Ultrastructural Changes in Coconut Calli Associated with the Acquisition of Embryogenic Competence

Ultrastructural studies of 2,4-D (2,4 dichlorophenoxyaceticacid) induced coconut calli and of untreated controls enabledus to characterize early events in cellular reorganization leadingto embryogenic cell individualization and subsequent developmentinto proembryos. Embryogenic cells were characterized by specialfeatures that chiefly affected the nucleus, cytoplasm and cellwall: deep invaginations of the nuclear envelope, proliferationof dictyosomes, with emission of Golgi vesicles, directly relatedto an increase in cell wall thickness. Modification of the cellwall structure was studied and particular attention was paidto the cytolocalization of ß-1,4-glucans, and of calloseand pectin epitopes, using gold-conjugated probes. The firstchanges (detected 7–14 d after 2,4-D increase) involvedthe closure of plasmodesmata, breaking of symplastic continuity,and callose deposition. The acquisition of embryogenic competencewas linked to the appearance of an outer layer of fibrillarmaterial containing pectin epitope (mainly un-methyl-esterified),fully coating the embryogenic cells (21 d after the inductiontreatment). Some of the ultrastructural changes observed duringthe reprogramming of somatic cells towards embryogenesis canbe likened to those accompanying the maturation of female gametecells in many plant species. The possible significance of theseobservations is discussed. Copyright 2001 Annals of Botany Company Callose, cell wall structure, Cocos nucifera L., cytological events, embryogenic cells, pectin, somatic embryogenesis     

An ultrastructural stereological analysis of the myocardium in homoiothermal animals during cooling]

By methods of electron microscopy and stereology the ultrastructural cardiomyocyte reorganization of rats exposed to influence of low temperatures (exposure during 16 days at -7 degrees C) was studied. It was shown, that with this regime of cooling the disturbance of intracellular regeneration in cardiomyocytes occurred and the complex of morphological changes typical for the syndrome of regenerative-plastic deficiency was developed. The most essential changes were seen in myofibrils and mitochondria. According to the stereologic data a change in spatial reorganization of cardiomyocytes was connected largely with these organelles. With the increase in cooling duration an increase in the volume density of myofibrils and a decrease in this parameter for mitochondria were marked. As a result of these changes the mitochondria/myofibrils volume ratio was essentially decreased. As a whole, ultrastructural reorganization of cardiomyocytes under the influence of low temperatures on rats was characterized by a decrease in the ratio of the organelle volume to myofibrillar volume. The same quantitative reorganization was revealed in atrophied cardiomyocytes under the conditions of the decreased inflow of plastic substances to cells.     

An Electron Microscope Study of Intranuclear Inclusions in Mouse Liver and Hepatoma

An electron microscope study of intranuclear inclusions which occur in giant cells in a transplantable mouse hepatoma and in enlarged liver cells in mice fed a diet containing bentonite demonstrates that these inclusions are formed by invaginations of the nuclear envelope, and corroborates a previous histochemical study which revealed that the contents of the inclusions are of cytoplasmic origin. In the hepatoma cells the intranuclear inclusions are abundant, small, and situated close to the border of the nucleus, and there are wide openings from the cytoplasm into the invaginations whose contents include lipid droplets, ergastoplasm, and structurally normal mitochondria. In the enlarged liver cells the inclusions are fewer in number, generally much larger than those in the hepatoma, hence they extend deeper into the nucleus, and the interior is continuous with the cytoplasm through only a small opening. Some normal ergastoplasm is present within the inclusions but all other constituents are abnormal. Both normal and degenerating mitochondria occur in the cytoplasm but only degenerating ones are found within the inclusions. Both types of inclusions arise in greatly enlarged cells in which an attempt is made to maintain the normal nuclear surface/nuclear volume ratio by the development of the invaginations of the nuclear envelope.     

Ultrastructure of spermiogenesis in Liza aurata Risso, 1810 (Teleostei,Mugilidae)

Summary Sequential cytological events in late spermatogenesis of Liza aurata were studied from the stage secondary spermatocytes to that of mature spermatozoa. Spermiogenesis involves preparatory morphological events followed by conspicuous modifications such as intracellular movements (diplosome and mitochondrion migration, spermatid lengthening, nuclear rotation) and structural changes (dense chromatin granules, increase in size of mitochondria, loss of cytoplasm). Mature spermatozoa, with a round nucleus and a pseudo-midpiece are of a primitive type, even compared with spermatozoa of other teleosts.     

Interdigitating cells in the thymus of the frog, Rana temporaria

Interdigitating cells (IDC) of the thymic medulla of the frog, Rana temporaria, collected in the summer, were examined by electron microscopy. The most characteristic cytological features of IDC are voluminous electron-lucent cytoplasm and widespread interdigitations and invaginations of the cell membrane. IDC possess an excentrically located nucleus with pronounced nucleoli and a thin rim of a dense chromatine as well as a perinuclear area with characteristic tubulo-vesicular complex. In our material Birbeck granules were absent. Some IDC contain phagocytized material. A few transitional forms between monocytes and IDC were observed. On the basis of these observations it is highly probable that the amphibian IDC belong to the mononuclear phagocyte system.     

Anomalies of autogamy in Paramecium tetraurelia temperature sensitive mutant induced by heat treatment.

The run of autogamy in temperature sensitive mutant ts401, carrying also the mutation of trichocysts nd3a, at permissive (26 degrees C) and non-permissive temperature (35 degrees C) was studied and compared with this process in wild d4-2 stock of Paramecium tetraurelia. The effect of heat treatment on programmed nuclear and cortical events was investigated using cytological silver impregnation Fernandez-Galiano method and immunofluorescence technique with the application of two anti-alpha-tubulin antibodies. The appearance of cells with some large macronuclear fragments and an excess number of micronuclei suggests that nuclear differentiation is inhibited at restrictive temperature and that macronuclear regeneration takes place in this thermosensitive mutant. In such cells impaired oral reorganization was observed. After slight refeeding at the late autogamy, cells stopped at first postautogamous division were induced by heat treatment. The most striking feature of these abnormal divisions was the lack of a basal body duplication wave, which suggests that production of gamma-tubulin, structural protein being the essential component of the microtubule organizing centres, is disturbed at high temperature.     

Archaebacteria (Archaea) and the origin of the eukaryotic nucleus

The eukaryotic nucleus is a unique structure. Because it lacks an obvious homologue or precursor among prokaryotes, ideas about its evolutionary origin are diverse. Current attempts to derive the nuclear membrane focus on invaginations of the plasma membrane in a prokaryote, endosymbiosis of an archaebacterium within a eubacterial host, or the origin of a genuinely new membrane system following the origin of mitochondria in an archaebacterial host. Recent reports point to ways in which different ideas regarding the origin of the nucleus might someday be discriminated.     

Focal regeneration in the rat myocardium following cold injury

Summary The sequential cytological events in the myocardium of the rat were followed for 3 weeks after cold injury by light and electron microscopy. The traumatized area was initially filled with leukocytes and undifferentiated mononucleated cells and subsequently mainly with fibroblasts surrounded by collagen fibers. However, in the margins of the necrotic area repair processes of damaged myocardial cells and probably also the appearance of newly formed cells were evident. The ultrastructural features of these cells were characterized by clusters of ribosomes, numerous mitochondria that were dispersed in the cytoplasm and formation of junctional complexes and transverse tubular systems. Fibrillogenesis was also clearly evident in these cardiomyocytes. The myofibrillar material was initially dispersed in the cytoplasm and associated with clusters of ribosomes and thereafter with presumptive Z-bands and intercalated discs. The myofibrils became further organized in the shape and orientation of those of mature cells two to three weeks after injury. It is concluded that following cold injury regeneration in the mammalian myocardium takes place but is limited to the perinecrotic area. The process resembles the sequential cytological events which occur in cardiomyocytes during embryonic and postnatal development of the ventricular myocardium.     

"Dark" and "pale" cells of the amygdala complex in dynamics of the estrous cycle

Data on cytological peculiarities of dorsomedial nucleus neurones of the amygdala complex, one of the main zones of sexual dimorphism, in dynamics of estrous cycle are reported. We show that structural and functional characteristics of "dark" and "pale" cells may change depending on the concentrations of gonadal steroids in estrous and metaestrous stages. This specifies the previous hypothesis about mutual reorganization of these cells.     

Cytological Mechanism of Cytoplasmic Inheritance in Pinus tabulaeformis: II. Transmission of Male and Female Organelles During Fertilization and Proembryo Development

In an earlier report the ultrastructure and nucleoid organelles of male gamete in Pinus tabulaeformis Carr. have been described. Presently, the ultrastructure of the cytoplasm of the egg cell and pollen tube—immediately before fertilization and during cytoplasmic transmission of male gametophyte—has been described for the same species. The fate of parental plastids and mitochondria in the proembryo has also been followed. The mature egg cell contains a large amount of mitochondria, but seems to lack normal plastids. Most plastids have transformed into large inclusions. Apart from the large inclusions, there are abundant small inclusions and other organelles in the egg cell. During fertilization, pollen tube penetrates into the egg cell at the micropylar end and thereafter the contents are released. Plastid and mitochondrion of male origin are lacking near the fusing sperm-egg nuclei. The second sperm nucleus—not involved in karyogamy—remains at a site near the receptive vacuole. This nucleus is surrounded by large amount of male cytoplasm containing mixed organelles from the sperm cell, tube cell, and egg cell. At the free nuclear proembryo stage, organelles of male and female origin are visible in the perinucleus-cytoplasmic zone. Most of the mitochondria have the same morphological features as those in the egg cell. Some of the mitochondria appear to have originated from the sperm and tube cells. Plastids are most likely of male gametophyte origin because they have similar appearance as those of the sperm and tube cell. Large inclusions in the egg cell become vacuole-like. Paternal plastids have been incorporated into the neocytoplasm of the proembryo. In the cellular proembryo, maternal mitochondria are more abundant. Plastids resembling those of the sperm and tube cell are still present. These cytological results clearly show that in P. tabulaeformis , plastids are inherited paternally and mitochondria bipaternally. The cytological mechanism of plastid and mitochondrion inheritance in gymnosperm is discussed.     

Balbiani's vitelline body in the oocytes of vitellogenic and nonvitellogenic females of the domestic fowl: a correlative cytological and histochemical study.

A cytological and histochemical study has been made of Balbiani's vitelline body in the oocytes of nonvitellogenic and vitellogenic females of the domestic fowl (Gallus domesticus), which consists of yolk nucleus, mitochondria, Golgi bodies and lipid bodies of diverse nature. The yolk nucleus consisting of RNA, protein and lipoprotein shows the greatest development and morphological complexity in the growing oocytes of vitellogenic females. Its possible significance has been discussed in relation, to the variable metabolism and hormonal status of the female during sexual maturation.     

Actin-filled nuclear invaginations indicate degree of cell de-differentiation

For years the existence of nuclear actin has been heavily debated, but recent data have clearly demonstrated that actin, as well as actin-binding proteins (ABPs), are located in the nucleus. We examined live EGFP-actin-expressing cells using confocal microscopy and saw the presence of structures strongly resembling actin filaments in the nuclei of MDA-MB-231 human mammary epithelial tumor cells. Many nuclei had more than one of these filamentous structures, some of which appeared to cross the entire nucleus. Extensive analysis, including fluorescence recovery after photobleaching (FRAP), showed that all EGFP-actin in the nucleus is monomeric (G-actin) rather than filamentous (F-actin) and that the apparent filaments seen in the nucleus are invaginations of cytoplasmic monomeric actin. Immunolocalization of nuclear pore complex proteins shows that similar invaginations are seen in cells that are not overexpressing EGFP-actin. To determine whether there is a correlation between increased levels of invagination in the cell nuclei and the state of de-differentiation of the cell, we examined a variety of cell types, including live Xenopus embryonic cells. Cells that were highly de-differentiated, or cancerous, had an increased incidence of invagination, while cells that were differentiated had few nuclear invaginations. The nuclei of embryonic cells that were not yet differentiated underwent multiple shape changes throughout interphase, and demonstrated numerous transient invaginations of varying sizes and shapes. Although the function of these actin-filled invaginations remains speculative, their presence correlates with cells that have increased levels of nuclear activity.     

Cytological mechanisms of gynogenesis and sperm incorporation in unreduced diploid eggs of the clonal loach, Misgurnus anguillicaudatus (Teleostei: Cobitidae)

The loach Misgurnus anguillicaudatus comprises diploid clonal, triploid and diploid-triploid mosaic individuals in a wild population on Hokkaido island, Japan. When diploid eggs of clonal loaches are fertilized by haploid sperm of normal bisexual loaches, both diploid clonal and non-diploid aclonal individuals occur in the progeny. Flow cytometry and microsatellite analyses revealed that the occurrence of triploid, diploid-triploid and other progeny was essentially due to the genetic incorporation of sperm to diploid clonal genomes of unreduced eggs. In this study, we examined the influence of water temperature from fertilization to early embryogenesis on frequencies of diploid clonal and other progeny and observed that progeny of three out of four clonal females examined exhibited approximately constant rates of diploid clonal individuals (54.2-68.9%) at hatching stage. Thus, no drastic increase of non-diploid progeny was detected. However, the 28 degrees C group of the fourth clonal female gave significantly lower rate (28.1%) of diploid clonal progeny, suggesting that this temperature might be a critical or a borderline temperature inducing sperm incorporation. We also examined the cytological process by which diploid clonal and other aclonal progeny develop after fertilization. In some fertilized eggs, the sperm nucleus remained condensed throughout fertilization and early embryogenesis and never fused with the female pronucleus. This cytological observation concludes that clonal eggs develop by the mechanism of gynogenesis. However, some other eggs showed the cytological process of syngamy between the female pronucleus and an accidentally formed male nucleus, suggesting the formation of triploid progeny. The syngamy between an accidentally activated sperm nucleus with a male pronucleus-like structure and nucleus of a blastomere of gynogenetically developing clonal diploid embryo might produce a diploid-triploid mosaic individual.     

Variation of ultrastructural organization of the mitochondria of pea cells in vitro

Unusual huge organelles together with mitochondria of typical organization were found in a part of population of callus cells of pea. They like mitochondria are surrounded by two biologic membranes, the inner one has invaginations. Inclusions in the matrix are a specific character of structural organization of these unusual mitochondria. These inclusions look as electronically dense bars that extend parallel to longitudinal axis of organelle and consist of fibril-like elements. Organization of inclusions and cristae as well as a shape of organelles are changing during cell development. Possible nature of the unusual mitochondria is discussed.     

Plant nuclei can contain extensive grooves and invaginations

Plant cells can exhibit highly complex nuclear organization. Through dye-labeling experiments in untransformed onion epidermal and tobacco culture cells and through the expression of green fluorescent protein targeted to either the nucleus or the lumen of the endoplasmic reticulum/nuclear envelope in these cells, we have visualized deep grooves and invaginations into the large nuclei of these cells. In onion, these structures, which are similar to invaginations seen in some animal cells, form tubular or planelike infoldings of the nuclear envelope. Both grooves and invaginations are stable structures, and both have cytoplasmic cores containing actin bundles that can support cytoplasmic streaming. In dividing tobacco cells, invaginations seem to form during cell division, possibly from strands of the endoplasmic reticulum trapped in the reforming nucleus. The substantial increase in nuclear surface area resulting from these grooves and invaginations, their apparent preference for association with nucleoli, and the presence in them of actin bundles that support vesicle motility suggest that the structures might function both in mRNA export from the nucleus and in protein import from the cytoplasm to the nucleus.     

Spatial reorganization of the organelles of catecholamine-synthesizing group A1 neurons during emotional stress

Dynamics of spatial reorganization of organellas in catecholamine-synthesizing neurons of A1 group, that participate in reactions of the cardiovascular control, have been studied under normal conditions and under stimulation of the ventromedial nucleus in the hypothalamus. During transition of the hypertensive reaction into the hypotensive one, concentration of various organellas in the Golgi complex area and in the perinuclear zone is intensively increasing. Deep mutual nucleocytoplasmic invaginations appear. The spatial reorganization of the organellas in the neuron demonstrates that some new integrations take place in its metabolic system and reflects its another qualitative state. This serves as an impulse for structural adaptive modifications of the synaptic apparatus. There is a reason to suppose that one of the ways for organization of the alive system stability to an emotional stress is the way of reorganization of the connections, creation of another physiological architecture of the functional system, that can ensure the adaptive reactions of the organism to emotional overstrains.     

Cytological Events during Zygote Formation of the Fern Ceratopteris thalictroides

The cytological events, including nuclear fusion, digestion of male organelles and rebuilding of the plasmalemma and cell wall, during zygote formation of the fern Ceratopteris thalictroides (L.) Brongn. are described based on the observations of transmission electron microscopy. When the spermatozoid enters the egg and contacts the cytoplasm, the male chromatin relaxes continually. The microtubular ribbon (MTr) is separated from the male nucleus and then an envelope reappears around the male nucleus. During nuclear fusion, the egg nucleus becomes highly irregular and extends some nuclear protrusions. It is proposed that the protrusions fuse with the male nucleus actively. After nuclear fusion the irregular zygotic nucleus contracts gradually. It becomes spherical before the zygote divides. The male chromatin is identifiable as fibrous structure in the zygotic nucleus in the beginning, but it gradually becomes diffused completely. The male organelles, including the MTr, multilayered structure, flagella and the male mitochondria are finally digested in the zygotic cytoplasm. Finally a new plasmalemma and cell wall are formed outside the protoplast. The organelles in the zygote are rearranged, which produces a horizontal polarity zygote. The zygote divides with an oblique-vertical cell plate facing the apical notch of the gametophyte.     

Studies on the transition of the cristal membrane from the orthodox to the aggregated configuration. I: Topology of bovine adrenal cortex mitochondria in the orthodox configuration

Bovine adrenal cortex mitochondria examined by electron microscopyin situ orin vitro in 0·25 M sucrose have an unusual cristal membrane structure. The cristae usually appear as unconnected vesicles within a double membrane system. A few of the vesicles appear to be attached to the inner boundary membrane or to one or more other vesicles. The configuration of such mitochondria will be defined as the orthodox configuration. In this communication we will provide evidence that the inner membrane is not composed of multiple vesicles, but is one continuous membrane with tubular invaginations, and that these invaginations alternately are ballooned out and squeezed down. A mechanism has been proposed to account for the differentiated structure of the cristae of adrenal cortex mitochondria.     

Physiological Reorganization and Post-Traumatic Regeneration in Stylonychia mytilus: Reflections on Developmental Constraints and the Evolutionary Origin of Alternative Modes of Asexual Morphogenesis

We investigated development of cortical ciliature in Stylonychia mytilus during starvation-induced physiological reorganization, and during regeneration following amputation of the anterior part of the cell. Cortical reorganization in the two processes is generally similar. The posterior part of the adoral zone of membranelles is resorbed and replaced with newly assembled membranelles. The pre-existing set of ventral cirri and dorsal bristles is entirely resorbed and replaced with new ones. Regenerants exhibit posterior displacement of the frontal-ventral-transverse cirri primordium and the undulating membrane primordium, and recruit basal bodies from ectopic locations for the development of these ciliature. This illustrates flexibility in the initiation site of ciliary primordia, and opportunism in utilizing building blocks. Such morphogenetic versatility of hypotrichs provides the basis for the operation of a global control of pattern formation, which governs cortical reorganization in dividers, and additionally, in the absence of the prerequisites for binary fission, alternative modes of cortical development such as physiological reorganization or regeneration. These considerations suggest that the three processes are homologous and that physiological reorganization and regeneration have evolved from binary fission. In physiological reorganization and regeneration, the micro- and macronuclei reorganize to resemble that in binary fission; these nuclear events are considered evolutionary relics of the nuclear development of binary fission. Tetrahymena also exhibits such morphogenetic flexibility; stomatogenesis is under global control, so that asexual cells can replace its oral apparatus without undergoing binary fission. Paramecium , on the other hand, adopts a more rigid strategy in relying heavily on pre-existing structures for morphogenetic cues; this could have imposed constraints in the exploration of alternative modes of asexual development.