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
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
Overall strength of the skeleton arises from a composite assemblage of elements
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
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.
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 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:
- 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
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
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
begins forming in two regions
- within the cartilage (perichondral bone)
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
from the continued production of osteons, and the ossification of the areas around trabeculae, two types of bone form:
- compact bone - dense peripheral bone tissue
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
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:
- 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)
- 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
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
- 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)
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
- lowering the neck to the ground, in contrast, requires muscular
contraction
Two general parts of the skeleton are the:
|
Mandible Hyoid Vertebral column Ribs Sternum |
Appendicular
skeleton:
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
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
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 is sometimes called the neurocranium and covers the ventral, lateral and posterior parts of the brain as well as the ear and nose
Development:
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
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 consists of the visceral arches composed of cartilage or cartilage replacement bone
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)
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
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
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):
- 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
- the prefrontals, postfrontals, and postorbital ring above and behind the orbit
- the jugal complete the lower margin
- intertemporal, sipratemporal, and tabular make up the medial part of the series - usually lost in advanced species
- squamosal and quadratojugal complete the lateral margins
- lateral elements vomer, palatine, and ectopterygoid
- teeth may be present on any or all of these bones
- 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
Agnathans:
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
- a ventral plate (cartilage?) streched across the floor acted as a suctioning device to pull water into the mouth and across the gills
Fish:
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 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 Meckels 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
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)
- arch V form the cartilage of the larynx, or cricoid cartilage
- other arches become the auditory ossicles or other cartilages
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
- 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
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
- 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
- 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
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