptiles are able to shed and replace large portions of their
skin at one time in the process of moulting. What is moulted is the epidermis. The inner
of the three layers reproduces the outer two layers (one living and one dead) from below
and white blood cells invade the space between the new and old skin causing it to slough
off.
If the scale is large and plate like it is called a scute. These occur on the bellies of
snakes and turtles (The scutes in turtles are distinct from the shell itself which is of
dermal and skeletal origin). They may be modified into crests, spines or hornlike
processes.
The epidermis in birds has the same basic structure as that of reptiles. Epidermal
scales remain on legs and feet and in the facial areas of some species.
In mammals, they are also usually confined to the legs and tail but in armadillos
they occur all over the body.
Claws, Hoofs and Nails
Plates of tightly compacted, cornified epithelial cells produced through
keratinization. Continuously produced at the base and worn down at the tips. Serve for
protection, gripping, grasping, tearing, and for cushioning and shock absorption
Horns and Antlers
While some reptiles have horns that are pointed epidermal scales, the terms horns
and antlers really only apply to mammals. True horns have a keratinized surface. Antlers
do not.
Horns occur in bovids (oxen, cows, sheep, goats, antelopes) and pronghorn
antelopes (not true antelopes). They have a core of dermal bone covered by a sheath of
horn. They occur in both sexes and are usually present year-round. The epidermal
component can often be far more extensive than the core of dermal bone.
The horns of the rhinoceros are only made of compact keratinized fibers of
epidermis (hair).
Antlers appear in cervids (deer family), usually only in males. Throughout much
of their growth they are covered in living skin (called velvet) which is eventually shed to
reveal the dead bone beneath. They are shed and replaced annually. This means that the
dermal bone must break off and this is a hormonally controlled process.
The horns of giraffes are ossified cartilage that fuse to the top of the skull.
Baleen
Keratinized plates that form from the epidermis in the mouths of many whales.
Dermal papillae extend and lengthen, carrying the overlying epidermis which forms
cornified frills which act as strainers to extract small invertebrates from the water they
gulp. They can then strain out the water and only swallow the food.
Differences in the arrangement and configuration of the sheets are correlated with
feeding habits. Baleen is continually worn away and replaced.