Vertebrate Structure and Development
Lecture Notes


Skin

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.

Skin functions:
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 territoriality.
8) Milk glands
9) Salt balance via gills and sweat
10) Sense organs - heat, pain and touch
11) Produce vitamin D

Skin anatomy:

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 mucus.

Jawed fishes have thin skin. Most lack keratin. All fishes secret a slimy protein. Scales of fishes are derived from dermis (mesoderm).

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 reptiles.

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 feathers.

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)

Mineralized Tissue

Bone

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 them.

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.

Cartilage

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 fish.

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 (the disks).
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 girdle.

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.

Head Skeleton

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 dermis).

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 the neurocranium.

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 occipital condyles.

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 cartilage.

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 sievertl@emporia.edu
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