DEVELOPMENT OF THE EMBRYONIC CHICKEN THYMUS III. UNEQUAL DIVISION OF LYMPHOID CELLS

Cell division of thymus lymphoid cells from 11- to 17-day old embryonic chickens, as well as chickens just after hatch was investigated on cell smears stained with Giemsa. Unequally dividing cells were observed in the developmental stage of thymocytes. At the telophase of such cells, the cytoplasm of one of two future daughter cells was apparently larger in amount and was sometimes stained deeper than the cytoplasm of its counterpart. Unequal division was also observed in pro-, meta- and anaphase; sometimes a dividing cell had a large cytoplasmic process belonging to one hemisphere, suggesting that only one of the two daughter cells would receive the cytoplasmic process through cell division.
The incidence of unequal division calculated by a rough estimation was around 10% of the total cell division between 11 and 13 days of embryonic development, and decreased progressively thereafter.     

THE ULTRASTRUCTURE OF PORPHYRIDIUM CRUENTUM

An electron microscopic examination of Porphyridium cruentum revealed the presence of mitochondria which had been reported absent in this aerobic organism. The chloroplast in this red alga was found to contain small granules (about 320 A) regularly arranged along the parallel chloroplast lamellae. The chloroplast granules differ in size and staining intensity from the ribosomes located in the cytoplasm. Two tubular elements are described. One type (450 to 550 A) is associated with the Golgi bodies. Another type (350 A), in the cell periphery, is believed to connect the endoplasmic reticulum and the cell membrane. Daughter nuclei were found to be positioned at opposite ends of the cell prior to commencement of cell division. Cytokinesis is accomplished by an annular median constriction causing the gradual separation of the chloroplast, pyrenoid, and other cell organelles, resulting in two equal daughter cells. No appreciable differences were observed between cells grown in high light (400 ft-c) and low light (40 ft-c). Structural differences between young and old cells were compared.     

TAXONOMIC STUDY OF BIGELOWIELLA LONGIFILA SP. NOV. (CHLORARACHNIOPHYTA) AND A TIME‐LAPSE VIDEO OBSERVATION OF THE UNIQUE MIGRATION OF AMOEBOID CELLS1

A new species of a chlorarachniophyte alga, Bigelowiella longifila sp. nov., is described. It is classified as a member of Bigelowiella as flagellate cells constitute the main stage of the life cycle. However, this alga is different from the only described species of the genus, B. natans Moestrup, in having a unique amoeboid stage in the life cycle. We observed an interesting behavior of amoeboid daughter cells after cell division: One of the two daughter cells inherits the long filopodium of the parental cell, and it subsequently transports its cell contents through the filopodium to develop at its opposite end. The other daughter cell forms a new filopodium. This unequal behavior of daughter cells may have evolved before the chlorarachniophytes and some colorless cercozoans diverged.     

Unequal cell division and chromatin differentiation in pollen grain cells

Pinus pollen grains, normally developing, were subjected to centrifugal force, low temperature and caffeine solution. In the former two treatments, daughter cells with some abnormal directions of division, abnormal volume and chromatin dispersion were induced in pollen grains treated. Regardless of the direction of division, of the two daughter cells produced by the unequal division, the larger one contained strongly dispersed chromatin and the smaller one weakly dispersed chromatin. In the two daughter cells produced by approximately equal division, the chromatin was dispersed strongly to a similar degree, and by halfway unequal division, chromatin in the larger cell was dispersed strongly and in the smaller one intermediately. Chromatin in bi-nucleate cells induced by caffeine treatment was dispersed strongly to an identical degree. It is suggested that for the occurrence of heteronomous chromatin configuration in natural pollen grains the unequal cell division was indispensable, although the axis of division didn't directly contribute. After both the treatments of centrifugation and low temperature during microspore and embryonal cell divisions, the affected daughter cells divided in terms of the certain fixed axis of division and chromatin dispersion, instead of exhibiting abnormal development.     

RELATIONSHIP BETWEEN NUCLEO-CYTOPLASMIC RATIO AND FINAL CELL VOLUME IN MICRASTERIAS CRUX-MELITENSIS (DESMIDIACEAE,CHLOROPHYTA)1

In Micrasterias crux-melitensis (Ehrbg.) Hass., small parental half-cells produced daughter half-cells larger than themselves. As the volume of the parental half-cells decreased, the volume ratio of daughter to parental half-cells increased; the larger the nucleo-cytoplasmic ratio of parental half-cells, the larger the volume of the daughter half-cells. Small daughter half-cells grew larger than expected when cytoplasm of other cells was incorporated. The number of dictyosomes was almost the same for cells of different volumes. Results obtained seem to suggest that the volume of daughter cells is determined by the balance between the gradient of mRNA and the quantity of cytoplasm.     

DEVELOPMENT OF STOMATA IN SELAGINELLA: DIVISION POLARITY AND PLASTID MOVEMENTS

The young guard cell of Selaginella inherits a single plastid from the division of the stomatal guard mother cell (GMC). During early stomatal development the single plastid undergoes a complex series of migrations and divisions. The regular pattern of plastid behavior appears to be an expression of the genetic program controlling division plane and cytomorphogenesis. The plastid in the GMC becomes precisely aligned with its midconstriction intersected by the plane of a preprophase band of microtubules (PPB) oriented parallel to the long axis of the leaf. This alignment with respect to the future division plane of the cytoplasm ensures equal plastid distribution to the daughter cells. Cytokinesis occurs in the plane previously marked by the PPB and the plastid in each daughter cell lies between the lateral wall and the newly formed nucleus. Following cytokinesis the plastid in each young guard cell develops a median constriction and migrates to the common ventral wall where the isthmus is associated with a system of microtubules in the vicinity of the developing pore region. Plastid division is completed while the plastid is adjacent to the common ventral wall. Following division, the two daughter plastids move back toward the lateral wall. Each plastid may divide again during guard cell maturation but no further migrations occur.     

TWO TYPES OF CYTOPLASMIC CLEAVAGE IN THE COENOCYTIC SOIL ALGA PROTOSIPHON BOTRYOIDES (CHLOROPHYCEAE)1

Cytokinesis in the coenocytic green alga Protosiphon botryoides (Kütz.) Klebs was studied with transmission electron microscopy. In vegetative cells, nuclei with associated basal bodies and dictyosomes are scattered throughout the cytoplasm. Mature cells may develop either multinucleate resting spores (coenocysts) or uninucleate zoospores. Cytokinesis may be preceded by contraction of the protoplast due to the disintegration of vacuoles that are present in larger, siphonous cells. The formation of coenocysts in ageing, siphonous cells, is signalled by cleavage of the chloroplast and the development of arrays of phycoplast microtubules in one or more transversely oriented planes through the cell. Nuclei with associated basal apparatuses stay dispersed throughout the cytoplasm; the basal bodies apparently are not involved in organization of the phycoplast. The plasma membrane invaginates, resulting in a centripetal cleavage of the protoplast into two or more multinucleate daughter protoplasts. Simultaneously, wall material is deposited along the outside of the daughter protoplasts by dictyosome-derived vesicles, and finally two or more thick-walled coenocysts are formed. The formation of zoospores, on the other hand, is signalled by clustering of the nuclei in one or more groups depending on the shape of the mother cell. The nuclei become arranged with the associated basal apparatuses facing toward the center of the cluster. Bundles of phycoplast microtubules develop between the nuclei, radiating from the center of a cluster toward the plasma membrane; basal apparatuses or associated structures apparently are involved in organization of the phycoplast. Cleavage furrows grow out centrifugally along these bundles of micro-tubules, fed by dictyosome-derived vesicles. No wall material is deposited. An additional mitotic division occurs during cleavage, and finally numerous uninucleate, wall-less, biflagellate zoospores are formed. The ultrastructural features of the two different types of cytoplasmic cleavage associated with two different types of daughter cells have not previously been reported for chlorophycean algae.     

VALVE MORPHOGENESIS AND THE MICROTUBULE CENTER IN THREE SPECIES OF THE DIATOM NITZSCHIA1

The relationship of cell organelles to valve morphogenesis was investigated in three species of Nitzschia. One, N. sigmoidea (Nitzsch) W. Sm., showed consistent ability to generate both nitzschioid and hantzschioid symmetry in daughter cells following cytokinesis; the other two maintained nitzschioid symmetry stably. From previous work with Hantzschia, a certain sequence of events could be anticipated in the cytoplasm. In two significant areas–the behavior of the Microtubule Center (MC) and its microtubule (MT) system, and the central origin of the silicalemma–not only were the results unexpected, but the three species showed fundamental differences among themselves. In N. sigmoidea, the silicalemma (and the future raphe region) arises centrally on the cleavage furrow, and after some lateral expansion, the silicalemmas and their associated organelles move in opposite directions in daughter cells, so that the raphe and the raphe canals end up along the girdle side of the cell as expected. However, the MCs never become associated with their silicalemma, remaining throughout near the girdle bands. In N. sigma (Kütz) W. Sm., the silicalemmas arise centrally and after lateral growth, move in opposite directions to generate nitzschioid symmetry. In this case, the MCs move to the vicinity of but never close to the silicalemmas, and follow them distantly during their lateral movement. In N. tryblionella Hantzsch, the new silicalemmas arise opposite one another, on one side of the daughter cells; each MC soon moves very close to its silicalemma, and remains thus through most of valve morphogenesis. Later, only one silicalemma/MC complex moves laterally, establishing the nitzschioid symmetry in both daughter cells. In all three species, as in Hantzschia, linear arrays of mitochondria aligned along MTs occupy the forming raphe canal, and microfilaments line the outer edge of the expanding silicalemma. The fibulae (the wall struts arching across the raphe canal) in Hantzschia always grow from the valve surface to the girdle surface of the forming valves. In these three Nitzschiae, this invariably happens in only one daughter cell of any pair; in the other, all the fibulae grow from the girdle surface to the valve surface. An explanation of these variations is proposed: that the morphogenetic machinery of Nitzschia and Hantzschia have a common origin, with present Nitzschiae having undergone considerable diversification at the intracellular level, causing the unstable cell symmetry exhibited by several modern species. Perhaps a taxonomic distinction between Hantzschia and Nitzschia lies in whether the morphogenetic machinery associated with valve morphogenesis moves laterally in the same or in opposite directions.     

ANALYSIS OF THE VARIABILITY IN CLEAVAGE TIMES AND DEMONSTRATION OF A MITOTIC GRADIENT DURING THE CLEAVAGE STAGES OF NOTHOBRANCHIUS GUENTHERI

Living embryos of the annual cyprinodont fish Nothobranchius guentheri were observed under the microscope. Detailed records were made of the time of cell division, disappearance of the nucleus and of the position of each cell within the blastoderm up to and including the sixth cleavage. Combination of these data revealed the presence of a mitotic gradient, a cell division gradient and a gradient of cell cycle duration in the 8-cell, 16-cell and 32-cell stage. Comparison of the variabilities in the duration of the interphase and mitosis reveals that differences between sister cell intercleavage times in the 8-, 16- and 32-cell stage are, for the most part, due to the variability in the duration of the mitotic process. It is concluded that the DNA-division cycle is composed of at least two parallel series of events. We found the random transition model of cell cycle control, originally based on the analysis of intermitotic times of mammalian cells in tissue culture, helpful also in analysing intercleavage time variability in embryonic cells.     

SURFACE CHARACTERS OF DIVIDING CELLS III. UNEQUAL DIVISION CAUSED BY STEEP TEMPERATURE GRADIENT IN GRASSHOPPER SPERMATOCYTE

Division of the spermatocytes of the grasshopper, Acrida lata, was studied under a steep temperature gradient. When a temperature gradient is applied along the spindle, the development of the aster on the warmer side is accelerated which, in turn, pushes the spindle toward the cooler side of the cell and the division becomes unequal. A condition to obtain such an unequal division is to shift the spindle by the temperature gradient and hold it eccentric during anaphase. Future cleavage plane is foreshadowed by the tongue of the mitochondria at anaphase before the cell departs from sphericity. If a temperature gradient is applied across the spindle, the spindle slides towards the cooler side sideways and the furrow is formed earlier, on the farther side of the cells from the spindle on the warmer side, although the size of the daughter cell is equal. The result indicates that the advance of the furrow is endothermically accelerated.     

ULTRASTRUCTURE OF ANOMALOUS POLLEN DEVELOPMENT IN EMBRYOGENIC ANTHER CULTURES OF HYOSCYAMUS NIGER

The formation of anomalous, binucleate pollen grains and their subsequent embryogenic development, induced by anther culture in Hyoscyamus niger, were analyzed by transmission electron microscopy (TEM). In culture, uninucleate pollen grains occasionally divided symmetrically giving rise to two apparently identical nuclei sharing a common cytoplasm. These nuclei divided once or twice unaccompanied by cell wall formation. After the daughter nuclei organized into cells, their subsequent division products contributed to embryoid formation. In conjunction with previous studies of pollen embryogenesis in H. niger, it appears that in contrast to the principle mode of embryogenesis (i.e., first asymmetric division forms typical two-celled pollen grain and the generative cell acts as the embryogenic precursor), anomalous pollen show no carry-over of gametophytic influences following embryogenic induction. This suggests that specific pathways of embryogenesis are correlated with the rate at which gametophytic gene activity is repressed following induction.     

萱草幼嫩花粉原生质体培养启动细胞分裂的超微结构研究

萱草(Hemerocallis fulva L.)幼嫩花粉,即后期小孢子原生质体在培养8天时进入有丝分裂或已形成二个细胞。此外,还观察到游离核分裂、无丝分裂、微核形成等现象。这显示了花粉原生质体分裂方式的多样性。在启动分裂时发生一系列变化:如细胞核移位、大液泡消失、细胞质电子密度增加、细胞器增多、质体不含淀粉等。再生的细胞壁含许多小泡,很少纤丝,表现出现有培养条件下壁的形成能力薄弱。这是今后改进培养技术需要特别注意的问题。     

THE PYRENOID OF SCENEDESMUS QUADRICAUDA

The development of the pyrenoid of Scenedesmus quadricauda from the time of its initiation and its subsequent activities is described in some detail. Correlation is made between the evidence from light and electron microscopy. The pyrenoid is a dynamic organelle which continues to change its appearance throughout the development of the algal cell due to the following factors: the deposition of starch platelets within the periphery of the expanding matrix; the separation of starch grains into individual pockets by the intrusive growth of the chloroplast lamellae in centripetal fashion; and the transition of the shape of the starch from concavo-convex platelets to lenticular grains. By these processes starch grains are continuously formed by deposition of carbohydrates within the matrix. The grains accumulate within the chloroplast, maintaining an organic connection with each other by slender starch bridges. Some parental starch grains are passed on to daughter cells during cell division. By taking into account the planes of cleavage during cell division, it is not difficult to see that pyrenoid starch grains could become distributed throughout the daughter chloroplast, regardless of their distance from the pyrenoid.     

ULTRASTRUCTURE OF THE NUCLEOLUS DURING THE CHINESE HAMSTER CELL CYCLE

Changes in the structure of the nucleolus during the cell cycle of the Chinese hamster cell in vitro were studied. Quantitative electron microscopic techniques were used to establish the size and volume changes in nucleolar structures. In mitosis, nucleolar remnants, "persistent nucleoli," consisting predominantly of ribosome-like granular material, and a granular coating on the chromosomes were observed. Persistent nucleoli were also observed in some daughter nuclei as they were leaving telophase and entering G₁. During very early G₁, a dense, fibrous material characteristic of interphase nucleoli was noted in the nucleoplasm of the cells. As the cells progressed through G₁, a granular component appeared which was intimately associated with the fibrous material. By the middle of G₁, complete, mature nucleoli were present. The nucleolar volume enlarged by a factor of two from the beginning of G₁ to the middle of S primarily due to the accumulation of the granular component. During the G₂ period, there was a dissolution or breakdown of the nucleolus prior to the entry of the cells into mitosis. Correlations between the quantitative aspects of this study and biochemical and cytochemical data available in the literature suggest the following: nucleolar reformation following division results from the activation of the nucleolar organizer regions which transcribe for RNA first appearing in association with protein as a fibrous component (45S RNA) and then later as a granular component (28S and 32S RNA).     

THE EFFECT OF METHANOL ON SPORE GERMINATION AND RHIZOID DIFFERENTIATION IN ONOCLEA SENSIBILIS

In germinating spores of Onoclea sensibilis, the nucleus migrates to one end prior to an asymmetric cell division that partitions each spore into two daughter cells of unequal size. The larger cell develops into a protonema, whereas the smaller cell immediately differentiates into a rhizoid. When spores were germinated in the presence of methanol, nuclear migration was inhibited and most nuclei moved only to the raphe on the proximal side of the spores. Subsequent cell division partitioned each spore into daughter cells of equal size of which both developed into a protonema and neither into a rhizoid. Spores became sensitive to methanol at a time just prior to or coincident with nuclear migration and the effects of the alcohol were rapidly reversible as long as the spores were removed from methanol prior to the completion of cell division. Exposure to methanol prior to, but not during, nuclear migration or after mitosis had no effect upon rhizoid differentiation. The alcohol disrupted the formation of crosswalls after mitosis and they were often convoluted and irregularly branched. These results are consistent with the interpretation that methanol may disrupt a membrane site that plays an essential role in nuclear movement and rhizoid differentiation.     

THE ONTOGENY OF CARBONIFEROUS ARTICULATES: THE APEX OF SPHENOPHYLLUM

Twig apices of Sphenophyllum lescurianum, S. constrictum, and two new Sphenophyllum taxa are described in transverse and longitudinal section from middle and upper Pennsylvanian age specimens. In all of the species the single apical cell has the shape of a tetrahedron, with a triangular upper surface and three internal cutting faces. Segment cells are produced from each of the cutting surfaces in a dextrorse or sinistrorse direction, depending upon the species. The central portion of each segment cell contributes to the initiation of the procambium, while the remaining outer portion undergoes a vertical and subsequent horizontal division to form segment cells. Segment cells are aligned in vertical tiers beneath the respective apical cell cutting faces, with the individual leaves positioned directly beneath a tier of segment cells. Leaf primordia are first observed as a series of surface undulations below the apex, with an intercalary meristem located directly beneath each primordium. The vegetative apical organization of Sphenophyllum is demonstrated to be very similar to the type of organization found at the stem tips of Catamites and Equisetum.     

Non‐equivalence in old‐ and new‐flagellum daughter cells of a proliferative division in Trypanosoma brucei

Differentiation of Trypanosoma brucei, a flagellated protozoan parasite, between life cycle stages typically occurs through an asymmetric cell division process, producing two morphologically distinct daughter cells. Conversely, proliferative cell divisions produce two daughter cells, which look similar but are not identical. To examine in detail differences between the daughter cells of a proliferative division of procyclic T. brucei we used the recently identified constituents of the flagella connector. These segregate asymmetrically during cytokinesis allowing the new‐flagellum and the old‐flagellum daughters to be distinguished. We discovered that there are distinct morphological differences between the two daughters, with the new‐flagellum daughter in particular re‐modelling rapidly and extensively in early G1. This re‐modelling process involves an increase in cell body, flagellum and flagellum attachment zone length and is accompanied by architectural changes to the anterior cell end. The old‐flagellum daughter undergoes a different G1 re‐modelling, however, despite this there was no difference in G1 duration of their respective cell cycles. This work demonstrates that the two daughters of a proliferative division of T. brucei are non‐equivalent and enables more refined morphological analysis of mutant phenotypes. We suggest all proliferative divisions in T. brucei and related organisms will involve non‐equivalence.     

Über die Abgrenzung derChlorosarcinales von denChlorococcales

It is proposed to use amongst other characters the type of cell division in order to delimit theChlorosarcinales from theChlorococcales. A definition of the two processes of division occuring in these orders is given. It differs from that of other authors. In theChlorosarcinales only those genera should be assembled in which vegetative daughter cells arise by bipartition followed by firm association of the wall between the daughter cells with that of the mother cell. In contrast, autospores, the vegetative daughter cells of a number ofChlorococcales, develop by multiple division, their cell walls are formed all around the protoplasts and are free from that of the mother cell. The chlorococcalean generaTrebouxia andDictyochloropsis incorporate species which multiply by zoo-, aplano- and autospores as well as others having no autospores. Autospores possibly have arisen more than once during evolution.

     

FINE STRUCTURE OF THE SNOW ALGA (CHLAMYDOMONAS NIVALIS) AND ASSOCIATED BACTERIA1

Chlamydomonas nivalis (Bau.) Wille is present in red snow as large spherical resting cells. Fine structural studies reveal an abundance of clear granules in the cytoplasm and occasional starch grains in the chloroplast. Individual cells display a thick cell wall with a smooth outer surface. Cells may be surrounded by a loose fibrous network in which encapsulated bacteria are seen. The bacteria have a characteristic Gram-negative cell wall and constrictive mode of division. The algal-bacterial association appears to be characteristic of red snow populations.     

A Cell Kinetic and Cytological Study on the Asymmetric Cell Division of Thymic Lymphoblasts of the Embryonic Rat

Cell division of thymus lymphoid cells from embrynonic and young rats was investigated cytologically on cell smears, focusing attention on asymmetric cell division. Some of thymic lymphoblasts displayed features implicating asymmetric cell division. At the telophase of such cells, two immature daughter cells looked dissimilar: one of them was smaller in size and possessed a more condensed nucleus, compared with the counterpart cell. Furthrmore, in most cases the cytoplasm of the smaller daughter cell was stained with Giemsa more deeply. It was suggested that the asymmetry of the nucleus emerges at anaphase and telophase probably due to some polarized situation of the cytoplasm. Asymmetrically-dividing cells were relatively frequently observed during the developmental period when large lymphoblasts actively transform into smaller lymphocytes :16% to 17% of whole dividing cells were under asymmetric cell division on days 16 and 17 of gestation, while less than 5% on day 19 or thereafter. In correlation with this observation, asymmetrically-dividing cells were more frequently observed among large lymphoblasts than among other smaller cell fractions. These results support the view that the asymmetric cell division may play some essential role in the transformation of large lymphoblasts into smaller lymphocytes.