Lecture 16: Germ Layer and Gastrulation Flashcards
What is morphogenesis?
Process that gives rise to the shapes and forms of tissue and organs
What are the 3 axes? When do they become apparent?
- Cranial/caudal axis (anterior/posterior) 2. Dorsal/ventral axis 3. Left/right axis
What are the 3 parts of the primitive streak?
- Pit 2. Node 3. Groove
What is the direction of formation of the primitive streak?
Caudal to cranial
What part of the amniotic cavity becomes the anus?
Cloacal membrane
What part of the amniotic cavity becomes the membrane between the primitive mouth and the pharynx?
Buccopharyngeal membrane
Why are certain organs on the left of the body? (4 steps)
- Cells nearest the primitive groove and pit produce special chemical substances known as nodal (proteins) 2. Cilia on the ectoderm create proper currents to direct the nodal to the left (lateral) side of the bilaminar disk 3. Sonic hedgehog (SHH) and FGF-8 diffuse to the left axis and activate nodal protein on the left axis (right side nodal is not activated) 4. Heart, spleen, stomach, and gut are on the left (lungs are different on the left)
What happens during gastrulation? 5 steps
- Epiblast cells on the central axis begin proliferating (dividing like cray)
- Primitive streak forms.
- Cells in neighboring area proliferate rapidly and migrate into the primitive groove and pit
- Epiblast cells move down (ingression) and spread laterally pushing the hypoblast cells to the side (which degenerate) and replacing them to form the endoderm. As they ingress they undergo major structural, physiological, and organizational changes
- Once the bottom layer is replaced, another layer of epiblast cells slide in between the top and bottom layers to form the mesoderm. The top layer is the ectoderm
Where does gastrulation take place?
At the bilaminar disk
What are the 2 basic outcomes of gastrulation?
- Axes 2. Germ layers
What is the basic function of the notochord?
To signal overlying dorsal ectoderm to differentiate into neural plate tissue
When is the bilaminar disk formed?
Week 2 of gastrulation
What is the bilaminar disk?
The layer of epiblast and hypoblast cells that divides the amniotic cavity from the secondary/definitive yolk sac
Describe the formation of the notochord (4 steps)
- Once all 3 germ layers have formed, cells from the mesoderm migrate through the primitive node to the cranial end of the bilaminar disk and create a tubular structure = notochordal process
- The notochordal process advances caudally to the prechordal plate
- The floor of the notochord process fuses with the endoderm bringing it in contact with the underlying yolk sac fluids (full of micro-RNA). The notochordal process becomes the notochordal plate in this state (with the neurenteric canal under it)
- The proliferating cells create a solid mass of notochordal cells called the definitive notochord
When do the neural tube’s ends fuse?
Anterior neuropore: day 25 of gastrulation Posteriar neuropore: day 27 of gastrulation
What does true gut endoderm arise from?
Ectoderm
In what direction does the neural plate develop? What happens during this?
Cranially-caudally
The primitive streak regresses crannially-caudally
What is the one organ that is generated from 2 different germ layers?
Adrenal glands come from both the ectoderm (adrenal medulla) and the mesoderm (adrenal cortex)
What is the difference between the extraembryonic mesoderm and the embryonic mesoderm? What is the one thing they have in common?
Extraembryonic is formed prior to gastrulation Embryonic forms as a result of gastrulation and makes up the proper embryo They are both derived from the epiblast
What does ingression mean?
The migration of individual cells from a surface layer into the interior of the embryo: epithelial to mesenchymal transformation
What do cancer and embryonic cells have in common?
They both undergo ingression, but the cancer cells do not know how to manipulate the cell-cell adhesion properties that regulate ingression
What makes ingression possible?
The cells on the surface layer alter their cell-cell adhesion properties
What determines what the cells migrating away from the primitive streak become?
The caudal/cranial location of the cells moving through the primitive streak
What prevent the mesoderm from aggregating too soon?
The embryonic epiblasts secrete hyaluronic acid into the space between the ecto and endoderms
What enhances the ability of the mesoderm cells to walk on a substrate?
Fibronectin on the basal side of the ectoderm (the substrate) allowing for spreading and convergent extension
What do we call the epiblast when cells are moving away from the primitive streak?
The embryonic primitive ectoderm
What happens to the mesoderm once it has filled most of the space between the ectoderm and the endoderm? What does this allow?
The cells travel through a mechanism called spreading/convergent extension by which they intercalate to narrow the tissue and move the entire bulk of the tissue forward. This allows the embryo to grow lengthwise.
What is Kartagener’s syndrome caused by? What is the result? Prevalence?
CAUSE: The cilia on the ectoderm near the primitive node are abnormal and immotile (issues with dynein arm), which allows Shh and FGF-8 to diffuse to the right side and activate nodal
RESULTS: embryonic lethality or situs inversus (all/some of the embryo’s organs are reversed)
Prevalence = 1:10,000 births
What are the 3 parts of the mesoderm that differentiate during notochord process formation? Through what process do they differentiate?
- Paraxial 2. Intermediate 3. Lateral plate
Stochastic process (random) + cell signals
Label the embryonic mesoderm in this diagram
Top: Paraxial
Middle: Intermediate
Bottom: Lateral Plate
Label the lateral plate mesoderm in this diagram
Top: Somatopleuric mesoderm
Bottom: Splanchnopleuric mesoderm
Label the embryonic mesoderm in this diagram
What is the invagination of the overlying ectoderm by the notochord caused by?
Signals sent out by the notochord to alter the cytoskeletal components of the cells
What are the 3 factors that influence gastrulation to occur? What are these called?
- Morphogen diffusion
- ECM of cells in contact (usually involving adhesion molecules in between cells)
- Direct cell-cell contact
What is the difference between gastrulation and morphogenesis?
Differentiation happens at the cell level vs morphogenesis happens at the tissue level
What is the role of hyaluronic acid in spreading the mesoderm? What is it secreted by?
It prevents early aggregation of the mesoderm and is secreted by the epiblast
What is another name for the bilaminar disk?
The germ disk
What are the 3 parts of the germ disk?
- Buccophyaryngeal membrane
- Cloacal membrane
- Primitive streak
In what other cells does ingression happen?
Cancer cells
When do you start calling the epiblast the ectoderm?
After the neural plate and the neural tube have formed
How short can the cell cycle be for ectoderm cells that are about to ingress? How does this compare to the cell cycle of cancer cells?
4.4 hrs (shortest ever)
Much faster than cancer cells!
What is the effect of Kartagener’s syndrome on men’s fertility?
Infertile because cilia on sperm do not work
Draw invagination
Draw ingression
Draw involution
Describe the formation of the neural plate and neural tube (5 steps)
- After the notochordal process comes in contact with the endoderm the definitive notochord releases growth factors to induce the above ectoderm cells to proliferate and differentiate into neuroepithelial cells creating the neural plate (the prechordal plate also induces this)
- Primitive streak and node regress caudally while the neural plate starts progressing caudally and folding/depressing down because of notochord signals (invagination) = neural groove
- Fold continues to elevate until the edges fuse to form the neural tube = involution
- Some cells from fold area slide off the edge and become the neural crest cells
- The tube’s ends fuse last: anterior neuropore (cranial end) and then posterior neuropore (caudal end)
What part of the mesoderm cells allow them to undergo spreading and convergent extension?
Lamellipodia
Where is the prechordal plate? What germ layer is it made of? What is it critical for?
Right next to the notochordal process (cranial side)
Mostly mesoderm
Critical for formation of forebrain
What is the difference between the notochord and the notochordal process?
The notochordal process has a physical hole in it
Explain the difference between the prechordal plate, the notochordal plate, and the neural plate.
- Prechordal plate: portion of mesoderm to the cranial side of the notochordal process that is important for forebrain formation
- Notochordal plate: notochordal process (mesoderm) once it has fused with endoderm during notochord formation (has the neurenteric canal under it)
- Neural plate: ectoderm that folds into the neural tube by notochord and prechordal plate induction
What is the invagination of the neural plate directed by?
Cytoskeleton changes induced by the notochord and underlying mesoderm proliferation
When do somites arise in mesoderm?
During neural plate formation
What does induction mean?
The signaling sent by cells that influence the development of other cells
How does induction control development?
With feedback loops (+ or -)
Why isn’t the ectoderm signaled to invaginate again by the notochord after formation of the neural tube?
Because the signals from notochord are too far away from epidermis for it to invaginate again
What are the crest cells important for?
Facial development
What does the effect of morphogens depend on?
The distance to the source of the signal (concentration decreases with increased distance)
Provide an example of the importance of germ layers communicating during organogenesis?
The heart is one of the first organs to form from the cardiogenic area made of mesoderm cells which communicate with endoderm cells, which is key
What is the basic function of embryonic folding?
Getting organs and tissues in the correct position
What is embryonic folding driven by?
- Slowed growth of yolk sac endoderm
- Rapid growth of embryonic disk and amniotic tissues around the yolk sac
What is the endoderm that is not part of the amniotic sac and surrounds the yolk sac?
Hypoblast = primitive endoderm
When does embryonic folding begin?
15-18 days post fertilization
Around what does folding occur?
The definitive yolk sac
By when can a definitive anterior head and tail region be morphologically delineated?
Day 24-25
What will form the outer layer of the umbilical cord? 2 parts
- The amniotic ectoderm2. The amniotic extraembryonic mesoderm
Which germ layer grows more rapidly than the mesoderm and endoderm?
The ectoderm
Describe the vasculature from the embryo to the mother
- One umbilical vein carries oxygenated blood from the placenta through the allantois to the embryo 2. Two umbilical arteries carry deoxygenated blood through the allantois to the placenta
Describe the blood flow in the fetal heart
- Blood flows into the fetal inferior vena cava and the liver portal vein into the right atrium2. Pulmonary circulation is bypassed as a majority of blood is pumped from the right to the left atrium through the foramen ovale (small portion does enter the right ventricle and is pumped through the pulmonary pathway)3. Once in left atrium, blood is pumped into the left ventricle and into the aorta and the whole fetus
When does the cardiogenic area start beating?
Within 3 weeks of fertilization
What is the role of the allantois?
Removal of waste before kidneys and amniotic sac develop
Which germ layers grows faster during embryonic folding?
Ectoderm
Describe the formation of the primitive streak.
- A longitudinal streak develops from caudal to cranial
- Primitive node forms
- Central depression occurs in node and streak forming the primitive groove and primitive pit
What germ layer is the prechordal plate made of?
Mesoderm
Where is involution seen in embryonic development?
Neural tube formation
What do we call the bilaminar disk after germ layer formation?
Trilaminar disk
What is fused at both the buccopharyngeal and cloacal membranes?
Epiblast and hypoblast
What is another name for the primitive node?
Hensen’s node
What are the somitomeres? What do the first seven pairs form?
The precursors to the somites
First 7 pairs: striated muscles of the face, jaw, and throat
What 2 structures induce the formation of the neural plate?
Notochord and prechordal plate
What types of cells is the neural plate made of?
Neuroepithethilial columnar cells
Describe the fate map of the epiblast based on its regions.
- Middle region (closest to primitive streak): endoderm and mesoderm
- Rostral region: NS and skin
What is a protein usually expressed during ingression?
Cadherin
Does ingression involve individual cell or sheet of cell movement?
Individual cell
Do invagination and involution involve individual cell or sheet of cells movement?
Sheet of cells
What is an example of morphogens?
SSH and FGF-8
What is the Regulator Hypethesis of Edelman?
Embryonic morphogenesis is directed by genes that control the timing of the expression of:
- adhesion proteins
- morphogens
- other inducers
⇒ this causes morphological differences between species
What is the amniotic fluid made by?
At first: secreted by amniotic membrane
Later: fetal urine
What is the septum transversum?
Band of mesoderm pushed because of cephalic folding that gives rise to part of the diaphragm
Label the diagram
Left (from top to bottom): neural fold, presumptive left lateral endocardial tube, presumptive left dorsal aorta
Right (from top to bottom): Cardiogenic area, presumptive venous inflow
What 3 structures are pushed toward the ventral region of the embryonic disk due to cephalic folding?
- Buccopharyngeal membrane
- Cardiogenic region
- Septum transversum
What 3 structures are pushed toward the ventral region of the embryonic disk due to caudal folding?
- Cloacal membrane
- Connecting stalk
- Allantois
What happens to the connecting stalk?
Umbilical cord
What does the folding of the lateral edges cause?
Ectoderm covers the whole embryo except for the yolk sac and connecting stalk
What 3 structures are formed due to the folding of the gut tube (previously part of the yolk sac)?
- Future foregut
- Future hundgut
- Vitelline duct (connecting yolk sac to GI tract)
Label the diagram.
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Label the diagram.
What is another name for the cranial-caudal axis?
Superior-inferior
Can a baby born from gestational diabetes mother have hypoglycemia? Why?
YES! In utero, the fetus was producing enough insulin to store the excess glucose that was crossing the placenta. However, the sudden cessation of access to maternal glucose supply during delivery results in hypoglycemia.
Describe the timing of embryological events.
- Week 1: fertilization to implantation
- Week 2: gastrulation
- Week 3: neuralation and heart field formation
