Vertebrate Structure and Development
The skin or integument is often overlooked when we think of
different systems, but it is the largest mass-wise in the body
and without it vertebrates can't survive. This is why burn
victims are at such risk.
1) Physical barrier - this is our 1st line of defense in
keeping out infections and UV light.
2) Water balance - most obvious in amphibians, but gills are
used in water balance in fishes, in terrestrial vertebrates it
inhibits water loss.
3) Thermoregulation - Changes in blood flow to the skin can
increase or decrease heat exchange.
4) Color is used in communication for aggression, sexual
behaviors, warning - don't eat me (Poison dart frogs) or to make
an animal cryptic. Color is used in thermoregulation by lizards
and roadrunners and many others.
5) Movement - thickened areas are specialized for weight
bearing or hanging on (prehensile tail), scales and claws are
used to secure an animal during movement. The foot pads of some
lizards (geckos) allows them to walk up glass and hang upside
down, feathers and gliding folds of some mammals, frogs and
lizards are used for flight.
6) Respiration - gills and skin respiration.
7) Skin glands produce pheromones to aid in courtship and
8) Milk glands
9) Salt balance via gills and sweat
10) Sense organs - heat, pain and touch
11) Produce vitamin D
The superficial epidermis comes from ectoderm and the deeper
dermis from mesoderm. This is true in all verts. The thickness
and subdivisions of these vary throughout the subphylum. If we
make a generic vertebrate we can look closer at the layers.
Starting with the epidermis and going from dermal to
external we have 2 layers. Stratum germinativum - cells formed
in this layer migrate out to form the stratum corneum. Mucus
cells produce mucus, poison and in some fish produce light.
Proteinaceous cells make various substances like slime, enamel
and other things and produce keratin which is used for
waterproofing and in feather, hair, claws, scales and the outer
stratum corneum. alpha-keratin is soft and found in skin. Beta-keratin
is hard (in horns and claws).
Chromatophores are color cells found in the skin. They can
be epidermal or dermal. Birds and mammals have melanophores with
melanin in the epidermis. Color change is slow and caused by the
production of more melanin - morphological color change. Fish,
amphibians and reptiles have several types of chromatophores
found in the dermis - melanophores, iridiophores - guanine
crystals reflect light, xanthophores - yellow and erythrophores -
red. These may be involved in morphological color change or
physiological color change which is rapid and caused by shifting
of the pigment or crystals in the chromatophore.
Cyclostomes (Agnatha) lack keratin and produce lots of
Jawed fishes have thin skin. Most lack keratin. All fishes
secret a slimy protein. Scales of fishes are derived from dermis
Amphibians have a thin stratum corneum. They produce a
mucousy substance that slows dehydration. Mucus glands keep the
skin moist and allow skin breathing. Granular glands secrete
toxins - This is why dogs spit out toads.
Reptiles use keratin to waterproof. In snakes and lizards
the old epidermis is shed periodically. The epidermis divides
and a new inner epidermal layer forms. The 2 epidermal layers
separate and the outer one is lost as a whole piece or in
patches. Turtles and crocs have scale-like plates called scutes.
The scales of snakes and lizards are also referred to as scutes.
There are a few scent glands found in patches on various
Birds have mostly thin skin with little keratin. The
feathers are keratinized as is the beak. The legs have thick
scales. The uropygial gland is used to produce oil to preen the
The typical feather is a contour feather. It functions in
flight. The central structure is hollow near the body (quill)
and solid away from the body (shaft). Barbs project from the
shaft. Barbules extend out from the barb and hook together to
keep the feathers in good shape. Birds will use their bills to
help keep the barbules in place.
Mammals have a thick dermis layer. There are a number of skin
derivatives - scales (rat tails), claws, hooves, baleen plates,
sweat glands, sebaceous or oil glands, scent glands, mammary
glands and hair.
Reptile scales come from the epidermis, but the dermis is
involved in their production. The epidermis in the place where
the scale will form becomes raised and dermis comes up into the
raised area. Later they retract which yields a flat scale made
of stratum corneum. (Fig. 10-13)
The formation of feathers occurs when the epithelium becomes
raised. It is filled with a dermal papilla. The future feather
grows out from the skin with an epidermal coating around a
mesodermal center filled with blood vessels. The epidermis sinks
into the skin and forms a follicle. The feather then develops
into either a contour or down feather. (Fig. 10-15)
Hair is strictly epidermal. It starts with the stratum
germinativum forming a nodule. This grows into the dermis. At
the bottom the dermal papilla forms. Everything above this is
epidermal. The hair grows out through the tube of epidermis.
Also, epidermal cells form the oil gland that supplies the hair
and the mesodermal cells around the hair form the smooth muscle
that attaches to the hair. (Fig 10-17)
There are a number of hard mineralized tissues in
vertebrates. Bone is the most common, but we also see cartilage,
dentin, and enamel.
Bone is formed by osteoblasts. The matrix is
hydroxyapatite. The osteoblasts sit in little lacunae which are
interconnected by canaliculi.
The hard bone which makes up the outer part of bones is
compact bone. The osteoblasts sit in circles around a Haversian
canal. It contains blood and lymph vessels and a nerve fiber.
These feed the bone cells. The very outer covering of the bone
is the periosteum. This covers everything except the
articulation. This Haversian system is typical of humans, but
many small mammals, amphibians, and some reptiles don't have
Spongy bone is found inside many long bones. These lack
Haversian systems. Within the spongy bone are trabeculae which
form cross bars. The hollow inner part houses the marrow. The
endosteum is the connective tissue separating the spongy bone
from the marrow.
Dentin is similar to bone, but is made by odontoblasts.
Today dentin is only in some fish scales and teeth.
All of the mineralized tissues originate from mesenchyme (connective tissue) cells. A blastema is a group of mesenchyme
cells that will differentiate into another tissue type.
Chondroblasts are cartilage cells that lie down cartilage.
Once they get trapped in the lacunae they become chondrocytes.
When cartilage is being formed in an embryo mesenchyme cells come
together. These cells then start to secrete a matrix. This
action pushes the cells apart and the end result is that the
chondrocytes become isolated in lacunae. Cartilage can grow by
adding more tissue to the outside of the existing cartilage or by
expanding internally. Once the cells are isolated in lacunae
they quit dividing, but we may see 2 or 4 cells in a lacuna.
Cartilage is the skeleton of vertebrate embryos and some
There are several types:
1) Hyaline cartilage: is found in joints and is the
precursor of replacement bone.
2) Fibrocartilage: makes up the cartilage between vertebrae
3) Elastic cartilage: flexible cartilage in the ear.
4) Calcified cartilage: has calcium salts and is bone-like.
Found in shark jaws.
Bone can be deposited in 2 ways:
1) Membrane (intramembranous) bone: is bone deposited
directly into the connective tissue blastema (scleroblast). The
mesenchyme cells congregate in strands at the site of the future
bone. These strands produce collagen and make a frame. At this
point the mesenchyme cells have differentiated into osteoblasts
which deposit calcium salts. The cells build a matrix and become
trapped in lacunae. Unlike cartilage cells they aren't isolated
from other cells. Extensions of the cytoplasm of the osteocytes
are connected via canaliculi. Once the cells become trapped in
lacunae they quit actually forming new bone and are called
osteocytes. Now their function is to maintain the bone.
This type of bone formation occurs in the skull and shoulder
2) Replacement bone: often cartilage is deposited into the
blastema. It can be replaced by bone. The cartilage cells are
destroyed and bone cells move in.
Bone growth occurs in 2 ways. In long bones that need to
grow, growth occurs as new cartilage is deposited and old
cartilage is replaced by bone. Adult animals no longer make new
cartilage and so growth in length stops. Long, skinny bones are
not sturdy, so bones need to grow in width too. The inner part
of the bone is destroyed by osteoclasts. These are bone cells
that tear down bone. This creates a hollow core. At the same
time cells in the periosteum are producing new bone.
Like teeth the skeleton is hard and can be preserved unlike
the soft tissues. Therefore, most of our knowledge of extinct
forms is based on teeth and bones. The skeletal system gives
lots of information concerning how an animal moved, relative
sizes of muscles, some information on size and position of nerves
and blood vessels, relative size of the brain and its subparts
and some information on the sense organs of the head.
Along with bone another hard tissue found in the skeleton is
cartilage. Unlike bone cartilage has no nerves or blood vessels.
Often times in an embryo cartilage forms the skeleton and it
is later replaced by bone. This bone is endochondral bone. Bone
that forms without preceding cartilage = membrane bone. Dermal
bone is associated with the outer surface of the body (from
The skull of the vertebrate is a unique structure and
developed to accommodate the needs of active predators. With
cephalization the need for a protective cover for the brain
arose. Initially this cover was made of cartilage.
We can divide the head skeleton into 3 parts.
1) Neurocranium (Chondrocranium - cartilage or Endocranium -
bone) - surrounds the brain and sense organs. Its function is to
protect the brain and special sense organs. It originates as
cartilage and in vertebrates is usually partially or completely
replaced by bone. Unlike other tissue nervous tissue can't stand
bruising and banging. The neurocranium protects it. This
structure forms the base of the braincase and back of the skull. Neural crest
cells and sclerotomes of somites provide the mesenchyme that makes up
In Agnatha and Chondrichthyes this is cartilage and in other
extant classes it's bone. The chondrocranium is one piece, but
as you've seen in lab it's made of numerous bones. The notochord
often becomes attached to the chondrocranium. Embryonically the
sense organs are housed in cartilage capsules. The nasal and
otic capsules fuse to the chondrocranium, but the optic
cartilage doesn't. In our eyes this becomes the sclera. In many
vertebrates it contains cartilage or bone.
This allows movement of the eyes.
We start in the embryo with 6 pairs of cartilages that will
form the chondrocranium. 1) Parachordal cartilages - around
notochord; 2) Prechordal cartilages - form in front of the
notochord; 3) Occipital cartilages - form where the brain meets
the spinal cord; 4) Otic capsule - will form the inner ear; 5)
Orbital cartilage - will form the eye; 6) Nasal capsule - will
form the olfactory organ.
The neurocranium around the foramen magnum ossifies to form
a basioocipital bone, 2 exoccipital bones and a supraoccipital
bone. In mammals these fuse into 1 occipital bone.
The neurocranium articulates with the 1st vertebra via
2) Dermatocranium - forms the plates and armor of several
vertebrate classes. It also forms the outer part of the
braincase. It is derived from membrane bone. It consists of the
roofing bones, dermal bones of the upper jaw, dermal bones of the
palate, and opercular bones.
The bones of the dermatocranium include the : nasals,
frontals, parietals, lacrimals, prefrontals, postfrontals,
jugals, quadratojugals, premaxillae, maxillae, vomers, and bones
of the operculum.
In the lower vertebrates there are a number of bones that
aren't present in humans. The lower jaw may consist of a number
of bones. As one progresses to mammals all but one of these is
lost. Some of the single bones we see in mammals (occipital) are
a number of bones in lower vertebrates.
Diversity in the neurocranium - dermatocranium
Agnatha: the chondrocranium stays cartilaginous.
Chondrichthyes: the chondrocranium is calcified
Osteichthyes: incredible variation. Sarcopterygii
(crossopterygians and dipnoans) have a chondrochranium and
Actinopterygii have an endocranium.
Amphibians: The skull is flattened compared to other
tetrapods. The columella is a bone that sits against the otic
capsule. It functions to transmit sound from the eardrum to the
inner ear. It is the only middle ear bone in amphibians. Many
of the bones of the dermatocranium are reduced or absent. The
area under the eyes lacks bones when a frog blinks the eyes
actually protrude into the oral cavity.
The two occipital condyles form from the exoccipitals.
Reptiles: Many reptiles have an opening - the parietal
foramen which houses the parietal eye. New in this group are
openings in the temporal area. These are bordered by bony
arches. Only the anapsids lack this arrangement.
In synapsids the single temporal fossa has an arch - the
zygomatic arch which is made from the squamosal and jugal bones.
This arrangement is still found in mammals.
In diapsids there are two temporal fossae and two arches. The
lower one corresponds to the zygomatic arch and the upper one,
the supratemporal arch comes from the postorbital and squamosal
bones. Modern lizards and snakes have lost part or all of the
arches and therefore have modified diapsid skulls.
The importance of the fossae is that they allow room for
powerful jaw muscles.
Reptiles are the 1st to have a secondary palate. It divides
the oral and nasal cavities of crocodiles and is made of bone.
It allows chewing and breathing to occur simultaneously.
The reptiles (and birds) have one occipital condyle derived
from the basiooccipital bone.
The jaw joint is formed by the articular and quadrate bones.
The columella is the only middle ear bone.
Birds: A bird skull is a modified diapsid skull and very
reptilian - with modifications for flight, feeding, and a large
brain. The eyes are placed so they have maximal view of the
surrounding world. There is no parietal foramen.
The zygomatic arch is made of the jugal and quadratojugal
bones. Unlike reptiles, the zygomatic arch is very thin.
The bulk of the beak comes from the dentary and premaxillae
bones. The maxillary and nasals often contribute to the beak.
The jaw joint is made of the articular and quadrate bones.
There is only 1 bone (columella) in the middle ear.
There are a number of patterns of palate structure in birds
and this is used for establishing major bird taxa.
Mammals: The dentary is the only bone of the lower jaw.
The skull is more domed to house the large brain. Now the middle
ear has 3 bones.
Because of the birth process the bones of the skull have
soft spots - fontanels. These fill in with bones after birth.
Some of the bones of lower vertebrates (squamosal etc.) fuse to
become the temporal bone. Part of this bone forms the tympanic
bulla which houses the middle ear.
In lower vertebrates the jaw and skull join at the quadrate
- articular joint. In mammals the dentary bone articulates with
the squamosal/temporal bone.
The skull of mammals has air-filled sinuses in the maxilla,
frontal, sphenoid, and ethmoid bones.
Mammals have three pairs of rolled up bones called turbinal
bones. One set of these is covered with olfactory epithelium.
Birds have two pairs and reptiles have one pair of turbinal bones.
3) Visceral skeleton (Splanchnocranium) - the skeleton of
the jaw and gill arches. The pharynx sits between the mouth and
esophagus. It is involved in feeding and in lower vertebrates in
respiration. Evolutionary and embryonically the pharynx has gill
bars and gill slits. The visceral skeleton is derived from
splanchnic mesoderm and neural crest cells.
The protochordates have many gills and they were used to
trap food particles. This worked well for filter feeders, but as
vertebrates evolved the number of gill slits decreased to 6 and
the number of gill bars (= visceral arches) decreased to 7.
The 1st arch becomes the mandibular arch and contributes to
the lower and upper jaw. The mandibular arch is made up of a
mandibular (Meckel's) cartilage and a quadrate cartilage.
The 2nd arch becomes the hyoid arch which contribute to the
hyoid apparatus which supports the jaw.
Arches 3 - 7 become branchial arches and are involved in
respiration. (distinguish between branchial, bronchial, and brachial)
In many tetrapods the branchial arches develop into
structures in the throat and ear. The middle ear bones of
tetrapods come from branchial arch 2 and in mammals branchial
arch 1. The styloid process of humans comes from branchial arch
2. The hyoid comes from branchial arches 2 and 3, and the larynx
comes from branchial arches 4 and 5.
Diversity within the visceral arches:
Fishes: the arches are used in respiration and the pectoral
girdle is joined to the skull.
Amphibia: no longer have gill bars and an operculum. The
larvae and gilled amphibians have 5 branchial arches used in
respiration. Others have 3 arches that support the tongue and
larynx. These arches are now called a hyoid apparatus. The
pectoral girdle is no longer joined to the skull. The
hyomandibula became the columella.
Reptiles and Birds: the jaw comes from the 1st visceral
arch. A bone for hearing and the hyoid apparatus come from
visceral arch 2. The 3rd and 4th arch may be involved in the
hyoid. The 6th and 7th form tracheal rings.
Mammals: the angular bone of the jaw becomes the tympanic
bulla and aids in hearing. Somewhere in the reptile -> mammal
transition a secondary palate formed.
In the stem reptiles (most primitive ones) the external and
internal nares are both at the front of the snout. Longer
chewing and higher metabolic rate and therefore oxygen demands
required a constant air flow to the lungs. As a result the
internal nares moved posteriorly.
Adaptations and bone shifts:
Quadrate - in bony fish -> birds it is part of the hinge of
the jaw. In mammals it becomes the incus - a middle ear bone.
Articular - another jaw bone, becomes the malleus.
Hyomandibula - the second visceral arch in fishes -> stapes(columella) of tetrapods.
See fig. 11-14, 11-16, 11-26.
Last updated on 1 March 2007
Provide comments to Lynnette Sievert at firstname.lastname@example.org
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