head/neck development

Question 1

morphogenesis is?

Answer 1

Coordinated and Generative

Question 2

what codes proteins

Answer 2

Genes (DNA) encode RNA and proteins

Question 3

what defines cell ID

Answer 3

The complement of RNA & protein produced defines the identity of each cell – its appearance and how it behaves.

Question 4

cells and their surroundings relation

Answer 4

Cells receive and process information from their surroundings – extracellular matrix, secreted molecules (growth factors) from other cells, hormones, contact with other cells (tension), nutrients, oxygen levels, etc.
•These in turn modify the genes expressed, thus allowing the cell to adapt to its situation and take on new properties / behaviors.

Question 5

determining phenotype

Answer 5

ØGenes (genome) provide the blueprint that ensures we all have a maxilla and mandible and an integrated oronasal cavity, etc. But gene mutation and differences in these non-genetic ‘instructions’ determines everything about our phenotype.

Question 6

Epithelia vs mesenchyme

Answer 6

epithelia: bound cells, organized
mesenchyme: non-bound/ mobile cells

Question 7

3 germ layers names and related to?

Answer 7

Ectoderm, mesoderm, endoderm

These are terms related to the embryonic origin of cells!

Question 8

All aspects of the craniofacial complex are defined by what axeses?

Answer 8

3 axes:
•Left-right
•Dorsal-ventral
•Anterior-posterior

Question 9

Clinical treatments always consider:

Answer 9

•form (eg. tooth shape, spacing, number, cusp pattern), and
•symmetry
because they are critical for function and esthetics.

Question 10

Dorsal-ventral axis apparent by what stage?

Answer 10

•apparent by blastocyst stage

Question 11

compaction

Answer 11

occurs with the morula to form trophectoderm and blastocyst/ ICM
ICM will form embryo (embryonic stem cells)

Question 12

implantation

Answer 12

allows blastocyst to form more distinct layers of cells

Question 13

what has formed by week 3?

Answer 13

all three germ layers

Question 14

The A-P and L-R Axes determined by and when?

Answer 14

Anterior-posterior (A-P) axis [head-tail] and left-right (L-R) axis determined at start of week 3, with appearance of the primitive streak

Question 15

primitive streak beginning

Answer 15

forms on dorsal side of bilaminar discs at midline and begins on the posterior end of the embyro
epiblastic cells will converge to form this

Question 16

streak appearance defines?

Answer 16

Appearance defines A-P and L-R axes

Question 17

elongation of streak

Answer 17

Furrow progressively elongates along midline

•through process of convergent extension

Question 18

Formation of 2nd& 3rd Germ Layer

how can cells be mobile?

Answer 18

Cells of the epiblast (layer 1) migrate through primitive
streak to form mesoderm & embryonic (gut) endoderm, called gastrulation

Through the process of epithelial to mesenchyme transformation (EMT) to form mesoderm, mesenchymal cells are mobile

Movement of epiblast (ectoderm) to form embryonic endoderm – both are epithelia

Question 19

Disproportionate growth of the germ layers?

Answer 19

> Greater proliferation of epiblast (ectoderm) because it also generates all mesoderm and embryonic endoderm.

Question 20

migration of proliferated mesoderm

Answer 20

Generated (proliferating) mesoderm migrates 
toward anterior (head) end

Question 21

The Node, used for?

Answer 21

Anterior-most end (dorsal side) of primitive streak is unique in both appearance and function
Essential for:
•patterning and induction of mesoderm and endoderm
•establishment of the left-right symmetry

Question 22

Factors secreted by cells of the Node induce?

Answer 22

Factors secreted by cells of the Node induce anteriorly migrating mesoderm to form:
•the prechordal plate and notochord

Question 23

prechrodal plate

Answer 23

• most anterior region of mesoderm

* provides the signals for induction of head structures

Question 24

The notochord

Answer 24

• transient epithelial-like rod structure along the midline.

* provides the signals for induction of the neural plate

Question 25

does the node induce body aixs? how so?
creates what symmetry in emryo? how does it do this?\
what will this symmetry result in?

Answer 25

the Node induces the body axis: > Induce and specify the fate of the mesoderm as it is formed
•Creation of asymmetry in symmetrical embryo
•First establishes molecular asymmetry
•Conversion into asymmetric organogenesis

Question 26

Establishing Molecular Asymmetry

Answer 26

Asymmetric expression of morphogenetic factors around Node
> initiates cascade of gene expression to promote asymmetric specialisation / commitment of mesoderm for organs

due to the rotational beat of nodal cells cilium (only a few hrs): generates leftward flow, resulting in asymmetric distribution of morphogens / growth factors

Question 27

Organ asymmetry, mutations?

Answer 27

Reproducible morphological & functional asymmetries in
nearly all internal organs.
» Evolutionarily conserved mechanism in vertebrates
» cilia function affected; gene expression around Node changes
1 in 8500-10000 people with situs inversus (normal health)
> cf. partial situs which can be deleterious

Question 28

Neurulation [Neural Tube Formation]
steps
precursor to?
patterning of neurons?

Answer 28

• **precursor to brain & spinal cord
  1. Factors from the mesoderm/ notochord induce thickening of overlying ectoderm&raquo_space; neural plate
  2. Signals from notochord induce a ‘hinge’ point (floor plate) to help drive folding
  3. D-V patterning of neurons begins

Question 29

Anterior (head) Specification
different regions of the brain?
morphological difference of the anterior-most region of the neural tube reflects?
where are the cues from?

Answer 29

The morphological difference of the anterior-most region of the neural tube reflects unique underlying cellular identities and thus different inductive cues (secreted factors): produce different regions of the brain
cues come from notochord, Anterior visceral endoderm (in direct contact with ectoderm), and Pre-chordal plate

Question 30

Neural Tube Closure, anterior end maturity and directionality

Answer 30

Advanced maturity of anterior end > expanded neural plate&raquo_space; future brain
Closure of neural tube proceeds anteriorly and posteriorly from mid-region

Question 31

head and neck comprise how much of embryo by wk 3?

Answer 31

1/2 (expanded nn plate)

Question 32

Defects in Neural Tube Closure

Answer 32

variety are possible, spina bifia common and viable

others may be lethal

Question 33

Cephalization (Head Formation)

hinge point?

Answer 33

The oropharyngeal membrane serves as the hinge point during cephalization, where endo and ecto are in direct contact
neural plate will fold downward/in to form the anterior neuropore, meso will give rise to heart, etc. and the mouth will begin to form (stomodeum), lateral folding is occurring as well during this
during this time the induction of head/facial structures is occurring as well

Question 34

Embryonic Folding occurs during?
concomitant formation with?
specialization of what from this?

Answer 34

Folding of whole embryo while cephalization occurs
•Concomitant with formation of pharyngeal arches (> face & neck)
•Foregut and hindgut specialization occurs

Question 35

NCC
stemness?
origin? 
migration pattern?
become what?

Answer 35

a unique population of (pluripotent) stem-like cells originating at the ‘crest’ of the enclosing neural tube between ectoderm and nn tube
•Migrate ‘ventro-laterally’ to populate the ventral side of the embryo
•Differentiate into a wide variety of cell types / tissues

Question 36

how NCC are generated with tobe folding/closure

why would this be important?

Answer 36

•NCC generated by process of epithelial-mesenchymal
transformation (EMT) that is analogous to that generating the
third germ layer (mesoderm)
ALLOWS THESE CELLS TO BE MOBILE

Question 37

Cranial Neural Crest Cells
migration of these drives?
potential?
contribute to what structures?

Answer 37

• CNCC migration drives outgrowth of the primitive tissue masses that will form much of the head and neck (branchial arches)
• Unique in developmental potential (distinct from other neural crest cells) “second wave” of development (vertebrate-specific)

> Major contributor to craniofacial structure, and specifically to mineralized tissues of the oral region

Question 38

CNCC Derivatives

Answer 38

conn tissue, skeleton and mm of the craniofacial region

Question 39

Rhombomeres & Cranial Patterning
how many rhomobmeres?
regions of brain involved?
rhombomeres define what?

Answer 39

• CNCC fate determined prior to departure from neural tube, originate at rhombomeres > ie. they have “positional identities”
• Multiple visible constrictions in neural tube=rhombomeres 1 - 8, the diencephalon and anterior mesencephalon
• define distinct populations of CNCC and their route of migration into early facial tissue.

Question 40

Directed CNCC Migration, manipulation?
reporducable?
defined by what germ layer?

Answer 40

•Reproducible paths of migration > defined by mesoderm

if CNCC from r1 placed in another r it may move into new branchail arch but will form same structure it was meant to be

Question 41

Coordinated Morphogenesis, what allows this?

Answer 41

All layers are differentiating and forming structures at the same times in development i.e: ectoderm, meso, branchail arches all working at same time
Early molecular patterning (est. during gastrulation) enables later coordinated tissue morphogenesis.

Question 42

Changing Cell Fates/phenotypes, what factors could play a role?

Answer 42

•Embryogenesis is a ‘generative’ process (not descriptive), many factors can play a role

Malformation and normal phenotypic variation can result from:

• single changes in gene sequence
• combinations of ‘normal’ gene variants
• changes in the ‘environment’ to which cells respond