The polarity orientation of cellular microtubules is widely regarded to be important in understanding the control of microtubule assembly and microtubule-based motility in vivo . We have used a modification of the method of Heidemann and McIntosh (Nature (Lond .) . 286:517-519) to determine the polarity orientation of axonal microtubules in postganglionic sympathetic fibers of the cat. In fibers from three cats we were able to visualize the polarity of 68% of the axonal microtubules ; of these, 96% showed the same polarity orientation . Our interpretation is that the rapidly growing end of all axonal microtubules is distal to the cell body . We support Kirschner's hypothesis on microtubule organizing centers (J . Cell Biol. 86 : 330-334), although this interpretation raises questions about the continuity of axonal microtubules. Our results are inconsistent with a number of models for axonal transport based on force production on the surface of microtubules in which the direction of force is determined by the polarity of microtubules .

Several workers have found that axonal microtubules have a uniform polarity orientation. It is the "+" end of the polymer that is distal to the cell body. The experiments reported here investigate whether this high degree of organization can be accounted for on the basis of structures or mechanisms within the axon. Substantial depolymerization of axonal microtubules was observed in isolated, postganglionic sympathetic nerve fibers of the cat subjected to cold treatment; generally <10% of the original number of microtubules//zm 2 remained in cross section. The number of cold stable MTs that remained was not correlated with axonal area and they were also found within Schwann cells. Microtubules were allowed to repolymerize and the polarity orientation of the reassembled microtubules was determined. In fibers from four cats, a majority of reassembled microtubules returned with the original polarity orientation. However, in no case was the polarity orientation as uniform as the original organization. The degree to which the original orientation returned in a fiber was correlated with the number of cold-stable microtubules in the fiber. We suggest that stable microtubule fragments serve as nucleating elements for microtubule assembly and play a role in the spatial organization of neuronal microtubules. The extremely rapid reassembly of microtubules that we observed, returning to near control levels within the first 5 min, supports microtubule elongation from a nucleus. However, in three of four fibers examined this initial assembly was followed by an equally rapid, but transient decline in microtubule number to a value that was significantly different than the initial peak. This observation is difficult to interpret; however, a similar transient peak has been reported upon repolymerization of spindle microtubules after pressure induced depolymerization.Axonal growth and form are crucial to the formation of the appropriate connections within the nervous system. Microtubules (MTs) play a major role in the specification and maintenance of axonal shape (4, 10, 23). In view of this, it is not surprising that the MTs of neurons are highly organized in space. We became interested in the spatial organization of MTs when we found that the polarity orientation of axonal MTs is uniform (7,13,19). However, the mechanisms responsible for the organization of microtubules in the axon are very poorly understood.In non-neuronal cells microtubule organization is generally thought to depend on microtubule organizing centers (31), the centrosome being the most familiar. Although the data of Spiegelman et al. (44), Gonatas and Robbins (18), and Tennyson (45) implicate the centrosome in the outgrowth of the axon, many experiments argue convincingly that axonal MTs are not organized by a centrosome or, indeed, any observable microtubule organizing center. Most recently, Sharp et al. (40) observed no consistent relationship between the location of centrioles and the position of the cell processes by serial section electron micros...

We measured several of the components involved in the thiol-disulfide balance of immature and mature oocytes of Xenopus laevis: total soluble native sulfhydryl, reduced and oxidized glutathione, native sulfhydryl content of soluble protein, and disulfide content of soluble protein. None of these values changed during maturation. Both the immature and mature oocyte were characterized by a relatively high disulfide to sulfhydryl ratio. We also measured native sulfhydryl content of germinal vesicle proteins and enucleated oocyte proteins in immature oocytes. Contrary to early cytochemical findings, our measurements suggest that the germinal vesicle has a lower concentration of protein sulfhydryl than does the cytoplasm.

Previous work indicated that immature oocytes of Xenopus were incapable of assembling microtubules but that competence was achieved during maturation. We report here that small numbers of microtubules do exist in immature oocytes. Consistent with this finding, ultrastructural observations indicate that brain microtubules injected into immature oocytes persist in large numbers for at least 30 min. We report that the tubulin dimers of mature and immature oocytes are equally capable of assembling with brain tubulin in vitro. We confirmed previous results that injection of taxol into immature oocytes has no effect when assayed by light microscopy. However, ultrastructural observations suggest that some microtubule assembly is stimulated by taxol. We tested for the ability of immature oocytes to elongate microtubules from ‘seeds’ by injecting deciliated pellicles of Tetrahymena. No elongation was observed either by light or electron microscopic observation. We conclude that the immature oocyte is capable of very limited microtubule assembly and that a marked increase in assembly competence occurs during maturation. Our data suggest that the change in assembly competence during maturation is due to the release, activation or synthesis of a stimulatory co-factor.