Embryology of the Head, Neck, Oral Cavity and Tongue Flashcards
Week 1 of embryonic development
Week 1: fertilisation, implantation
- Ovum and sperm meet in uterine tube zygote forms morula (through mitosis and cleavage) blastocyst (increase in cell size w/ no change in cell number; consists of 2 types of cells = trophoblast and embryoblast)
o Trophoblast helps embryo attach to the uterus wall; embryo’s contribution to placenta
o Embryoblast: becomes the embryo
Implantation occurs on the 7th day
Week 2 of embryonic development
Week 2: bilaminar embryonic disc
- Blastocyst proliferates, forming the bilaminar embryonic disc. Process is as follows:
o Cells receive external induction signals to differentiate into epiblast or hypoblast cell
o Epiblast, primary ectoderm and dorsal columnar cells form the amniotic cavity
o Hypoblast, primary endoderm and ventral cuboidal cells form the yolk sac
o A disc is suspended between these two cavities (amniotic cavity and yolk sac)
- Prochordal plate: thickened area where the epiblast and hypoblast meet: first visual sign of embryonic axis development
Week 3 of embryonic development
Week 3: trilaminar disc; neurulation and NCC migration; mesoderm differentiation
- Cells of the amniotic cavity differentiate and burrow to form the primitive streak
o Rostral end of streak ends in a pit, which is called the primitive node
- Epiblast cells (soon to be ectoderm) move towards the hypoblast (soon to be endoderm) to form the mesoderm: these are the three germ layers of the trilaminar disc
- Neurulation
o Ectoderm cells differentiate and thicken to form the neural plate
o As the plate continues to grow and thicken, it invaginates inwards to form the neural groove and neural folds
o Neural folds undergo fusion and form the neural tube, which eventually separates from the ectoderm and become the brain and spinal cord
- Neural crest cells: these cells know what to do because of signalling events
o Develop from the neuroectoderm (which is the ectoderm immediately above the notochord)
o Undergo epithelial-mesenchymal transformation; migrates from the crest of the neural folds (i.e., where the neural folds meet during the closure of neural tube) to join the mesoderm
o Mesenchyme formed
o Ectomesenchyme formed
Week 4 of embryonic development
Week 4: formation of stomodeum, forebrain, and tongue
- 2x folding is going on
o Rostrocaudal axis folding forms primitive stomodeum, which is separated from the gut through the buccopharyngeal membrane
o Lateral folding allows ectoderm lining upon all aspects of the embryo
- Neural tube expands and forms primitive forebrain produces the frontal prominence
- Somites form from the paraxial mesoderm, e.g., occipital somites
- Stomodeum forms between the frontal prominence and developing cardiac bulge; this will become the oral cavity
- 6 branchial arches form on each side on the lateral aspect of the embryo from head to tail
- NCC migrate to the branchial arches
- First branchial arch extends into two processes: maxillary process (above the stomodeum) and the mandibular process (below the mandibular process)
- The mandibular process on each side grow together and fuse into a single structure very early on in embryonic development
- Two patches of ectoderm on the frontal prominence start to proliferate forming nasal placodes: starts embryonic development of the face
- Tongue formation: local proliferation of mesenchyme in the floor of the mouth
o Tuberculum impar and lingual swellings develop in the middle of the 1st branchial arch: they form the future 2/3 of external mucous membrane of the anterior of the tongue
§ Sensory information from lingual branch of the mandibular division of CN V (trigeminal nerve)
o Hypobranchial eminence develops to form the posterior 1/3 of mucous membrane. It fuses with tuberculum impar and lingual swellings to form the tongue by growing over the 2nd branchial arch
§ Sensory information from CN IX = glossopharyngeal nerve and X = vagus nerve)
o The (skeletal) muscles of the tongue are from the occipital somites (which migrate and proliferate into the tongue from the neural tube)
Week 5 of embryonic development
Week 5: olfactory placode and nasal process development, formation of the lower lip
- Olfactory placode: formed from localised thickenings within the ectoderm of the frontal prominence
- Nasal process and pit: rapid proliferation of mesenchyme around the nasal placodes, which form the horseshoe-shaped ridges
o Lateral nasal process (outer half)
o Medial nasal process (inner half)
- Nasal placodes sink downwards, forming nasal pits
- Formation of the lower lip through apparent fusion of the mandibular processes
o Apparent/ faux fusion: tissues were originally connected with continuous epithelium, lateral extension eliminates the furrow e.g., fusion of facial processes, formation of lateral commissures of lips
Week 6 of embryonic development
Week 6: rudimentary mandible formation
- Maxillary processes start to proliferate towards the midline
- Meckel’s cartilage (ecto-mesenchymal migration into the 1st branchial arch) acts as a scaffold to facilitate intramembranous ossification of the mandible
o The cartilage is separated at the midline by mesenchyme
o Mandibular nerve begins 2/3 along the cartilage and divides into inferior alveolar nerve (IAN) and lingual nerves
- Ossification occurs where the IAN divides into the incisive and mental branch; progresses anteriorly and posteriorly
o Left and right parts remain separated until mandibular symphysis after birth
- Meckel’s cartilage is then resorbed, except for at the malleus (in the ear)
Week 7 of embryonic development
Week 7: formation of upper lip
- Formation of upper lip
o Middle portion of the upper lip (+ philtrum, primary palate, anterior maxilla): from merging of the medial nasal processes and frontonasal processes
o Lateral aspect of upper lip: anterior aspect of the maxillary process fusing with the lateral aspect of the medial nasal process
- Medial nasal processes come together forming the intermaxillary segment
o This develops into bridge of nose, philtrum, 4 upper incisors and primary palate
- Lateral nasal processes form the lateral nasal wall
- Mandibular processes form the lower jaw
- Mandibular and maxillary processes fuse to form the cheeks
- Two nasal pits burrow deeper and backwards above the stomodeum, forming nasal cavity
Weeks 7-9 of embryonic development
Weeks 7-9: secondary palate formation
- Nasal septum grows downwards from the frontonasal process
- Palatine shelves grow vertically downwards from the maxillary processes, on either side of the developing tongue
o Initially growing downwards, until the tongue descends and moves away from the shelves
o After the tongue descends, creating room for the palatine shelves ascend:
§ Merge with the primary palate
§ Grow towards each other and fuse with each other (true fusion occurs in week 9)
· True fusion: two epitheliums meet, causing their breakdown: loss of epithelial tissue and leads to a continuous underlying mesoderm/ allows continuity of the underlying mesenchyme, e.g., palatal processes undergo true fusion to secondary palate, neural folds to neural tube
§ Fuse with nasal septum
o Oral and nasal cavities are now separated
- Week 8: taste buds of tongue begin to develop; finish differentiating weeks 11-13
What has occurred by week 12 of embryonic development?
- by this point, final palate is formed
- taste buds finish differentiating weeks 11-13
What are epithelia-mesenchymal transformations?
EMT – epithelia-mesenchymal transformation
- Epithelial cells lose cell polarity and cell-cell adhesion gaining migratory properties to become mesenchymal stem cells and differentiate into a range of cell types (multipotent)
- Different to epithelial-mesenchyme interactions: programmed communication between epithelial and mesenchymal cells, resulting in further cell differentiation
Reciprocal induction in amelogenesis (interaction between pre-dentine and pre-ameloblasts)
What day of after fertilisation does implantation occur?
On the 7th day
What is the blastocyst? What is it comprised of? When does this form?
- Increase in cell size w/ no change in cell number
- Consists of 2 types of cells: trophoblast and embryoblast
- Forms in first week of embryonic development
What is the trophoblast?
Helps embryo attach to the uterus wall; embryo’s contribution to the placenta
What is the embryoblast?
Becomes the embryo
How does the bilaminar embryonic disc form? When does this occur?
- Blastocyst proliferates, forming the bilaminar embryonic disc. Process is as follows:
o Cells receive external induction signals to differentiate into epiblast or hypoblast cell
o Epiblast, primary ectoderm and dorsal columnar cells form the amniotic cavity
o Hypoblast, primary endoderm and ventral cuboidal cells form the yolk sac
A disc is suspended between these two cavities (amniotic cavity and yolk sac) - Forms in week 2 of embryonic development
What is the prochordal plate? When is it seen?
Prochordal plate: thickened area where the epiblast and hypoblast meet: first visual sign of embryonic axis development
Seen in week 2 of embryonic development
What forms the amniotic cavity and yolk sac?
o Cells receive external induction signals to differentiate into epiblast or hypoblast cell
o Epiblast, primary ectoderm and dorsal columnar cells form the amniotic cavity
o Hypoblast, primary endoderm and ventral cuboidal cells form the yolk sac
o A disc is suspended between these two cavities (amniotic cavity and yolk sac)
These are from the bilaminar disc
What does the amniotic cavity form?
- Cells of the amniotic cavity differentiate and burrow to form the primitive streak
o Rostral end of streak ends in a pit, which is called the primitive node
Three layers of the trilaminar disc. When is it formed?
- Epiblast cells (soon to be ectoderm) move towards the hypoblast (soon to be endoderm) to form the mesoderm: these are the three germ layers of the trilaminar disc
- Formed during week 3 of embryonic development
What is neurulation? When does it occur?
- Occurs during week 3 of embryonic development
o Ectoderm cells differentiate and thicken to form the neural plate
o As the plate continues to grow and thicken, it invaginates inwards to form the neural groove and neural folds
o Neural folds undergo fusion and form the neural tube, which eventually separates from the ectoderm and become the brain and spinal cord
How do neural crest cells know what to do?
Because of signalling events
Where do neural crest cells develop from?
Develop from the neuroectoderm (which is the ectoderm immediately above the notochord)
What happens to neural crest cells? When does this occur?
o Undergo epithelial-mesenchymal transformation; migrates from the crest of the neural folds (i.e., where the neural folds meet during the closure of neural tube) to join the mesoderm
o Mesenchyme formed
Ectomesenchyme formed
Occurs during week 3 of embryonic development
They migrate to the branchial arches during week 4 of development
What folding is occurring during week 4 of embryonic development?
- 2x folding is going on
o Rostrocaudal axis folding forms primitive stomodeum, which is separated from the gut through the buccopharyngeal membrane
o Lateral folding allows ectoderm lining upon all aspects of the embryo
What does the first branchial arch give rise to?
- First branchial arch extends into two processes: maxillary process (above the stomodeum) and the mandibular process (below the mandibular process)
Tongue formation
- Tongue formation: local proliferation of mesenchyme in the floor of the mouth
o Tuberculum impar and lingual swellings develop in the middle of the 1st branchial arch: they form the future 2/3 of external mucous membrane of the anterior of the tongue
§ Sensory information from lingual branch of the mandibular division of CN V (trigeminal nerve)
o Hypobranchial eminence develops to form the posterior 1/3 of mucous membrane. It fuses with tuberculum impar and lingual swellings to form the tongue by growing over the 2nd branchial arch
§ Sensory information from CN IX = glossopharyngeal nerve and X = vagus nerve)
o The (skeletal) muscles of the tongue are from the occipital somites (which migrate and proliferate into the tongue from the neural tube)
What is the olfactory placode formed from? When does this occur?
Olfactory placode: formed from localised thickenings within the ectoderm of the frontal prominenc
Occurs during week 5 of embryonic development
What happens to nasal placodes? When does this occur?
- Nasal process and pit: rapid proliferation of mesenchyme around the nasal placodes, which form the horseshoe-shaped ridges
o Lateral nasal process (outer half)
o Medial nasal process (inner half) - Nasal placodes sink downwards, forming nasal pits
- Formation of the lower lip through apparent fusion of the mandibular processes
o Apparent/ faux fusion: tissues were originally connected with continuous epithelium, lateral extension eliminates the furrow e.g., fusion of facial processes, formation of lateral commissures of lips - occurs during week 5 of embryonic development
Apparent/ faux fusion
Apparent/ faux fusion: tissues were originally connected with continuous epithelium, lateral extension eliminates the furrow e.g., fusion of facial processes, formation of lateral commissures of lips
What is the role of Meckel’s cartilage?
- Meckel’s cartilage (ecto-mesenchymal migration into the 1st branchial arch) acts as a scaffold to facilitate intramembranous ossification of the mandible
o The cartilage is separated at the midline by mesenchyme
o Mandibular nerve begins 2/3 along the cartilage and divides into inferior alveolar nerve (IAN) and lingual nerves - Ossification occurs where the IAN divides into the incisive and mental branch; progresses anteriorly and posteriorly
o Left and right parts remain separated until mandibular symphysis after birth
Meckel’s cartilage is then resorbed, except for at the malleus (in the ear)
When does tongue formation begin?
During week 4 of embryonic development
What gives rise to the inferior alveolar nerve? Relate it to ossification
o Mandibular nerve begins 2/3 along the cartilage and divides into inferior alveolar nerve (IAN) and lingual nerves
- Ossification occurs where the IAN divides into the incisive and mental branch; progresses anteriorly and posteriorly
o Left and right parts remain separated until mandibular symphysis after birth
Formation of the upper lip. When does this occur?
Week 7 of embryonic development
- Formation of upper lip
o Middle portion of the upper lip (+ philtrum, primary palate, anterior maxilla): from merging of the medial nasal processes and frontonasal processes
Lateral aspect of upper lip: anterior aspect of the maxillary process fusing with the lateral aspect of the medial nasal process
What do the medial nasal processes form when they come together? When does this occur?
Week 7
- Medial nasal processes come together forming the intermaxillary segment
o This develops into bridge of nose, philtrum, 4 upper incisors and primary palate
What do the lateral nasal processes form? When does this occur?
- Lateral nasal processes form the lateral nasal wall
- Week 7
What do mandibular process form? When does this occur?
Week 7
- Mandibular processes form the lower jaw
What is formed when the mandibular and maxillary processes fuse? When does this occur?
Week 7
Fuse to form the cheeks
How is the nasal cavity formed? When does this occur?
Week 7
Two nasal pits burrow deeper and backwards above the stomodeum, forming nasal cavity
Describe secondary palate formation. When does this occur?
- Nasal septum grows downwards from the frontonasal process
- Palatine shelves grow vertically downwards from the maxillary processes, on either side of the developing tongue
o Initially growing downwards, until the tongue descends and moves away from the shelves
o After the tongue descends, creating room for the palatine shelves ascend:
§ Merge with the primary palate
§ Grow towards each other and fuse with each other (true fusion occurs in week 9)
· True fusion: two epitheliums meet, causing their breakdown: loss of epithelial tissue and leads to a continuous underlying mesoderm/ allows continuity of the underlying mesenchyme, e.g., palatal processes undergo true fusion to secondary palate, neural folds to neural tube
§ Fuse with nasal septum
o Oral and nasal cavities are now separated
When do taste buds begin and finish developing?
Week 8: taste buds of tongue begin to develop; finish differentiating weeks 11-13
