a) cranial ("most modified (i.e., complex) part of skeletal system")
b) post cranial (vertebral column and associated ribs and sternum)
2) appendicular
a) forelimb (pectoral limb)
b) hindlimb (pelvic limb)
Skull is defining characteristic of "vertebrates"
modern term for phylum of animals to which humans belong is Craniates
Bone growth
bone always forms (ossification) in a precursor tissue
a) endochondral ossification - precursor tissue is cartilage
found in both cranial and post-cranial skeleton
b) intramembranous ossification - precursor tissue is membrane (dense connective tissue)
process forms (1) dermal, (2) sesamoid and (3) periosteal bone
dermal bone largely restricted to skull but includes portion of clavicle
remnant of ancestral exoskeleton
dermis layer of skin provides membrane for ossification
Divisions of skull
1) functional
a) neurocranium - houses the brain
b) facial skeleton (viscerocranium) - surrounds oral and nasal cavities
2) developmental - skull divided into components by embryological / phylogenetic origin
a) chondrocranium - forms base of skull; as name suggests endochondral ossifications
b) splachnocranium - derived from pharyngeal arches; also endochondral ossifications
c) dermatocranium - remnant of ancestral exoskeleton; as name suggests dermal ossifications
Chondrocranium
phylogenetically second oldest part of skull
as name suggests, series of cartilages which form base of cranial cavity
derives from
1) series of paraxial mesodermal condensations (names not important), and
2) sensory capsules which form which form around the nasal, eyes and the inner ear organs
capsule and condensations turn to cartilage, expand and then ossify to form bones of the skull base
bones include: portions of occipital, temporal, and sphenoid and all of ethmoid and inferior nasal concha
Splanchnocranium
phylogentically oldest part of skull, derived from pharyngeal arches
pharyngeal arches present in chordates (sister group of craniates (vertebrates))
pharynx ancestral respiratory and feeding organ
series of U-shaped bars separated by slits (incomplete in tetrapods), each bar containing a cartilaginous skeleton with associated muscles and nerves
these cartilaginous rods give rise to the bones and cartilages of the splanchnocranium
small but important part of cranial skeleton
derivatives of splanchnocranium
first or mandibular arch: cartilage splits into cranial (palatoquadrate) and caudal (mandibular) arches around which the dermal bones of the upper and lower form
palatoquadrate: incus (quadrate) and alisphenoid (greater wing of sphenoid)
mandibular cartilage: malleus (articular)
second or hyoid arch: stapes, styloid process, body and lesser horn of hyoid
third arch: greater horn of hyoid
fourth and fifth (sixth) arches: laryngeal cartilages
Evolution of mammalian ear ossicles
in earliest mammals (pelycosaurs), lower jaw reptilian like (see iguana)
1) lower jaw formed by multiple bones
tooth bearing dentary
multiple post-dentary bones
2) jaw joint (primary) formed by quadrate and articular
unique to mammals, however, is that tympanum carried not by quadrate (upper jaw) but by post-dentary angular bone
thus lower jaw functioned in both feeding and hearing
during evolution (and development) note two trends:
1) expansion of dentary
ultimately contacts squamosal and forms new (secondary) jaw joint (TMJ or sq/d)
2) reduction of post-dentary bones with eventual transferal from "jaw" to temporal bone
basically separates original dual functions of jaws (hearing and feeding) to increase acuity of hearing
Dermatocranium
phylogenetically, remnant of ancestral exoskeleton
primitive condition for all vertebrates is to be encased in a bony exoskeleton of dermal origin
secondarily evolved exoskeletons common in many terrestrial vertebrates
as name suggests, dermatocranium represents intramembranous ossifications within the dermis of the skin
gives rise to the vast majority of bones covering the cranial cavity and face
dermal bones: frontal, parietal, zygomatic, nasal, vomer, lacrimal, palantine, maxilla, dentary (mandible) and portions of occipital, temporal and sphenoid
Composite bones
result from fusion between separate centers of ossification
found in mammals and birds (endotherms)
phenomenon accounts for apparent complex morphology of many cranial "bones"
temporal
fusion of elements from all three developmental components
dermatocranium > squamosal and tympanic annulus (angular)
chondrocranium > petrous (prootic and opisthotic) and mastoid
splanchnocranium > styloid process
sphenoid
also fusion of elements from all three developmental components
dermatocranium > pterygoid process (pterygoid, ectopterygoid and parasphenoid)
chondrocranium > oriboto-, pre- and basi-sphenoid
splanchnocranium > alisphenoid (greater wing)
occipital
fusion of chondocranial and dermatocranial elements
joint (G, arthroses) = articulation between two or more bones
joint classifications
1) relative mobility: syn-, amphi- and vs di-arthroses
2) structural:
a) fibrous (solid or non-cavitated joints) - collagenous fibers link articulating bones; sub-types distinguished by location and slight differences in histology
iii) gomphoses (periodontal ligament) - bind teeth to alveolar bone
b) cartilaginous (solid or non-cavitated joints) - pad of cartilage interposed between bony elements
i) synchondroses (primary cartilaginous joint) - hyaline pad between
e.g., metaphyses, costal cartilage
ii) symphyses (secondary cartilaginous joint) - pad of fibrocartilage between hyaline covered bony elements
limited to median sagittal plane
c) synovial (cavitated joints)
actual gap (synovial cavity) between bony elements which are covered with thin layer of articular (hyaline cartilage)
encased within fibrous articular capsule line by synovial membrane
skull joints - site of bone growth
predominately but not all sutural
a) sutural - typically take names of articulating bones (e.g., spheno-parietal) but few exceptions with descriptive names: coronal (fronto-parietal), sagittal (inter-parietal) , lambdoidal (occipito-parietal)
fontanelles - expanded sutures in fetus and infants which persist until age 2
allow movement of skull during parturition and rapid growth of brain following birth
during growth, fibrous sutural joints of skull simultaneously allow for growth of skull while binding bones together
following cessation of growth, bone-forming cells in suture replace fibrous tissue with bone and thus obliterate margins of adjacent bones