Vertebrate Structure and Development
Lecture Notes


Muscle System

The skeletal system is very useful in determining phylogeny, but the muscular system gives use very few clues about relationships. They are very plastic and adapt readily to serve new functions.

Muscles function in: 1) movement; 2) blood flow - heart, smooth muscle and skeletal muscles; 3) eye movement; 4) speech; 5) produce heat; 6) make body shape; 7) protect some internal organs.
Muscle attachment varies with function and location. Muscles may attach directly to skin or bone (face). Tendons attach bone to muscle. Ligaments attach bone to bone. An aponeurosis attaches to muscle and is a strong flat sheet of connective tissue. Fascia is tissue that attaches muscle to muscle or muscle to skin.

An insertion of a muscle is the more movable end and the origin is fixed. Antagonists are muscles that have opposite functions. Synergists work together.

There are 3 categories of vertebrate muscles:, cardiac, smooth and skeletal. Skeletal and cardiac muscles are actually a syncytium. They appear multinucleated, but what happened in the embryo was that when muscle cells divided we had mitosis without cytokinesis.

Skeletal muscle anatomy:

Each fiber is surrounded by connective tissue called endomysium. The fibers are grouped into bundles. Each bundle is surrounded by connective tissue called perimysium. Groups of bundles make up the entire muscle. Each muscle is covered by connective tissue called epimysium. The epimysium may blend into the tendons or fascia. Another name unique to muscle cells is sarcoplasmic reticulum. This is the endoplasmic reticulum of the cell.

All a skeletal muscle cell can do is contract or relax. Within a given cell the contraction is complete or not-at-all. This is called the all-or-none law. Contraction occurs because of nervous input. A nerve cell and the skeletal muscle cells it innervates form a motor unit. All of the muscle cells of a motor unit contract together. The amount of contraction in the entire muscle is a function of how many motor units are involved.

Muscle comes from mesoderm. The exact mesoderm it comes from depends on where the muscle is located. If you think back to the chick models remember that you found somitic mesoderm along the midline and it gave rise to the somites. Along the lateral part of the embryo you found lateral mesoderm (lateral plate). This tissue split early in development into somatic and splanchnic mesoderm except in the head area.

Skeletal muscles are derived from both the somitic and lateral plate mesoderm. The somitic mesoderm of interest to us is the myotome.

Let's look first at the myotome contribution to skeletal muscles. In the head region the pre-otic myotomes give rise to the voluntary muscles of the eye. The post-otic myotomes give rise to the muscles of the pharyngeal region. The occipito-cervical myotomes give rise to the skeletal muscles of the tongue, head and neck. The trunk and tail myotomes give rise to muscles of the trunk, tail, and limbs.

The lateral plate divides into the somatic and splanchnic parts and the somatic mesoderm gives rise to the limb skeletal muscles. In some vertebrates the ventral and lateral trunk muscles may come from here too. In the head region the lateral mesoderm doesn't split into 2 layers. The lateral plate in this region forms the muscles of the visceral arch region.

That is the general picture for vertebrate skeletal muscles.

Fishes and tetrapods show a different pattern in where the skeletal muscles of the limbs come from. In fishes the fin muscles come from trunk myotomes that spread into the mesenchyme of the embryonic fin. In tetrapods the limb muscles come from mesenchyme in the developing limb bud. The limb bud is made of somatopleure and the mesodermal part (mesenchyme) becomes the skeletal muscle, bone (cartilage), and dermis. The ectodermal part becomes the epidermis.

The shoulder and hip girdles get their skeletal muscles from both myotomes and mesenchyme that's in the area.

The trunk muscles on the dorsal 1/3 of the body come from myotomes. The ventral half come from the somatic mesoderm. The muscles in-between come from both.

We can see the remnants of the myotome positions in the muscles of fish. If you look at the muscles (myomeres) each one is made from an original myotome. Connective tissue separates each myomere from its neighbor and so you can see this arrangement. These large muscles in fish provide the main propulsion as the fish swims.

In tetrapods the shift is away from swimming to walking on the limbs. Therefore, the back muscles become reduced and are used for support of the vertebral column. Also, the number of muscles becomes reduced as the original myotomes no longer give rise to single myomeres, but rather larger muscles that go the length of more than one segment evolved. We still see a great deal of segmentation of the small back muscles, but not to the degree found in fishes.

The smooth muscles of the skin are derived from the dermatome. These are only found in birds to erect their feathers and in mammals to erect hair.

There are a number of muscles that you didn't look at in the cat that we will look at in the frog.
We'll look at the smooth and cardiac muscles when we get to the digestive and circulatory units.

The electric organs of some fishes is a modified muscle. These produce, store, and release electricity. This can be used to subdue prey (eels). Most fish have much weaker electric organs that are used for communication and sensing objects in the water.

The electric organ is made of modified skeletal muscle fibers sitting in a jelly like substance. The whole thing is surrounded by a connective tissue covering. These disks of muscle fibers sit together. Like muscles the electric organ is activated by nervous input.

Digestive Tract and Coelom

Coelom

Functions: provide room for internal organs to move - stomach, urinary bladder and uterus expand. Lungs can move when the heart beats, the liver and stomach can move down when the lungs fill.

The coelom is a body cavity that is different from other cavities in that it is lined with epithelium derived from mesoderm. The mesoderm starts as lateral plate mesoderm and it splits into somatic and splanchnic mesoderm. In between is a cavity - the coelom. Remember how the spinal cord and notochord separate the mesoderm on each side? Initially, we start with a pair of cavities. They merge over time and in the adult extend from behind the pharynx to the cloaca.

The somatic part of the mesoderm forms the outer wall of the coelom and in adults we call it parietal peritoneum. The splanchnic layer comes to surround the organs and is called visceral peritoneum. Its purpose is to secrete a lubricant so organs can slide by each other. Where the visceral peritoneum connects organs to the wall of the coelom we call it a mesentery. Mesenteries are reinforced with elastic or collagenous fibers.

Often we find vessels and nerves embedded in the mesenteries. Mammals have fat in them also. The mesenteries above the gut are the dorsal mesenteries. Those below are the ventral mesenteries. The ventral mesentery degenerates.

Some mesenteries connect 2 organs to each other. These are called omenta, but some are called ligaments. They aren't the same ligaments we've already discussed.

In vertebrates we see a transverse septum that divides the coelom. The heart is separated from the more posterior organs. We see this in some Agnathans and fishes and salamanders.

The heart is in the pericardial cavity and in fish the other organs are in the peritoneal cavity. Salamanders have lungs sitting behind the transverse septum and so we call their main cavity a pleuroperitoneal cavity. In this case the lungs are attached to the side of the coelom so we call the mesenteries lateral mesenteries.

In most tetrapods there is a membrane separating the heart and lungs - pleuropericardial membrane.

Mammals - the heart sits in a pericardial cavity and the lungs sit in pleural cavities. These 2 organs are completely walled off from the rest of the coelomic organs by a muscular diaphragm. Part of it comes from the transverse septum and part from the dorsal mesentery. Part comes from the pleuroperitoneal membranes. The skeletal muscle portion (which is why you have control over it) originates in the neck region. This is why it is innervated by cervical nerves.

In mammals the heart and several other structures separate the 2 lungs. This structure is called the mediastinum.

The mesenteries of fish are usually pigmented to protect the gonads from UV light. In reptiles we see either pigmentation in mesenteries or in the layers of the skin.

Digestive System

Functions: 1) Take in food; 2) Store food; 3) Break up food; 4) Absorb nutrients; 5) Process and remove solid waste.

Unlike a filter feeder vertebrates normally eat meals. The food can't be immediately processed so it must be stored. For most vertebrates this is in the stomach. Birds have a crop and some rodents have cheek pouches - these may have functions other than immediate consumption of food.

Breaking down food involves both physical and chemical components. The teeth grind, rip and chew food, the stomach may further mash food. Fluids in the digestive system dissolve food. Some fats are broken by emulsification. Digestive enzymes break down food chemically. Some mammals even have bacteria to help break down plant material.

Absorption means moving nutrients from the gut to the blood vessels and lymph vessels.

The digestive system is readily modified to meet the dietary needs of the animal, so from an evolutionary perspective it doesn't tell us much.

Embryonically the gut either sits above the yolk or is surrounded by the yolk-filled endoderm. Very early on the gut has a blind pouch in front and one in back. The central part is open to the yolk stalk. As time goes on, the foregut and hindgut continue to grow and the midgut basically disappears. A stomodeum forms at the anterior end separate from the foregut. When the membrane between the stomatodeum and foregut disappears it leaves the oral cavity. A proctodeum forms at the posterior end, separated from the hindgut. It becomes the anal cavity.

The foregut is all of the digestive tract to the intestine. The hindgut is the intestine.

Mouth

Probably the 1st vertebrates were filter-feeders with small mouths and oral cavities and a large pharynx. Agnathans have a small oral cavity and eat soft, small food or attach to fish as parasites.

Chondrichthyes and Osteichthyes have a tongue that isn't very movable, they have limited salivary glands. The actual mouth varies throughout these groups.

Tetrapods need salivary glands to moisten food. We now see the mouth used to break down food and add digestive enzymes used to break down starch. The tongue is involved in food manipulation. In frogs and anteaters the tongue is sticky so that insects stick to it. Some snakes, lizards and shrews have salivary glands that have been modified to secrete poison. Vampire bats and lampreys which feed on blood secrete anticoagulants in their saliva. Marine reptiles and birds have modified salivary glands to help them get rid of excess salt from their salty diets.

Teeth

We think of tissue as soft, but there are four types of hard tissue. Bone is widespread in the vertebrates and for many makes up the endoskeleton. Cartilage is widespread in vertebrates. Dentin is harder than bone and occurs in teeth, denticles (tooth- like and project from the body surface), scales and armor. Enamel is the hardest. It occurs in teeth, denticles, scales and plates.

Because teeth are very hard they show up in fossil records. Teeth have to be suited for the diet of the animal, so examining teeth tells us about the habits of an extinct animal. Because teeth are typically species specific they can be used for identification.

The crown of the tooth is the exposed area and the root is buried within the gum. The hard external surface of the crown is made of enamel. The bulk of the tooth is dentin. The internal core is the pulp cavity. It contains nerves and blood vessels. Cement is a modified bone that lines the root of the tooth.

Embryonically a tooth starts where there is an enamel organ. It secretes enamel. A dental papilla forms under the enamel from mesenchyme that came from neural crest cells. It will secrete the dentin. The cells that make the dentine are odontoblasts. The papilla itself will become the pulp once after the dentin is layed down. The enamel organ becomes 3 layers, but only the inner layer produces the enamel. Once the tooth is completed, the enamel organ disappears. The odontoblasts produce dentin, but they stay behind the dentin so they aren't walled in. The pulp becomes a support for nerves and blood vessels.

We can divide teeth by how they are attached. Pleurodont teeth are attached to the jaw on their lateral side. Acrodont teeth sit on the rim of the jaw and have limited roots. Thecodont teeth sit in sockets. Some fishes have teeth attached to skin.

In some groups tooth replacement is constant. These animals are polyphyodont. Mammals typically have 2 sets of teeth throughout life and are diphyodont. The 1st set may only occur in the embryo or in the juvenile. In diphyodont species the molars may only be represented in 1 set. In polyphyodont species the teeth typically occur in 2 sets. Only 1 set is replaced at a time and replacement is in a wavelike motion from back -> front.

Cyclostomes - teeth are cone-shaped and found along the mouth.

Other fishes - many cone-shaped or sharp teeth that are all the same (homodont). Teeth may be found all over the mouth. The teeth are acrodont.

Amphibians - this group shows a reduction in tooth number (some anurans lack them). The teeth are little and pleurodont.

Reptiles - These have mostly homodont teeth. Depending on the species they are pleurodont, acrodont or thecodont. Turtles lack teeth, but crush food with their beak.

Birds - These are toothless although ancestral birds had reptilian teeth.

Mammals are heterodont and the shape of a tooth is related to its function. The teeth are thecodont. The number of teeth tends to be less in mammals than in other vertebrates. Mammals have 4 types of teeth. Incisors hold food and are used in grooming or cutting vegetation. They have 1 root and sit anterior in the mouth. Canines are used for holding and piercing. They also have 1 root. No animal has more than 1 canine tooth in a row of teeth. Premolars and Molars have several roots and cusps. Only 1 set of molars is produced.

Dental formula describes the teeth on 1 side of the mouth. It goes in order from front to back. 
          2, 1, 3, 3
          -  -  -  -
          2  1  2  2
Gut Wall

The gut has several layers which are found throughout most of the system. The inner layer is the mucosa. This lines the lumen. Digestive enzymes are secreted from this layer.

Next is the submucosa. This layer has nerves, blood vessels and lymph vessels. Surrounding this is the muscularis externa. This is a smooth muscle layer that has two parts. The one part has circular fibers and the other has longitudinal fibers. By alternately contracting they cause peristalsis and segmentation.

Peristalsis - pushes chyme in one direction.
Segmentation - mixes chyme

The outer layer is the serosa which is synonymous with the visceral peritoneum.

Esophagus

The esophagus of vertebrates is a tube that connects the pharynx and stomach. It secretes mucus to help move food. Animals that swallow rough food have a cornified surface. In mammals the muscular layer is partly skeletal muscle.

Stomach

There are sphincter muscles at either end of the stomach. The cardiac sphincter prevents reflux of acidified food into the esophagus. The pyloric sphincter prevents food from entering the intestine prematurely.

Agnathans lack a stomach. Other fish, amphibians and reptiles have a straight or J shaped stomach. Their esophaguses are fairly short except in reptiles. Birds have a long esophagus with a crop. It sits far forward to allow flight. Their stomach has 2 parts the anterior proventriculus
produces digestive enzymes. The posterior ventriculus
(gizzard) is used for grinding. Mammals have a long esophagus. In some orders the stomach is baglike and in ruminants it is complex.

Intestine

The 1st portion of the intestine is where most digestion occurs and absorption also occurs. The process of absorption requires great surface area. Making the small intestine longer isn't feasible so instead the inner surface is folded. The inner surface has visible folds, on these folds are small microscopic folds (villi) and on these folds are even tinier microvilli. This inner folding will increase surface area up to 600x.

Tetrapods have a small and large intestine. For most of them the duodenum is a distinct portion of the small intestine. The mammalian small intestine can be divided into the duodenum, jejunum and ileum.

Within the small intestine proteins, fats and carbohydrates are broken down to their component parts. Hormones that influence digestion are also released.

The large intestine absorbs water and electrolytes and forms feces. The part that leaves the coelom is the rectum.

Many vertebrates have a cloaca which is a common area for dumping products of the digestive, excretory and reproductive systems. Frequently there are baglike structures protruding from the intestines where the small and large intestine unite. This bag is a caecum. It is used for storage, fermentation and vitamin concentration.

Agnatha - intestine goes from pharynx -> cloaca. Sometimes the posterior portion of the pharynx is called the esophagus.

Chondrichthyes - the intestine is N shaped. The last part of the N has the spiral valve. It functions to increase surface area. They also have a rectal gland for salt balance.

Osteichthyes - also has a spiral valve in some species. Most species have 1 or more ceca. They lack a rectal gland.

Amphibians - adults are carnivorous and have a short intestine. Frog tadpoles are vegetarians and have a long intestine. They often have a caecum.

Reptiles - the intestine is relatively short except in turtles which are usually herbivorous or omnivorous as adults. Reptiles often have a caecum.

Birds - the intestine is long and there are 2 cloacal bursae. These are part of the immune system.

Mammals - in carnivores (insectivores) the intestine is relatively short. Herbivores have very long intestines. A caecum is usually present.

The digestive tract mirrors the feeding of a vertebrate. Animals that eat continuously or eat nutritious fast digesting food have no need for a storage organ and often the gut is short. Carnivores also have a short gut, but eat big meals, so they need a storage organ. Herbivores eat low quality food. They need lots of storage room. Bacteria in the stomach may also digest cellulose.

Accessory Digestive Organs

There are several accessory digestive organs. The liver is a storage organ for carbohydrates (glycogen) and fats. It converts protein to fat or carbohydrate. It is involved in yolk production. It produces blood cells in embryos and destroys old RBCs in adults. It detoxifies the blood. It manufactures and stores vitamins. It emulsifies fat.
Blood goes into the liver from the intestine and enters sinusoids. These literally bathe the cells in blood. Only 1 or 2 cells sit between sinusoids. Blood then goes into hepatic veins and leaves the liver.

The liver produces bile, an emulsifier. When it is not needed for digestion it is shunted into the gallbladder - if the species has one. When food is in the duodenum bile is released. The ducts that carry bile are the cystic duct which carries bile into and out of the gallbladder, the hepatic ducts which carry bile from the liver, and the common bile duct which carries bile into the duodenum.

Pancreas - this organ is present in all vertebrates. It is exocrine in that it secretes digestive juices and endocrine because it produces hormones. The groups of cells that are endocrine are the à cells and á cells. In the embryo of a vertebrate the pancreas starts as one or two ventral buds and a dorsal bud. The ventral buds form the body of the pancreas and the dorsal bud forms the tail.

Feeding Strategies

An animal must feed, so it is not surprising that locomotion, defense and various other behaviors are correlated to feeding. Sensory equipment needs to be adequate to detect prey.

Feeding In Water:
This provides real problems because water is harder to move through. Filters need to allow water through, but still filter food. The predator's movements may move the prey too far.
A) Filter feeders - often use the gills for filtering. The gill structures are covered with mucus and food sticks to it. We see this in lamprey larvae, many fishes, tadpoles, ducks and baleen whales. Fishes move food from the gill area to the esophagus as do tadpoles. Ducks and flamingos have modified bills for straining food. Baleen whales have baleen plates attached to the upper jaw.

B) Suction feeders - suck in relatively large prey. The animal rapidly expands the back of the oral cavity and this causes water to rush in taking the prey with it. This is seen in sharks, bony fish, salamanders, turtles and walruses.

C) Jaw Protrusion allows the front of the head to move out and then back into place. Often this is used to enhance filter feeding by making the mouth tubular. It makes sealing the mouth after suction more easy.

Cranial Kinesis:
This is the ability to move one part of the skull relative to another. This only involves up and down movements. This may involve hinges or joints that allow the snout or beak to move up and down relative to more posterior parts. Found in some bony fish, amphibians and lizards. Found in all snakes and birds.

Projectile Feeding:
This involves hurling the front part of the body at the prey. This may involve hurling the entire front part of the body (snakes), hurling a head on a long neck (turtles, herons, loons) or hurling just the tongue (amphibians, lizards, woodpeckers, anteaters). This is called a lingual flip. The protractor muscles hurl the tongue out and the retractors bring it back in.

The food of vertebrates is often not easily accessible. Invertebrates often hide under rocks, leaf litter, mud or tree bark. We see many adaptations for uncovering food that includes both anatomy and behavior.

Many vertebrates have special structures for reaching hard to get high structures or inaccessible structures. Tool use has shown up in a number of vertebrates - esp. birds and mammals. Lures have evolved in fish and turtles to attract bait to the mouth. Raptors have feet adapted for quick grabs and securing prey in flight.

Getting the food is just the 1st step. Next it must be processed in the oral cavity. For aquatic vertebrates water keeps the food buoyant and teeth help move it along. Terrestrial vertebrates use the tongue to position food.

Terrestrial vertebrates with teeth have a distinct bite cycle that is repeated while they are chewing on food. Since the tongue is so important they are often called lingual feeders. In general they have a mobile, large tongue, the roof of the mouth is domed and frequently rough. The tongue helps cycle food through the mouth so it can all be chewed.

A modification of this is inertial feeding. The head is held still with the food clamped in the jaws. Rapidly the jaws open and the food sits still while the head goes forward. Dogs chewing a piece of meat do this. These animals have light heads that are easy to dart forward.

We can define a number of different types of feeding modes.
Carnivores - eat other animals.
Scavengers - eat carrion as the bulk of their diet.
Insectivores - eat insects.
Herbivores - eat vegetation.
Omnivores - eat both plant and animal material.
Granivores - eat seeds.
Piscivores - fish eater.

Carnivores - need either feet (talons of raptors) or teeth (sharp) for killing or holding prey. In carnivorous mammals carnassials are specialized cheek teeth that are enlarged and act to shear food. These work so well that in some species other cheek teeth are reduced. Birds lack teeth, but a sharp beak and sharp claws can shred prey.

Animals that eat fruits, seeds or nuts need to crush the food. Their teeth resemble a mortar and pestle. Birds use their gizzards to crush food.
Insectivores often swallow their prey whole, but some need to chew their food. This aids digestion by breaking up the food or puncturing it to allow digestive juices in. The teeth will match the preferred prey. Bugs with tough chitin coverings need to be pierced with sharp teeth and wormish prey need to be ground with a mortar and pestle. Animals that eat a variety of inverts have both types of teeth.

Omnivores usually have generalized teeth.

Herbivores need to cut plant parts with their beaks or teeth. Mammalian herbivores then grind their food in the mouth. Nonmammalian herbivores don't.

In mammalian herbivores there's a gap between the front cutting teeth and the back grinding teeth - the diastema. When the incisors are cutting, the cheek teeth don't come together. When the cheek teeth are grinding the incisors don't meet. Many herbivores spend a great deal of time chewing so the teeth need to stand up to it.

Very large herbivores have large, flat teeth. Often the food is course yet the teeth have to last a lifetime. In young animals the roots run deep and the crown is high. Over time chewing wears the crown down. As this happens the roots move up and bone fills in underneath. By the time the tooth is completely gone the animal is very old.

Elephants have 6 teeth in each side of each jaw. The teeth are positioned to maximize the amount of wear they endure. As each tooth wears out it is lost and replaced by another. By the time the animal gets to the 6th tooth it is very old.

All herbivores have ridges and valleys in the cheek teeth that increase their grinding ability. Some small mammalian herbivores have ever-growing teeth.

We've been talking about animals that chew or rip food. Many fish, amphibians, reptiles, birds and a few mammals swallow prey whole. Most of them are careful to align the prey so it slides down without roughing up the inside of the oral cavity. In these animals teeth serve to shove food down to the throat or keep food from getting away.

Snakes and some other animals swallow large prey. Snakes can move the bones of the lower jaw laterally to enlarge the oral cavity. They can also move the 2 sides of the jaws independently to walk teeth down the prey. Egg-eating snakes normally have projections from the throat or esophagus to open the egg.

Birds that swallow seeds or nuts often eat grit to enhance processing the food in the gizzard.

Birds and bats that feed on nectar and pollen ofetn have specialized beaks or long snouts for getting in flowers. Their teeth and jaw muscles are weak, but the tongue is long and able to get to the nectar.

Ant and swarming insect eaters often have reduced teeth - or no teeth, a projectile tongue and tough skin or thick fur to prevent bites. The nostrils may close and even the eyelids and cornea may be thick to resist bites.

Last updated on 1 March 2007
Provide comments to Lynnette Sievert at sievertl@emporia.edu
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