We will begin our discussion of the skeletal system by talking about the tissues that will go into the construction of skeletal elements. The two primary components are cartilage and bone, with additional support coming from fibrous materials such as ligaments and tendons.

These structural materials must be able to

• withstand tremendous forces that affect an organism

• support the mass of the body and all of the muscles and organs that are part of the body

• remain strong under the stresses of locomotion, such as when the feet strike the ground, sending the force of the impact through the body frame

• be strong at the junction where two bones meet, where stress is applied and felt

• protect against impact to soft tissues, such as the skull protects the brain

Altogether, bones and other skeletal materials must be resistant to such stresses, or they may break or distort. The types of forces experienced on different parts of the body will influence the structural material that is used.

Overall strength of the skeleton arises from a composite assemblage of elements

• heterogeneous materials composed of many different elements, that is generally much stronger than homogenous materials

• composite materials dissipates fractures and breakage, such that the overall material is much stronger - cartilage without associated supportive materials would be mushy and flatten rapidly

Cartilage: • defined as a firm but elastic skeletal tissue whose matrix contains chondroitin sulfate (ground substance) and collagen or elastic protein (fibers) molecules that bind with water

• the cellular elements of cartilage are called chondrocytes which lie in spaces called lacunae (Fig. 5.20, p. 173) surrounded by the perichondrium, fibrous connective tissue that lies on the outside of cartilaginous tissue

• because cartilage is very watery, it is highly flexible and can change drastically under stress but snaps back into its original shape.

• may be found in several forms (ranked from least dense to most dense): Hyaline cartilage - cartilage with a clear translucent matrix; found primarily on the ends of ribs and on the trachea

Elastic cartilage - cartilage containing elastin fibers that appears yellowish; found primarily on external ear and epiglottis

Fibrocartilage - cartilage containing collagen fibers; found in the intervertebral disks and pubic symphysis

Calcified cartilage - cartilage containing deposited calcium salts; found in the vertebrae of cartilaginous fish

Bone: • also a composite tissue similar to cartilage, but with a greater mineral component, primarily consisting of calcium (Fig. 5.21, p. 174)

• bone consists of calcium phosphate and other organic salts deposited in a matrix

• may be deposited in highly ordered units called osteons (Haversian canal system) - each a series of concentric rings made up of bone cells and layers of bone matric surrounding a central canal through which nervess and blood and lymphatic vessels travel

• bone cells are identified based on their activity:

- osteoblasts produce new bone

- osteoclasts remove and resorb existing bone by secreting acid to break down mineral component of bone and enzymes to break down the collagen component of bone

- osteocytes maintain equilibrium in fully formed bone

• bones that bear more weight, such as the femur, have about 67 percent mineral, whereas antlers contain only 59 percent mineral
Skeletal Development

During embryonic development, the skeleton remains primarily cartilagenous to form the basic structural components and framework of the body

After the basic structure of the embryo is formed, bone begins to be deposited in one of two ways:

1) Membrane/dermal bone

• formed through the deposition of calcium salts and osteoblasts within the connective tissue located near skin surfaces

• formation of membrane bone begins with the formation of trabeculae - small rods or tubes through connective tissue that provide reinforcement

• trabeculae then fuse together and then bony tissue forms around them

2) Cartilage replacement bone • formed in and around the cartilage of the embryonic endoskeleton

• begins forming in two regions

- around the cartilage (endochondral bone), such as occurs on the diaphysis of the bone

- within the cartilage (perichondral bone)

• trabeculae are formed (as they are in membrane bone formation) and act as an interconnected structural network within the bone - the spaces within and between trabeculae form the bone marrow
As ossification proceeds, the replacement of cartilage by bony tissue forms from the diaphysis to the epiphysis (end of bone) • in anamniotes the epiphysis remains cartilagenous until adult size is reached

• in mammals and birds, epiphyseal plates (space between the diaphysis and epiphysis) are formed which remain cartilagenous to allow for room to grow until the organism has reached full adult size

To grow in girth, bones must continually be rebuilt through the combination of osteoblasts and osteoclasts (Fig. 5.24, p. 176). • the resulting structure is columnar, with each osteocyte surrounded by bone to form the osteon , with a vascularized center

• from the continued production of osteons, and the ossification of the areas around trabeculae, two types of bone form:

- cancellous bone - spongelike tissue that lies at the interior part of bone and at the ends

- compact bone - dense peripheral bone tissue

In addition to structural support, bone • can be mobilized for other uses by the body - i.e. calcium source for egg shell production in birds

• is responsible for red and white blood cell production - red bone marrow is the hemopoietic tissue of bone which is replaced by yellow bone marrow later in life, containing mostly fat cells

Joints, tendons, and ligaments:

Each independent part of the skeletal system must articulate with another part, or with other parts of the body (such as muscles)

The joint is a point of articulation between elements, including:

• synarthroses - joints at which there is limited movement but room for growth - sutures where two dermal bones meet, such as in the skull or in the shell of turtles

- synchondroses where sheets of cartilage ossify, but remain separatedby a plate of cartilage (such as in long bone production and the epiphyseal plateor ribs)

- symphyses where two individual halves of bone meet and are separated by cartilage (such as in the jaw or pubic symphysis)

• diarthroses are joints at which there is considerable movement, i.e. hinge joints (elbow), ball and socket joints (shoulder, hip), saddle joints (thumb). - diarthroses are connected by strong ligaments, or sheets of connective tissue composed of collagen

- between bones is a viscous synovial fluid surrounded by a synovial membrane, as well as articular cartilages that help to maintain the fluid movement of the joint

Tendons: • are tightly packed bundles of parallel collagen fibers that connect muscle to bone

• those that move against adjacent tissues often have fluid-filled sheaths surrounding them

• concentrate the force of a muscle onto a relatively small area of the skeleton

• the narrowness of tendons allows many of them to be packed in a small space

• those that must pass around corners behave much like a pulley system, either a bony tunnel or process, or a ligament loop

- generally the forces are tensile

- if shearing forces occur, as when a tendon passes over a bending joint, there may be a sesamoid bone within the tendon to withstand the shearing forces (example the patella or kneecap)

• collagen fibers of a tendon merges with the perimysium and endomysium of the muscle - perimysium of muscles that attach to large areas of bone, such as the supraspinatus muscle, merges with the periosteum of the bone
Ligaments: • join bone to bone, binding the skeleton together in a passive fashion

• are similar to tendons, but may be more variable, having more irregularly arranged fibers, and some elasticity

• nuchal ligaments, for example, are prominent in animals with large heads and/or long necks

- hold the head and neck in a normal posture without muscular effort

- lowering the neck to the ground, in contrast, requires muscular contraction

Breakdown of the skeletal system

Two general parts of the skeleton are the:
Axial skeleton: Skull



Vertebral column



Appendicular skeleton: Pectoral girdle

Forelimbs (or fins)

Pelvic girdle

Hindlimbs (or fins)


The Skull and Visceral Skeleton

General function of the skull and visceral skeleton:

When describing the characteristics of vertebrates, one important characteristic was cephalization, or the evolution of a head region distinct from the body that acted as a centralized location of sensory, nervous and trophic (feeding) function

• these structures require associated protective structures to prevent them from being damaged

• other structures can be derived from the skull (i.e. antlers and horns) for combat or for mating displays

• teeth and tusks are also derived from structures related to the skull

Generally when describing the skull, we divide it into three parts: 1. Chondrocranium - anterior part of the axial skeleton that encases the special sense organs and contributes to the skeletal elements encasing the brain.

2. Splanchnocranium - or visceral arches that support and move the gills and contribute to production of the jaws in gnathostomes.

3. Dermatocranium - dermal bones that encase the chondrocranium and splanchnocranium and contribute to the braincase, jaws, and skeletal elements of the mouth (teeth)

The Chondrocranium:

The chondrocranium is sometimes called the neurocranium and covers the ventral, lateral and posterior parts of the brain as well as the ear and nose


• the chondrocranium is formed by a combination of mesodermal sclerotome and neural crest cells

• during development, cartilage forms around the brain beginning at the notochord

• starts with the development of parachordals (cartilagenous rods) that run anteriorly and meet to form the basal plate

• sense organs are then surrounded by cartilage to form the optic capsules (for the eyes), nasal capsules (for the olfactory organs) and auditory capsules (for the ear) (Fig. 7.1, p. 206 in text).

• in the posterior region of the chondrocranium the occipital arch develops, which is perforated by the foramen magnum to allow for passage of the spinal cord to the developing brain

Parts of the chondrocranium: • the chondrocranium is most visible in more primitive species, such as the cartilagenous fishes

• specific regions, such as the rostrum, denotes the anterior portion of the cranium

• the occipital condyle is in the posterior region of the chondrocranium and articulates with the notochord/vertebral column

• in more advanced vertebrates, the chondrocranium is later ossified and becomes a more minor part of the skull

The Splanchnocranium

The splanchnocranium consists of the visceral arches composed of cartilage or cartilage replacement bone

• visceral arches are the most visible in agnathans

• generally seven visceral arches grow to support the developing pharyngeal pouches that are formed during early embryonic development

• each visceral arch is a > shaped structure composed of two parts, the epibranchial cartilage and a ceratobranchial cartilage (Fig. 7.5, p. 227)

• in more advanced vertebrates, parts of the splanchnocranium are modified to form derived structures such as jaws, ears and parts of the hyoid apparatus and pharyngeal cartilage (Fig. 7.7, p. 228)

Origin of jaws and jaw suspension • jaws are primarily derived from the cartilage of the first visceral arch

• in primitive species such as lampreys, food was moved through the pharynx by moving the first visceral arch to create a pump-like action

• jaw formation probably evolved from selection favoring fish that utilized the first visceral arch to help it seize prey

In the first jawed fishes (Chondrichthyes) the first visceral arch was renamed the mandibular arch, which consisted of two cartilages: the palatoquadrate cartilage (upper jaw) and mandibular or Meckel’s cartilage (lower jaw) (Fig. 7.18, p. 239) • the dorsal part of the second gill arch (hyoid arch) - called the hyomandibular cartilage - articulates with the chondrocranium

• these arches work together in different ways to create jaw movement, based on the ways in which these arches articulate with the chondrocranium (Fig. 7.8, p. 229):

Amphistylic (primitive cartilaginous fishes) - jaw is supported both by the hyomandibular and by a direct connection between the jaw and the chondrocranium

Hyostylic (elasmobranchs and most bony fishes) - upper jaw loses any major direct connection with the chondrocranium and the upper and lower jaws are supported solely by the hyomandibular

Autostylic (lungfishes and in tetrapod ancestors) - upper jaw (pterygoquadrate cartilage) articulates or is fused with the chondrocranium, lower jaw forms from the mandibular cartilage, and the jaw remains unsupported by the hyomandibular

In mammals, the pterygoquadrate cartilage is modified further to form the incus and malleus - the hyomandibular becomes part of the hyoid and the stapes

The fate of the branchial arches is shown in Table 7.2 (p. 228) - LEARN THIS


The Dermatocranium

The dermatocranium is composed of plates of dermal bone that cover the head and protect the brain and gills

Six basic groups of dermal bones make up the dermatocranium (Table 7.3 and Fig. 7.10, p. 230):

1. Facial series - encircles the external nares and collectively form the snout - the maxilla and premaxilla (incisive) define the margins of the snout and usually bear teeth

- the nasal lies medially to the naris

- the septomaxilla a small dermal bone which, when present, sinks below the surface bones and aids in forming the nasal cavity

2. Orbital series - encircles the eye and superficially defines the orbit - the lacrimal is associated with the nasolacrimal duct

- the prefrontals, postfrontals, and postorbital ring above and behind the orbit

- the jugal complete the lower margin

3. Temporal series - lies behind the orbit and completes the posterior wall of the braincase - the otic (temporal) notch suspends the tympanic membrane

- intertemporal, sipratemporal, and tabular make up the medial part of the series - usually lost in advanced species

- squamosal and quadratojugal complete the lateral margins

4. Vault series - the roofing bones that run across the top of the skull and cover the brain beneath - include the frontal (anteriorly), parietal (medially) and postparietal or interparietal (posteriorly) 5. Palatal series - form the roof of the mouth - largest and most medial pterygoid

- lateral elements vomer, palatine, and ectopterygoid

- teeth may be present on any or all of these bones

6. Mandibular series - Encases mandibular cartilage - Meckel’s cartilage is usually encased in dermal bones of this series

- laterally, the wall includes the tooth-bearing dentary and one or two splenials, angular, and surangular

- many wrap around to the medial side of the mandible and meet the prearticular and coronoids

- left and right mandibles usually meet anteriorly at the midline in a mandibular symphasis

An Overview of Skull Morphology


Early ostracoderms posessed a flattened head shield formed from a single piece of of arched dermal bone, two close-set eyes dorsally set and a pineal opening between them

- formed the roof over the pharynx and held the sequential branchial arches

- a ventral plate (cartilage?) streched across the floor acted as a suctioning device to pull water into the mouth and across the gills

Lamprey and hagfish as heirs to ostracoderms lost all bone and specialized for parasitic or scavenging lifestyleswith a rasping tongue - the braincase is entirely cartilage and branchial arches form an unjointed branchial basket All vertebrates, with the exception of the agnathans have jaws and form the group Gnathostomes ("jaw mouth") - modification of the branchial arches into biting or grasping devices


Much of the information describing the three regions of the skull is generally characteristic of the primitive fishes and Chondricthyes, and is the precursor for skull and visceral skeletal formation in tetrapods

In Chondrichthyans, the braincase is an elaborate cartilagenous case around the brain. The only modification in the Osteicthyes is that the skull region and its associated cartilagenous structures are ossified.

- the dermatochranium is absent, reflecting a loss of all bone from the skeleton

- the otic capsue rests on the posterior part of the endocranium and encloses the sensory organs of the ear

- the upper jaw consists of the fully functional endoskeletal palatoquadrate - makes limited contributions to higher vertebrates as the epipterygoid (fuses to the cranium) and quadrate (suspends the lower jaw); replaced by maxilla and premaxilla as the upper jaw

- the lower jaw (mandible) consists only of Meckel’s cartilage - encased in exoskeletal bone of the dermatocranium and supports teeth; may be ossified as the mental (anterior) and articular (posterior)

- the hyoid apparatus derives ventrally from the splanchnocranium as a support for the floor of the mouth and functional gills

In the movement from fishes to tetrapods, several general changes in the skull may be noted: 1. A gradual reduction in the number of the separate bony elements by elimination and fusion - there may be as many as 180 bones in a fish’s skull whereas the human skull contains only 28

2. Autostylic method of jaw attachment - creates changes in the articulation of the jaw and the evolution of the hard palate

3. Shift in gas exchange mechanism from gills to lungs - requires the evolution of a pair of internal nostrils, or choanae that pass from the external nares to the lungs

4. Creates shift in the function of the visceral arches - no longer used to support gills and are often modified to perform different functions (Table 7.2, p. 228)

- hyobranchial apparatus evolves to support the tongue and larynx

- arch V form the cartilage of the larynx, or cricoid cartilage

- other arches become the auditory ossicles or other cartilages

5. Movement of the dermatocranium from a close relationship with the integument to a deeper position in the head where it articulates more closely with the chondrocranium
Stages of dermatocranium development include the formation and fusion of the different cranial bones to surround the endocranium • include a stage in which there are temporary gaps between bones (fontanelles) and fusion to form sutures

• sometimes results in the production of intermediary Wormian bones that are small islands of bone that are between sutures


General characteristics of the amphibian skull are strong deviations from the generalized tetrapod skull

- only partial ossification of the skull occurs, with much of the chondrocranium remaining cartilagenous

- the skull is broad and flat (Figure 7.30, p. 246)

- in aquatic salamanders, as with fish, there is a unidirectional flow of food and water into the mouth and out the gill slits; in metamorphosed salamanders and adult frogs, this is replaced by bidirectional flow or a sticky tongue

Reptiles and Birds - Share common feature of a completely ossified skull and openings (fenestrae) in the outer dermatocranium in the temporal region 1. Diapsida: Superior and inferior temporal fenestrae, above and below the postorbital-squamosal bar. The condition in squamates and birds is highly modified. The lower arch is lost in squamates and in snakes the postorbital bar is also lost.

2. Synapsida: Inferior temporal fenestra only. Found in Mammalia and modified in that the postorbital bar is lost.

3. Euryapsida: Superior temporal fenestra only (Plesiosaurs and ichthyosaurs, both extinct groups)

4. Anapsida: No opening: found in turtles and related fossil forms emarginations of the posterior margin of the skull roof in recent turtles replace fenestrae to allow temporal muscle expansion

- in reptiles the dermatocranium tends to be heavier than that of birds, which is thinner and has air spaces

- birds also have more highly developed vision, which results in reinforcement of the eyeball with a ring of bones (sclerotic bones) that ring the orbit, but do not articulate with it (Figure 7-14, p. 233).

- birds tend to have larger brain-to-body size ratios, requiring increased braincase size

- birds have modification of the jaws into the beak and loss of articulated teeth

Mammals - marked by a loss of the sclerotic bones and an increase in other structures associated with sense organs and change in the feeding apparatus

- increased dependence on the senses of smell and hearing; evolution of turbinate bones (also called nasal conchae) that increase the surface area available for olfaction; further modification of branchial arches into the auditory ossicles: malleus, incus and stapes

- formation of the hard palate or roof of the mouth that continues as the soft palate that allow for feeding while still being able to breathe; epiglottis also forms to deflect food away from larynx entrance and into esophagus

- a hard palate also leads to shift in dentition; teeth located at jaw margins and socketed in the jaw itself; primitive condition of teeth is homodont thecodont, which is undifferentiated tooth structure in single alveoli (scokets); derived condition is heterodont, where teeth are modified for different functions: incisors used for nipping, canines for grasping or tearing, and premolars and molars used for cutting and grinding

Definitions Acoelous - centrum flat on posterior and anterior surface, characteristic of some reptiles and mammals

Amphicoelous - centrum hollowed at anterior and posterior end, characteristic of teleosts and early reptiles

Amphistylic - jaw is supported both by the hyomandibular and by a direct connection between the jaw and the chondrocranium; found in primitive cartilaginous fishes

Atlas - the first cervical vertebra of terrestrial vertebrates, which articulates with the skull; nodding movements of the head occur between the atlas and the skull

Autostylic - upper jaw (pterygoquadrate cartilage) articulates or is fused with the chondrocranium, lower jaw forms from the mandibular cartilage, and the jaw remains unsupported by the hyomandibular found in lungfishes, and in the tetrapod ancestors

Axis - the second cervical vertebra of mammals; rotary movements of the head occur between the atlas and axis

Calcified cartilage - cartilage containing deposited calcium salts. Found in the vertebrae of cartilaginous fish

Carinate - having a massively enlarged sternum to support flight muscles

Cartilage - a firm but elastic skeletal tissue whose matrix contains proteoglycan molecules that bind with water.

Choanae - internal nostrils that pass from the external nares to the lungs

Chondrocranium - anterior part of the axial skeleton that encases the special sense organs and contributes to the skeletal elements encasing the brain

Chondrocyte - a mature cartilage cell

Composite materials - heterogeneous materials composed of many different elements, that is generally much stronger than homogenous materials.

Cricoid cartilage - cartilage of the larynx that develops from the hyobranchial apparatus

Dermatocranium - consists of dermal bones that encase the chondrocranium and splanchnocranium and contribute to the braincase, jaws, and skeletal elements of the mouth (teeth)

Elastic cartilage - cartilage containing elastin fibers that appears yellowish. Found primarily on external ear and epiglottis.

Fibrocartilage - cartilage containing collagen fibers. Found in the intervertebral disks and pubic symphysis

Fontanelle - temporary gaps between bones, such as those that occur during the formation and fusion of the cranial bones

Heterodont - teeth modified for different functions

Hyaline cartilage - cartilage with a clear translucent matrix. Found primarily on the ends of ribs and on the trachea

Hyomandibular cartilage - dorsal part of the hyoid arch that articulates with the chondrocranium

Hyostylic - upper jaw loses any major direct connection with the chondrocranium and the upper and lower jaws are supported solely by the hyomandibular; found in elasmobranchs and most bony fishes

Lacunae - small cavities, such as those in bone or cartilage, that contain osteocytes or chondrocytes

Membrane bone - superficial bones that lie in or just beneath the skin and develop from the direct deposition of bone in connective tissue

Opisthocoelous - centrum convex on anterior surface, concave on posterior surface, characteristic of some of the vertebrae of reptiles and mammals

Perichondrium - the connective tissue that lies on the outside of cartilaginous tissue

Procoelous - centrum concave on anterior surface, convex on posterior surface, characteristic of some reptiles and amphibians

Pygostyle - the fused caudal vertebrae of a bird that support the tail feathers

Ratite - lacking a distinctive keel or sternum for attachment of breast muscles

Sacrum - the union of two or more vertebrae and their ribs, by which the pelvis articulates with the vertebral column

Sclerotic bones - bones that surround the orbit but do not articulate with it, and provide reinforcement for the eye

Splanchnocranium - or visceral arches that support and move the gills and contribute to production of the jaws in gnathostomes

Thecodont - also called homodont, or undifferentiated tooth structure

Turbinate bones - bones of the nasal cavity that increase the surface area available for olfaction

Wormian bones - intermediary bones, or small islands of bone that occur between sutures in the skull

Zygaphophysis - articular processes that extend forward and backward of neural arches and help to strengthen union between vertebrae