Astral microtubule

Schaar That is interesting, because in normal epithelium vertical divisions usually occur. Would you think that there could be some sort of epistasis that would put Pins closer to being able to anchor these astral microtubules from the spindle Have you tried to mislocalize Pins ... 

Nurse There are yeast mutants that capture the astral microtubules and orient the nucleus. It may be interesting to look at homologues of these. 

FIG. 5. Distribution of microtubules during anaphase in wild-type and %yg-8 t165G) mutant embryos as revealed by staining with anti-tubulin antibodies. White contour marks the anterior cortex. In wild-type, astral microtubules are long, extending all the way to the cell cortex. Astral microtubules in %yg-8 mutant embryos are shorter, and do not reach the vicinity of the cell cortex. Bar=10/tm. 

Goncyy That is true in embryos that manage rotation as in wild-type. But in those 20% of embryos that don t, the spindle usually drifts slowly towards the anterior pole. We think this is because astral microtubules are not long enough to reach the posterior cortex. 

Nurse With respect to the astral microtubules emanating towards the cortex, which are shorter in the gyg-8 mutant, are the pulling forces a consequence of the microtubules in wild-type being attached to the cortex, or are they due to attachment sites in the cytoplasm and associated motors, which means that if you extend longer into the cortex you would have a bigger pulling force than if you are shorter ... 

Gonc y We don t know. Indeed, there could be either length-dependent forces which do not require astral microtubules touching the cortex, or forces that require such contact. 

Gonc y We know of a requirement for astral microtubules, cytoplasmic dynein, the dynactin components p50and pi 50, as well as a protein called LET-99 (Hyman White 1987, Gonczy et al 1999b, Rose Kemphues 1998). 

Goncgy One of the popular models to explain the 90° rotation of centrosomes in PI invokes capture of astral microtubules at an anterior cortical site by a localized minus-end directed motor. This would generate a torque on the centrosome/ nuclear complex and reel one of the centrosomes in the direction of the cortical site (Hyman 1989). 

Nurse In that context, in budding yeast there is nuclear migration and there are astral microtubules captured by the bud tip. The bud tip will have also been marking where cytokinesis will have been initiated. There might be some links there. 

Three types of microtubule can readily be defined in the mitotic spindle. Polar microtubules overlap (and probably interact) between the poles and are involved in pushing the poles apart in anaphase. Astral microtubules radiate in all directions and also help separate the poles. Kinetochore microtubules attach themselves to specialized protein structures (kinetochores) located on each side of the centromere of each chromosome. These microtubules are involved in moving the chromosomes to the metaphase plate and in separating sister chromatids at anaphase. The microtubules in the spindle are very dynamic and have a half-life of only a few seconds. This appears to be especially important in the capture of chromosomes by the kinetochore microtubules. Microtubules that miss the target kinetochores are quickly lost because their dynamic instability soon leads to depolymerization. The new microtubules that form may hit the target and be partially stabilized through plus-end capping. 

In each half of the spindle, a single centrosome at the pole organizes three distinct sets of microtubules whose (—) ends all point toward the centrosome (Figure 20-3lb). One set, the astral microtubules, forms the aster they radiate outward from the centrosome toward the cortex of the cell, where they help position the mitotic apparatus and later help to determine the cleavage plane in cytokinesis. The other two sets of microtubules compose the spindle. The kinetochore microtubules attach to chromosomes at specialized attachment sites on the chromosomes called kinetochores. Polar microtubules do not interact with chromosomes but instead overlap with polar microtubules from the opposite pole. Two types of interactions hold the spindle halves together to form... 

When the duplicated centrosomes have become aligned, formation of the spindle proceeds, driven by simultaneous events at centrosomes and chromosomes. As just discussed, the centrosome facilitates spindle formation by nucleating the assembly of the spindle microtubules. In addition, the (—) ends of microtubules are gathered and stabilized at the pole by dynein-dynactin working with the nuclear/mitotic apparatus protein. The role of dynein in spindle pole formation has been demonstrated by reconstitution studies in which bipolar spindles form in Xenopus egg extracts in the presence of centrosomes, microtubules, and sperm nuclei. The addition of antibodies against cytosolic dynein to this in vitro system releases and splays the spindle microtubules but leaves the cen-trosomal astral microtubules in position (Figure 20-35). 

The mitotic apparatus of animal cells comprises the astral microtubules forming the asters, the polar and kine-tochore microtubules forming the football-shaped spindle, the spindle poles derived from the duplicated centrosomes, and chromosomes attached to the kinetochore microtubules (see Figure 20-31). 

Around stage 13, the oocyte nuclear envelope breaks down and the meiotic spindle assembles around the chromatin. The meiotic spindle arrests at metaphase I until fertilization. The metaphase I spindle is highly tapered and lacks astral microtubules. The chromatin is stretched on the spindle but is held together by chiasmata. The always nonexchange fourth chromosomes are found adjacent to the main mass that contains the exchange chromosomes. 

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