A cytoplasmic clock with the same period as the division cycle in Xenopus eggs.

Abstract

In most species the cell cycle is arrested in the unfertilized egg. After fertilization the cell cycle is reestablished and a rapid series of cleavages ensues. Preceding the first cleavage in Xenopus the egg undergoes a contraction of its cortex, called the "surface contraction wave," which can be visualized by time-lapse cinematography. This wave of contraction is propagated in a circular manner from the animal pole to the equator. We have found that eggs prevented from cleaving by treatment with antimitotic drugs undergo a sequence of periodic surface contraction waves timed with the cleavage cycle in untreated eggs. In addition, artificially activated eggs, which fail to cleave presumably for lack of a functioning centriole, undergo the same periodic contractions. No nuclear material is required for the periodic waves because a separated egg fragment, produced by constricting a fertilized egg, still undergoes contraction waves with the same period as the cleaving nucleated fragment. These results demonstrate that some expression of the cell cycle persists in the absence of any nuclear material or centrioles, suggesting to us that a biological clock exists in the cytoplasm or cortex of vertebrate eggs, which may be involved in timing the cell cycle.In most species, the cell cycle is arrested in the unfertilized egg and is then reinstated by fertilization. In amphibians, after fertilization there is a somewhat long period leading up to the first cleavage and then cells in the animal, equatorial, and vegetal regions in sequence proceed through a rapid series of 10 or 11 metachronous cleavages until the midblastula stage is reached. During the metachronous cleavage period, the cells within each region divide synchronously. Thereafter, the cell cycle variably lengthens and cleavage becomes totally asynchronous (1-3). Thus, in amphibian development, the cell cycle is acquired in steps after fertilization, beginning with a short rudimentary cycle containing only M and S phases and characterized by little if any transcriptional activity and followed by a longer cell cycle having G1 and G2 phases and active RNA synthesis (4-6). Because the high degree of synchrony at the early stages is preserved when the individual blastomeres are dissociated and separated (ref. 2 and unpublished data), the machinery for timing accurately the alternate waves of DNA synthesis and mitosis must be partitioned to each of the daughter cells. An understanding of the mechanism of timing of the simple cell cycle in the early cleavage period may be useful for understanding the overall control of the cell cycle in somatic cells.Although many cell types have been used for studies of the cell cycle, the amphibian egg offers some special advantages. Its large size permits enucleation and nuclear transplantation, allowing a clear-cut distinction between the contributions of the nucleus and the cytoplasm. In general, the lack of activity of the nucleus in the early cleavage period makes the egg a good system for studying the autonomy of the...

A functional analysis of p53 during early development of xenopus laevis

“…We investigated the localization of p53 in relation to the cell cycle by microinjection of demembranated sperm nuclei into Xenopus eggs. Microinjection into unfertilized eggs triggers the activation of a cell cycle clock (Hara et al, 1980) and induces micro-injected nuclei to synthesize DNA (Gurdon, 1986). We observed that, when sperm nuclei are injected, all the morphological changes previously described in vitro are reproduced Masui, 1983, 1984), including the entry in S phase (Blow and Laskey, 1986).…”

Pre-M Phase-promoting Factor Associates with Annulate Lamellae inXenopusOocytes and Egg Extracts

“…Cycles of Cyclin B accumulation and destruction sufficient to drive synchronous cell cycles in the absence of any other protein synthesis occur in both fertilized or activated eggs and in egg extracts (Murray and Kirschner, 1989;Hartley et al, 1996). Although MPF activation cycles in amphibian oocytes and eggs can continue in the absence of nuclei and microtubules (Hara et al, 1980;Shinagawa, 1983;Gerhart et al, 1984;Newport and Kirschner, 1984;Kimelman et al, 1987;Shinagawa, 1992), two lines of evidence have demonstrated that subcellular structure is important for MPF activation, even in cytoplasmic extracts. First, nuclei, centrosomes and microtubules have all been shown to stimulate MPF activation Article published online ahead of print.…”

“…Cycles of Cyclin B accumulation and destruction sufficient to drive synchronous cell cycles in the absence of any other protein synthesis occur in both fertilized or activated eggs and in egg extracts (Murray and Kirschner, 1989;Hartley et al, 1996). Although MPF activation cycles in amphibian oocytes and eggs can continue in the absence of nuclei and microtubules (Hara et al, 1980;Shinagawa, 1983;Gerhart et al, 1984;Newport and Kirschner, 1984;Kimelman et al, 1987;Shinagawa, 1992) both in manipulated cells and extracts (Detlaff et al, 1964 and references therein, Gautier, 1987;Iwashita et al, 1998;Pérez-Mongiovi et al, 2000). Second, M-phase entry and exit proceed inhomogeneously within the cell (Masui, 1972;Iwao and Elinson, 1990) and travel as waves from the animal hemisphere, where the nucleus and centrosomes are located, to the vegetal hemisphere (Rankin and Kirschner, 1997;Pérez-Mongiovi et al, 1998).…”

Organization of early frog embryos by chemical waves emanating from centrosomes

“…For example, a typical protein with a cytoplasmic diffusion coefficient of D ¼ 500 mm 2 min 21 [25,26] would take hours to travel from the centre to the periphery of a frog egg with radius 600 mm (time t ¼ L 2 /6D ¼ 120 min). By contrast, cell cycle waves traverse similar distances in minutes [1,27] and calcium waves in tens of seconds [7]. This can be understood by assuming an autocatalytic reaction of rate approximately 1 min 21 , resulting in a chemical wave of…”

“…In time-lapse images of fertilized eggs made in the 1970s, Hara observed a travelling wave of cortical contraction, visualized by movement of pigment granules. These SCWs occurred just before the first cleavage and proceeded as circular waves from the animal to the vegetal pole [27,28] (figure 2a). SCWs consist of two distinct waves: SCWa was the result of the relaxation of the cortical tension, whereas the succeeding SCWb was a stiffening of the cortex [30].…”