Role ofp27Kipl

It is uncertain whether the intrinsic timer in OPCs primarily controls the onset of differentiation, the cessation of proliferation, or both. It seems likely, however, that the timer at some point interacts with the cell-cycle control system that regulates progress through the cell cycle. As the cells stop dividing and differentiate in G1, it is the part of the control system that operates in G1 that is most likely to be relevant. In principle, the components of the cell-cycle control system...

Xenopus oocyte maturation

From a cell cycle point of view, oocyte maturation has been an interesting system for many years. In the ovary all post vitellogenic oocytes are physiologically arrested in late diplotene at the G2 prophase border in meiosis I. The meiotic genome of the resting oocyte is transcriptionally active, especially for ribosome synthesis, but growth over several months occurs via uptake of vitellogenin from the blood. The oocyte is one of the clearest cases where cell growth is completely uncoupled...

A cellintrinsic timer

The timing of OPC differentiation can be reconstituted in cultures of dissociated embryonic optic nerve cells, as long as there is sufficient mitogen to drive OPC proliferation (Raff et al 1985, 1988). As we discuss below, the main mitogen for these cells is platelet-derived growth factor (PDGF). The reason that OPCs stop dividing and differentiate in these cultures is not because PDGF becomes limiting or that PDGF receptors disappear or can no longer be activated. The addition of excessive...

C elegans cell cycle machinery

Components of the conserved cell cycle machinery are rapidly being identified in C. elegans and their roles are being tested in the context of the developing worm. For example, a number of Cdc2-related protein kinases were identified from the genomic sequence, and the gene encoding one of these, ncc-1 was found by RNA-mediated interference (RNAi) experiments, and by mutant phenotypes, to be required for mitosis in embryonic and postembryonic cell divisions (Boxem et al 1999). ncc-1 is not...

The midblastula transition

Once an egg is fertilized, intracellular Ca2+ is elevated, cyclin B degradation is stimulated, and CSF activity declines in concert with loss of MAPK and Rsk activity. The first 12 cleavage cycles occur with a cycle time of 26 min and display only M and S phase, except for the first cycle which is longer and has a detectable G2 phase (Fig. 1). Despite these short cycles, the enzymes that control Cdc2 Tyr15 phosphorylation, Wee1 and Cdc25A, are rapidly synthesized after fertilization and new...

From meiosis to mitosis

While the nurse cells endoreduplicate, the oocyte within the egg chamber enters meiosis and arrests in metaphase of the first meiotic division. While recent progress has provided first insights into the role of cell cycle regulators during oocyte choice and has uncovered an interesting meiotic checkpoint co-ordinating this process with embryonic axes predetermination (Gonzalez-Reyes et al 1997, Ghabrial & Schupbach 1999, Lilly et al 2000), we still know very little about the regulation of...

Poles dictate spindle position along the AP axis

The mechanisms by which overall cell polarity, as established by the PAR proteins, is communicated to the cytoskeleton to mediate proper spindle positioning are not understood. Below we discuss two experimental approaches that we have taken to address this question. In the first set of experiments, we sought to identify the forces that act on spindle poles to drive spindle positioning during anaphase. Experiments in other systems have revealed that two types of microtubule-dependent forces can...

Regulation of the entry into the early mitotic M phases

The entry into the first mitotic M phase at the end of the first embryonic cell cycle requires activation of MPF. In the mouse one-cell embryo this activation is fully autonomous from the nucleus (Ciemerych 1995, Ciemerych et al 1998). It proceeds within the cytoplasts obtained either by enucleation or by bisection of the embryo. Other autonomous phenomena are the cortical activity, or the deformation of the one-cell embryo, directly preceding the entry into first mitosis (Waksmundzka et al...

Discussion

Gonc y Do you know whether the differences between mitosis and meiosis that you told us about in budding yeast also apply in mammals Nasmyth In the second meiotic division the arms have come apart in every organism. As you saw in the yeast, they come apart at the first division because that is the mechanism by which chiasmata are resolved, but in humans the arms don't completely come apart. In a normal mitosis the arms are still glued together, but there is only a tenuous connection at the...

Cell cycledependent cell fate choices

In many developing systems, decisions about cell fate are influenced by cell cycle status (McConnell & Kaznowski 1991, Weigmann & Lehner 1995, Lehner & Lane FIG. 3. Two cell cycle phase-dependent choices of cell fate for VPCs. Two VPCs are shown, one near to the anchor cell (AC) the source of inductive signal, and one further away. The VPC closest to the AC receives a high LIN-3 inductive signal (transduced by the Ras signal transduction pathway) and initiates expression of the 1 fate...

Other interacting factors

The conflict theory proposes that paternally and maternally expressed imprinted genes oppose each others actions on fetal growth. Thus far we have considered how this can happen at the level of the growth factors and their receptors themselves (e.g. paternal IGF2 and maternal IGF2R). However opposing interactions may occur at many different levels. For example most imprinted genes are clustered in the genome and the Igf2 gene is located in one of the major clusters, being largely surrounded by...

Embryonic cell cycles regulation at G2M by patterning cues

Mitosis in most Drosophila cells is triggered by bursts of transcription of string (stg), a Cdc25-type phosphatase that activates the mitotic kinase, Cdk1. During the post-blastoderm embryonic cell cycles (14-16) string is transcribed in dynamic, invariant patterns that correspond to mitotic patterns, but which precede mitoses by 10-20 minutes (Edgar et al 1994). During this period all other factors required for cell cycle progression, including cdks 1 and 2 and cjclins A, B and E, are...

General discussion I

Nurse As we move to a general discussion, I have three questions which I suggest we could focus on, going back to this thorny question of growth and the cell cycle. First, is the cell cycle itself limiting growth This addresses questions to do with cell size can you be too big, or too small, and what influence does this have on overall growth of the cell Second, what actually regulates overall growth of the cell Third, what do we mean by 'in' and 'out' of cycle Does the term 'G0' mean anything...

Splitting the chromosome cutting the ties that bind sister chromatids

Kim Nasmyth, Jan-Michael Peters and Frank Uhlmann Research Institute of Molecular Pathology (IMP), Dr. Bohr-Gasse 7, A-1030 Vienna, A ustria Abstract. In eukaryotic cells, replicated DNA molecules remain physically connected from their synthesis in S phase until they are separated during anaphase. This phenomenon, called sister chromatid cohesion, is essential for the temporal separation of DNA replication and mitosis and for the equal separation of the duplicated genome. Recent work has...

Cell polarity and anaphase spindle positioning in the wildtype onecell stage C elegans embryo

In C. elegans, polarity along the AP embryonic axis is established shortly after fertilization (Goldstein & Hird 1996). A sperm component, which remains to be identified, provides an initial polarity cue that determines the future posterior of the embryo. This initial cue is then translated by the concerted action of six maternally required par genes (for artitioning-defective) to establish polarity along the AP axis (for review see Kemphues & Strome 1997). All six par genes have been...

Results and discussion

Induction of DNA damage to the cleavingXenopus eggdoes not block cytokinesis We used two different experimental means for inducing DNA damage in Xenopus eggs during cleavage. Fertilized eggs were injected at the one or two cell embryo stage with restriction enzymes, or embryos were subjected to y-irradiation at various stages following fertilization. In either case, we did not observe a delay in cell cycle progression when compared to control uninjected embryos (Hensey & Gautier 1997). Cell...

References

Ahlgren SC, Wallace H, Bishop J, Neophytou C, Raff MC 1997 Effects of thyroid hormone on embryonic oligodendrocyte precursor cell development in vivo and in vitro. Mol Cell Neurosci 9 420-432 Barres BA, Hart IK, Coles HS et al 1992 Cell death and control of cell survival in the oligodendrocyte lineage. Cell 70 31-46 Barres BA, Lazar MA, Raff MC 1994 A novel role for thyroid hormone, glucocorticoids and retinoic acid in timing oligodendrocyte development. Development 120 1097-1108 de Nooij JC,...

A tense period in the cell cycle

Changes in the interaction between sister chromatids, as opposed to changes in the activity of spindle fibres, is thought to trigger the sudden movement of chromatids to the poles at the metaphase to anaphase transition. Destroying the spindle fibre that connects a chromosome to one pole using UV or laser microbeams (McNeill & Berns 1981) causes the entire chromosome (i.e. both chromatids) to move rapidly to the opposite pole. The implication is that sister chromatid pairs on the metaphase...

Spindle positioning during the asymmetric first cell division of Caenorhabditis elegans embryos

Pierre G nczy*f1, Stephan Grill*f, Ernst H. K. Stelzer*, Matthew Kirkhamf and Anthony A. Hyman*f *'European Molecular Biology Laboratory (EMBL), Meyerhofstrasse 1, D-69117 Heidelberg, and fMax-Planck--Institute for Cell Biology and Genetics (MPI-CBG), D-01307 Dresden, Germany A bstract. Cell division during development in many cases generates daughter cells that differ not only in fate, but also in size. We investigate the mechanisms that ensure proper spindle positioning during such asymmetric...

Control of cell fate in plant meristems

Martin Hobe, Ulrike Brand, Richard Waites and R diger Simon1 Institut f r Entwicklungsbiologie der Universit t u K ln, Gyrhofstrasse 17, 50931 K ln, Germany .Abstract. In contrast to animals, plants do not stop to initiate organs with the end of embryogenesis. Instead, most of the growth and development of higher plants will take place during later phases following germination of the seed. Plant development depends on the activity of two meristems, the root meristem and the shoot apical...

Role of placenta

Mutations of several genes in mice, which are associated with placenta development, result in embryonic lethality or fetal growth retardation (reviewed by Ihle 2000, Cross 2000). In the case of the imprinted Mash2 gene the lethality is immediate and there is not time for the manifestation of growth retardation. Mash2 is an essential regulator of trophoblast giant cell differentiation (Guillemot et al 1995). Mice lacking Mash2 fail to develop a spongiotrophoblast layer and the labyrinthine layer...

Pattern and growthlinked cell cycles in Drosophila development

Edgar, Jessica Britton, Aida Flor A. de la Cruz, Laura A. Johnston, Dara Lehman, Cristina Martin-Castellanos and David Prober 'Division of Basic Sciences, Fred Hutchinson Cancer Research Center, 1100 Fairview Avenue North, Seattle, WA 98109, USA Abstract. During Drosophila development the cell cycle is subject to diverse regulatory inputs. In embryos, cells divide in stereotypic patterns that correspond to the cell fate map. There is little cell growth during this period, and cell...

Coordination of fetal growth endocrine versus paracrine

The INS IGF system is a very ancient system that integrates metabolic signals with growth. Various effects on organism growth have been documented, including the control of cell proliferation, cell death and extracellular fluid accumulation (Gardner et al 1999). Although this has not been studied in detail, it is not likely that in mammals there are major effects on cell growth (i.e. increase in cell mass itself), though curiously in other organisms the same system is involved in cell growth....

Imaginal discs dual control at G1 S and G2M

Cells in the developing imaginal discs, which form much of the adult body during metamorphosis, exhibit a cell cycle that incorporates both modes of control described above (Fig. 2C). Each imaginal disc has 10 50 cells in the newly hatched larva, and these proliferate to as many as 100 000 cells before differentiating into an adult structure such as a wing, leg, or eye (Bryant & Simpson 1984). Imaginal cells arrest in G1 during mid-embryogenesis and require the influx of nutrients from...

Growth in advance oogenesis and endoreduplication

The size of egg cells demonstrates that higher eukaryotes have evolved ways to uncouple cell growth and cell cycle progression. The Drosophila egg is about 10 000-fold larger than the stem cells in the ovary. Much of the egg content is synthesized in other organs (fat body) and imported into the egg cell. Moreover, in Drosophila, the egg is generated from an egg chamber in which the oocyte is connected to 15 highly polytene nurse cells via ring canals. These nurse cells which produce most of...

Growth factors controlling imaginal disc growth in Drosophila

Developmental Biology Center, University of Californialrvine, Irvine, CA 92697, USA Abstract. In the imaginal discs of Drosophila, contact-dependent cell interactions are important both for promoting cell proliferation and for limiting it at the end of the growth period. However, recent work indicates that diffusible growth factors are also important in regulating growth and proliferation. We have identified a family of five imaginal disc growth factors (IDGFs) by purifying mitogenic proteins...

Cutting the Gordian knot

Could proteolytic cleavage of a cohesin subunit really be a universal trigger for sister separation If so, how does one explain the dissociation of the bulk of cohesin from chromosomes during pro-metaphase in organisms other than yeast (Losada et al 1998) In vertebrates, this process clearly occurs in the absence of APC activity and is therefore presumably not due to separin activity (I. Sumara & J.-M. Peters, Personal communication). The implication is that there must exist two separate...

Ties that bind chromatids together

In many organisms, the regions around centromeres have a special role in holding sister chromatids together during metaphase. Fluorescence in situ hybridization (FISH) shows that most sister DNA sequences separate from each other (at least a short distance) soon after DNA replication (Selig et al 1992). Nevertheless, sister chromatids usually do not acquire morphologically separate axes until prometaphase, well after the onset of chromosome condensation. Human chromosomes, for example, appear...

Role of imprinted genes in fetal growth

A number of genes affect fetal and early postnatal growth and these fall into different categories (Efstratiadis 1998). We only consider genes as growth controlling (Table 1) if they have a direct effect on fetal size or organ size. We exclude, for example, genes that result in aberrant functioning of the heart leading to poor circulation, which may cause reduced growth. However, in a following section we also consider genes with an indirect effect on growth due to their action in development...

Separin an endopeptidase necessary for separating chromatids

The budding yeast securin has what appears to be a single stable partner, a 180 kDa protein called Espl (Ciosk et al 1998). In fission yeast, Cut2 had previously been found to be associated with Cut 1, an Espl homologue (Funabiki et al 1996b). Vertebrate securins are likewise associated with an Esp1 homologue (Zou et al 1999). Esp1 Cut1-like proteins, now known as separins, are found in most if not all eukaryotes. They are usually large proteins, with molecular weights from 180 to 200 kDa,...

Determining organ size

Nurse I wondered whether it might be worth revisiting the issue of what determines organ size, because this is obviously relevant to a number of issues that have risen. Martin Raff, do you have any thoughts about overall organ size and how that is regulated Raff I think there are several things to be said. First, the extent to which an organ depends on local controls versus systemic controls for its final size varies greatly. The thymus is apparently dependent on local controls. Don Metcalf did...

Stopping the cell division cycles

Each of the cell divisions after cellularization is triggered by a brief pulse of zygotic string cdc25 expression (Edgar & Lehner 1996). Entry into S phase follows after mitosis without an intervening G1 phase. This immediate entry into S phase is most likely explained by the presence of maternal stores. All components required for S phase appear to be present in amounts sufficient to allow all of the embryonic S phases in most of the tissues. Zygotic expression of various S phase genes...

Imaginal disc growth factors

We have purified and identified the active fraction from medium conditioned by imaginal disc cells from the C1.8+ line (Kawamura et al 1999). After size-exclusion filtration, anion exchange chromatography, gel filtration HPLC and preparative electrophoresis, the protein was subjected to N-terminal microsequencing. The data led to the identification of a family of five genes encoding similar glycoproteins that we call imaginal disc growth factors (IDGFs). One of them is the 47 kDa glycoprotein...

Do later embryonic cell cycles differ from the somatic ones

Cyclin A2 knockout mice are not viable (Murphy et al 1997). However, a few early embryonic divisions proceed despite the absence of any cyclin A2 gene product in homozygous embryos obtained by crossing + heterozygotes (Winston etal FIG. 4. Hypothetical action of the cyclin A2-dependent mechanism retarding exit from the first embryonic M phase in the mouse. FIG. 4. Hypothetical action of the cyclin A2-dependent mechanism retarding exit from the first embryonic M phase in the mouse. 2000). This...

Possible interaction of IDGFs with the insulin pathway

Like other growth factors, the IDGFs presumably activate a signal transduction pathway that ultimately controls transcription and replication. One possibility is that they interact in some way with the insulin receptor (InsR) pathway, which has FIG. 3. Expression of IDGF genes in the embryonic and larval fat body. Embryos and larval tissues (wholemounts) were used for in situ hybridization using digoxigenin-labelled antisense RNA probes. (A) Late embryo showing fat body expression of IDGF2....

Developmental pathways controlling cell cycle progression

Work from Drosophila (Edgar & Lehner 1996, Lehner & Lane 1997) established the paradigm that the timing of cell cycle progression during development could result from the coupling of limiting components of the cell cycle machinery to the activity of developmental signals or regulatory molecules. In C. elegans, genetic pathways affecting cell cycle progress have been identified, and in some cases, the targeted cell cycle machinery has been identified. TABLE 1 Stage- and tissue-specific...

Regulation of Drosophila imaginal disc growth by the insulinIGF signalling pathway

Leevers I would like to give a brief summary of what is known about the role of signalling by the insulin insulin-like growth factor IGF pathway during the growth of Drosophila imaginal discs, and then contrast this with what we have heard in the previous papers in this book. There is a pathway in Drosophila that is highly homologous to the insulin IGF signalling pathway in mammals Fig. 1 Leevers . A number of different labs have studied various genes on this pathway Edgar 1999, Leevers 1999,...

Structure of the shoot apical meristem

The angiosperm shoot apical meristem SAM is initiated during embryogenesis and subsequently produces the basic elements of the plant shoot structure, leaves and stems. In Arabidopsisand many other dicotyledonous plants, the SAM consists of a small dome of cells that are organised into regions with different functions and fates Steeves amp Sussex 1989, Vaughan 1952 . At a first level, cells are organized into the outer tunica layers and the inner corpus layer Fig. 1 . In many species, the tunica...

IDGFs as lectins

An intriguing possibility is that the IDGFs could interact as lectins with the insulin receptor itself, which is already known to bind other lectins. Wheat germ agglutinin WGA , a mitogenic lectin for which the receptor sugar is N-acetyl-glucosamine, binds to the mammalian insulin receptor see http suggesting that its mitogenic activity could be mediated in part through the insulin receptor pathway. Further evidence supporting interaction between mitogenic lectins and the insulin receptor comes...