development of head and mandible

• Growth

  An increase in mass, of the whole organism, or a given tissue/organ
• Development

  An increase in complexity, in anatomical form or function
• Growth [quantitative] and development [qualitative] are coordinated and thus correlated
• Allometry

  Coordinates the relationships between size and shape as we grow, and has important implications for development of function – this is particularly important in the head
• Skull function

  • Brain protection
  • Sensory organs; balance
  • Airway
  • Mastication
  • Speech
• The human skull has 22 bones (paired/single) connected by sutures (fibrous joints) or synchondroses (cartilaginous joint)

  
• Diversity of skull function

  • Reflected by its complex and highly variable morphology
• Unique features of the adult human head

  • Large cranial vault - brain size
  • Gracile - musculature
  • Flat facial profile - sensory
  • Orbits located medially - depth perception
  • Inferior foramen magnum - bipedal
  • Large mastoid processes - musculature
  • Parabolic dental arches and small teeth - diet
  • Relatively slow-growing (especially dental arches) - brain development/parental investment
• These adaptations are a function of mosaic evolution…not a "march of forms"
• Skull needs to increase in volume progressively

  To accommodate increases in brain size (early adolescence)
• Head growth/development is influenced/integrated with other aspects of physiology

  (e.g. surface area to volume ratios of brain influence gas exchange and hence vascularity)…allometry
• Skull features become functional at different times


  (e.g.protection = pre-natally, speech = early childhood)

  
• Large relative head size (even at birth)

  Presents challenges for vaginal birth…concurrent evolution of pelvic girdle, and key temporal features of cranial vault development
• Different components of the skull grow and develop at different rates…and over different time periods

  
• Remodelling also occurs throughout life in response to factors such as growth, trauma, and dental development
• There is population variation in skull size (minor sexual heterogeneity) and facial shape…classifications

  
• Three essential processes of craniofacial growth

  • Size increases
  • Remodelling
  • Displacement
• Remodelling

  A combination of deposition and resorption
• Displacement

  Movement of bones away from each other at their articular junctions; role of soft tissues
• Displacement is synergistic with size increase/remodelling
• Components of the craniofacial skeleton

  • Neurocranium
  • Viscerocranium
• Neurocranium

  • Protective; surrounds brain and sensory organs
  • Derived from neural crest cells, somites and somitomeres
  • Includes cranial vault (Dermatocranium; forms from intramembranous ossification) and cranial base (Chondrocranium; forms from endochondral ossification)
  • Three basic types – dolicocephalic, brachycephalic, mesocephalic…two extremes have characteristic facial features
• Viscerocranium


  • Bones of the face and related structures
  • Develops primarily from 1st and 2nd branchial arches
  • Derived from neural crest cells, occipital somites
  • A combination of intramembranous and endochondral ossification
  • Three basic profiles – orthognathic, retrognathic, prognathic
  • Sexual heterogeneity
  • Throughout life, growth and shape influenced by functional factors (masticatory forces; breathing patterns; facial muscle activity)…can affect development of jaws, position of the teeth, overall shape of the face
• Stages of neurocranium development

  1. Embryonic

  2. Foetal
• Embryonic stage

  • Mesenchyme (derived from neural crest of paraxial mesoderm) surrounding the developing brain; contains osteoprogenitor cells
  • 4th and 5th weeks of gestation - mesenchyme condensation where bone is to be formed
  • Vascularisation, membrane (ectomeningeal capsule) formation… earliest evidence of skull formation…condensation greatest in primordial cranial base
  • Membranous precursor to cranial vault appears around day 30; consists of curved plates of mesenchyme at sides of skull; gradually extend cranially to blend with each other; also extend towards primordial cranial base (chondrocranium)
  • Beginning of selective genes specific for either intramembranous or endochondral osteogenesis
  • Capsule (meninx primitiva) composed of endomeninx (gives rise to pia mater and arachnoid mater) and ectomeninx (gives rise to inner dura mater and outer membrane with osteogenic/chondrogenic potential)
  • Membrane formation complete by 37 days
  • Brain expands dramatically, capsule is totally responsive to expanding spatial demands
  • Growth of the brain and surrounding mesenchyme occurs simultaneously in all three dimensions; biodynamic differentiation, however, cause some areas of the developing brain to grow more rapidly than others…allometry
• Foetal stage

  • Ectomeninx initiates ossification
  • Intramembranous ossification of cranial vault starts in isolated ossification centres as early as 7-8 weeks…bone formation spreads centrifugally
  • Subsequent calcification to primitive trabecular bone, initially present in 'woven state', gradually converted to 'lamellar state' after birth
  • During fetal and postnatal growth, flat bones enlarge by apposition of new layers on outer surface and by simultaneous osteoclastic resorption from the inside
  • Skeletal structure of skull: mesenchyme for structures derived from neural crest (white) and paraxial mesoderm (grey)
  • Small line of neural crest-derived mesenchyme remains between two parietal bones; contributes to signaling system governing growth of cranial vault at sutures and development of underlying meninges
  • In floor of the brain, bones of cranial base are formed initially in cartilage and are later transformed by endochondral ossification into bone
  • Conversion of ectomeninx mesenchyme into cartilage constitutes beginning of the chondrocranium…commences from 40th day of gestation and onwards
  • By 8 weeks, through endochondral ossification, cartilage is replaced by bone in the cranial base
• Ossification

  1. Ossification centres as early as 7-8 weeks

  2. Bone formation spreads centrifugally
• Bone formation

  1. Subsequent calcification to primitive trabecular bone

  2. Gradually converted to 'lamellar state' after birth
• Flat bone enlargement

  1. Apposition of new layers on outer surface

  2. Simultaneous osteoclastic resorption from the inside
• Skeletal structure of skull

  • Mesenchyme for structures derived from neural crest (white)
  • Paraxial mesoderm (grey)
• Small line of neural crest-derived mesenchyme

  • Remains between two parietal bones
  • Contributes to signaling system governing growth of cranial vault at sutures and development of underlying meninges
• Bones of cranial base formation

  1. Initially in cartilage

  2. Later transformed by endochondral ossification into bone
• Conversion of ectomeninx mesenchyme

  1. Into cartilage

  2. Constitutes beginning of the chondrocranium

  3. Commences from 40th day of gestation and onwards
• Desmocranium replacement

  1. By 8 weeks, through endochondral ossification

  2. Ossification begins at precisely 12 weeks and 4 days of gestation, initially within the chondrocranium, forming part of the occipital bone
• Sutures and synchondroses

  Non-ossified tissue barriers that allow the skull to change shape and grow in response to brain development
• Fontanelles

  • Gaps/spaces at birth that allow for the compression of the skull during birth and rapid brain growth in infancy
  • Gradually close as growth centres expand
• Suture fusion

  1. Some sutures fuse completely (e.g. metopic)

  2. Others (e.g. coronal) retain patency = fibrous connective tissue
• Premature fusion/loss of patency


  Can result in craniosynostosis
• Cranium at birth

  • ~65% of adult dimensions, increasing to ~90% at 5 years
• Calvarial bones translation

  Away from one another, synergistically promoting further bone growth