Developmental cytology of the genus Vaucheria I. Organisation of the vegetative filament

The developmental cytology of the vegetative filament of Vaucheria has been investigated with light and electron microscopes. The vegetative filament consists of three distinct zones: the apical zone, the sub-apical zone and the zone of vacuolation. Organelle distribution and associations in these zones have a primary role in controlling morphogenetic events which influence growth and induce differentiation of sexual and asexual organs. Cell elongation in the polarised vegetative filament occurs by vesicular addition in the apical zone. Two types of active cyclosis are found exclusively in the zone of vacuolation. Nuclear cyclosis involves microtubular bands which associate with persistent centrioles found adjacent to the outer nuclear membrane. A specialised mitochondrial-dictyosome association is transported by a microfilament-like system of anastomosing strands. This latter system is extremely labile to conventional fixatives.     

CYTOLOGICAL BASIS FOR CYTOPLASMIC INHERITANCE IN PSEUDOTSUGA MENZIESII. II. FERTILIZATION AND PROEMBRYO DEVELOPMENT

Douglas fir (Pseudotsuga menziesii [Mirb.] Franco) ovules were used to study male gamete formation, insemination of the egg, and free nuclear and cellular proembryo development. Two male nuclei form as the pollen tube either reaches the megaspore wall or as it enters the archegonial chamber. No cell wall separates them. They are contained within the body-cell cytoplasm. A narrow extension of the pollen tube separates the neck cells and penetrates the ventral canal cell. The pollen tube then releases its contents into the egg cytoplasm. The two male gametes and a cluster of paternal organelles (plastids and mitochondria) migrate within the remains of the body-cell cytoplasm toward the egg nucleus. Microtubules are associated with this complex. The leading male gamete fuses with the egg nucleus. The zygote nucleus undergoes free nuclear division, but the cluster of paternal organelles remains discrete. Free nuclei, paternal and maternal nucleoplasm, maternal perinuclear cytoplasm, and the cluster of paternal organelles migrate en masse to the chalazal end of the archegonium. There, paternal and maternal organelles intermingle to form the neocytoplasm, the nuclei divide, and a 12-cell proembryo is formed. The importance of male nuclei or cells, the perinuclear zone, and large inclusions in cytoplasmic inheritance are discussed in the Pinaceae and in other conifer families. This completes a two-part study to determine the fate of paternal and maternal plastids and mitochondria during gamete formation, fertilization, and proembryo development in Douglas fir.     

Development and cellular differentiation of an insect telotrophic ovary (Rhodnius prolixus)

Differentiation events accompanying the larval-adult ovarian transformation in Rhodnius prolixus can be divided into three phases: proliferative phase (unfed to 3 days post-feed or DPF), early differentiation phase (9–15 DPF) and late differentiation phase (16 DPF to moult at 21 DPF). Ovarioles remain morphologically larval until feeding initiates development. The unfed ovariole contains germ cells surrounding a central trophic core region with the ‘germarial lumen’ occupying the basal region of the tropharium immediately above the pre-follicular tissue. Mitosis of germ cells during the proliferative phase results in a progressive increase in tropharial size with no differentiation of tissues. Regional specialization within the ovariole marks the beginning of the early differentiation phase. A zone of oocytes is established at the base of the tropharium with nuclei containing synaptonemal complexes and condensing chromosomes. Nurse cell differentiation is characterized by nucleolar elaboration and nucleo-cytoplasmic transport, the cytoplasm becoming rich in ribosomes. Autoradiographic results suggest that functional nurse cell-oocyte divergence occurs concurrently with morphological divergence. Pre-follicular tissue is divided into apical and basal zones with apical zone differentiation occurring during early and late differentiation phases.     

红毛菜丝状体核分裂研究

选择具异型世代交替的福建人工栽培的红毛菜为研究材料,对红毛菜丝状体世代的丝状藻丝及孢子囊枝等阶段进行了较系统的核分裂观察研究,探讨红毛菜核分裂特征.结果显示:红毛菜营养藻丝和孢子囊枝细胞均为二倍体细胞,2n=8,其核分裂显示为有丝分裂的过程;同时,丝状体阶段细胞核分裂至前期末均会出现同源染色体配对现象,显示有丝分裂同源染色体配对行为是红毛菜丝状体核分裂的一个重要特征.     

NUCLEAR BEHAVIOR IN THE BASIDIOMYCETE,VOLVARIELLA VOLVACEA

The basidiospores of the straw mushroom are typically uninucleate and its vegetative hyphae are generally multinucleate. There is a marked reduction of nuclear number in the trama and subhymenium. Interphase nuclei exist in two forms, each of which undertakes a particular mode of division. The “diffused” nuclei divide by conventional mitosis while the “constricted” ones divide amitotically. In metaphase of mitosis nine chromosomes were seen both in polar and lateral view. This haploid number confirms the nine bivalents found in basidia during meiosis. A unique characteristic of this fungus is that the diploid nucleus, the two postkaryotic nuclei and the four postkaryotic nuclei may be enclosed by a well-defined nuclear envelope during division.     

Immunofluorescence microscopy of the microtubule cytoskeleton during conjugate division in the dikaryon Pleurotus ostreatus N001

Fluorescence microscopy was used to describe the distribution of nuclei and the organization of the microtubule network in hyphae of Pleurotus ostreatus. Dikaryotic hyphae of P. ostreatus N001 grow by tip extension with two closely spaced nuclei moving slowly forward with the growing hyphal tip. During vegetative growth of the hyphae, cytoplasmic microtubules are found as long filaments oriented longitudinally within fungal hyphae. When the apical cell reaches a length of approximately 150 μm, the two nuclei divide synchronously. Mitosis occurs in association with clamp connection formation, with one of the nuclei dividing in the hook of the developing clamp connection and the other in the main hypha. After mitosis, two daughter nuclei move forward to approximately the center of the apical cell, while the other two move backward to a central position in the subapical cell. Two septa are formed, one in the clamp and the other across the main axis of the hypha to delimit the apical cell. The use of fluorescence microscopy made it possible to examine the changes in the cytoplasmic microtubules, the configuration of the mitotic apparatus, the site of septation and the post-mitotic nuclear migrations during conjugate division in P. ostreatus dikaryotic hyphae.     

CELL DIVISION STUDIES OF THE SHOOT APEX OF DATURA STRAMONIUM DURING TRANSITION TO FLOWERING

Seedlings of Datura stramonium L., although not photoperiodically sensitive, are useful for floral transition studies when raised in a growth chamber at a constant temperature of 25 C with a photoperiod of 8 hr of light (1,600-2,000 ft-c) and 16 hr of darkness. A terminal flower is formed after the seventh or eighth leaf primordium is produced. A constant rate of leaf initiation up to the time of flowering enables specific apical stages to be obtained and studied. Changes in the mitotic index, substantiated with calculated rates of cell division (measured by the accumulation of metaphases following treatment with colchicine) were studied in shoot apical zones during transition to flowering. Fluctuations in the mitotic index of each zone in the vegetative and transition apex with respect to apical stage as well as time of day were not statistically significant. The mitotic index of the summit zone of the vegetative apex was significantly lower than in the other zones whose mitotic indices were not significantly different from one another. During floral transition the mitotic index of the summit zone as well as the central zone (just below the summit zone) significantly increased while no significant changes were detected in the flank zones. It was shown that the mitotic index could be considered representative of the rates of cell division in Datura.     

hyp loci control cell pattern formation in the vegetative mycelium of Aspergillus nidulans.

Aspergillus nidulans grows by apical extension of multinucleate cells called hyphae that are subdivided by the insertion of crosswalls called septa. Apical cells vary in length and number of nuclei, whereas subapical cells are typically 40 microm long with three to four nuclei. Apical cells have active mitotic cycles, whereas subapical cells are arrested for growth and mitosis until branch formation reinitiates tip growth and nuclear divisions. This multicellular growth pattern requires coordination between localized growth, nuclear division, and septation. We searched a temperature-sensitive mutant collection for strains with conditional defects in growth patterning and identified six mutants (designated hyp for hypercellular). The identified hyp mutations are nonlethal, recessive defects in five unlinked genes (hypA-hypE). Phenotypic analyses showed that these hyp mutants have aberrant patterns of septation and show defects in polarity establishment and tip growth, but they have normal nuclear division cycles and can complete the asexual growth cycle at restrictive temperature. Temperature shift analysis revealed that hypD and hypE play general roles in hyphal morphogenesis, since inactivation of these genes resulted in a general widening of apical and subapical cells. Interestingly, loss of hypA or hypB function lead to a cessation of apical cell growth but activated isotropic growth and mitosis in subapical cells. The inferred functions of hypA and hypB suggest a mechanism for coordinating apical growth, subapical cell arrest, and mitosis in A. nidulans.     

Structure of ovarioles in adult queens and workers of the common wasp, Vespula germanica (Hymenoptera: Vespidae)

The ovaries of the common wasp, Vespula germanica are polytrophic-meroistic and consist of 2-3 (workers) or 7 (queens) ovarioles. The ovarioles are differentiated into three regions: a terminal filament, a germarium, and a vitellarium. The germaria of both castes consist of two zones: an anterior zone of germ-cell cluster formation and a posterior one of germ-cell cluster differentiation. The vitellaria comprise 4-6 (workers) or 7-10 (queens) ovarian follicles (egg chambers). Each chamber consists of an oocyte and about 60 isodiametric nurse cells (trophocytes). The egg chambers have been arbitrarily classified into four developmental categories: early and late previtellogenic, vitellogenic, and choriogenic. The process of oogenesis in workers proceeds only up to the onset of the late previtellogenesis. Neither vitellogenic nor choriogenic egg chambers were observed in this caste. During early and late previtellogenesis the envelope of the oocyte nucleus proliferates and becomes highly folded. This process leads to the formation of characteristic organelles, termed accessory nuclei (AN). Although AN arise in the oocytes of both queens and workers, their number in the latter caste is always considerably lower. At the onset of the late previtellogenesis AN start to migrate towards the periphery of the oocyte where they reside till the end of oogenesis. The physiological state of the worker ovaries is discussed in the light of the presented results.     

The dissociation of nuclear and centrosomal division in gnu, a mutation causing giant nuclei in Drosophila

We describe a recessive, maternal-effect lethal mutation of Drosophila, gnu. gnu uncouples nuclear division from many cytoplasmic events of mitosis in the Drosophila embryo. Embryos from homozygous females are defective in nuclear division, but not in DNA replication, and therefore develop a small number of giant nuclei. Centrosomes divide independently of nuclear division and migrate to the surface of the syncytial blastoderm. There they nucleate microtubules into asters, which appear normal at first but become very large. Only later, when the giant nuclei begin to break down, are spindles sometimes formed. The cortical actin of these embryos develops into a characteristic network.     

Nuclear cytoplasmic domains,microtubules and organelles in microsporocytes of the slipper orchid Cypripedium californicum A. Gray dividing by simultaneous cytokinesis

Microsporocytes of the slipper orchidCypripedium californicum A. Gray divide simultaneously after second meiosis. The organization and apportionment of the cytoplasm throughout meiosis are functions of nuclear-based radial microtubule systems (RMSs) that define domains of cytoplasm - a single sporocyte domain before meiosis, dyad domains within the undivided cytoplasm after first meiosis, and four spore domains after second meiosis. Organelles migrate to the interface of dyad domains in the undivided cytoplasm after first meiotic division, and second meiotic division takes place simultaneously on both sides of the equatorial organelle band. Microtubules emanating from the telophase II nuclei interact to form columnar arrrays that interconnect all four nuclei, non-sister as well as sister. Cell plates are initiated in these columns of microtubules and expand centrifugally along the interface of opposing RMSs, coalescing in the center of the sporocyte and joining with the original sporocyte wall at the periphery to form the tetrad of microspores. Organelles are distributed into the spore domains in conjunction with RMSs. These data, demonstrating that cytokinesis in microsporogenesis can occur in the absence of both components of the typical cytokinetic apparatus (the preprophase band of microtubules which predicts the division site and the phragmoplast which controls cell-plate deposition), suggest that plant nuclei have an inherent ability to establish a domain of cytoplasm via radial microtubule systems and to regulate wall deposition independently of the more complex cytokinetic apparatus of vegetative cells.     

Real-time lineage analysis reveals oriented cell divisions associated with morphogenesis at the shoot apex of Arabidopsis thaliana

Precise knowledge of spatial and temporal patterns of cell division, including number and orientation of divisions, and knowledge of cell expansion, is central to understanding morphogenesis. Our current knowledge of cell division patterns during plant and animal morphogenesis is largely deduced from analysis of clonal shapes and sizes. But such an analysis can reveal only the number, not the orientation or exact rate, of cell divisions. In this study, we have analyzed growth in real time by monitoring individual cell divisions in the shoot apical meristems (SAMs) of Arabidopsis thaliana. The live imaging technique has led to the development of a spatial and temporal map of cell division patterns. We have integrated cell behavior over time to visualize growth. Our analysis reveals temporal variation in mitotic activity and the cell division is coordinated across clonally distinct layers of cells. Temporal variation in mitotic activity is not correlated to the estimated plastochron length and diurnal rhythms. Cell division rates vary across the SAM surface. Cells in the peripheral zone (PZ) divide at a faster rate than in the central zone (CZ). Cell division rates in the CZ are relatively heterogeneous when compared with PZ cells. We have analyzed the cell behavior associated with flower primordium development starting from a stage at which the future flower comprises four cells in the L1 epidermal layer. Primordium development is a sequential process linked to distinct cellular behavior. Oriented cell divisions, in primordial progenitors and in cells located proximal to them, are associated with initial primordial outgrowth. The oriented cell divisions are followed by a rapid burst of cell expansion and cell division, which transforms a flower primordium into a three-dimensional flower bud. Distinct lack of cell expansion is seen in a narrow band of cells, which forms the boundary region between developing flower bud and the SAM. We discuss these results in the context of SAM morphogenesis.     

NUCLEAR STRUCTURE AND DIVISION IN THE VEGETATIVE MYCELIUM OF THE SAPROLEGNIACEAE

Bakerspigel , Alexander . (The New Mount Sinai Hospital, Toronto.) Nuclear structure and division in the vegetative mycelium of the Saprolegniaceae. Amer. Jour. Bot. 47(2): 94—100. Illus. 1960.–Saprolegnia parasitica, S. ferax and Achlya racemosa are additional examples of fungi in whose vegetative mycelium the nuclei do not divide in a manner directly comparable to ordinary mitosis. During division the entire nucleus, which consists of a crescent, ring or cap of chromatin and the nucleolus, becomes angular and elongates. The chromatin separates into 2 portions which are situated at opposite ends of the elongating nucleus. As division proceeds the elongated nucleus constricts at the midregion and the sister nuclei separate. Each sister nucleus consists of a crescent of chromatin and a portion of the divided nucleolus. Spindles, chromosomal filaments or metaphase plates could not be demonstrated during division.     

PHYSIOLOGICAL AND PHOTOSYNTHETIC CHANGES DURING THE FORMATION OF RED APLANOSPORES IN THE CHILOROPHYTE HAEMATOCOCCUS PLUVIALIS1

Changes in physiological and photosynthetic parameters were followed in the freshwater chorophyte Haematococcus pluvialis Flotow (Volvocales) during the transformation of green vegetative cells to red aplanospores.Formation of aplanospores was induced by exposure to a nitrogen-deficient medium. In spite of an increase in cellular volume (from 6.6 to 41 pL) and amassive accumulation of astaxanthin, chlorophyll content of the mature aplanospore decreased only slightly (from 16 to 14.8 pg'cell^?1)as compared to the vegetative cell. Aplanospore formation was characterized by a gradual reduction in the maximal photosynthetic rate and increases in the photosynthetic quantum requirement and minimal turnover time for photosynthetic O₂ evolution.Respiration rate increased (4.2 times)and excretion rate decreased (up to 8.8 times) during aplanospore formation. Measurements of photosynthetic unit “size” and estimation of the cellular content of photosystem II reaction centers suggest that the photosynthetic complex remains relatively centers stable during the formation process and in the mature aplanospore.A functional relationship between the describe changes in the physiology of the cells and their photosynthetic parameters is proposed.     

Observations On Nuclear Division in vegetative hyphae of aquatic fungi

Summary Achlya aquatica, A. prolifera andA. racemosa are additional examples of fungi in whose vegetative mycelium, the nuclei do not divide in a manner directly comparable to ordinary mitosis. During division the entire nucleus, which consists of variously shaped chromatin and the nucleolus, becomes angular and elongates. The chromatin separates into two portions, which are situated at opposite ends of the elongating nucleus. As division proceeds the nuclei separate. Each sister nucleus sonsists of chromatin and a portion of the divided nucleolus. Spindless, chromosomal filaments or metaphase plates could not be observed during division.     

Study of the behavior of the vegetative meristem of alfa (Stipa tenacissima L.). Cytological and histological approaches

The biological and cytological studies of the vegetative meristem of Stipa tenacissima L. gave clear indication about its structure. It was similar to what was previously described in several species. This meristem showed an axial apical zone constituted by sommital cells of both tunica and corpus, a sub-apical lateral zone, very chromophilous, representing the initial ring and a medullar meristem. The cytofluorimetric determination of DNA in interphasic nuclei of these three zones revealed that the nuclei of the apical and lateral zones were in S phase, announcing the beginning of mitosis and meaning that these zones were the centers of the foliar initiation. The medullar meristem was in dormancy: all the nuclei were in G1 phase.     

Single-cell gene profiling defines differential progenitor subclasses in mammalian neurogenesis

Cellular diversity of the brain is largely attributed to the spatial and temporal heterogeneity of progenitor cells. In mammalian cerebral development, it has been difficult to determine how heterogeneous the neural progenitor cells are, owing to dynamic changes in their nuclear position and gene expression. To address this issue, we systematically analyzed the cDNA profiles of a large number of single progenitor cells at the mid-embryonic stage in mouse. By cluster analysis and in situ hybridization, we have identified a set of genes that distinguishes between the apical and basal progenitors. Despite their relatively homogeneous global gene expression profiles, the apical progenitors exhibit highly variable expression patterns of Notch signaling components, raising the possibility that this causes the heterogeneous division patterns of these cells. Furthermore, we successfully captured the nascent state of basal progenitor cells. These cells are generated shortly after birth from the division of the apical progenitors, and show strong expression of the major Notch ligand delta-like 1, which soon fades away as the cells migrate in the ventricular zone. We also demonstrated that attenuation of Notch signals immediately induces differentiation of apical progenitors into nascent basal progenitors. Thus, a Notch-dependent feedback loop is likely to be in operation to maintain both progenitor populations.     

CELL POPULATION KINETICS OF AN OSTEOGENIC TISSUE · II

A study of the cell kinetics on the actively growing periosteal surface of the femur of rabbits aged 2 weeks has been continued. A single injection of tritiated thymidine was given and the rabbits killed from 1 hour to 4 days after injection. The grain count spectra of the different cell types, pre-osteoblast, osteoblast, and osteocyte, have been compared at different times after injection. The results showed evidence for the uptake of thymidine in nuclei which is not associated with cell division. A small percentage of osteoblasts was initially labeled at 1 hour and there was evidence that the majority of these had not divided by 3 or 4 days after injection. Some thymidine-labeled cells had also become osteocytes without division. Furthermore, it appeared that a considerable fraction of the initially labeled pre-osteoblasts did not divide. The S period for the pre-osteoblasts and osteoblasts was measured using a double-labeled thymidine technique.     

Cytoskeleton and morphogenesis in brown algae

BACKGROUND: Morphogenesis on a cellular level includes processes in which cytoskeleton and cell wall expansion are strongly involved. In brown algal zygotes, microtubules (MTs) and actin filaments (AFs) participate in polarity axis fixation, cell division and tip growth. Brown algal vegetative cells lack a cortical MT cytoskeleton, and are characterized by centriole-bearing centrosomes, which function as microtubule organizing centres. SCOPE: Extensive electron microscope and immunofluorescence studies of MT organization in different types of brown algal cells have shown that MTs constitute a major cytoskeletal component, indispensable for cell morphogenesis. Apart from participating in mitosis and cytokinesis, they are also involved in the expression and maintenance of polarity of particular cell types. Disruption of MTs after Nocodazole treatment inhibits cell growth, causing bulging and/or bending of apical cells, thickening of the tip cell wall, and affecting the nuclear positioning. Staining of F-actin using Rhodamine-Phalloidin, revealed a rich network consisting of perinuclear, endoplasmic and cortical AFs. AFs participate in mitosis by the organization of an F-actin spindle and in cytokinesis by an F-actin disc. They are also involved in the maintenance of polarity of apical cells, as well as in lateral branch initiation. The cortical system of AFs was found related to the orientation of cellulose microfibrils (MFs), and therefore to cell wall morphogenesis. This is expressed by the coincidence in the orientation between cortical AFs and the depositing MFs. Treatment with cytochalasin B inhibits mitosis and cytokinesis, as well as tip growth of apical cells, and causes abnormal deposition of MFs. CONCLUSIONS: Both the cytoskeletal elements studied so far, i.e. MTs and AFs are implicated in brown algal cell morphogenesis, expressed in their relationship with cell wall morphogenesis, polarization, spindle organization and cytokinetic mechanism. The novelty is the role of AFs and their possible co-operation with MTs.