Unit 2: The Skull

Functions of bone

support, protection, movement, mineral homeostasis, hemopoiesis, lipid storage

Support

shape, muscle, against gravity. Durable an resistant to a lot of forces

Protection

The role of flat bones, such as the skull and sternum, in protecting our most vital organs.

Movement

The function of long bones in the appendicular skeleton that act as levers for movement.

Mineral homeostasis

The process by which bones build up and break down based on blood calcium levels.

Hemopoiesis

The production of blood cells that occurs in red bone marrow found in various bones.

Lipid storage

The function of yellow bone marrow, which develops from red bone marrow as an individual matures.

Name these bones

1. flat bones

2. irregular bones

3. sesamoid bone

4. short bones

5. long bones

Flat bones

Thin bones primarily found in the skull and sternum, serving mainly for protection.

Irregular bones

Bones like vertebrae that provide attachment points for ligaments and muscles, contributing to support and movement.

Sesamoid bone

A type of bone, such as the patella, that protects ligaments of the knee as we flex and stand so the ligament doesn’t get compressed. Some people have extra ones in their toes.

Short bones

Cubed-shaped bones found in the wrists and ankles that help transfer forces.

Long bones

Bones located in the appendages that are essential for movement.

Osteogenic cell

The stem cell that divides to form osteoblasts. Initial bone cell (baby cell)

Osteoblasts

Bone building active cells that construct the inorganic bone matrix. Juvenile cells

Osteocytes

Mature bone cells that maintain bone tissue after osteoblasts become trapped in the matrix.

Osteoclast

Cells that break down bone tissue, working with osteoblasts to maintain calcium homeostasis.

Compact bone

Dense bone structure containing osteons, which resemble tree rings. Individual rings are called lamellae. Center of osteon is called the central/haversian canal and is where blood vessels pass through.

2 structures made by osteocytes

1. Lacunae: openings the osteocyte is trapped in, within the lamellae

2. Canaliculi: canals containing projections of the osteocytes stretching into the lamellae

Spongy bone

Bone structure characterized by trabeculae that provide scaffolding for bone marrow. Less compressed canaliculi and osteocytes

Parts of a bone

• Diaphysis

• epiphysis

• articular cartilage

• metaphysis

Diaphysis

The shaft of a bone, composed of compact bone on the outside and spongy bone on the inside. Middle of the shaft is the medullary cavity

Epiphysis

The ends of a bone, with proximal and distal regions relative to the body.

Articular cartilage

Hyaline cartilage that protects the ends of bones and can break down during arthritis.

Metaphysis

The region between the diaphysis and epiphysis. Where the epiphyseal line is and where growth is going to occur

Xrays of juvenile vs adult

In juveniles there will be a gap between the diaphysis and epiphysis because the metaphysis is cartilage (metaphysis plate. In adults, it ossifies and turns into bone (metaphysis line)

Intramembranous ossification

The process of bone formation for flat bones, starting before birth and finishing around maturity. General shape is created by multiple layers of fibrous connective tissue

Endochondral ossification

The process responsible for forming all the other bones (not flat), beginning with a cartilage model. Starts before birth in the diaphysis of the developing bone and works its way towards the ends. 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

• supports the voicebox and acts as a resonating chamber

• vital part of feeding systems of vertebrates

3 parts that make up the skull

chondrocranium, splanchnocranium, dermatocranium

Chondrocranium

Retained in Chondrichthyes to support and protect the brain. Embryonic structure in Osteichthyes. Cartilaginous foundation then it ossifies. Serves as scaffold for developing brain and supports sensory capsules.

Development of chondrocranium

1. Mesenchyme (embryonic tissue) of the head condenses to form elongate cartilages along the notochord

2. Sensory capsules, associated with the eyes, nose, and ears develop supporting cartilages laterally (towards the outside of the body

3. Cartilages fuse into a plate that serves as the bases of the chondrocranium

Cartilages (all paired)

• anterior trabeculae cartilage

• posterior parachordals cartilage

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

• occipital cartilage (more posterior)

Sensory capsules

• nasal capsule

• otic capsule

• optic capsule

Plates

• 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

The oldest part of the skull, associated with filter feeding and later functions in vertebrates such as gill support and attachment for respiratory muscles

Development of splanchnocranium

neural crest cells migrate to the pharynx between the pharyngeal slits to form the pharyngeal/gill/branchial arches. Support structures

Elements that make up each gill arch

• pharyngobranchial

• epibranchial

• ceratobranchial

• hypobranchial

• basibranchial

Mandibular arch

The first gill arch that gives rise to the jaw structures in vertebrates and bears teeth. Made up of palatoquadrate (precursor to maxilla) and Meckel’s cartilage (precursor to mandible)

Hyoid arch

second gill arch, follows mandibular arch. Hyomandibula is the prominent element. Gives rise to hyoid bone and the inner ear of tetrapods

types of jaw attachments

• Amphistylic

• hyostylic

• metatostylic

• craniostylic

Amphistylic

in chondrichthyans and some actinoptergians. Jaw is attached to the skull by way of ligaments to the palatoquadrate bone and hyomandibula. Very flexible because it’s just ligaments

Hyostylic

Majority of bony fishes. Jaw is attached by the hyomandibula (much closer to the brain case compared to amphistylic). Tighter connection

Metatostylic

Amphibians. Hyomandibula turns into stapes so it doesn’t play a role in jaw attachment. The palatoquadrate turned into the quadrate bone and attaches directly to the skull

Craniostylic

Mammals. Entire maxilla is now apart of the skull itself. Most rigid structure, not flexible

Dermatocranium

Made of dermal bones that forms the sides, roof of the skull, and hard palate.

Series of the dermatocranium

• Facial

• orbital

• temportal

• vault

• palatal

• mandibular

Facial series

encircles the external nares, forming the snout

made up of

• maxilla and premaxilla

• nasal (sits right next to the nares)

• septomaxilla (adjacent to the nasal bones), not always present

Orbital series

encircles the eye, defining the superficial orbit

Made up of:

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

• prefrontal, postfrontal, and postorbital (fused in tetrapods)

• jugal: associated with lower rim of the orbit

• scleral ossicles (found in turtles and birds): sitting towards back of the orbit

Temporal series

behind the orbit and completes posterior wall of braincase

made up of:

• intertemporal, supratemporal, and tabular (tends to fuse in evolutionary history)

• squamosal and quadratojugal: near the cheek area

Vault series

roofing bones that cover the top of the skull

made up of:

• frontal: forehead

• parietal: major vault bone, primary bone at the top of the head

• postparietal

Parietal foreman

Only in amphibians and reptiles. “Third eye” a photosensitive hole through the top of the skull that strikes the pineal gland which regulates sleep/wake cycle. Allows animal to be aware of changes in light intensity around them.

Palatal series

covers much of the roof of the mouth

made up of:

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

• vomer, palatine, and ectopterygoid: associated with 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

Made up of:

• lateral side (side facing the outside of the skull (fused in mammals)

• medial side

• mandibular symphysis: where the 2 halves of the mandibles meet. (fused in mammals)

Lateral side of mandibular series parts

• Dentary bone: bares teeth

• splenial: 1 or 2 sitting underneath dentary bone

• Angular: behind dentary bone

• surangular bones: above angular bone

Medial side of mandibular series parts

• coronoids: other side of teeth but not holding them in like the dentary bone

• prearticular: rest of the mandible

Chondrichthyans skull overview

elaborative cartilage case around the brain (chondrocranium never ossifies), dermatocranium is absent (reflects absense of bone in skeleton)

Bony fishes and tetrapods skull overview

braincase is extensively ossified. Braincase is a bony box with an endoskeletal platform and everything encased by exoskeleton bones

Chondrocranium contribution (embryonic)

• provides scaffolding

• sphenoid: can be in a series, platform for the brain. Makes up parts of the orbits and the side

  • occipital: can have up to 4, humans only have 1 at the back of the skull, foramen magnum and occipital condyles

• otic capsule: ossifies and surrounds auditory organs and bones

Foramen magnum

where the spinal cord passes, inside the skull the spinal cord is called the medulla oblongata. Basically a 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

Splanchnocranium contribution

epipterygoid (alisphenoid in mammals): a platform that partly becomes the stapes, incus, and malleus

Dermatocranium contribution

endoskeletal elements, brain, and sensory organs are encased by the exoskeletal elements of it.

Cranial kinesis

Movement between upper jaw and the braincase at the joints between them. The ability of the skull to move, allowing for flexible feeding strategies.

Kinetic skull

majority of vertebrates have this type. Movable skull

Akinetic skull

A skull type that is immovable. Most mammals, crocodiles, and testudines have this type

Advantages of kinetic skull

• suction or filter feeding: method of prey capture where the animal gulps water along with the food

• Allows tooth bearing (dentary) bones to move rapidly into strategic positions and walk over prey items to push it down the throat

ex) teleost fishes with protrusible jaws and venomous snakes

Unidirectional flow

In animals with gills, excess water exits gill slits at the back of the mouth. Drops pressure in mouth and water flows in. Close mouth to expel water

Bidirectional flow

in animals without gills, excess water must exit back out the mouth. Have a specialized esophagus that stores the water to slowly expel it after they close their mouth without losing food.

Advantages of akinetic skulls

• greater jaw strength and crushing

• allows offspring to suckle easily (in mammals)

• Mastication, more efficiently chew with specialized teeth

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

Major transitions in the skull: 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. Allows for rapid suction feeding/pipette feeding where the buccal cavity expands to suck in the prey.

• Have nares but only do unidirectional flow

Major transitions in the skull: 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

• Nasolacrimal duct connected to the nasal sac which drains excess secretions from the lacrimal gland.

• External nare meets with internal nare that goes to the mouth and links up with respiratory system

Major transitions of the skull: 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

Major transitions of the skull: Reptiles (non avian)

• Begin to see fenestrae and emarginations

• changes in skeletal elements are reflected in degrees of skull mobility

• Lizards: ability to rotate parts of the skull and alter the angle of rows of teeth to reduce prey loss

• Snakes: have kinetic skull and flexible mandibular symphysis that allow lateral bones to move apart and move independently

Fenestrae

Openings through the cavities of the skull

Emarginations

notches in the skull to accommodate muscles attachment while not fully enclosing them

Anapsids

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

Diapsids

have 2 fenestrae paired on each side in addition to the eye sockets. ex) crocodiles, dinosaurs, squamates

Lingual feeding

chameleons do this. Skull rotation puts pressure on the muscles and squeezes the tongue to make it just out and capture prey

Streptostyly

mandibles move apart and expand

Major transitions in the skull: 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

Major transitions in the skull: mammals

• single occipital bone with foramen magnum and 2 occipital condyles. Nuchal crest point of attachment for neck muscles on the crown of the skull towards the back

• single fused large temporal bone found laterally and moves up a bit to make up part of the eye orbits. Mastoid process attaches to the mandible for mastication

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

• typically perform mastication (chewing food)

• exhibit diphyodonty: only 2 sets of teeth erupt. Have specialized teeth

• evolution of hard and soft palate so that we can breathe while we eat

Tympanic bulla

bony structure that encloses the middle and inner ear (auditory ossicles)

Splanchnocranium produces 3 ossicles

stapes, incus, and malleus

ethmoid bone

contains 3 sets of scroll like turbinates that increase surface are to condition the air. Cribiform plate is perforated by olfactory nerves, collecting chemical signals and taking the info to the brain. Very important for sense of smell