sea urchin earlydev

Type of cleavage in sea urchins

Radial holoblastic cleavage

Define radial cleavage

Spindle axes are parallel or perpendicular to the animal-vegetal axis

How conserved are early sea urchin cleavages?

First seven divisions are invariant across individuals

How many cells are present after seven divisions?

128 blastomeres

What structure is formed at the 128-cell stage?

Blastula

Structure of sea urchin blastula

Single epithelial layer with tight junctions surrounding a blastocoel

What is the blastocoel?

Fluid-filled cavity in the center of the blastula

What forms the vegetal plate?

Thickening of cells at vegetal pole

What causes hatching of the blastula?

Digestion of the fertilization envelope

Type of cleavages in early sea urchin embryo

Meridional followed by equatorial divisions

What special event occurs at the 4th cleavage?

Vegetal cells divide unequally forming micromeres

Which cells are autonomously specified?

Micromeres

Which cells are conditionally specified?

All other cells

What maternal factor drives autonomous micromere specification?

β-catenin

Where is Disheveled localized?

Vegetal cortex

Role of Disheveled in sea urchins

Promotes nuclear accumulation of β-catenin

Where does β-catenin accumulate at highest levels?

Micromere nuclei

Function of β-catenin in micromeres

Acts as a transcription factor specifying micromere fate

Which transcription factor works with β-catenin?

Otx

What happens when β-catenin is in all nuclei?

Endomesodermal fates expand

What treatment experimentally causes this?

Lithium chloride treatment

What signaling molecule do micromeres express?

Delta

What receptor binds Delta in neighboring cells?

Notch receptor

Primary role of Delta-Notch signaling

Specification of secondary mesenchyme cells

What cell population is influenced by Notch signaling?

veg2 cells

What fates arise from veg2 cells?

Mesoderm and endoderm

What level of β-catenin is present in veg2 cells?

Moderate nuclear β-catenin

What mechanism distinguishes mesoderm from endoderm in veg2?

Notch signaling downregulates β-catenin in some cells

Kinase downstream of Notch signaling

Nemo-like kinase (NLK)

Evolutionary significance of NLK

Conserved β-catenin regulation mechanism

Define secondary mesenchyme cells

Mesodermal cells specified via Notch signaling

Abbreviation SMC

Secondary mesenchyme cell

Define a gene regulatory network (GRN)

All DNA interactions controlling spatial and temporal gene expression

Primary components of GRNs

Transcription factors and signaling pathway components

How is a GRN experimentally defined?

Gene expression analysis knockdowns and binding assays

Key early regulator in micromere GRN

Pmar1

Animal-vegetal axis in sea urchins

Established early and leads to A-P axis

What is the oral-aboral axis equivalent?

Ventral-dorsal axis

What signaling pathway specifies oral-aboral axis?

TGF-beta signaling

Which signal promotes oral fate?

Nodal

Which signals promote aboral fate?

BMP2 and BMP4

Left-right axis specification

Controlled by Nodal later in development

What initiates sea urchin gastrulation?

Ingression of primary mesenchyme

Origin of primary mesenchyme

Micromeres

Cell behavior during primary mesenchyme ingression

Epithelial to mesenchymal transition

What changes during EMT?

Cytoskeleton morphology and adhesion patterns

How do primary mesenchyme cells move?

Using filopodia and lamellipodia

What are filopodia?

Thin actin-based protrusions for probing and movement

What are lamellipodia?

Broad actin-based projections that anchor and pull cells

Function of primary mesenchyme cells

Migration and skeleton formation

What initiates archenteron formation?

Thickening of the vegetal plate

How far does initial archenteron invagination extend?

About one quarter to one half of blastocoel length

What cells lead archenteron elongation?

Secondary mesenchyme cells

What structure is formed by the archenteron?

Primitive gut

How does the archenteron elongate?

Convergent extension

Key genes involved in endomesoderm patterning

Wnt8 Blimp1 and Brachyury

Are Wnt8 Blimp1 and Brachyury regulated by Pmar1?

No

Major theme in sea urchin development

Sequential use of β-catenin Notch and GRNs to specify fate

Comparison with C. elegans

Uses similar signaling logic but distinct GRN architecture

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