Lecture 3: Axis formation Flashcards
How does C. elegans development occur?
Development occurs in a controlled way.
1) Sperm enters the oocyte.
2) The sperm is moved to the future posterior pole of the embryo.
3) Simple ball/sheet of cells is converted into a 3-layered structure called the gastrula.
Why are asymmetrical divisions important? How do they occur?
Asymmetry is an important part of development in C. elegans. Asymmetry occurs in 5 steps.
1) Symmetry is broken (usually because of an outside signal).
2) Polarity is established.
3) Determinants are segregated.
4) Spindle position moves.
5) Distinct daughter cells are formed.
How is the A-P axis determined? Draw a diagram.
The A/P axis is the most fundamental axis. In C. elegans it is established with the sperm cell.
• Sperm polarise the cytoskeleton and organise cytoplasmic flows. Sperm that lack DNA are sill able to do this.
• The egg and sperm pronucleus are both found in the oocyte.
• Partitioning-defective (PAR) proteins are found uniformly across the cytoplasm. PAR mutants give mutations that produce cells of equal size or cells that have P granules in both cells.
• PAR-3, PAR-6 and PKC-3 are uniformly distributed in the cortical cytoplasm.
• PAR-1 and PAR-2 are restricted to the internal cytoplasm through phosphorylation by PKC-3.
• When the sperm enters, it induces cytoplasmic movements that place the sperm pronucleus at the nearest end of the oblong oocyte. This is done through CYK-4. This end becomes the posterior.
• The microtubules protect PAR-1 and PAR-2 from phosphorylation, thereby allowing them into the cortex near the centrosome.
• PAR-1 phosphorylates PAR-3 when it is in the cortex. This induces PAR-3 and its binding partners PAR-6 and PKC-3 to move from the posterior. At the same time, the cytoskeleton contracts, moving these proteins to the anterior.
• The two nuclei fuse, during anaphase the spindle becomes displaced towards the posterior. The force difference depends on PAR-2 and PAR-3. In par-2 mutants, both asters are round. Low pulling force at each pole. In par-3 mutants, both asters are flat-high pulling force at each pole.
• The first cell division gives a larger AB cell (anterior founder cell) and a smaller P1 cell (posterior stem cell).
How is the D/V axis determined?
D/V axis is determined straight after the A/P axis is determined.
• The division of the AB cell is orthogonal to the first one. One becomes the ABa (anterior) and one becomes ABp (posterior). This is done through PAR-3, PAR-6 and PKC-3 which prevent nucleocentrosomal rotation.
• The P1 cell divides into the EMS cell and the P2 cell. P1 divides in the same direction as before. The nucleocentrosomal rotation of P1 ensures parallel cleavage.
• The ABp cell is forced above the EMS cell and it therefore marks the dorsal side of the developing embryo.
• Extensive cell-cell interactions determine blastomere fate. Not just asymmetric distribution of gene products.
• ABa and ABp are initially equivalent, made non-equivalent by cell-cell interactions.
We can test this experimentally by manipulating the AB cleavage plane.
• We can rotate the future Aba cell.
• ABp is produced in the anterior and Aba is produced in the posterior.
• EMS becomes dorsal.
• This means that cell-cell interactions are involved in D/V specification.
• Diagram demonstrates this. Left side is the control, right side is manipulated side.
How is the left-right axis determined?
The mechanism of left/right axis determination is not entirely understood.
• Axis determined after D/V.
• ABa divide at the third cleavage to produce laterally disposed left and right cells.
• The left daughter lies anterior to the right-hand sister. If the right-hand daughter is made anterior, the handedness of the whole animal is reversed.
• The spindle is skewed in order to achieve this. This involves G-alpha GPA-16, also involved in spindle asymmetry in earlier divisions. Temperature sensitive gpa-16 mutants raised at higher temperatures have variable ABa and ABp spindle positions.
