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Flashcards in skull development (A11) Deck (42):
1

neonate

first 4 weeks of life

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intramembranous ossification

-mesenchymal cell differentiate into osteoblasts and begin to synthesize and secrete osteoid at multiple sites
-as cells become trapped they become osteocytes
-membrane bone formation from mesenchyme
-this process begins at particular sites called centres of ossification, these then grow outwards.
-the immature woven bone laid down initially will be remodeled to lamellar bone in time

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osteoblasts

lay down bone

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osteocytes

surrounded by bone

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location of start of intramembranous ossification

centres of ossification

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endochondral ossification/ bone formation

-mesenchyme -> chondroblasts
-embryo 5-6 weeks old- just hyaline cartilage
-embryo 6-8 weeks old- begins to develop periosteal/compact bone at secondary ossification centre
-featus 8-12 weeks- primary ossification centre develops along with blood vessels and cartilage begins to calcify?

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chondroblasts

lay down a cartilage model

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divisions of the neurocranium

-membranous neurocranium/dermatocranium
-cartilaginous neurocranium/chondrocranium

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dermatocranium

-membranous neurocranium
-flat bones of the skull eg. frontal and parietal bones
-formed by intramembranous ossification

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chondrocranium

-cartilaginous neurocranium
-bones of base of skull eg. ethmoid and sphenoid bones
-formed by endochondral ossification

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endochondral bone

-more supportive
-bones of the base of the skull (chondrocranium) eg. ethmoid and sphenoid bones
-support the brain above
-more complex shapes (general rule)
-long bones of the limbs
-under tight genetic control (achondroplasia)

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achondroplasia

-most common cause of dwarfism, or significantly abnormal short stature
-form of dwarfism

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membrane bone

-more protective
-flat bones of the membranous neurocranium eg. frontal and parietal bones
-the zygoma, maxilla and mandible of the viscerocranium
-mainly flat shapes (general rule)
-more easily modified by environmental factors (eg.hydrocephaly)

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hydrocephalus

a condition in which fluid accumulates in the brain, typically in young children, enlarging the head and sometimes causing brain damage

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hydrocephaly

-condition in which the brain grows within the cranial cavity, expanding rather like a balloon being inflated
-the increasing brain volume simply lifts the membrane bones of the neurocranium (frontal, parietal etc) apart, applying tension to the sutures between the bones and stimulating new bone formation at the sutural edges of the bones
-they grow more if the intracranial pressure rises, as in hydrocephaly ('water on the brain')

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skull growth rate (refer to diagram)

-driven by 4 factors (neural, dental, muscle, gonad)
-neurocranium dictated in utero by growth of brain (neural development is fast initially) 20% developed at birth
-brain has to be well enough developed at birth to enable protective reflexes (coughing) and suckling/swallowing
-neurocranium growth normally around 25% complete at birth
->brain is fully developed around about the age of 7
->dental development begins at 20% and increases steadily until the age of 2(40%) before plateauing, then increasing steadily to the age of 7 (60%) before gradually increasing, at age of 12 dental development is around 80% and by the age of around 20 reaches 100%
->muscle and gonad development increasly slowly until the age of around 12 when gonad begins to steadily increase

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gonad

an organ that produces sex cells; a testis or an ovary

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the birth canal

-at the opening into the birth canal the transverse diameter is wider than the AP/anterior-posterior diameter (13cm)
-at the exit from the birth canal the AP diameter is wider than the transverse diameter (12.5cm)

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design features of the neonatal skull

-allow ease at which baby is born and ensure both mum and baby are well
-there is conflict between:
->size of head (bigger head=problem for mum during labour however bigger head = bigger brain = better survival chances for baby (as they will have better developed protective reflexes)
-therefore the design features of the neonatal skull have to facilitate birth:
1.the babies head is deformable ('moulds' within the birth canal' - this is due to the wide sutures, fontanelles and the thin, deformable bones of the neurocranium
2.the neonate skull has minimal development of inessential parts - small jaws, as they have a liquid diet and no chewing is required, and small nose as they have no sense of smell

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frontal and parietal eminences of neonatal skull

design feature of neonate skull

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fontanelles of neonate skull

-anterior fontanelle (due to wide coronal suture, and presence of frontal suture)
->anterior fontanelle is usually closed by 18months
->frontal suture is not present in adults
-anterolateral fontanelle and posterolateral fontanelle (due to wide squamosal suture)

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cuteness factor of the neonatal skull

-arouses mothering instincts
-beneficial to survival
-large eyes, large forehead, small nose and chin, wide smile

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ratio of neonate skull to face

8:1

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ratio of adult skull to face

2.5 : 1

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growth of neonate skull and face to reach adult dimensions/ratio

-neonate skull has to grow 4 times its size
-neonate face has to grow 12 times its size

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post natal growth of the skull and face

-after birth the neurocranium has to grow 4x and the viscerocranium has to grow 12x to reach adult size and proportions :
->further brain growth until the age of 10 is mainly responsible for skull growth
->the jaws grow under the influence of the teeth developing within them
->as muscles attaching to bones become stronger they pull on the bones further developing their various processes
->sex hormones at puberty further encourage male/female patterns of growth
->growth at primary cartilaginous joints (synchondroses) in the base of the skull allow the face to be carried forwards as it grows

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synchondroses

primary cartilaginous joints capable of genetically programmed growth

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location of the base of the skull synchondroses

-sphenoethmoidal synchondrosis (closes at the age of 6)
-sphenooccipital synchondrosis (becomes active after birth and closes at adolescence)

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development of occipital bone

-basal part near the sphenoid develops by endochondral ossification
-the rest develops by intramembranous ossification

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design features of the adult skull

1. lightness
-2. strength
-3. keeping airway open
-4. warming and humidifying air breathed in through nose
-5. supporting muscles involved in chewing and swallowing
-6. attractiveness

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how is the 'lightness' design feature of adult skull achieved

-we have large brains for our size and are quite weak
-therefore the lightness of skull is optimized by:
->diploe
->orbits
->cavities
->paranasal sinuses (eg. maxillae are hollow inside)

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how is the 'strength' design feature of the adult skull achieved

-for protection of brain and sense organs
-achieved by:
->the near-spherical shape of the neurocranium allowing maximum volume and strength with minimum weight and surface area
->the suture joints between many bones of the neurocranium acting as 'crack-stoppers'
->the 'crumble zone' of the viscerocranium involved in le fort fractures
->the 'buttress system'
->design feature of the orbital rims normally located on the same coronal plane as the surface of the eyeballs

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diploe of dermatocranium

-the flat bones of the dermatocranium are made from an outer and inner plate of thin compact bone, which 'sandwich' between them a 'filling' of light cancellous bone
-the filling of the sandwich structure is known as the diploe
-the membrane bones of the dermatocranium contain diploe that confers lightness and strength as well as some flexibility

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dermatocranium

= flat bones of the neurocranium

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restriction of crack propagation in the dermatocranium

-if the skull was made up of a hypothetical single large bone, as cracks spread out from point of injury the full skull would fracture
-however, as the skull is made up of multiple small bones (eg frontal, 2 x parietal) joined together by sutures/fibrous joints, the sutures act as 'crack stoppers'
-additionally, the presence of the 'diploe' means that only the outer plate of bone may fracture, helping to protect the brain inside

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sutures

fibrous joints

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brain protection design features of the viscerocranium

-force dispersal
->after traumatic forces to the facial skeleton
->of masticatory forces
-buttress system

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brain protection by force dispersal following traumatic forces to the facial skeleton

-blows to the mid-face (maxillary region):
->walls of the maxillae are very thin (bones crumple rather than remaining intact to be driven towards and through the base of the skull
->le fort fractures separate face from base of skull rather than damaging the base of the skull
-blows to the lower face (mandible)
->the mandible is made up of dense bone therefore can absorb force to a certain extent
->mandible will fracture itself rather than be driven via its condyle through the base of the skull

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brain protection by force dispersal of masticatory forces

-world record for force generated between teeth in occlusion is >50kg (held by an Alaskan eskimo)
-the mandible is made of thick, strong bone and can dissipate these occlusal forces by spreading them across itself
-the maxilla is made of thin bone and is hollow, it has to dissipate biting (occlusal) forces away from itself onto the neurocranium via 3 pathways (the 'buttress system') or else it might fracture

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buttress system

-the maxilla has to dissipate biting (occlusal) forces away from itself onto the neurocranium via 3 pathways (the 'buttress system') or else it might fracture
-the 3 pathways are:
->frontal buttress
->zygomatic buttress
->pterygoid buttress (on inside of skull)

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temporomandibular joint

condyl of mandible articulates with mandibular fossae on temporal bone

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the elderly patient

-the alveolar processes of the right and left maxillae and the mandible resorb (are removed by remodeling) when the teeth are lost (through trauma, gum disease or decay; are knocked out, fall out or are pulled out)
-mental foramen sometimes resorbs and disappears
-patient can present with very thin mandible in old age due to bone resorption