Embryology Lecture 10 Establishment of Body Plan II Flashcards
What are the three zones of the mesoderm?
Paraxial mesoderm
Intermediate Mesoderm
Lateral Plate mesoderm
What is described by a thick column of mesoderms, closest to and parallel with the notochord, and becomes segmented into somites?
Paraxial Mesoderm
What is described by a narrow column of mesoderm, lateral to the paraxial mesoderm, and gives rise to urogential system?
Intermediate mesoderm
What is described by a thing plate of mesoderm, lateral to intermediate mesoderm, splits to form the lining of body cavities and mesoderm of most internal organs as well as limbs?
Lateral plate mesoderm
Where do the somitomeres begin to develop and where are they added?
Develop in the paraxial mesoderm and new ones are added caudally
What event happens simultaneously as somites are being created?
Creation of new somitomeres at the anterior end of the primitive node (primitive streak: caudal end)
How many pairs of somitomeres are kept constant at the caudal end of the paraxial mesoderm?
11 pairs (Possible 12)
Which vertebrae are made from the cells migrating through the primitive streak?
Cervical, thoracic, and lumbar vertebrae
At what point does the somitomere become a somite?
At the 7th somitomere at the expense of the 8th
Cells migrating from the tail bud will form ____
The sacrum and coccyx
What are the 4 steps of somite history and organization?
- Epithelial Stage
- Epitheliomesenchymal Transformation stage
- Separation of the myotome
- Breakup of epithelial dermatome into dermal fibroblasts
What is happening during the epithelial stage of somite organization?
Cells are bunched together
What is happening during the epitheliomesenchyal transformation stage of somite organization?
Some of the cells lose their binding to each other
They become more mobile and diffuse out
What happens during the separation of the myotome during somite organization?
Split up into three regions
Dermatome, myotome, and sclerotome
What signaling balance causes myotome portion to commit to the myogenic lineage?
Shh from the notochord and Wnt from the dorsal tube
What is the function of BMP-4 and what inhibits it?
Stimulates somites to migrate to the limb bud
Inhibited by Noggin
What signaling molecule from the myotome tells the sclerotome to produce scleraxis?
FGF
What is the purpose of scleraxis
Causes the anterior and posterior border of somties to form the syndetome (The precursor of tendons)
What creates gaps that allow nerves to grow in the neural tube?
Sclerotomal break up
It breaks up into an anterior and posterior portion
In sclerotomal breakup, what portions fuse?
The posterior portion fuses with the adjacent anterior portion
What model of somite differentiation is described below?
Be able to pick out the defining characteristics.
-Expressed along a timeline: 90 minutes/somite
-At each critical location that will serve to divide adjacent somites, lunatic fringe will become more concentrated at the future anterior border of a somite and c-hairy will be concentrated at the future posterior border of a somite
-cells at the anterior border will express Eph A (receptor)
-cells at the posterior border will express Eph B (ligand)
(These two will result in a fissure between the two adjacent somites)
Segmental Clock model
Refer to fig. 6.9
What model of somite differentiation is described below?
Be able to pick out the defining characteristics.
- There is an increase in [FGF-8] that leads to mitosis in the mesenchymal cells in posterior PRIMITIVE STREAK
- Increase in [RETINOIC ACID] more anteriorly as opposed to FGF-8 (which opposes mitosis)
- A balance becomes of the two which leads to a determination front. (this then leads to differentiation)
- Mesp-2 is expressed because of the balance
Wavefront Mechanism
Retinoic acid opposes what in the wavefront mechanism of somite differentiation?
FGF-8 which leads to the inhibition of mitosis
What is cessation of somitogenesis?
A balance between FGF-8 and Retinoic Acid in the wavefront model of somite differentiation
What are the steps in somite differentiation?
- Ectoderm dorsal to somites –> Wnt -6 –> Paraxis in somite
- Paraxis + decrease in Snail –> Mesenchymal cell to epithelial cell conversion
- Formation of somitocoel
- Notochord –> Shh + Noggin –> Pax1 + Pax9 in ventral part of somite
(=sclerotome: Note that it forms around the notochord) - Wnt genes in dorsal neural tube –> transformation of dorsal half of somite into dermomyotome
- Demomyotome –> Expression of Pax3, Pax7, paraxis –> Separation of dermomyotome into dorsal and ventral myotome
What does the future intermediate mesoderm respond to?
BMP from lateral ectoderm
Activin from paraxial mesoderm
These two will make it become the intermediate mesoderm
What does the intermediate mesoderm express?
Pax-2
What is the intermediate mesoderm associated with?
The formation of pronephros and, later, the mesonephros
What is the cranial and caudal extent dependent on in the intermediate mesoderm?
Hox-4 through Hox-11 genes
Think Hox genes
What does the formation of the heart arise from?
Epiblast
In the formation of the heart, cells that migrate through the ANTERIOR part of the primitive streak become____
The outflow track
In the formation of the heart, cells that migrate through the MIDDLE part of the primitive streak become____
Ventricles
In the formation of the heart, cells that migrate through the POSTERIOR part of the primitive streak become____
Atria
What are the steps in heart formation?
- Anterior visceral endoderm is acted upon by BMPs and FGFs
- Cardiac Crest forms
(movement through the anterior, middle, and posterior primitive streak) - Expression of Nkx2-5, MEF2, and GATA4 genes
Expression of these genes by cells allow for heart formation
What are the cardiac tubes?
Bilaterally paired tubes that form the cariogenic mesoderm (Fuse beneath foregut to form one tube)`
What are the two layers of the cardiac tube?
Inner layer: Endocardium
Outer layer: Myocardium
Cardiac Jelly forms between them (A specialized matrix)
What is the source of pericardium and myocardium fibroblasts?
Proepicardial primordium
