Centriole Replication and Nuclear Division in Saprolegnia
TL;DR: The processes of centriole replication, spindle formation and nuclear division are discussed in relation to reports of the ultrastructure of these processes in other organisms.
Abstract: SUMMARY: Paired centrioles in the hyphae and young sporangia of Saprolegnia ferax and Dictyuchus sterile were aligned end to end at 180° to each other. At the end of interphase new centrioles were replicated on the proximal end of each parent centriole. Each pair of centrioles was associated with a characteristic region of the nuclear envelope, termed the pocket. As the centriole pairs moved apart the mitotic spindle developed between these pockets. Kineto-chores, initially found at the equator of the spindle, became polarized as the nucleus elongated. A characteristic association between the nuclear envelope and astral microtubules is described and its role in nuclear division discussed. Mitochondria frequently lay along tracts of microtubules. The processes of centriole replication, spindle formation and nuclear division are discussed in relation to reports of the ultrastructure of these processes in other organisms.
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TL;DR: This chapter discusses the characteristics and evolution of mitosis, a types of nuclear division that produce two, or rarely more, daughter nuclei, each containing a chromosome complement approximately similar to that of the original nucleus.
Abstract: Publisher Summary Mitosis is defined as all those types of nuclear division that produce two, or rarely more, daughter nuclei, each containing a chromosome complement approximately similar to that of the original nucleus. The greatest range of variations by which mitosis is accomplished, occurs in the protistan and fungal kingdoms, some members of which are probably most similar to the ancestors of higher plants and animals. The variations in the higher organisms are secondarily derived from the division patterns of typical plants and animals. This chapter discusses the characteristics and evolution of mitosis. The efficiency of mitosis consists of two basic components; the frequency with which each daughter nucleus receives the necessary complete genome complement (genetic efficiency) and the amount of energy and materials expended in the synthesis and operation of the mitotic apparatus. The chapter also discusses the use of mitosis as a phylogenetic marker. It is applicable to all eukaryotic cells and thus is valuable across boundaries where other structures are absent on one side and present in various forms on the other side.
318 citations
296 citations
TL;DR: The earlier view that oomycetes evolved from the largely saprotrophic “saprolegnian line” is not supported and current evidence shows these organisms evolved from simple holocarpic marine parasites.
Abstract: Molecular sequencing has helped resolve the phylogenetic relationships amongst the diverse groups of algal, fungal-like and protist organisms that constitute the Chromalveolate "superkingdom" clade. It is thought that the whole clade evolved from a photosynthetic ancestor and that there have been at least three independent plastid losses during their evolutionary history. The fungal-like oomycetes and hyphochytrids, together with the marine flagellates Pirsonia and Developayella, form part of the clade defined by Cavalier-Smith and Chao (2006) as the phylum "Pseudofungi", which is a sister to the photosynthetic chromistan algae (phylum Ochrophyta). Within the oomycetes, a number of predominantly marine holocarpic genera appear to diverge before the main "saprolegnian" and "peronosporalean" lines, into which all oomycetes had been traditionally placed. It is now clear that oomycetes have their evolutionary roots in the sea. The earliest diverging oomycete genera so far documented, Eurychasma and Haptoglossa, are both obligate parasites that show a high degree of complexity and sophistication in their host parasite interactions and infection structures. Key morphological and cytological features of the oomycetes will be reviewed in the context of our revised understanding of their likely phylogeny. Recent genomic studies have revealed a number of intriguing similarities in host-pathogen interactions between the oomycetes with their distant apicocomplexan cousins. Therefore, the earlier view that oomycetes evolved from the largely saprotrophic "saprolegnian line" is not supported and current evidence shows these organisms evolved from simple holocarpic marine parasites. Both the hyphal-like pattern of growth and the acquisition of oogamous sexual reproduction probably developed largely after the migration of these organisms from the sea to land.
270 citations
Cites background from "Centriole Replication and Nuclear D..."
...Most oomycete fungi have a closed mitosis with an intranuclear spindle which forms between pairs of apically or sub-apically located centrioles (Heath and Greenwood 1970)....
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TL;DR: It is proposed that at mitosis the separation of the K CEs and their attached chromosomes is initiated by a differential expansion of the nuclear envelope restricted to the region between recently divided KCEs and that expansion ofThe nuclear envelope later becomes general, resulting in a marked elongation of the nucleus.
Abstract: Mitosis in Schizosaccharomyces pombe has been followed in living cells by phase-contrast microscopy and studied in fixed and suitably stained preparations by light microscopy. Successful preservation of nuclear fine structure in this yeast, not previously achieved, has allowed us to confirm and extend the observations made with light microscopy. Without first arranging themselves on a metaphase plate, mitotic chromosomes become grouped in 2 clusters radiating, finger-like, from 2 points of attachment at opposite poles of an elongating nucleus. At these 2 sites electron microscopy reveals the presence of disk-shaped electron-dense organelles which we have called kinetochore equivalents (KCE). At mitosis the KCEs are connected across the nucleus by a narrow bundle of parallel microtubules which we refer to as the spindle. Integration of our observations has led us to propose that at mitosis the separation of the KCEs and their attached chromosomes is initiated by a differential expansion of the nuclear envelope restricted to the region between recently divided KCEs and that expansion of the nuclear envelope later becomes general, resulting in a marked elongation of the nucleus. Displacement of the nuclear contents to the ends of the elongated nucleus gives it the shape of a dumbbell. The elongation of the microtubule bundle keeps in step with the elongation of the nucleus but does not appear to be the cause of it. It may have the function of keeping the separated KCEs rigidly apart. During mitosis the nucleolus persists and stretches out within the unbroken envelope of the nucleus as it elongates. Towards the end of division equal amounts of nucleolar material are found in the rounded ends of the dumbbell-shaped nucleus. The break up of the dumbbell shape into daughter nuclei seems to involve the breaking of its tenuous middle part and a pivoting of its 2 ends in opposite directions. In the course of our work on mitosis we have become aware of features in the cytoplasm of growing S. pombe cells which are described here for the first time. The cells invariably contain several prominent vacuoles containing an extremely electron-dense material which stains metachromatically with toluidine blue and may be polyphosphate. The mitochondria are of special interest for 2 reasons. First, because they have unique mesosome-like membrane invaginations and secondly, because a mitochondrion is regularly associated with the single KCE by the side of the interphase nucleus, as well as with each one of the 2 KCEs that occupy opposite ends of the intranuclear spindle during mitosis.
212 citations
Cites background from "Centriole Replication and Nuclear D..."
...Apparently, no discontinuous microtubules are present like the ones shown by Heath & Greenwood (1970) in Saprolegnia (in which, as in S. pombe, chromosomes are not visible with electron microscopy)....
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TL;DR: In an attempt to provide a unified hypothesis to account for the various aspects of oriented cellulose fibril synthesis it is proposed that plasmalemma located cellulose synthetase enzyme complexes are free to move in the plane of the membrane.
203 citations
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TL;DR: After glutaraldehyde is used as a fixative in this present study, the general preservation of cortical fine structure is greatly improved, shown by the first evidence of slender tubules, 230 to 270 A in diameter and of indeterminate length, in plant cells of this type.
Abstract: This paper reports an electron microscope examination of the cortices of some plant cells engaged in wall formation. Previous studies of similar material fixed in OSO4 alone have disclosed discontinuities in the plasma membrane and other evidence of inadequate fixation. After glutaraldehyde, used as a fixative in this present study, the general preservation of cortical fine structure is greatly improved. This is shown, for example, by the first evidence of slender tubules, 230 to 270 A in diameter and of indeterminate length, in plant cells of this type. They have been found in the cortical regions of cells of two angiosperms and one gymnosperm, representing all the material so far studied following this method of fixation. The tubules are identical in morphology to those also observed here in the mitotic spindles of plant cells, except that the latter have a somewhat smaller diameter. It is noted that the cortical tubules are in a favored position to govern cytoplasmic streaming and to exert an influence over the disposition of cell wall materials. In this regard it may be of some significance that the tubules just beneath the surface of the protoplast mirror the orientation of the cellulose microfibrils of the adjacent cell walls.
909 citations
TL;DR: Reconstruction of the processes of centriolar formation and ciliogenesis based on evidence found in electron micrographs of tissues and organ cultures obtained chiefly from the lungs of foetal rats leads to an interpretation of the centriole as a semi-autonomous organelle whose replicative capacity is separable from the characteristic triplet fibre structure of its wall.
Abstract: This study presents reconstructions of the processes of centriolar formation and ciliogenesis based on evidence found in electron micrographs of tissues and organ cultures obtained chiefly from the lungs of foetal rats. A few observations on living cultures supplement the major findings. In this material, centrioles are generated by two pathways. Those centrioles that are destined to participate in forming the achromatic figure, or to sprout transitory, rudimentary (primary) cilia, arise directly off the walls of pre-existing centrioles. In pulmonary cells of all types this direct pathway operates during interphase. The daughter centrioles are first recognizable as annular structures (procentrioles) which lengthen into cylinders through acropetal deposition of osmiophilic material in the procentriolar walls. Triplet fibres develop in these walls from singlet and doublet fibres that first appear near the procentriolar bases and thereafter extend apically. When little more than half grown, the daughter centrioles are released into the cytoplasm, where they complete their maturation. A parent centriole usually produces one daughter at a time. Exceptionally, up to 8 have been observed to develop simultaneously about 1 parent centriole. Primary cilia arise from directly produced centrioles in differentiating pulmonary cells of all types throughout the foetal period. In the bronchial epithelium they appear before the time when the ciliated border is generated. Fairly late in foetal life, centrioles destined to become kinetosomes in ciliated cells of the epithelium become assembled from masses of fibrogranular material located in the apical cytoplasm. Formation of these centrioles may be under the remote influence of the diplosomal centrioles. More certainly, the precursor material accumulates in close proximity to Golgi elements. Within the fibrogranular areas, osmiophilic granules (400-800A) increase in size and eventually become consolidated into dense spheroidal bodies (deuterosomes), which organize the growth of procentrioles around them. When mature, the newly formed centrioles become aligned in rows beneath the apical plasma membrane. There each centriole produces satellites from its sides, a root from its base, and a cilium from its apex. Early stages in the formation of both primary cilia and those of the ciliated border are similar. In developing cilia of the ciliated border, however, the outer ciliary fibres rapidly reach the tips of the elongating shafts, and a central pair of fibres is formed (9 + 2 arrangement). In primary cilia, development of the fibres seems to lag behind the elongation of the shafts, and only the outer ciliary fibres appear (9 + 0 arrangement). The strengths and weaknesses of the proposed reconstructions of centriolar formation and ciliogenesis are discussed, and the occurrence in other living forms of similar pathways for centriolar formation is noted. Further discussion leads to an interpretation of the centriole as a semi-autonomous organelle whose replicative capacity is separable from the characteristic triplet fibre structure of its wall.
743 citations
TL;DR: The structure of the flagella, basal bodies, and some of the associated fibre systems in three genera of complex flagellates, Trichonymspha, Pseudotrichonympha, and Holomastigotoides is described.
Abstract: This paper describes the structure of the flagella, basal bodies, and some of the associated fibre systems in three genera of complex flagellates, Trichonympha, Pseudotrichonympha, and Holomastigotoides. Three groups of longitudinal fibres occur in a flagellum: two central and nine outer fibres such as have been repeatedly described in other material, and an additional set of nine smaller secondary fibres not previously identified as such. Each central fibre shows a helical substructure; the pair of them are enveloped in a common sheath. Each outer fibre is a doublet with one subfibre bearing projections-called arms-that extend toward the adjacent outer fibre. The basal body is formed by a cylinder of nine triplet outer fibres. Two subfibres of each triplet continue into the flagellum and constitute the doublets. The third subfibre terminates at the transition of basal body to flagellum, possibly giving rise to the nine radial transitional fibres that seem to attach the end of the basal body to the surface of the organism. The central and secondary flagellar fibres are not present in the lumen of the basal body, but other complex structures occur there. The form of these intraluminal structures differs from genus to genus. The flagellar unit is highly asymmetrical. All the flagella examined have possessed the same one of the two possible enantiomorphic forms. At least two systems of fibres are associated with the basal bodies of all three genera.
666 citations
01 Jan 1961
TL;DR: The development of a multicellular organism, cell division, beginning with the egg, is the essential step toward differentiation, and the chromosomes may be viewed as a system in which the numerous genes are packaged into a small number of units.
Abstract: Publisher Summary The reproduction of biological systems may be referred to the reproduction of cells Subcellular entities such as viruses cannot maintain themselves indefinitely without parallel reproduction of the cells in which they live The asexual reproduction of organisms such as flatworms is limited by the production of new cells, and all cases of sexual reproduction imply the generation of new cells after the mixing of genetic material from the parent cells An evolutionary consequence of cellularity is implicit in the designation of multicellular organisms as higher organisms Single cells can develop a high level of variety and complexity of structure and function In the development of a multicellular organism, cell division, beginning with the egg, is the essential step toward differentiation If mitosis is a device for the distribution of sister genes to sister cells, the chromosomes may be viewed as a system in which the numerous genes are packaged into a small number of units Mitosis is accompanied by some changes in the physical state or texture of the cytoplasm, which have been assessed as viscosity changes
537 citations
TL;DR: The precise selection of cells during the various stages of anaphase has made it possible to follow chronologically the morphological features of the initiation of nuclear membrane reformation.
Abstract: With a technique of preselecting the mitotic cell in the living state for subsequent electron microscopy, it has been possible to examine the ultrastructure of the various stages of mitosis with greater precision than has been reported previously. The early dissolution of the nuclear envelope has been found to be preceded by a marked undulation of this structure within the nuclear "hof." This undulation appears to be intimately related to the spindle-forming activity of the centriole at this time. Marked pericentriolar osmiophilia and extensive arrays of vesicles are also prominent at this stage, the former continuing into anaphase. Progression of the cell through prophase is accompanied by a disappearance of these vesicles. A complex that first makes its appearance in prophase but becomes most prominent in metaphase is a partially membrane-bounded cluster of dense osmiophilic bodies. These clusters which have a circumferential distribution in the mitotic cell are shown to be derived from multivesicular bodies and are acid phosphatase-positive. The precise selection of cells during the various stages of anaphase has made it possible to follow chronologically the morphological features of the initiation of nuclear membrane reformation. The nuclear membrane appears to be derived from polar aggregates of endoplasmic reticulum, and the process begins less than 2 minutes after the onset of karyokinesis. While formation of the nuclear envelope is initiated on the polar aspects of the chromatin mass, envelope elements appear on the equatorial aspect long before the polar elements fuse. Apparently interfering with this fusion are continuous spindle tubules which traverse the chromatin mass in striking density at characteristic points. Several cortical changes, also most pronounced in anaphase, have been described, as has the kinetochore which is seen to good advantage only in this stage. The Golgi complex has been found to disappear both morphologically and histochemically during mitosis and to reappear rapidly in telophase. Evidence is presented which implicates the continuous spindle tubules in certain phases of chromosome movement.
531 citations