Unit 2: The Skull

Functions of bone

Support: shape, muscle, against gravity. It’s durable and resistant to a lot of forces

Protection: flat bones (skull and sternum), protect our most vital organs

Movement: levers of movement, long bones, appendicular skeleton

Mineral homeostasis: calcium, we will build up and break down bone based on calcium level of the blood

Hemopoiesis: red bone marrow, found in a variety of our bones, where blood cell production takes place

Lipid storage: in a variety of different bones, red bone marrow starts to turn into yellow bone marrow at maturity.

5 bone shapes:

Flat bones: found in the sternum, skull, face. Thin. Main function is for protection

Irregular bones: vertebrae. Main function is points for attachment for ligaments and muscles. Goes into support and movements

Sesamoid bone: patella (knee cap), some people have extra ones in their toes. Protects ligament of the knee as we flex and stand so the ligament doesn’t get compressed.

Short bones: cubed shaped bones (kind of), small, found in wrists and ankles. Help to transfer forces

Long bones: found in the appendages

98% of our bones is in inorganic matrix, the other 2% is cells.

Cells:

Osteogenic cell (stem cell): initial bone cell, going through division, daughter cells develop into osteoblasts, baby cells

Osteoblasts (forms bone matrix): bone building cells, active, building bone, constructing all on the inorganic matrix, juvenile cells

Osteocytes: once the osteoblasts trap themselves (completely surrounded by matrix) they become osteocytes, maintains bone tissue, mature cells

Osteoclast (resorbs bones): break bones down, osteoclasts and osteoblasts go back and forth to maintain homeostasis of calcium levels.

Compact bone:

Have osteons, look like tree rings, rings that make up osteon are called lamellae.

At every osteon, there is a center where the blood vessels pass through called the central/haversian canal

Osteocytes make 2 structures:

Within the lamellae, there are little openings that the osteocyte is trapped in. The openings are called lacunae.

Canals are coming off of the lacunae and stretch into the lamellae. The canals contain the projections of the osteocytes. The canals are called canaliculi

Spongy bones

Trabeculae make a scaffolding where the bone marrow is. There are still osteocytes and canaliculi but its not as compressed

Parts of a bone:

Diaphysis: shaft of the bone, compact bone on the outside, spongy bone on the inside. In the middle shaft there is a medullary cavity

Epiphysis is the ends. Proximal epiphysis is where the bone attaches to the body, distal epiphysis is away from the body

Articular cartilage protects the end of the bone (breaks down during arthritis) where Hyaline cartilage is.

Metaphysis is the transition between the shaft and the ends. This is where the epiphyseal line is. Where growth is going to occur. If you take an x-ray of a juvenile, there will be a gap between the diaphysis and the epiphysis because the metaphysis is cartilage (metaphysis plate). In adults, it ossifies and turns to bone (metaphysis line)

Ossification

Intramembranous ossification (only flat)

A general shape is created by multiple layers of fibrous connective tissue

Ossification starts before birth and finishes around maturity

How our flat bones are going to be ossified

Endochondral ossification (everything but flat)

Process responsible for forming the rest of the bones

Starts before birth in the diaphysis of the developing bone and works its way towards the ends

Start as a cartilage model

Shortly after birth ossification begins in the end of the bone.

Skull functions

Protects the brain and sense organs (taste, smell, sight, hearing)

Flat bone

Possibly cools the brain when temperatures rise

In many animals, supports the voicebox and acts as a resonating chamber

Vital part of feeding systems of vertebrates

3 parts that make up the skull:

Chondrocranium:

In chondrichthyes, this is retained and supports and protects the brains

In osteichthyes, primarily an embryonic structure, this is the foundation and it becomes ossified

Serves as scaffold for developing brain and supports sensory capsules

Development

Mesenchyme (embryonic tissue) of the head condenses to form elongate cartilages along the notochord (flexible rod that becomes the spinal cord)

Paired anterior trabeculae cartilage

Paired posterior parachordals cartilage

Paired polar cartilage (in some vertebrates) sit in between the trabeculae and parachordals

Paired Occipital cartilage (more posterior)

Sensory capsules, associated with the eyes, nose, ears, also develop supporting cartilages laterally (towards the outside of the body)

Nasal capsule

Otic capsule

Optic capsule

Cartilages fuse into plate- the basis of the chondrocranium

Ethmoid plate: front of the skull, fusion of nasal capsule and front portion of the trabeculae

Basal plate: in the middle, coming from the parachordals

Occipital arch: occipital cartilages expand and grow around the nerve cord/notochord

Splanchnocranium

Oldest part (also in chondrichthyes)

Ancient chordate structure associated with filter feeding, later in other vertebrates for other functions

In modern vertebrates, generally supports the gills and provides attachment for respiratory muscles

Development

Cells migrate from the neural tube to the walls of the pharynx, between the pharyngeal slits and differentiate into the pharyngeal arches

Neural crest cells migrate to the pharynx between the pharyngeal slits and form the pharyngeal arches. The support structures.

In aquatic vertebrates, pharyngeal arches are referred to as gill (branchial) arches as these are associated with the gill system.

Each gill arch is composed of a series of elements:

Pharyngobranchial (dorsal) (top)

Epibranchial

Ceratobranchial

Hypobranchial

Basibranchial (bottom)

Anterior arches may border mouth (jaws), support soft tissue, or bear teeth

Mandibular arch and hyoid arch are the first and second gill arches

Mandibular (1st) arch - largest/most anterior

Made up of palatoquadrate (precursor to maxilla) and Meckel’s cartilage (precursor to the mandible)

Hyoid (2nd) arch- follows mandibular arch; hyomandibula is prominent element. Give rise to the hyoid bone and the inner ear of tetrapods

Types of jaw attachments

paleostylic (hagfish)

Euautostylic (extinct vertebrates)

Amphistylic (chondrichthyans and some actinopterygians): jaw is attached to the skull by way of ligaments to the palatoquadrate bone and hyoid. Very flexible jaw. The ligaments are attached

Hyostylic (majority of bony fishes): similar to amphistylic but jaw is attached by the hyomandibula. Both still have hyomandibula but it's much more closely connected to the brain case. Tighter connection. The jaws are attached.

Metatostylic (amphibians): shift in hyomandibula doesn’t play any role because it is a new bone called the stapes (much much smaller) has nothing to do with the jaw. The palatoquadrate changed into the quadrate bone (much smaller) and attaches directly to the skull.

Craniostylic (mammals): entire maxilla is now part of the skull itself. Not how jaw connects, the jaw is in the skull. Most rigid structure. Not flexible

Dermatocranium

Made up of dermal bones that contribute to the skull

Forms the sides and roof of the skull as well as the hard palate

Facial Series: encircles the external nares, forming the snout

Maxilla and premaxilla

Nasal (sits right next to the nares)

Septomaxilla (adjacent to nasal bones), not always present

Orbital Series: encircle the eye, defining the superficial orbit

Lacrimal bone (nasal lacrimal duct/tear duct in tetrapods)

Prefrontal, postfrontal, and postorbital (fused in tetrapods)

Jugal: associated with the lower rim of the orbit

Scleral ossicles: not present in everything, mostly found in birds and turtles, sitting towards the back of the orbit

Temporal series: behind orbit and completes posterior wall of braincase

Intertemporal, supratemporal, and tabular: tends to fuse in evolutionary history

Squamosal and quadratojugal: near the cheek area

Vault series- “roofing bones” and cover the top of the skull

Frontal: forehead

Parietal: major vault bone, primary bone at the top of the head

Parietal foreman: not in all vertebrates, mostly amphibians and reptiles. “Third eye”; a photosensitive hole through the top of the skull that strikes the pineal gland that regulates sleep/wake cycle and be aware of changes in light intensity around them. Lost in mammals

Postparietal

Palatal Series: cover much of the roof of the mouth

Pterygoid: tends to be the largest and most medial (along the midline of the body) of the bones

Vomer, palatine, and ectopterygoid: associated with the maxillary teeth

Parasphenoid: medial, towards the back of the skull. Found in the lower vertebrates (fishes and some amphibians).

Mandibular series: generally made up of ossified Meckel’s cartilage

Lateral side: side facing the outside of the skull (fused in mammals)

Dentary bone: bares teeth

Splenial (1 or 2) sitting underneath dentary bone

Angular (behind dentary bone)

surangular bones (above angular bone)

Medial side:

Prearticular: rest of the mandible

Coronoids (other side of the teeth but not holding them in like the dentary bone it)

Mandibular symphysis: where the 2 halves of the mandible meet

Mammals have 1 fused mandibular symphysis

Overview of the braincase

In chondrichthyans, this is an elaborate cartilage case around the brain (never ossifies)

The dermatocranium is absent

Reflects the absence of bone in the skeleton

In most bony fishes and tetrapods the braincase is extensively ossified

Braincase is a bony box, with an endoskeletal platform and everything encased by exoskeleton bones

Chondrocranium contribution (embryonic): providing the scaffolding

Sphenoid (may be a series): the platform for the brain. Starts as cartilage, eventually ossifies. Makes up part of the orbits and the side

Occipital (up to four): humans just have a singular one at the back of the skull

Foramen magnum: where the spinal cord passes, inside the skull it is called the medulla oblongata outside its called spinal cord. Giant hole in the skull

Occipital condyles: articulate (make the joint) with the first cervical vertebrae in the column. Allows for rocking back and forth on the vertebra

Otic capsule: ossifies and surrounds the auditory organs and bones

Splanchnocranium contribution

Epipterygoid (alisphenoid in mammals): platform. Part of it becomes the

Stapes (first), incus, and malleus

Dermatocranium

In most vertebrates, the endoskeletal elements, brain, and sensory organs are encased by the exoskeletal elements of the dermatocranium (specific bones mentioned earlier)

Cranial kinesis: movement between the upper jaw and the braincase at joints between them

Kinetic (movable) vs Akinetic (immovable) skulls

Majority of vertebrates have a kinetic skull

Most Mammals, crocodiles, and testudines have akinetic skulls

Advantages of kinetic skulls

Suction or filter feeding: method of prey capture in which the animal gulps water, carrying food

In animals with gills, excess water exits gill slits at the back of the mouth (unidirectional flow). Drops pressure in the mouth and water flows in. They close their mouth to expel the water

In animals without gills, excess water must exit back out the mouth (bidirectional flow). Specialized esophagus that stores the water to slowly expel it after they close their mouth without losing their food.

Allows tooth beating bones to move rapidly into strategic positions

ex) teleost fishes protrusible jaw and venomous snakes

Dentary bones walk over the prey item to push it down the throat.

The fangs move in snakes

Advantages of akinetic skulls

Results in greater jaw strength and crushing

Allows offspring to suckle easily (in mammals)

Can also more efficiently chew with specialized teeth when the skull is immovable. Mastication

Major transitions in the skull

Chondrichthyans

Dermatocranium is absent and chondrocranium has expanded

Anterior regions of ethmoid and orbital and posterior oticooccipital merge into an undivided braincase

Hyomandibular and palatoquadrate is loosely attached to the braincase. Makes the jaws very flexible and allow them to jut the mouth out during feeding

Allows sharks to use suction to draw in small prey or to protrude the jaws forward/downward to capture prey

Actinopterygians

See a proliferation (massive amount) of facial bones

Fishes also have opercula bones (operculum, bony structure that covers the gills) and extrascapulars (series of bones at the back of the skull)

Lots of joints between the different bones, why they have a protrusible jaws

Most fishes employ rapid suction feeding (pipette feeding, called pipette because mouth doesn’t open very wide but sucks it in like a straw) very fast 40 milliseconds

Explosive expansion of the buccal cavity to create a vacuum (negative pressure) for swift capture of prey

Once the item is brought in the buccal cavity is closed and the water is pushed through the gills

Sarcopterygians

In lungfishes, the upper jaw is fused to the braincase and teeth are flattened into plates

Coelacants have strong jaws with large teeth and the braincase is divided into the ethmosphenoid and oticooccipital units. Joint at the top of the skull allow the mouth to open really wide

Nasal capsules contain olfactory epithelium in the form of paired nasal sacs

Actinopterygians have nares but its just a 1 directional flow

Also connected to the nasal sac is the nasolacrimal duct, which drains excess secretions from lacrimal gland

External nair meets with an internal nare that goes to the mouth and links up with the respiratory system.

They have this because they are exposed to the air more, getting toward terrestrial life.

Nasolacrimal duct drains the secretions from the eye into the throat, why you get all snotty when you cry

Amphibians

Bones of snout reduced, start to fuse

Hyomandibula no longer functions in jaw suspension- now used for hearing as stapes (auditory ossicle/bone that is conserved throughout all tetrapods, middle ear)

Opercular bones and extrascapulars (because we see evolution of the neck) disappear and the cranial kinesis is reduced

Reptiles (non avian)

Begin to see fenestrae (openings through to the cavities of the skull) and emarginations (notches in the skull to accommodate muscles and not fully enclose them)

Anapsids (ex. turtles) only opening in the skull is the eye sockets. Thought to have been lost over evolutionary history

Diapsids (ex. Crocodilians, dinosaurs, squamates). 2 fenestrae in addition to the eye sockets. Paired on each side

Changes to skeletal elements are reflected in degrees of skull mobility (Not on the test)

Metakinesis: joint in back of the skull

Mesokinesis: joint in the middle of the skull

Prokinesis (tend to be more common): in front of the eye socket

Streptostyly: mandibles move apart and expand

Can be monokinetic (one joint) or dikinetic (2 joints)

Lizards- ability to rotate parts allows them to alter the angle of rows of teeth, reducing prey loss

Lingual feeding (chameleons), skull rotation puts pressure on the muscle and squeezes the tongue to jut out

Snakes- kinetic skull and flexible mandibular symphysis allow lateral bones to move apart and move independently

Birds

Braincase is inflated (domed, accommodates for a larger brain theoretically), lost the fenestrae (like anapsids)

Bones are hard to differentiate (total fusion of the skull like mammals)

Jaws are drawn out into a beak that is toothless, though may be serrated to help with feeding

Mammals

Now see a single occipital bone with foramen magnum and 2 occipital condyles (axis is the spinal column)

Nuchal crest point of attachment for neck muscles. On the crown of the skull towards the back (occipital bone)

Single fused Large temporal bone found laterally and moves up a bit and makes up part of the eye orbits

Mastoid process attaches to the mandible for mastication

Tympanic bulla is a bony structure that encloses the middle and inner ear (auditory ossicles)

Splanchnocranium produces the 3 ossicles - stapes, incus, and malleus

The nasal capsule remains largely unossified with the exception of the ethmoid bone (base/back of the bones)

Contains 3 sets of scroll like turbinates that increase surface area to condition the air

Cribiform plate is perforated by olfactory nerves, collecting chemical signals and take the info to the brain. Very important for sense of smell

Typically perform mastication- chewing of food

Associated changes with feeding style include specialized dentition and evolution of hard and soft palate. Separates breathing and feeding. Animal can chew and breathe

Akinetic skull helps with the process of mastication. Strong platform for the animal to grind upon

Exhibit diphyodonty - only 2 sets of teeth erupt and specialized teeth