DB - L12 - Mesoderm A-P patterning - mesoderm segmentation

what is the primitive streak?

• a structure that forms on the dorsal surface of the epiblast in early vertebrate embryos

• site of gastrulation

• formation of the primitive streak establishes the anterior–posterior axis

how is the primitive streak formed?

1. Epiblast cells converge toward the midline of the posterior epiblast

2. Cells at the midline elongate and ingress, creating a groove — the primitive streak

3. The streak extends anteriorly, establishing the anterior–posterior axis

4. The anterior end of the streak thickens to form the primitive node (Hensen’s node) which acts as an organizer for axial development

what happens during gastrulation?

1. Epiblast cells migrate centrally toward the primitive streak

2. Cells migrate into the groove at the midline of the streak

3. Once in the groove, cells emerge into the space beneath (the cavity)

4. Cells populate this space and start forming different cell types

5. Deepest migrating cells form the endoderm

6. More lateral (shallower) cells form the mesoderm

7. Mesoderm cells spread out and differentiate into different mesodermal regions depending on their migration path

what forms at the primitive streak during gastrulation?

mesoderm

what are the 4 types of mesoderm?

1.      axial mesoderm

2.      paraxial mesoderm

3.      intermediate mesoderm

4.      lateral mesoderm

what is the axial mesoderm and what does it form?

• Enters at the node, the anterior-most part of the primitive streak

• Anteriorly forms the pre-chordal mesoderm

• Posteriorly forms the notochord

• Lies in the centre of the embryo along the midline

what is the paraxial mesoderm and what does it form?

1. Emerges from cells that migrate slightly posterior to the axial mesoderm

2. Located next to the axial mesoderm

3. Contributes to the head and forms somites (transient embryonic structures)

4. Somites further differentiate into:

   • Myotome → skeletal muscle

   • Sclerotome → cartilage

what is the intermediate mesoderm and what does it form?

1. Emerges from cells that migrate even more posteriorly

2. Located lateral to the paraxial mesoderm

3. Gives rise to the kidneys and gonads

what is the lateral mesoderm and what does it form?

1. Posterior-most migrating cells, forming the most lateral mesoderm.

2. Gives rise to:

   • Extra-embryonic tissues

   • Limb tissues

   • Heart

what are somites?

• Highly organized, repeated, separated segments of paraxial mesoderm along the A–P axis

• Segmentation occurs anteriorly first and progresses posteriorly, with no segmentation initially at the posterior

what is the function of somites?

• give rise to skeletal muscles (from myotome)

• form cartilage and vertebrae (from sclerotome)

• contribute to dermis of the skin (from dermatome)

• serve as segmental units for body organization along the anterior–posterior axis

what is segmentation in embryonic development?

• division of embryonic tissues into repeated, organized units along an axis

• in vertebrates, occurs in the paraxial mesoderm to form somites

• establishes body plan and repeated structures

how do somites relate to vertebrate spine structure?

• spine consists of repeated vertebral structures

• vertebrae perform similar functions but differ slightly along the A–P axis

• number of somites determines the number of vertebrae

how are somites formed from the paraxial mesoderm?

1. somites form in pairs from the paraxial mesoderm in a continuous manner until the species-specific number is reached

2. the number and timing of somite formation are fixed within the species

3. paraxial mesoderm is continually produced from the posterior primitive streak until all somites form

4. the primitive streak persists until somite formation is complete

5. segmentation occurs as cells insert into the primitive streak and new mesoderm emerges, driving posterior elongation of the embryo

what are the physical mechanisms underlying somite formation?

1. cell ingression into the posterior primitive streak drives posterior elongation and adds new axial mesoderm

2. a band of paraxial mesoderm (pre-somitic mesoderm) form enough for ~12 somites before segmentation begins

3. in the anterior region, segmentation occurs, producing new pairs of somites in a timed manner (every 90 min in chick)

4. each somite has an anterior-posterior boundary and a cleft between somites

5. results in repeated, organized somite units along the axis

what needs to happen for somite formation to occur?

cell within the paraxial mesoderm must be able to respond to:

1. positional information along the A-P axis

2. mechanism that coordinates left and right somite pairs

3. mechanisms that generate anterior and posterior boundaries of each somite

4. formation of cleft to separate somites

what does periodicity of somite formation refer to?

• somites being made in a periodic manner as a clock

what is the clock and wavefront model of somite formation?

1. predicts two key features dictate somite formation: a molecular clock and a wavefront

2. a clock in the posterior PSM drives a molecular oscillator dictating somite periodicity

3. when cells enter the pre-somitic mesoderm (PSM), a timer is set, indicating how long they remain unsegmented

4. cells encounter a travelling wavefront, which moves opposite to somite formation

5. upon reaching the wavefront, cells receive a signal to change state and start organizing into somites

what was the first evidence for the somite formation clock?

1. oscillating gene expression was observed in the posterior part of the embryo

2. this oscillation moves posterior to anterior

3. the oscillation correlates with the timing of new somite formation

how does oscillating gene expression in the presomitic mesoderm occur?

1. Hairy/Hes/Her proteins have a short half-life and repress their own transcription expression (negative feedback loop)

2. a gene is turned ON by an instructive signal, then the protein represses itself, turning the gene OFF quickly

3. other genes with short half-lives and negative feedback (Wnt, Notch, FGF) also oscillate

4. presomitic mesoderm cells undergo ~12 cycles of oscillation before forming somites

what is position S-1 in somite formation?

• presomitic region just before a somite forms

• marks the specification of the next boundary – the determination front

• when cells encounter the wavefront, their oscillations stop

how is the position of the somite wavefront determined?

1. determined by two opposing gradients of signalling molecules: Retinoic Acid (RA) and FGF8

2. RA: produced by the neural tube and formed somites, diffuses posteriorly → high anterior, low posterior

3. FGF8: produced posteriorly, diffuses anteriorly → high posterior, low anterior

4. the intersection of low levels of RA and FGF8 defines the determination front, where oscillations stop and segment is specified

how do RA and FGF8 gradients maintain their opposing relationship?

1. negative feedback loop regulates the gradients

2. high RA levels → inhibit FGF8 signalling

3. mutual repression ensures RA and FGF8 gradients remain opposite, maintaining proper somite patterning

what is the role of Mesp2 in somite formation?

1. Mesp2 is key for cells to know they need to form a somite

2. FGF8 and Notch signalling initiate Mesp2 expression

3. high Notch → Mesp2 expressed

4. Mesp2 is first expressed throughout the S-1 region

5. Mesp2 blocks Dll1, reducing Notch activity

6. activates Ripply2, forming a feedback loop that inhibits its own expression

7. S-1 region has lower Notch levels next to a somite with higher Notch → creates a molecular boundary between formed and forming somites

how do somite boundaries form?

1. boundary cells induce somite boundary formation

2. they instruct anterior cells to form a boundary

3. Notch genes are expressed at the somite boundary → Notch signalling is involved

4. Mesp2 induces EphA4 expression in the anterior part of the cell

5. EphA4–Ephrin interaction triggers transcriptional changes that establish the boundary