Embryology - Growth & Development Flashcards
What are the functional roles of the skull?
- 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.
Compare and Contrast Growth and Development
- 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.
Describe diversity of human skull
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.
How do the features of brain influence development?
- 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
Define growth in the context of craniofacial development.
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.
What does allometry refer to?
The relationship between size and shape during growth
Allometry is critical for coordinating size and shape relationships, particularly during head development.
What are the functions of skulls?
- Brian protection
- Sensory organs; balance
- Airway
- Mastication
- Speech
What are the three essential processes of craniofacial growth?
- size increase & 2. remodelling
- A combination of deposition and resorption - Displacement -> synergistic with size increase/remodelling
- Movement of bones away from each other at their articular junctions
- Roles of soft tissues
List the types of skull classifications.
- Dolicocephalic
- Brachycephalic
- Mesocephalic
Skull types are characterized by their shape, which influences facial profiles.
What are the two types of ossification processes?
- Intramembranous (IM)
- Endochondral (EC)
IM ossification occurs in the cranial vault, while EC ossification occurs in the cranial base.
What is the role of sutures and synchondroses in skull growth?
Allow skull shape change and growth in response to brain development
Sutures are fibrous joints, and synchondroses are cartilaginous joints that facilitate growth.
What is craniosynostosis?
Premature suture fusion
This condition alters skull shape and can lead to complications.
What are the primary components of the neurocranium?
- Cranial Vault (Dermatocranium)
- Cranial Base (Chondrocranium)
The neurocranium protects the brain and sensory organs and is derived from neural crest cells and somites.
What are the three essential processes of craniofacial growth?
- Size Increase
- Remodeling
- Displacement
These processes work synergistically to facilitate growth and development.
True or False: The human skull has 22 bones.
True
The skull consists of paired and single bones.
Fill in the blank: The _______ protects the brain and sensory organs.
[neurocranium]
The neurocranium is a key component of the craniofacial skeleton.
What are the main influences on the development of the viscerocranium?
- 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
What are the 3 different profiles of viscerocranium?
- Orthognathic, retrogathic, and prognathic
What changes occur to the face during aging?
- 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.
What is the significance of fontanelles in skull development?
Allow skull compression and rapid brain growth at birth
Fontanelles are gaps between calvarial bones that close as the skull matures.
What is meant by ‘mosaic evolution’ in the context of skull features?
Features evolved independently, not as a linear progression
This concept reflects the diverse adaptations seen in skull morphology across species.
Identify the embryonic origins of craniofacial components.
- Neural crest cells
- Somites
- 1st and 2nd branchial arches
These tissues contribute to the formation of various skull structures.
What is the primary factor driving cranial vault growth?
Brain growth
The cranial vault expands to accommodate the developing brain, particularly in early adolescence.
What are the stages of neurocranium development?
- Embryonic (4th–5th Weeks)
- Fetal (7–12 Weeks)
- Postnatal
Each stage involves specific changes and processes leading to the mature skull.
Detail the embryonic stage of neurocranium development
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 biodWynamic differentiation as some brain regions grow more rapidly than others.
Detail the Foetal stage of neurocranium development
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.
What are the roles of sutures & synchondroses?
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.
Detail the post-natal stage of neurocranium development
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.
What are the stages of viscerocranium development?
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.
Describe the signs of aging
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.
What is the definition of Brachycephalic?
Short, wide skull
Brachycephalic skulls are often seen in certain dog breeds.
What does Chondrocranium refer to?
Cartilage-based cranial base
The chondrocranium forms the base of the skull during embryonic development.
What is Craniosynostosis?
Premature suture fusion
This condition can lead to abnormal skull shapes and increased intracranial pressure.
Define Dolicocephalic.
Long, narrow skull
Dolicocephalic skulls are often associated with certain physical traits.
What is Endochondral Ossification?
Bone formation from cartilage
This process is crucial for the development of long bones.
What is a Fontanelle?
Gaps between calvarial bones at birth
Fontanelles allow for skull compression during childbirth.
Define Intramembranous Ossification.
Bone formation from mesenchyme
This type of ossification occurs in flat bones like the skull.
What does Mesocephalic mean?
Intermediate skull shape
Mesocephalic skulls have balanced proportions.
What are Neural Crest Cells?
Embryonic cells forming skull components
These cells contribute to the development of various facial structures.
Define Orthognathic.
Straight jaw profile
Orthognathic profiles are considered ideal in dental occlusion.
What is Resorption in the context of bone?
Bone breakdown by osteoclasts
This process is essential for bone remodeling and calcium homeostasis.
What is a Suture?
Fibrous joint between cranial bones
Sutures allow for growth and changes in the skull shape.
Define Synchondrosis.
Cartilaginous joint in cranial base
Synchondroses provide flexibility during growth.
What is the Viscerocranium?
Bones of the face and related structures, including: facial skeleton, ossicles, hyoid bone, laryngeal / tracheal cartilages, certain skull processes
List five functions of the skull.
- Protects the brain
- Supports facial structure
- Houses sensory organs
- Facilitates chewing
- Assists in respiration
Each function has evolutionary significance for survival.
Define allometry.
The study of the relationship of body size to shape, anatomy, physiology, and behavior
In head development, allometry explains how different parts grow at different rates.
Compare dolicocephalic, brachycephalic, and mesocephalic skulls.
- Dolicocephalic: Long, narrow
- Brachycephalic: Short, wide
- Mesocephalic: Intermediate
Each type has distinct evolutionary adaptations.
Differentiate between Intramembranous and Endochondral ossification.
- Intramembranous: Directly from mesenchyme
- Endochondral: From cartilage models
Examples include the flat bones of the skull (IM) and long bones (EC).
Describe the structure and function of sutures and synchondroses.
- Sutures: Fibrous joints, allow growth
- Synchondroses: Cartilaginous joints, provide flexibility
Clinical issues include craniosynostosis affecting skull shape.
Outline the developmental stages for neurocranium and viscerocranium.
- Embryonic stage
- Fetal stage
- Postnatal stage
Each stage has distinct developmental milestones.
Define orthognathic, retrognathic, and prognathic profiles.
- Orthognathic: Straight jaw
- Retrognathic: Receding jaw
- Prognathic: Protruding jaw
These profiles are important in orthodontic assessments.
List six age-related changes in the face.
- Wrinkles
- Bone resorption
- Loss of elasticity
- Changes in fat distribution
- Altered jaw position
- Tooth loss
These changes impact facial aesthetics and function.
Identify tissues responsible for skull components.
- Neural crest
- Somites
These tissues contribute to the formation of various skull structures.
Explain the growth processes in craniofacial development.
- Size increase
- Remodeling
- Displacement
These processes are crucial for proper craniofacial growth and function.
