Sperm assembly

Tilney, L.G., Bonder, E.M., Coluccio, L.M., Moosekar, M.S. (1983). Actin from Thyone sperm assembles on only one end of an actin filament A behavior regulated by profilin. J. Cell Biol. 97, 112-124. 

III. Locomotion in Nematode Sperm is Coupled to Assembly and Disassembly of the Cytoskeleton... 

Fig. 3. Leading edge dynamics can be reconstituted in vitro using cell-free extracts of Ascaris sperm in which vesicles derived from the plasma membrane induce the assembly of MSP filament meshworks called fibers that push the vesicles forward as they elongate. The two images were taken 10 sec apart. Bar, 2.5 /im. Reproduced from The Journal of Cell Biology, 1999, vol. 146, pp. 1087-1095 by copyright permission of the Rockefeller University Press.

Miao et al. (2003) have been able to reconstitute the essential features of retraction in vitro by adding Yersinia phosphatase (YOP) to the MSP-based motility apparatus assembled from cell-free extracts from Ascaris sperm... 

Fig. 6. Proposed push-pull model for nematode sperm locomotion. Assembly and bundling of MSP filaments into fiber complexes (dark band spanning the lamellipo-dium) pushes the membrane at the leading edge forward. At the same time a second force, associated with disassembly of the fiber complexes at the base of the lamellipo-dium, pulls the cell body forward. In this model, attachments where the cytoskeleton is linked to the membrane and the membrane anchored to the substratum establish traction and separate mechanically the forces produced at opposite ends of the fiber complexes. Thus, rather than canceling each other, these forces can be exerted independently against the substratum. Reproduced from The Journal of Cell Biology, 2000, vol. 149, pp. 7-12 by copyright permission of the Rockefeller University Press.

The principles of the push-pull model probably apply generally to amoeboid cell motility. Indeed, a consensus is developing that in both sperm and actin-based crawling cells the force for protrusion is derived from localized cytoskeletal assembly (reviewed by Pollard and Borisy, 2003). However, as applied to nematode sperm locomotion, the model envisions that lamellipod extension and cell body retraction are linked reciprocally to the polymerization state of the cytoskeleton. The lack of structural polarity of MSP filaments, the precise localization of cytoskeletal polymerization and depolymerization at opposite ends of the fiber complexes, and insights gained from reconstitution of cytoskeletal dynamics and motility in vitro and in vivo all support the conclusion that nematode sperm move without using motor proteins and that, instead, they rely on... 

Italiano, J. E.,Jr., Roberts, T. M., Stewart, M., and Fontana, C. A. (1996). Reconstitution in vitro of the motile apparatus from the amoeboid sperm of Ascaris shows that filament assembly and bundling move membranes. Cell 84, 105-114. 

In contrast to Xenopus laevis, the maternal histone pool in the mouse one-cell embryo (based on synthetic rates for histones) is probably sufficient for only one to two rounds of DNA replication (Wassarman and Mrozak, 1981). Consistent with such a small histone pool is the observation that poly spermic eggs have the capacity to transform up to three to four sperm nuclei into metaphase chromosomes (Clarke and Masui, 1986) a similar capacity was also determined from experiments that manipulated the cytoplasmic volume by either bisection or cell fusion (Clarke and Masui, 1987). This small pool of maternal histones may hence be insufficient to prevent effectively the assembly of stable basal transcription complexes. Thus, titration of the maternal histone pool by an increase in the mass of DNA due to blasto-mere proliferation may not be a critical factor in regulating the onset of transcription in the mouse embryo and other mammalian eggs this is because the maternal transcription factors may be able to outcompete successfully maternal histones for the newly replicated chromatin. This could, at least in part, account for the early onset of transcription in mammalian embryos ranging from rodents to humans (Telford et al., 1990). Moreover, the lack of arapid S phase in the mouse embryo and other mammalian embryos would permit sufficient time for productively assembled transcription complexes to generate full-length transcripts. In contrast to mammalian embryos, S phase is very short prior to the midblastula transition in Xenopus laevis (Newport and Kirschner, 1982) and hence these rapid rounds of DNA replication could prematurely terminate the transcription of genes for which transcription had initiated. 

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). 

In these experiments, chromosome decondensation and nuclear envelope assembly (late mitotic events) coincided with decreases in the cyclin B level and MPF activity. To determine whether degradation of cyclin B is required for exit from mitosis, researchers added a mutant mRNA encoding a nondegradable cyclin B to a mixture of RNase-treated Xenopus egg extract and sperm nuclei. As shown in Figure 2 l-9d, MPF activity Increased in parallel with the level of the mutant cyclin B, triggering condensation of the sperm chromatin and nuclear envelope disassembly (early mitotic events). However, the mutant cyclin B produced in this reaction never was degraded. As a consequence, MPF activity remained elevated, and the late mitotic events of chromosome decondensation and nuclear envelope formation were both blocked. This experiment demonstrates that the fall in MPF activity and exit from mitosis depend on degradation of cyclin B. 

The chromosomes are threadlike assemblies that are extremely important because they contain the genes that transmit the hereditary information. Every species has a definite number of chromosomes. The human species has 23 pairs, one chromosome of each pair being contributed by each parent. Every cell has 23 pairs of chromosomes with the exception of the egg and the sperm, which have 23 chromosomes each. When fertilization occurs, the first cell of the new organism contains 23 pairs of chromosomes, equally contributed by each parent. 

Scheme 10 Convergent assembly of the GPI anchor of sperm CD52 antigen.

Oko, R. and Clermont, Y. (1990). Mammalian spermatozoa structure and assembly of the tail, in Controls of Sperm Motility Biological and Clinical Aspects (C. Gagnon, ed.), pp. 3-27. CRC Press, Roca Raton, FL. 

Prepare Xenopus egg extracts (Newmeyer and Wilson, 1991) and incubate extract with demembranated Xenopus sperm chromatin to assemble nuclei. ER and Golgi will also assemble in the same extract. 

Lohka. M. J., and Masui, Y. (1984). Roles of cytosol and cytoplasmic particles in nuclear envelope assembly and sperm pronuclear formation in cell free preparations from amphibian eggs. J. Cell Biol. 9si 1222-1230. 

The reassembly of the NE at the end of mitosis is the best recognized example of NE assembly in eukaryotes. Another, perhaps less appreciated, example of NE assembly in metazoans is that which occurs around sperm chromatin during the formation of the male pronucleus after fertilization (for review, see Longo... 

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