4. Sea Urchin Development Flashcards
Advantages of Sea Urchin as models
- Used for fertilization and cleavage studies
- Eggs and sperms spawned in large quantities
- Easy to grow under laboratory conditions
- Transparent embryos
Diffrent meres formed in cleavage
- Mesomere (Animal pole)
- Macromere (Vegetal Tier)
- Micromere (Vegetal Pole)
Fate Maps of An1 An2 Veg1 Veg2 large micrometers and small micrometers
- An1 : ectoderm
- An2 : ectoderm
- Veg1 : Base of archentron and ectoderm (hindgut)
- Veg2: Gut (endoderm) midgut foregut, and secondary mesenchyme (non skeltogentic mesoderm)
- large micromeres: primary mesenchyme cells (skeltogenic mesoderm)
- small micromeres: primordial germ cells
Experiment:
What would develop if you:
•isolate animal hem only
•isolate A.hem with micrometers
A/V.hem + micromers
-complete Pluteus larva
A.hem:
- forms dauterblastula
- ectodermal cells formed only
A.hem + micromers
- a recognizable pleats formed but all endoderm derived from A.hem
- micromers can change the fate of the animal hem.
Experiment:
take micromeres and transplant onto the top of a different organism
- a new gastrulation site is induced
- ingression starts forming skeletal rods
- they do not contribute at all to the induced vegetal plate
- ectodermal cells had been respecified to become vegetal plate cells
- micromers are the organizers in sea urchins
Fate of micromeres
- Large micromeres are specified through autonomous specification.
- They are destined to become skeletogenic mesenchyme.
- They produce paracrine and juxtacrine factors that conditionally specify fate of neighbouring cells to form endoderm and the mesoderm.
Skeletogenic micromeres specified by β-
catenin (a maternal transcription factor)
•β-catenin is a maternal transcription factor polypeptide
found in the egg and zygote during cleavage—it is
protected from being degraded by another maternal protein
—disheveled (DSH).
•fated to become endoderm and mesoderm
• It seems to specify the vegetal half of the embryo
• if the entry of β-catenin into nuclei is inhibited, then no mesoderm or endoderm form-embryo is a ciliated ball of ectodermal cells
• if β-catenin enters the nuclei of all cells, then the ectoderm becomes mesoderm and endoderm, and an “exogastrula” forms
Experiment:
•noraml development ß-cat in micromers
•iLiCl (GSK-3 blocked) treatment to embryo, β- catenin in all the cells of embryo
•Nuclear transport of β-catenin blocked so it stays in cytoplasm
- predominant in micromeres, in Veg2 tier its prevented from degradation
- Animal cells become endoderm and mesoderm (exogastrula)
- Vegetal cell fate not specified and embryo develops as a ciliated ectodermal ball. (dauerblastula)
Double-Negative Gated circuit - Pmar1 Gene
- Pmar is regulating transcription of HesC by inhibiting HesC (a repressor of a repressor)
- HesC is a repressor of various downstream genes (repressor of delta)
- if Pmar present, HesC not expressed so delta will be expressed
- this is a double negative gated circuit
- delta binds with notch (juxtacrine signaling)
- Pmar1 only in micromers of sea urchin
- in Veg2 macromers Pmar1 is off
Autocrine Signalling
•Micromeres secrete an autocrine factor Wnt8.
• β-catenin and Otx activates Blimp1 gene transcription.
• Blimp1 and β-catenin activates Wnt8.
• The control of genes that differentiate the skeleton in sea
urchin operate on feed-forward process
Feed forward
- Regulatory A codes for protein
- RGA & RGB effect gene C
- if A not active, B active, and C D E inactive
- helps in determining specificity of cell fate in one direction
- amplifies the signal in one direction
Specification of vegetal cells
- Skeletal micromeres produce signaling factor-Activin that induces endoderm formation.
- Activin expression is also under the regulation of Pmar1 and HesC double negative gate
Nodal Expression
- nodel is expressed on the cells that will become the oral
- after the endoderm reaches the nodal area nodal expression shifts to the right side
Axis Specification
- Animal-vegetal axis established in egg cytoplasm.
- Dorsal-ventral and left-right axes is specified after fertilization.
- Oral pole of the future oral-aboral (mouth-anus, dorsal-ventral) axis lies 45O clockwise from the 1st cleavage
- Oral-aboral axis forms through Nodal genes in oral ectoderm.
- Nodal transcription is initiated by a small difference in redox state of ectoderm.
- Prospective ectoderm of the oral side has higher mitochondrial respiration than prospective aboral side.
- Nodal proteins become prominent on one side of the blastula that develops into oral ectoderm.
Gastulation
Ingression of primary mesenchymal cells
• Epithelial-mesenchyme transition
• Skeletognic cells extend and contract thin
and long processes called Filopodia
• Filopodia helps in moving cells from one spot to another
• Ingression is due to loss of affinity for
neighbours and hyaline membrane.
• Forms the spicules (skeletal rods).
• Vegtal plate invaginates to form the archentron.
• Secondary mesenchyme cells extend filopodia towards the animal pole. Mouth forms at the juction where the secondary mesenchyme contacts the ectoderm.
