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
