sea urchin early development

"What type of cleavage do sea urchins exhibit?"

"Radial holoblastic cleavage."

"What defines radial cleavage?"

"Spindle axes are parallel or perpendicular to the egg axis."

"How many divisions occur before the blastula stage?"

"Seven divisions produce a 128‑cell blastula."

"What is the structure of the blastula?"

"A single epithelial sheet surrounding a blastocoel."

"What forms at the vegetal pole during blastula stage?"

"Cells thicken to form the vegetal plate."

"What happens to the fertilization envelope?"

"It is digested

"What is unique about the 4th cleavage?"

"Vegetal cells divide unequally

"What are micromeres?"

"Small vegetal blastomeres that autonomously specify skeletogenic mesenchyme."

"What specifies micromere fate?"

"Maternal β‑catenin accumulation in micromere nuclei."

"What localizes Disheveled (Dsh) to the vegetal cortex?"

"Specific motifs that anchor Dsh at the vegetal pole."

"What is the role of Dsh in micromeres?"

"Stabilizes β‑catenin by inhibiting its degradation."

"What happens when β‑catenin is present in all nuclei?"

"All cells adopt vegetal fates (LiCl treatment phenotype)."

"What happens when β‑catenin is excluded from nuclei?"

"Vegetal fates fail to form."

"What transcription factors act with β‑catenin in micromeres?"

"Otx and other endomesodermal regulators."

"What does high nuclear β‑catenin + Otx specify?"

"Primary mesenchyme (skeletogenic) fate."

"What does moderate β‑catenin + Otx specify?"

"Veg2 endomesodermal fates."

"What ligand do micromeres express to signal veg2 cells?"

"Delta."

"What receptor do veg2 cells express?"

"Notch."

"What does Delta–Notch signaling induce in veg2 cells?"

"Secondary mesenchyme (mesoderm) specification."

"What is HesC?"

"A repressor that blocks skeletogenic genes."

"How do micromeres overcome HesC?"

"They express Pmar1

"What is the result of repressing HesC?"

"Activation of skeletogenic genes and Delta expression."

"What maintains endomesodermal fate in veg2 cells?"

"Autocrine Wnt signaling."

"What later distinguishes mesoderm from endoderm in veg2?"

"Notch signaling reduces β‑catenin in some veg2 cells."

"What kinase acts downstream of Notch to reduce β‑catenin?"

"Nemo‑like kinase (NLK)."

"What is the effect of NLK on β‑catenin?"

"Promotes its nuclear export

"What happens when NLK is inhibited?"

"β‑catenin remains nuclear and mesoderm fails to form."

"What is a gene regulatory network (GRN)?"

"A set of DNA elements controlling spatial and temporal gene expression for a developmental fate."

"What are the steps to identify a GRN?"

"Identify candidate genes

"What is the role of Pmar1 in the GRN?"

"Activates skeletogenic program by repressing HesC."

"What is the fate map of the 60‑cell embryo?"

"Micromeres autonomously specify; other fates are conditional."

"What axis does the animal–vegetal axis become?"

"The anterior–posterior axis."

"What specifies the oral–aboral axis?"

"TGF‑β signaling: Nodal promotes oral fates; BMP2/4 promotes aboral fates."

"What specifies left–right axis later in development?"

"Nodal signaling asymmetry."

"What initiates sea urchin gastrulation?"

"Ingression of primary mesenchyme cells (PMCs)."

"What are PMCs derived from?"

"Micromeres."

"What cellular changes accompany PMC ingression?"

"Altered cytoskeleton

"What structures do PMCs use to migrate?"

"Filopodia."

"What are filopodia made of?"

"Long

"What is the function of filopodia?"

"Dynamic extension and retraction to crawl along ECM."

"What are lamellipodia?"

"Broad

"What do PMCs do after migrating?"

"Fuse and initiate spicule (skeletal) formation."

"What happens after PMC ingression?"

"Vegetal plate cells invaginate to form the archenteron."

"What drives initial archenteron invagination?"

"Cell shape changes and apical constriction."

"What leads the archenteron during elongation?"

"Secondary mesenchyme cells."

"What process elongates the archenteron?"

"Convergent extension."

"What does the archenteron become?"

"The primitive gut."

"What signaling pathway is involved in endoderm ingression?"

"Wnt/Planar Cell Polarity (PCP) pathway."

"What does PCP signaling regulate?"

"Actin cytoskeleton and cell motility."