SKELETAL SYSTEM: SKULL & VISCERAL

SKELETON

Mrs. Ofelia Solano Saludar

Department of Natural Sciences University of St. La Salle

Bacolod City

The vertebrate skull consists of:Neurocranium (also called endocranium, chondrocranium or primary braincase)

Dermatocranium (membrane or dermal bones)

Splanchnocranium (visceral skeleton)

1. CHONDROCRANIUM or NEUROCRANIUM Cartilaginous stage protects the brain

Begins as a pair of parachordal cartilages alongside the notochord (derived from sclerotome), and the prechordal cartilages or trabeculae cranii (derived from neural crests) anterior to these.

Cartilages derived from neural crest also appears in the: 1. olfactory capsules partially surrounding

the nasal epithelium, 2. otic capsule surrounding the inner ear, 3. orbital/ optic capsules around the eyes

Completion of floor, walls, & roof: Parachordals join the notochord and expand to

form the basal plate, floor of hindbrain, occipital condyles (1-2), and foramen magnum

Ethmoid plate – prechordals fuse with olfactory capsules; optic capsules remain independent

Basal plate - fuses with otic capsules Development of cartilaginous walls (sides

of braincase) and a cartilaginous roof over the brain in cartilaginous fishes

Foramina remain for nerves & blood vessels

Hypophyseal fenestra remains for pituitary gland and carotid arteries

Cartilaginous fishes - retain a cartilaginous neurocranium throughout

life; completes skull by forming a cartilaginous roof (tectum) over

neurocranium 

Bony fishes, lungfishes, & most ganoids - retain highly cartilaginous neurocranium that is covered by membrane bone

Cyclostomes- the cartilaginous components of the embryonic neurocranium remain in adults as independent cartilages

Reptiles: Embryonic Development of Lizard Chondrocranium: Parachordal and trabecular

cartilages grow up around brain and sense organs

Consists of membrane bones that encase the chondrocranium and jaws.

Formed a complete roof for the skull of extinct tetrapods, but became reduced in number through loss.

Vacuities also tend to arise in the posterior part of the roof, and these temporal fossae are of importance in the evolution of the various amniotes.

2. DERMATOCRANIUM

Temporal fossae (plus mammalian

zygomatic arch) provide space

and surfaces in advantageous positions for

accommodating the large powerful muscles

(adductor, masseter,

temporalis) that operate the lower jaw of amniotes.

Differentiation of synapsid

adductor mandibulae

into temporalis, superficial

masseter, and deep masseter, opposed along the jaw by the pterygoideus

The dermatocranium lies superficial to the neurocranium & forms the bones of: Roof of the brain:

nasals, frontals, parietals, post- parietals

Posterior angle of skull: intertemporal, supratemporal, tabular, squamosal, quadratojugal

Around orbits: lacrimal, prefrontal, postfrontal, jugal (infraorbital), postorbital

The upper jaw: premaxilla, maxilla

The lower jaw: splenial, postsplenial, angular (tympanic bulla), surangular, prearticular (anterior malleus in mammals), coronoids, dentary

The palate: para- sphenoids, vomer palatines, pterygoids, ectopterygoids (cover palato- quadrate)

The operculum in fishes

PALATAL BONES – the primary palate is the floor on which the brain rests, & the roof of the oral cavity in fishes & amphibians; remain cartilaginous in sharks. Birds, mammals, some reptiles: A secondary palate (plus a

soft palate in mammals) develops from processes of the premaxillae, maxillae, and palatines, creating a horizontal partition that separates the oral cavity into nasal & oral passages

Allows chewing and breathing simultaneously

Parasphenoid is lost and internal nares is displaced caudad when palate forms

Evolution of the mammalian bony palate: dermal bones of the margin of the oral cavity expand medially

to house nasal passages from external nares to choanae.

3. SPLANCHNOCRANIUM Cartilage blastema origin is neural crest

Consists of typically 7 gill or skeletal visceral/ branchial arches

1st MANDIBULAR ARCH Dorsal half forms the

primitive upper jaw, the palatoquadrate or pterygoquadrate

Lower half forms the lower jaw, the Meckel’s cartilage

The upper jaw becomes incorporated into the skull, while the lower jaw forms a movable joint with it.

1. PALATOQUADRATE CARTILAGES: Unossified in tetrapods, function is taken

over by dermal bones Ossifications occur only in the ascending

process (epipterygoid bone), and in the otic process (quadrate bone) which becomes an immovable part of auditory region (except in streptostylic conditions which permits wide gape for swallowing large prey)

2. MECKEL’S CARTILAGES: Anterior mentomeckelian bone of

amphibians At the rear is the articular bone which

articulates with the quadrate bone of the upper jaw (autostylic suspension)

2nd HYOID ARCH and other gill arches: Composed of dorsal paired hyomandibular

cartilages, and lateral gill-bearing ceratohyals of elasmobranchs.

The remainder and the other 5 arches contribute to the hyoid apparatus and laryngeal cartilages of tetrapods.

Operculum is the fold of the hyoid arch that extends over the gill slits in holocephalans & bony fishes; in tetrapods no vestiges of opercular bones remain

HYOID CARTILAGES

Hyomandibular cartilage ossifies to form hyomandibula of fishes and suspend lower jaw (hyostylic suspension); in tetrapods, it gives rise partly to the columella of the ear

Remainder of hyoid arch fuse with gill arches to form hyobranchial skeleton consisting of:

Hyoid apparatus - serves as support for tongue and larynx, muscle attachment, buccal respiration of amphibians

Laryngeal cartilages- support voice box chamber

3. FORMATION OF THE COMPLETE SKULLCENTERS OF OSSIFICATION appear which

converts the chondrocranium into a complete skull that consists of: Cartilage bones ossified in the

chondrocranium, sense capsules, hyoid and mandibular arches

Dermal bones covering the cartilage bones everywhere except on midventral surface and posterior end of the skull.

Degree of ossification is greater in higher members of each group; cartilaginous skulls

result from retrogressive processes.

Occipital group: encircling the foramen magnum are the: basioccipital, exoccipital (2), supraoccipital

In mammals, all 4 occipital elements typically fuse to form a single occipital bone surrounding the foramen magnum

OCCIPITAL CENTERS

Occipital condyles are projections by which the skull articulates with the atlas.

Fishes and primitive tetra- pods have only 1 condyle formed by basioccipital and partly by exoccipital.

2 condyles present in amphibians and mammals result from reduction of basioccipital and enlargement of exoccipital.

SPHENOID CENTERS Posterior sphenoid group: basisphenoid,

pleurosphenoid (not the mammalian alisphenoid, epipterygoid location)

Orbitosphenethmoid region: presphenoid, orbitosphenoids, mesethmoid.

In mammals, the basicranial axis is occupied by: basioccipital, basisphenoid, presphenoid, mesethmoid (absent in some)

Remain cartilaginous & form anterior to sphenoid In most mammals, the nasal chamber is large &

filled with ridges from the ethmoid bones called the turbinals or ethmoturbinals.

These bones are covered with olfactory epithelium in life and serve to increase the surface area for a more acute sense of smell (olfaction).

Another ethmoid bone, the cribriform plate, separates the nasal chamber from the brain cavity within the skull.

ETHMOID CENTERS

SENSE CAPSULES:1. OTIC - the cartilaginous otic capsule is replaced

in lower vertebrates by several bones: prootic, opisthotic, epiotic, pterotic, sphenotic

• One or more of these may unite with adjacent replacement or membrane bones: Frogs & most reptiles - opisthotics fuse with

exoccipitals Birds & mammals - prootic, opisthotic, &

epiotic unite to form a single petrosal (periotic or petromastoid) bone; the petrosal, in turn, sometimes fuses with the squamosal to form the temporal bone

2. OPTIC – gives rise to sclerotic bones around pupil of reptiles and birds (absent in mammals)

blue- chondrocranium; pink-dermatocranium; yellow-

splanchnocranium

JAW SUSPENSIONS Autostyly (left) - hyomandibula has no role in

bracing the jaws (lungfish & tetrapods) Amphistyly (middle) - jaws & hyomandibula

both braced directly against the braincase (extinct sharks)

Hyostyly (right) - mandibular cartilage is braced against the otic capsule; jaws braced against hyomandibula (sharks & present day bony fishes)

PLACODERMS

PHYLOGENY OF THE VERTEBRATE SKULL

CROSSOPTERYGIANS- the dermatocranium forms a series of paired and unpaired bones along mid-dorsal line of skullLABYRINTHODONTS- these unpaired bones are lost but a series of paired bones resulted (nasals, frontals, parietals, & dermoccipitals)

AGNATHA

Chondrocranium: remains cartilaginous throughout life; skull roof is fibrous and protects brain & sensory structures

Splanchnocranium: no ancestral branchial skeleton; lingual cartilage bears horny teeth; continuous basket with branchial function

Dermatocranium: no dermal armor

CHONDRICTHYESChondrocranium: calcified; with 2

occipital condyles and foramen magnum; otic and nasal capsules fused to neurocranium

Splanchnocranium: mandibular arch gives rise to palatoquadrate and Meckel’s cartilages; hyoid arch composed of hyomandibula, ceratohyal, basihyal

cartilages

TELEOSTS

Neurocranium: remain cartilaginous in chondrosteans, neopterygians, dipnoans; ossifies via the four ossification centers in most fishes

Dermatocranium: numerous dermal bones overlying neurocranium

Splanchnocranium: resembles that of sharks except that bone is added; anterior part of palatoquadrate ensheathed by dermal maxilla and premaxilla bones

Caudal ends undergo endochondral ossification & become the quadrate bone; the remainder becomes the palatine & pterygoid bones.

Caudal part of Meckel’s cartilage ossifies as articular bones; remainder becomes invested by dentary and angular membrane bones

Hyomandibula ossifies to become symplectic and interhyal bones

Moveable bony operculum

Hyostylic suspension (ray-finned fish); Autostylic suspension (Dipnoans); Amphistylic suspension (Crossopterygians)

AMPHIBIANS Neurocranium: remains cartilaginous except

for sphenethmoid, prootics, exoccipitals Dermatocranium

incomplete (lacrimals and prefrontals only)o lacks temporal regiono 2 occipital condyles

Splanchnocranium: larval stages have fish-like gills supported by gill archeso forms altered primary

palate with large vacuities to allow retraction of eyeballs

Jaw suspension: Quadrate of upper jaw articulates with articular of lower jaw (autostylic suspension)

Hyomandibula is no longer needed since the jaw has an autostylic suspension

It is freed up and becomes a rudimentary stapes called the columella

The rest of the hyoid arch plus arches III and IV become the hyoid apparatus for tongue support

Visceral arch V is no longer needed and becomes the new larynx; arches VI and VII are absent

REPTILESNeurocranium: Well ossified, with

fewer bones, and single occipital condyle

Dermatocranium: Many bones, but

fewer than bony fish; crocodilians retain the largest number

In many lizards, a parietal foramen houses a median eye

Splanchnocranium: Similar to amphibians; snakes

have vestigial branchial skeleton Stapes – functional columella Hyoid apparatus: larynx Quadrate-articular joint forms

autostylic suspension; forms part of the kinetic mechanism of the skull

The hyoid consists of a body and 2 or 3 horns (cornua) in the pharyngeal walls.

The entoglossus, a long bony process extends from the hyoid body forward into a long darting tongue (snakes, lizards, birds).

EARLY TETRAPOD SKULL

top: dermatochranium removed

red: dermatocranium; blue:

chondrocranium; green: splanchnocranium

Formation of partial or complete secondary palate

Development of temporal fossae bounded by arches: infratemporal arch (below ventral fossa); zygomatic arch (infratemporal arch); supratemporal arch (below dorsal fossa)

CRANIAL KINESIS Independent movement of one or more skull

bones, especially between the upper jaw and braincase; e.g., a pivoting quadrate

Results from reduction or loss of arches along with presence of intracranial joints

Advantages: o provides a way to change

the size and configuration of the mouth rapidly

o optimize biting and rapid feeding

Disadvantages: lose force, hard to optimize apposition of occlusive surfaces

These fossae and

arches provide room for

huge chewing muscles which allows rotary

chewing

BIRDS Neurocranium- thin,

highly vaulted or domed, but basically a reptile skull

Dermatocranium: Modified diapsid: supratemporal arch is lost, one

big opening confluent with orbitBeak instead of teeth; premaxilla & dentary

elongated Splanchnocranium- similar to reptiles

Pivoting quadrates allow cranial kinesis although ectopterygoids are absent, and immobile parasphenoid is fused to basisphenoid.

When quadrate is pushed forward, the motion is transmitted to upper beak via a movable palate, a movable zygomatic arch, or both.

MAMMALSNeurocranium: Larger, fewer bones due to fusion;

sutures found between skull bones

Skull increasingly domed as cerebral hemispheres increase is size

Neurocranium is incomplete dorsally, resulting to fontanels (a bregmatic bone ossifies and forms an anomaly in human skulls)

Petrosal (periotic) bones form in the otic capsules

2 occipital condyles

Dermatocranium: Decreased number of

bones Synapsid skull; zygomatic

arch varies from massive to slender, even incomplete in insectivores

Air-filled cranial sinuses: frontals sinuses extend into horns; sinusitis is a common aliment in humans

Present in Homo erectus and Mongolians, is a postparietal or Inca bone

Temporal complex has intramembranous and endochondral origin: 1. Squamous portion- squamosal of lower

tetrapods2. Tympanic bulla- unique to mammals

and encloses the middle ear; tympanic bone surrounds the eardrum, entotympanic bone represents the bulla

3. Petrous portion- ossified otic capsule4. Mastoid portion- new in mammals;

dorsal part of hyoid arch may fuse to mastoid to form styloid process

Tympanic and petrous portions unite to form petrotympanic bone

Tympanic bulla

Squamosal

Mastoid Otic capsule

Pterygoids become reduced as winglike processes of the sphenoid

3 pairs of turbinal bones (nasal conchae) develop in the nasal passageways: superior concha is covered with olfactory epithelium; the 2 lower conchae are covered by nasal epithelium with venous plexuses that warm the air en route to the lungs

Squamosal articulates with dentary bone, which is sole lower jaw bone

Splanchnocranium: unossified tips of palato-quadrate and Meckelian cartilages give rise to middle ear ossicles, along with the columella: quadrate becomes incus articular becomes part of malleus hyomandibula has already became stapes

Hyoid apparatus and

larynx: Consists of a

body & 2 or 3 horns (cornua);

Anchors tongue, provides Attachment for some extrinsic muscles of larynx

Site of attachment of muscles that aid in swallowing

In addition to cricoid

and arytenoid cartilages common

to tetrapods, mammals

have thyroid

cartilages arising

from the 4th and

5th arches

ARCH SHARK TELEOST FROG REPTILE MAMMAL

1 PalatoquadrateMeckel’s cartilage

QuadrateEpipterygoidMetapterygoidArticular

QuadrateAmmulus tympanicusArticularMentomeckelian

QuadrateEpipterygoidArticular

IncusAlisphenoidMalleus

2 HyomandibulaCeratohyalBasihyal

HyomandibulaSymplecticInterhyalEpihyalCeratohyalHypohyalBasihyal

Columella (stapes)Anterior horn of hyoidBody of hyoid

Columella (stapes)Anterior horn of hyoidEntoglossus

Columella (stapes)Styloid processBody of hyoid

3 PharyngobranchialEpibranchialCeratobranchialHypobranchial

PharyngobranchialEpibranchialCeratobranchialHypobranchial

Body of hyoid 2nd horn of hyoidBody of hyoid

2nd horn of hyoidBody of hyoid

4 Branchial skeleton Branchial skeleton 2nd horn of hyoid

Last horn of hyoid

Thyroid cartilages

5 Branchial skeleton Branchial skeleton Cricoid and arytenoids

Cricoid and arytenoids

Cricoid and arytenoids

6 Branchial skeleton Branchial skeleton Not present Not present Not present

7 Branchial skeleton Branchial skeleton Not present Not present Not present

EVOLUTIONARY CHANGES IN MAMMALIAN SKULL:

Loss of connection between head and pectoral girdle to create neck in primitive tetrapods

Increasing skull strength through simplification by loss of bones and articulations

Braincase evolution reflecting enlarged brain Changes in sense organs (e.g., in median pineal-

parietal eye complex and loss of pineal foramen) Development of secondary palate and respiratory

passages Temporal fenestration and jaw adductor

differentiation Change from quadrate-articular to dentary-

squamosal jaw joint with concomitant development of three middle ear bones

Homologies among bones are difficult to establish

Sagittal section through skull of ancestral amniote and mammal to show evolution of bones that form braincase; note that the epipterygoid of the primitive amniote splanchnocranium is homologous with alisphenoid in mammals.

REGIONAL SERIES OF DERMATOCRANIAL BONES

IN EARLY TETRAPOD

DERMAL ROOF BONES LOST IN MAMMALS: Circumorbital series: prefrontal, postfrontal, postorbitalTemporal series: intertemporal, supratemporal, tabular (?)Cheek series: quadratojugalLower Jaw: splenials, surangular, coronoids

TEETH: teeth of vertebrates are homologous to the placoid scale of elasmobranchs.

Usually of simple form and all alike among lower tetrapods, they become heterodont (several kinds), and thecodont (set in sockets in jaw bones).

Borne in lower tetrapods on various jaw and palatal bones, they become limited in higher ones to the jaw margins.

Teeth of mammals: incisors, canines, premolars, molars; juvenile (no molars), and permanent sets

Trituberculate theory of mammalian tooth origin: 2 cusps arising from a ridge (cingulum) from neck of tooth are added to simple reptilian tooth. Teeth of upper jaw are slightly behind those of lower jaw.

HUMAN AND ANTHROPOID APE SKULL:

large, rounded cranial portion flattened facial portion, vertical

orientation complete separation of the orbits from

the temporal fossae reduction of the nasal cavities and

turbinals large mastoid process absence of a tympanic bulla extensive fusions of the skull bones unspecialized teeth: 2 incisors, slightly

enlarged canines, bunodont (separated rounded cusp) molars

1.Name the bones of the neurocranium of a basal craniate, and their fused derivatives.

2.What major steps occurred in the phylogenetic development of the “complete” cranium?

3.Tabulate: regions of ossification in the cranium, and the bones present in each region of the teleost, amphibian, reptilian, and mammalian skulls.

4.Tabulate: pharyngeal arches of basal vertebrate, and their homologues in teleost, amphibians, reptiles, mammals

5.Tabulate: dermal bones in each of these regions of the teleost, amphibian, reptilian, mammalian skull: roof, upper and lower jaw, palate

6.Describe the phylogenetic patterns for the craniate temporal fossae, and their fuctional role in each group.

7.Name the bones that contribute to the formation of the primary and secondary palate.

8.List the types of jaw suspension, and their participating bones.

9.Discuss the phylogeny of the mammalian middle ear bones.10.What is the functional significance of cranial kinesis?

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APPENDICULAR

SKELETON