Embryology - Growth & Development

Question 1

What are the functional roles of the skull?

Answer 1

• Brain Protection
• Sensory Organs
• Airway
• Mastication
• Speech

The skull encases the brain and houses sensory organs for vision, hearing, and balance, while also facilitating breathing and supporting chewing.

Question 2

Compare and Contrast Growth and Development

Answer 2

• Growth is an increase in mass, of the whole organism, or given tissue/organ
• Development is an increase in complexity, in anatomical forms or function.
• Growth is quantitative and development is qualitative, 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.

Question 3

Describe diversity of human skull

Answer 3

The adult human head exhibits unique features shaped by mosaic evolution, reflecting diverse functional adaptations:
1. a large cranial vault accommodates an expanded brain for enhanced cognitive abilities;
2. a gracile structure corresponds to reduced musculature demands;
3. a flat facial profile and medially located orbits enhance sensory perception and depth perception for improved environmental interaction;
4. an inferior foramen magnum supports bipedal posture; large mastoid processes anchor neck musculature;
5. parabolic dental arches with small teeth align with dietary shifts toward softer, processed foods;
6. and relatively slow growth, particularly of dental arches, correlates with prolonged brain development and increased parental investment.

Question 4

How do the features of brain influence development?

Answer 4

• 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

Consequently, different components 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. Population variation in skull size (minor sexual heterogeneity) and facial shape…classifications

Question 5

Define growth in the context of craniofacial development.

Answer 5

Quantitative increase in mass (whole organism or specific tissue/organ)

Growth is distinct from development, which involves a qualitative increase in anatomical form or function complexity.

Question 6

What does allometry refer to?

Answer 6

The relationship between size and shape during growth

Allometry is critical for coordinating size and shape relationships, particularly during head development.

Question 7

What are the functions of skulls?

Answer 7

• Brian protection
• Sensory organs; balance
• Airway
• Mastication
• Speech

Question 8

What are the three essential processes of craniofacial growth?

Answer 8

1. size increase & 2. remodelling
   - A combination of deposition and resorption
2. Displacement -> synergistic with size increase/remodelling
   - Movement of bones away from each other at their articular junctions
   - Roles of soft tissues

Question 9

List the types of skull classifications.

Answer 9

• Dolicocephalic
• Brachycephalic
• Mesocephalic

Skull types are characterized by their shape, which influences facial profiles.

Question 10

What are the two types of ossification processes?

Answer 10

• Intramembranous (IM)
• Endochondral (EC)

IM ossification occurs in the cranial vault, while EC ossification occurs in the cranial base.

Question 11

What is the role of sutures and synchondroses in skull growth?

Answer 11

Allow skull shape change and growth in response to brain development

Sutures are fibrous joints, and synchondroses are cartilaginous joints that facilitate growth.

Question 12

What is craniosynostosis?

Answer 12

Premature suture fusion

This condition alters skull shape and can lead to complications.

Question 13

What are the primary components of the neurocranium?

Answer 13

• Cranial Vault (Dermatocranium)
• Cranial Base (Chondrocranium)

The neurocranium protects the brain and sensory organs and is derived from neural crest cells and somites.

Question 14

What are the three essential processes of craniofacial growth?

Answer 14

• Size Increase
• Remodeling
• Displacement

These processes work synergistically to facilitate growth and development.

Question 15

True or False: The human skull has 22 bones.

Answer 15

True

The skull consists of paired and single bones.

Question 16

Fill in the blank: The _______ protects the brain and sensory organs.

Answer 16

[neurocranium]

The neurocranium is a key component of the craniofacial skeleton.

Question 17

What are the main influences on the development of the viscerocranium?

Answer 17

• Masticatory forces
• Breathing patterns
• Facial muscle activity
• These factors affect jaw development, tooth position, and facial shape.
• Develops from 1st and 2nd branchial arches
• Derived from neural crestal cells, occipital somites
• A combination of IM and EC ossification
• Sexual heterogeneity

Question 18

What are the 3 different profiles of viscerocranium?

Answer 18

• Orthognathic, retrogathic, and prognathic

Question 19

What changes occur to the face during aging?

Answer 19

• Skin loses elasticity
• Fat pads shrink/shift
• Muscle weakens
• Bone resorption
• Ligaments weaken
• Vasculature becomes fragile

These changes contribute to the appearance of aging in facial structures.

Question 20

What is the significance of fontanelles in skull development?

Answer 20

Allow skull compression and rapid brain growth at birth

Fontanelles are gaps between calvarial bones that close as the skull matures.

Question 21

What is meant by ‘mosaic evolution’ in the context of skull features?

Answer 21

Features evolved independently, not as a linear progression

This concept reflects the diverse adaptations seen in skull morphology across species.

Question 22

Identify the embryonic origins of craniofacial components.

Answer 22

• Neural crest cells
• Somites
• 1st and 2nd branchial arches

These tissues contribute to the formation of various skull structures.

Question 23

What is the primary factor driving cranial vault growth?

Answer 23

Brain growth

The cranial vault expands to accommodate the developing brain, particularly in early adolescence.

Question 24

What are the stages of neurocranium development?

Answer 24

• Embryonic (4th–5th Weeks)
• Fetal (7–12 Weeks)
• Postnatal

Each stage involves specific changes and processes leading to the mature skull.

Question 25

Detail the embryonic stage of neurocranium development

Answer 25

During the 4th and 5th weeks of gestation, mesenchyme derived from the neural crest of paraxial mesoderm, containing osteoprogenitor cells, surrounds the developing brain and undergoes condensation in areas destined for bone formation, with the greatest concentration in the primordial cranial base. By day 30, a membranous precursor to the cranial vault forms as curved mesenchymal plates at the skull's sides, extending cranially to blend with each other and toward the chondrocranium, while vascularization and the ectomeningeal capsule, comprising the endomeninx (forming pia and arachnoid mater) and ectomeninx (forming inner dura mater and an outer osteogenic/chondrogenic membrane), mark the earliest skull formation. By day 37, membrane formation is complete, and the capsule (meninx primitiva) dynamically responds to the brain's rapid three-dimensional expansion, driven by allometric growth where selective genes initiate either intramembranous or endochondral osteogenesis, accommodating biod Wynamic differentiation as some brain regions grow more rapidly than others.

Question 26

Detail the Foetal stage of neurocranium development

Answer 26

The ectomeninx initiates ossification, with intramembranous (IM) ossification of the cranial vault beginning in isolated ossification centers as early as 7-8 weeks, spreading centrifugally and forming primitive trabecular bone in a woven state, which transitions to a lamellar state postnatally. Flat bones of the skull enlarge during fetal and postnatal growth through apposition of new layers on the outer surface and simultaneous osteoclastic resorption internally. The skull’s skeletal structure derives from neural crest (white) and paraxial mesoderm (grey) mesenchyme, with a small line of neural crest-derived mesenchyme persisting between the parietal bones, contributing to signaling for cranial vault growth at sutures and meningeal development. In the cranial base, bones form initially as cartilage from the 40th day of gestation, marking the onset of the chondrocranium, and undergo endochondral ossification starting precisely at 12 weeks and 4 days, replacing the desmocranium with cartilage and forming parts of the occipital bone by 8 weeks.

Question 27

What are the roles of sutures & synchondroses?

Answer 27

At the end of the embryonic stage, ossification fronts of the calvaria and skull base are separated by non-ossified tissue barriers, such as sutures and synchondroses, which permit skull shape changes and growth in response to brain development. At birth, the calvaria's growth centers remain partially isolated, forming six fontanelles that allow skull compression during delivery and accommodate rapid brain growth in infancy. These fontanelles gradually close as growth centers expand, and skull bones fuse along sutures, with some, like the metopic suture, fusing completely, while others, like the coronal suture, retain fibrous connective tissue patency. Premature fusion or loss of suture patency can lead to craniosynostosis, disrupting normal skull development.

Question 28

Detail the post-natal stage of neurocranium development

Answer 28

At birth, the cranium is approximately 65% of adult size, reaching 90% by age 5, with calvarial bones translating apart to promote further bone growth while fontanelles, including the largest anterior fontanelle (closing at 12 months) and the last posterolateral fontanelle (closing at 18 months), reduce in size. Continuous remodeling alters the skull’s contour to accommodate brain growth, while the cranial base expands predominantly laterally through surface deposition and extensive remodeling as cerebral hemispheres grow. Three synchondroses (spheno-occipital, intersphenoid, and spheno-ethmoidal) facilitate some anteroposterior growth, with the spheno-occipital synchondrosis significantly influencing facial growth by affecting the maxillary-mandibular relationship due to its position relative to the condylar fossa and the nasomaxillary complex; it ceases growth around age 15 in boys (earlier in girls) and fully closes by approximately age 20.

Question 29

What are the stages of viscerocranium development?

Answer 29

During embryonic development, the head and oral cavity form from mesenchyme of the frontonasal prominence and first two pharyngeal arches. The frontonasal process shapes the forehead, nasal bridge, dorsum, and nasal bones, while medial nasal processes form the columella, philtrum, ethmoid perpendicular plate, vomer, cribriform plates, and primary palate, and lateral nasal processes create the nasal sides and alae. Maxillary processes develop into the upper cheeks, most of the upper lip, maxilla, zygoma, and secondary palate, and mandibular processes form the chin, lower lip, lower cheeks, and mandible. In the early fetal period, the nose is flat, the mandible small, and orbits face laterally, with the stomodeum spanning nearly the entire embryonic face; by weeks 7-8, differential growth repositions the eyes and maxilla/mandible more frontally, and cheek formation via maxillary-mandibular fusion reduces mouth size, though cranial vault expansion makes the mouth appear smaller. Postnatally, rapid growth continues through infancy and childhood, with the facial skeleton growing downward and forward from the cranial base, facilitated by sutures and growth at the spheno-occipital synchondroses and nasal septum, translating the maxilla. Mandibular growth, coordinated with the maxilla, involves bone translation and ramus/condyle growth to match maxillary translation and cranial base expansion, complicated by the mandibular condyles’ articulation in the glenoid fossa behind the spheno-occipital synchondrosis. Unlike the cranium, which tracks brain growth, the nasomaxillary complex and mandible peak during the pubertal growth spurt, aligning with musculoskeletal development.

Question 30

Describe the signs of aging

Answer 30

The development and aging of the human head and face involve a complex interplay of embryonic, fetal, postnatal, and pubertal processes, followed by age-related changes. Embryonic skull formation begins with neural crest and paraxial mesoderm-derived mesenchyme condensing around the brain by the 4th-5th weeks, forming the ectomeningeal capsule and initiating intramembranous ossification of the cranial vault by 7-8 weeks and endochondral ossification of the cranial base by 12 weeks, driven by allometric brain growth. Fetal facial structures arise from the frontonasal prominence and pharyngeal arches, with processes forming the forehead, nose, maxilla, and mandible, while differential growth by weeks 7-8 refines facial proportions. Postnatally, the cranium grows to 65% of adult size at birth and 90% by age 5, with sutures, fontanelles (closing by 12-18 months), and synchondroses (e.g., spheno-occipital, ceasing growth by 15-20 years) facilitating brain and facial expansion, particularly downward and forward maxillary and mandibular growth during the pubertal spurt, coordinated with cranial base changes. In aging, genetics and environment (e.g., sun exposure, smoking) drive skin elasticity loss, texture roughening, soft tissue volume shifts, muscle weakening, bone resorption, tooth loss, ligament stretching, and vascular fragility, resulting in wrinkles, sagging, age spots, and drooping features, collectively altering facial appearance over time.

Question 31

What is the definition of Brachycephalic?

Answer 31

Short, wide skull ## Footnote Brachycephalic skulls are often seen in certain dog breeds.

Question 32

What does Chondrocranium refer to?

Answer 32

Cartilage-based cranial base ## Footnote The chondrocranium forms the base of the skull during embryonic development.

Question 33

What is Craniosynostosis?

Answer 33

Premature suture fusion ## Footnote This condition can lead to abnormal skull shapes and increased intracranial pressure.

Question 34

Define Dolicocephalic.

Answer 34

Long, narrow skull ## Footnote Dolicocephalic skulls are often associated with certain physical traits.

Question 35

What is Endochondral Ossification?

Answer 35

Bone formation from cartilage ## Footnote This process is crucial for the development of long bones.

Question 36

What is a Fontanelle?

Answer 36

Gaps between calvarial bones at birth ## Footnote Fontanelles allow for skull compression during childbirth.

Question 37

Define Intramembranous Ossification.

Answer 37

Bone formation from mesenchyme ## Footnote This type of ossification occurs in flat bones like the skull.

Question 38

What does Mesocephalic mean?

Answer 38

Intermediate skull shape ## Footnote Mesocephalic skulls have balanced proportions.

Question 39

What are Neural Crest Cells?

Answer 39

Embryonic cells forming skull components ## Footnote These cells contribute to the development of various facial structures.

Question 40

Define Orthognathic.

Answer 40

Straight jaw profile ## Footnote Orthognathic profiles are considered ideal in dental occlusion.

Question 41

What is Resorption in the context of bone?

Answer 41

Bone breakdown by osteoclasts ## Footnote This process is essential for bone remodeling and calcium homeostasis.

Question 42

What is a Suture?

Answer 42

Fibrous joint between cranial bones ## Footnote Sutures allow for growth and changes in the skull shape.

Question 43

Define Synchondrosis.

Answer 43

Cartilaginous joint in cranial base ## Footnote Synchondroses provide flexibility during growth.

Question 44

What is the Viscerocranium?

Answer 44

Bones of the face and related structures, including: facial skeleton, ossicles, hyoid bone, laryngeal / tracheal cartilages, certain skull processes

Question 45

List five functions of the skull.

Answer 45

* Protects the brain * Supports facial structure * Houses sensory organs * Facilitates chewing * Assists in respiration ## Footnote Each function has evolutionary significance for survival.

Question 46

Define allometry.

Answer 46

The study of the relationship of body size to shape, anatomy, physiology, and behavior ## Footnote In head development, allometry explains how different parts grow at different rates.

Question 47

Compare dolicocephalic, brachycephalic, and mesocephalic skulls.

Answer 47

* Dolicocephalic: Long, narrow * Brachycephalic: Short, wide * Mesocephalic: Intermediate ## Footnote Each type has distinct evolutionary adaptations.

Question 48

Differentiate between Intramembranous and Endochondral ossification.

Answer 48

* Intramembranous: Directly from mesenchyme * Endochondral: From cartilage models ## Footnote Examples include the flat bones of the skull (IM) and long bones (EC).

Question 49

Describe the structure and function of sutures and synchondroses.

Answer 49

* Sutures: Fibrous joints, allow growth * Synchondroses: Cartilaginous joints, provide flexibility ## Footnote Clinical issues include craniosynostosis affecting skull shape.

Question 50

Outline the developmental stages for neurocranium and viscerocranium.

Answer 50

* Embryonic stage * Fetal stage * Postnatal stage ## Footnote Each stage has distinct developmental milestones.

Question 51

Define orthognathic, retrognathic, and prognathic profiles.

Answer 51

* Orthognathic: Straight jaw * Retrognathic: Receding jaw * Prognathic: Protruding jaw ## Footnote These profiles are important in orthodontic assessments.

Question 52

List six age-related changes in the face.

Answer 52

* Wrinkles * Bone resorption * Loss of elasticity * Changes in fat distribution * Altered jaw position * Tooth loss ## Footnote These changes impact facial aesthetics and function.

Question 53

Identify tissues responsible for skull components.

Answer 53

* Neural crest * Somites ## Footnote These tissues contribute to the formation of various skull structures.

Question 54

Explain the growth processes in craniofacial development.

Answer 54

* Size increase * Remodeling * Displacement ## Footnote These processes are crucial for proper craniofacial growth and function.