lec 13 - mesoderm segmentation

1st major embryonic event

establishment of axes

1st axis is AP

sign of AP development in chick, zebrafish, mice

primitive streak

primitive streak + epiblast migration

groove running down AP axis that forms during gastrulation (chick, mouse, zebrafish)

during gastrulation, epiblast cells migrate toward the primitive streak, ingress (fall into the groove), and emerge underneath as mesoderm or ectoderm, depending on how deeply they ingress.

ingression

falling into the primitive streak

Hensen’s node

anteriormost part of the primitive streak

how to identify what type of mesoderm an epiblast cell would turn into

label cells at different levels along the AP axis, follow the dye

4 types of mesoderm

Axial, Paraxial, Intermediate, Lateral

Axial mesoderm originates from cells that are ____, and ends up in ____

close to/within hanssen’s node

the center

two regions of the axial mesoderm

1. anterior - contributes to anterior CNS (forebrain structures)

2. posterior - notochord (trunk)

the paraxial mesoderm originates from cells that were ____ and ends up ____

just posterior to axial/hanssens’s node

imediately lateral on either side of notochord

the intermediate mesoderm originates from cells that were ____ and ends up ____

posterior to paraxial mesoderm

immediately lateral to paraxial mesoderm

the lateral mesoderm originates from cells that were ____ and ends up in ____

most posterior to hanssen’s node

most lateral

is paraxial mesoderm segmented or not

it’s not near the head, but it is posterior to it (formation of somites)

are there somites in a newborn?

no - transient structures

somites contributes to

skeletal muscles

axial skeleton/vertebrae

tendons

arteries

axial skin

somites form which way?

posterior → anterior

pre-somitic mesoderm

mesoderm that hasn’t formed a somite yet

somites form

• symmetrically, one on either side

• at the same time

somites have a ____ structure

repeated

mesoderm segmentation is _____ throughout evolution

conserved

engrailed1 expression in invertebrates and vertebrates

how do we get so many different structures from identical somites between species

small adjustments to somites → big adjustment to organism

_____ dictates the number of vertebrae

sominte #

humans have ___ vertebrae

33 newborn, 24 + 9 fused adult

human embryo has ____ somites

38-45

chick embryo has ____ somites

55

mouse embryo has ____ somites

65

zebrafish embryo has ____ somites

33

do somite #s change

not within species, but yes between species

paroxial mesoderm forms in a ____ manner until

continual

the proper somite number is reached

PSM can make up to __ somites

12

primitive streak is present until

somites don’t form anymore

somites form by

budding off pre-somatic mesoderm

____ occurs at the same time as somatogenesis

elongation/SC development

somites are added after [fixed/variable] time intervals

fixed

t/f - somites coordinate with each other to form symmetrically and simultaneously

f - spinal cord is in the way

cells with PSM must respond to

• positional info

• mech that coordinates left and ritght somites

• mech that generates anterior and posterior boundaries

• formation of cleft

cooke and zeeman

proposed the clock/wavefront model

describe the clock/wavefront model

• there is a “clock” in the posterior PSM

  • drives a molecular oscillator - provides timing info to cells

• there is a wavefront that tells cells where they are on the AP axis and triggers the decision to form a somite

c-hairy-1 in chick is what in mouse and zebrafish

mouse: hes

zebrafish: her

evidence for clock/wavefront theory

• c-hairy-1 expression oscillates, and completes each cycle in a fixed timeframe

• the timeframe of each oscillation corresponds to the formation of a somite

• the mesoderm

clock genes belong to which signaling pathways

Notch

Wnt

FGF

how to get a gene to turn on and off in a pattern

• mRNA and protein half-life

• negative feedback loop

describe the molecular mechanism behind the Hairy/Hes/Her oscillatory pattern

• Notch/Wnt/FGF signaling → transcribe Hairy/Hes/Her genes and create mRNA

• mRNA → protein (rise, peak)

• Hairy/Hes/Her target protein has the ability to block its own transcription (decline, trough)

• protein has a short half-life - degraded, gene transcription is on again

how do cells move posterior → anterior

cells are continuously being made at the posterior site - they just get pushed up

wavefront occurs at the

determination front

determination front is located at

somite -1 - two presumtive somites into the non-segmented pairs

how is the determination front precicely formed

positioned at the interface of two opposing gradients - RA and FGF8

RA is produced by

spinal cord

RA diffuses anteriorly or posteriorly?

posteriorly (a → p)

FGF8 diffuses anteriorly or posteriorly?

anteriorly (p → a)

____ maintains the relationship between RA and FGF8 gradients, as well as the location of the determination point

negative feedback

explain the negative feedback regulations between RA and FGF8

1. FGF8 activates Erk; RA represses it

2. FGF8 activates Cyp26, which inhibits RA

3. FGF8 inhibits Raldh2, which activates RA

RA signaling cannot happen where there is FGF8

____ is expressed only at the determination front

Mesp2

how does Mesp2 activate only at the determination front

activation: needs FGF8 and Notch (oscillating) to turn on

how are boundaries formed?

Mesp2 initially expresses throughout the presumptive somite region, but is very rapidly restricted to the anteriormost part of the somite

• Mesp2 —| delta (Notch ligand)

• Mesp2 → Ripply2 —| Mesp2

• only happens in posterior somite - important in boundary formation

how do we know the narrow bands of Notch and Mesp2 signaling are important for boundary formation?

transplant a whole presumptive chick somite and place it in the middle of a presumptive quail somite - you create a new boundary

• can be replicated by forcing Notch expression in the middle of a somite

delta-like 3 mutation in humans causes

spondylocostal dysplasia - defects in the segmentation of the vertebral column

• bones are misshapen and abnormally fused

• ossification centers do not align

how does Mesp2 signaling lead to the formation of an actual physical barrier between somites?

• Mesp2 → EphrinA4 expression

• EphrinA4 interacts with its receptor, EphrinB2, to alter cell adhesion

• mesenchymal → epithelial transition (MET) - loose mesenchymal cells come together to form a compact epithelial ball

• intersomitic furrow (cleft) forms, splitting the somite