THE INTEGUMENT AND ITS
DERIVATIVES
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Return to IndexEPIDERMIS and
its DERIVATIVES
The epidermis, derived from
somatic ectoderm, is the exterior-most covering of the chordate body. It
provides protection against the invasion of microorganisms, provides
flexibility in motion, and seals in moisture. As will be seen, it also gives
rise to a variety of differentiated structures such as feathers, hair, horns,
claws, nails and glands. Begin by looking at a cross-section of
Amphioxus
integument. Amphioxus possesses the simplest possible form of epidermis - a SINGLE
layer of columnar epithelium covered by a thin film of cuticle. All true
vertebrates, however, have developed a MULTI-LAYERED epithelium. Note the
simple, multicellular, epithelium of the lamprey, which has no scales. Fishes
and amphibians have a mucus layer for bacterial and mechanical protection and
to prevent drying on land. Terrestrial vertebrates have replaced the cuticle
with keratin. Epidermal Derivatives of the Integument
A) Keratin Structures New epidermal cells are formed
continuously in the lower layers of the epidermis. In terrestrial
vertebrates, new epidermal cells push more superficial ones to the
stratum corneum,
the outer-most epithelial layer. In the process of self-destruction, these
exterior epidermal cells accumulate protein products called keratin.
Keratinized or cornified skin serves to prevent water escape and to protect
against friction and direct mechanical stimulation (e.g. calluses in humans). The production of
all of the following structures involves keratinization: Epidermal Scales: a continuous layer of repetitious thickenings of the
stratum corneum; you cannot dissect an individual epidermal scale out of the
skin! These scales may be shed entirely or in small flakes. Examine preserved
specimen of snake and dried specimens of bird legs and feet. Claws and Talons: curved, laterally compressed keratinized
projections from the tips of digits. See dried specimen of cat claws and bird
talons. What are the possible functions of
these structures? Hooves: enlarged keratinized plates found on the
ends of ungulate digits. Examine the hooves of pig and horse. Nails: keratinized epithelial cells produced
at the name base by pushing the existing nail forward. Protect from
mechanical injury and stabilize skin for better grasping. Found only in
primates. Horns: a tough, cornified layer of the
integument covers horns. Their core, however, is bone, of dermal origin. Horns are found in bovines (cattle,
antelope, sheep, goats, bison, wildebeest). They are retained year-round and
grow throughout the animal’s lifetime. Baleen: found in some whales, is a series of
keratinized plates that arise from oral epithelium. These sheets hang from
the palate along its length. See the display. Of what use would the
sieve-like action of these plates be? Beaks: epidermal structures, jaws are covered
by keratinized sheaths in birds. Feathers: are believed to have evolved from
reptilian scales. Columns of epidermal cells project into the skin initially
to form an invagination called the feather follicle. Continued growth results
in their projection out of the skin to form the feather shaft. These columns
then separate from each other, become keratinized and develop into barbs.
Feather growth is initiated by mesodermal papillae, which die in the grown
feather to form feather pulp. Examine the dried specimens. Note the quill,
which attaches to the body and extends as a rachis. From the rachis project
many veins with barbs and barbules to hold them together. Hair: just as in feathers, there is an
initial ingrowth of epidermal cells to form the hair follicle, followed by an
outward growth of keratinized cells to form the hair shaft. Dermal papillae
die to form the core substance of hair follicles. Note the similarities
between hair and feathers both in development and in general anatomy. They
both possess dermal papillae, shafts, an inner pulp and columns of
specialized keratinized cells. Hair is characteristic of mammals. B) Glands Specialized to secrete specific products
(oil, sweat, milk, etc.), these cells are derived by an infolding of the
epidermis. In many cases they retain a connection to the stratum
corneum whereby
their secretions can be released at the skin surface. What do you think
is responsible for the slimy feeling of fish skin? The glands of the Frog can be
seen in the Frog Integument Slide (cross-section). The human scalp
cross-section has oil (sebaceous) and sweat glands. Which two layers constitute
the epidermis of the human scalp? Which is living and which is
non-living? What is the primary function
of each layer? The DERMIS
and its DERIVATIVES
The dermis is generally
much thicker than the epidermis and lies more deeply. It is made of a fibrous
mass of connective tissue (collagen) and is of mesodermal origin. It may directly produce dermal bone. The
dermis is important in defense against injury and in the maintenance of body
heat. Deeper regions of the dermis often contain fatty deposits, smooth
muscle, blood vessels and nerves. Chromatophore cells are sometimes epidermal, but
usually dermal in origin. They secrete melanin, which can be passed to the
stratum corneum of skin and to hair shafts to produce colour and block
harmful sunlight. Dermal Bone Once present in some extinct fish - Ostracoderms with a complete head shield, Placoderms with a broken head shield and body
armor. Now demonstrated in Turtle dermal bone, antlers, and in the dermal
armor of Armadillo. In antlers the velvet is epidermal in origin and shapes
and provides blood to the dermal bone. Once grown, the velvet is shed and
only the bone remains. Antlers are found in deer, elk, moose and their
relatives, often only in males. They are shed annually. In most modern vertebrates, dermal bone
is formed from embryonic mesenchyme by extra membranous ossification, and
contributes to the skull and skeleton, rather than being manifested
externally. An exception is teeth, which are partly derived from dermal bone. Fish Scales Fish scales are also called dermal
scales since they are derived mainly from the dermis. 1) Cosmoid Scales: Found in Placoderms (extinct) as plates, and also typical
of the Lobe Finned Fishes or Sarcopterygii, (Choanichthyes). Extinct fish had
scales of enamel, cosmine and bone with pulp cavities. Modern ones, like
Coelocanth and the lung
fish have calcified fibers so this type of scale is almost extinct. No
specimens available. Placoid Scales: See bioplastic mounts and dogfish
slides. Made of enamel (epidermal) and the dermal derivatives, dentine and
bone with a pulp core. Typical of cartilaginous fishes. Placoid scales are
responsible for the rough feeling of dogfish skin. Ganoid Scales: See bioplastic mounts, slides, the
plates of Sturgeon, called scutes, and the scales of the Gar Pike on display. Made of
multi-layered enamel called ganoin over lamellar bone. Primitive (now extinct) species also had a
cosmine layer and vascular bone with pulp, but these were lost in modern day
examples. Teleost (bony fish) scales These are thin scales of dermal bone.
They have a thin covering of epidermal tissue over them. It is derived by
reduction (loss) of parts of a ganoid scale. There are two types depending on
their shape. Cycloid Scales: See bioplastic mounts and slides. A
round ended scale. Ctenoid Scales: See bioplastic mounts and slides. A comb
shaped end is characteristic of this scale type. Referring to the bioplastic
mount and slides, make a sketch of the placoid, ganoid, cycloid and ctenoid scales. This sketch
is for your own reference. TEETH Teeth are composed of three main parts.
Enamel, the hardest
substance in the body, covers the tooth surface. It is epidermal in origin. Dentin is similar to bone in structure but is
harder. It is located beneath the enamel and forms the walls of the third
component of teeth, the pulp cavity. These are of dermal origin. Teeth are used to catch and hold prey,
to crush hard shells and, in some higher vertebrates, to carry out mechanical
digestion of food in the mouth. How does breaking of food into smaller
pieces improve its digestion? Examine the teeth of Shark on demonstration. Note that mollusk-eating sharks have blunt teeth, while others have a cutting edge. Despite such variations, the scales formed on dogfish skin and the teeth found in its mouth are both made of enamel, dentin and bone with a pulp core, i.e. shark teeth are modified placoid scales. Draw both the cross-section and the flat-mount of dogfish skin. Teeth of higher vertebrates are thought to have evolved from bony dermal scales similar to dogfish placoid scales. They have a complex embryonic origin involving both the epidermis and the dermis. Interestingly, their development bears some resemblance to that of hair and feathers. Mesenchyme cells collect in the dermis to form dermal papillae, which are instrumental in the production of dentin and go on to form the pulp of the tooth. Enamel is produced by the epidermis. The tooth in mammals is held in place by cement, which is a non-vascular form of bone. To compare the various types of
dermal scales with teeth, fill in the chart indicating the presence (+) or
absence (-) of the indicated substances. |
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Type of Dermal Scale |
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ganoin |
dentine |
cosmine |
bone |
pulp core |
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Cosmoid |
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Placoid |
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Shark Tooth |
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Ganoid |
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Cycloid |
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Ctenoid |
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Mammalian
Tooth |
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1) Fishes
Teeth or dermal structures, which are
tooth-like, may be found wherever ectoderm occurs in the mouth area. For this
reason teeth can occur outside the jaws as in the pharyngeal teeth on the bony elements of the branchial
bars, or outside the buccal cavity as teeth in the sawfish. See Amia, Pike and Sawfish.
How does this relate to deuterostome
development in vertebrates (recall the development of the mouth)?
2) Amphibians
Teeth occur mainly on the jaw bones, with
some occurrence on the palate. Examine the skulls of Frog (Anurans) and the
Salamander Necturus (Urodeles) on demonstration.
3) Birds
A few birds have teeth on their beaks.
4) Mammals
Teeth occur only on the jaw bones, i.e. on the dentary, premaxilla and maxilla.
This is the case in reptiles also.
What general trend in tooth
disposition do you
observe?
There are three methods of tooth
insertion. Examine the demonstration material and identify the following
methods of insertion:
1) Acrodont (acro=end)
The teeth are fused by their bases to the
outer surface of the jaw. This condition occurs in most Teleosts (bony fish)
and can be also seen in the sharks. The teeth are not firmly rooted and are
easily lost and replaced. Teeth that are continually replaced are called polyphylodont (poly=many).
2) Pleurodont (pleur=side)
The teeth are fused by one side to the
inner surface of the jaw. This occurs in the Salamander Necturus, frogs, and in the Lizards. Most birds
have no teeth but mergansers, Peking duck and a few others have pleurodont
teeth. Pleurodont teeth are usually polyphylodont.
What is the main reason for the loss of
teeth in birds?
How do they compensate for this loss?
3) Thecodont (theca=cup)
The teeth are placed on the crown of the jaw in a socket. A tooth may have a single root, as in the alligators, or several roots as in the molars of Mammals.
What general trend in tooth insertion do you observe?
Most thecodont vertebrates
replace their teeth only once in their lifetime. What would the reason for this
be?
What advantage(s) do acrodonts
have over pleurodonts and thecodonts? What disadvantage(s)?
The shape of the teeth in the Fishes,
Amphibia and Reptiles is relatively constant in any one group. Functional
adaptations occur in the size, and in specialization such as poison ducts in the fangs of
snakes. Study the demonstration material in the above groups and notice the similarity
in shape - called the Isodont or Homodont (iso/homo = equal) condition.
Dentition in Mammals is generally Heterodont (hetero=different). The teeth are
modified in shape and size to serve specialized functions. Study this
modification in the Wolf skull. Notice incisors (cutting), canines (piercing),
premolars (grinding) and carnassials (shearing), and molars (crushing).
1) Fishes
Teeth or dermal structures, which are
tooth-like, may be found wherever ectoderm occurs in the mouth area. For this
reason teeth can occur outside the jaws as in the pharyngeal teeth on the bony elements of the branchial
bars, or outside the buccal cavity as teeth in the sawfish. See Amia, Pike and Sawfish.
How does this relate to deuterostome
development in vertebrates (recall the development of the mouth)?
2) Amphibians
Teeth occur mainly on the jaw bones, with
some occurrence on the palate. Examine the skulls of Frog (Anurans) and the
Salamander Necturus (Urodeles) on demonstration.
3) Birds
A few birds have teeth on their beaks.
4) Mammals
Teeth occur only on the jaw bones, i.e. on the dentary, premaxilla and maxilla.
This is the case in reptiles also.
What general trend in tooth
disposition do you
observe?
There are three methods of tooth
insertion. Examine the demonstration material and identify the following
methods of insertion:
1) Acrodont (acro=end)
The teeth are fused by their bases to the
outer surface of the jaw. This condition occurs in most Teleosts (bony fish)
and can be also seen in the sharks. The teeth are not firmly rooted and are
easily lost and replaced. Teeth that are continually replaced are called polyphylodont (poly=many).
2) Pleurodont (pleur=side)
The teeth are fused by one side to the
inner surface of the jaw. This occurs in the Salamander Necturus, frogs, and in the Lizards. Most birds
have no teeth but mergansers, Peking duck and a few others have pleurodont
teeth. Pleurodont teeth are usually polyphylodont.
What is the main reason for the loss of
teeth in birds?
How do they compensate for this loss?
3) Thecodont (theca=cup)
The teeth are placed on the crown of the
jaw in a socket. A tooth may have a single root, as in the alligators, or
several roots as in the molars of Mammals.
What general trend in tooth insertion do you observe?
Most thecodont vertebrates
replace their teeth only once in their lifetime. What would the reason for this
be?
What advantage(s) do acrodonts
have over pleurodonts and thecodonts? What disadvantage(s)?
The shape of the teeth in the
Fishes, Amphibia and Reptiles is relatively constant in any one group.
Functional adaptations occur in the size, and in specialization such as poison ducts in the fangs
of snakes. Study the demonstration material in the above groups and notice the similarity
in shape - called the Isodont or Homodont (iso/homo = equal) condition.
Dentition in Mammals is generally Heterodont (hetero=different). The teeth are
modified in shape and size to serve specialized functions. Study this
modification in the Wolf skull. Notice incisors (cutting), canines (piercing),
premolars (grinding) and carnassials (shearing), and molars (crushing).
Examine the general demonstration
of mammalian teeth. Notice the adaptations are correlated with food habits.
Compare the Carnivores, Herbivores, Omnivores (human and pig) and Insectivores (mole and shrew).
In the Carnivores note the specialized long and sharp
canine teeth, and the last premolar of upper jaw and first molar of lower jaw,
which have been modified as carnassials used for cutting. Within the group
notice variations of teeth used for cutting, piercing, incising and gripping.
How do you think the
carnassials function?
Examine upper and lower jaws of
the Herbivores, and
note the differences, a large gap, the diastema (=internal) and often a horny upper pad -
check the horse, deer, rabbit, ox, and beaver. Examine the Omnivore dentition and compare with Carnivores,
Herbivores and
Insectivores.
Bears are Omnivores. Note, however, that their overall tooth
differentiation resembles
that of carnivores. Why do you think this is so?
Why would insectivores need such long and sharp incisors?
Updated by Sandra Millen, August/03