Biomedical subjects

S J Pickering

Publications and source records attributed to S J Pickering.

46 records · Page 3Linked to original sources

PubMed1987

The influence of cooling on the organization of the meiotic spindle of the mouse oocyte.

The effect of cooling on the organization of the microtubule system of the mouse oocyte has been investigated. Cooling to 25, 18 or 4 degrees C for varying periods of time resulted in a progressive disassembly of the spindle and the dispersal of the chromosomes. The extent of the changes observed was greater at lower temperatures and with longer periods of exposure. Transfer of oocytes from either 37 or 4 to 24 degrees C resulted in the rapid and transient appearance of polar asters and of multiple cytoplasmic asters associated with the pericentriolar material present at the spindle poles and in the cytocortex of the oocyte. This transient aster formation appeared to be driven by the elevated levels of free tubulin released during spindle disassembly. An apparent reversal of many of the changes induced by low temperature was observed in many but not all oocytes on their restoration to 37 degrees C for 1 h. These results have implications for our understanding of microtubule organization in the oocyte, and for the handling of oocytes during IVF and GIFT therapeutic procedures and during the cryopreservation of oocytes.

Animals↗

PubMed1987

The effect of dimethylsulphoxide on the microtubular system of the mouse oocyte.

The effect of dimethylsulphoxide (DMSO) on the organization of the microtubular system of the mouse oocyte has been examined. Exposure to DMSO causes the immediate appearance of multiple, cold-resistant microtubular asters associated with the foci of pericentriolar material (PCM) normally present in the oocyte. More prolonged exposure to DMSO leads to progressive disassembly of the spindle, and as a result dispersal of the chromosomes and polar PCM foci occurs, and tubulin polymerization becomes confined to PCM-organized asters. Those astral microtubules located between the PCM foci and the cortex of the oocyte appear to be particularly stable, resulting in the development of lengthening radial bundles of microtubules between the PCM and the surface and the progressive movement of the PCM foci towards the centre of the cell. In contrast, after activation of the oocyte the microtubules generated in the presence of DMSO remain located in a cortical mesh. The effects of DMSO do not appear to be fully reversible in most oocytes. We discuss the implications of these results both for the cytoplasmic organization of the oocyte and zygote, and for the attempts at cryopreservation of human oocytes for therapeutic use in infertility programmes.

Animals↗

PubMed1986

The cytoskeleton, endocytosis and cell polarity in the mouse preimplantation embryo.

The process of cell polarization in mouse 8-cell embryos includes the formation of a polar cluster of cytoplasmic endocytotic organelles (endosomes) subjacent to an apical surface pole of microvilli. A similar polar morphology, supplemented by basally localized secondary lysosomes, is evident following division to the 16-cell stage in outside blastomeres, precursors of the trophectodermal lineage. The roles of microfilaments and microtubules in generating and stabilizing endocytotic and surface features of polarity (visualized by horseradish peroxidase incubation and indirect immunofluorescence labeling, respectively) have been evaluated by exposure of 8- and 16-cell embryos and 8-cell couplets to drugs (cytochalasin D, colcemid, nocodazole) that disrupt the cytoskeleton. The generation of endocytotic polarity is dependent upon intact microtubules and microfilaments, but the newly established endocytotic pole in blastomeres from compacted 8-cell embryos appears to be stabilized exclusively by microtubules. Polarized endocytotic organelles at the 16-cell stage are more resistant to drug treatment than at the 8-cell stage (probably due to microfilament interactions) indicating a maturation phase in the polar cell lineage. Microtubules are also responsible for the orientation of endocytotic clusters along the cell's axis of polarity. In contrast, the generation and stability of polarity at the cell surface appears relatively independent of cytoskeletal integrity. The results are discussed in relation to the mechanisms that may control the development and stabilization of polarization during cleavage.

Animals↗

PubMed1986

The generation of cell surface polarity in mouse 8-cell blastomeres: the role of cortical microfilaments analysed using cytochalasin D.

The mechanism by which a surface pole of microvilli is generated in mouse 8-cell blastomeres has been investigated. 4-cell and 8-cell embryos (or cell couplets) were incubated for precise times during their respective cell cycles in medium containing cytochalasin D (CCD) to disrupt the microfilament system. The blastomeres were analysed immediately for the distribution and state of organization of their microvilli, using three morphological techniques. The results indicate that the surface pole, characterized by microvilli containing CCD-resistant core filaments, is not generated by the gradual segregation of stable microvilli to the apical surface. An alternative model is proposed, based upon the stabilization of the apical cytocortex prior to the elongation of apical microvilli and the destabilization of cytocortical elements in contact areas and the shortening and loss of basolateral microvilli.

Actin Cytoskeleton↗

PubMed1985

Changes in the distribution of membranous organelles during mouse early development.

The unfertilized oocyte, fertilized egg and early embryo (2-cell to 16-cell) of the mouse have been examined immunocytochemically for the distribution of antigens associated with the endoplasmic reticulum, the lysosomal and acidic vesicle fraction (100 kD antigen), Golgi apparatus (135 kD antigen) and coated vesicles (clathrin). The distribution of these antigens has also been examined in isolated 8-cell and 16-cell-stage blastomeres of various ages and phenotypes. Endoplasmic reticulum is detected only weakly in the oocyte and egg, but is seen abundantly at later stages both in association with the nuclear membrane and evenly distributed throughout the cytoplasm, except in regions of cell:cell apposition from which it is excluded. Intracellular clathrin is associated with the spindle in mitotic and meiotic cells. During interphase, clathrin is distributed throughout the cell until the mid-8-cell stage when it is concentrated into the apical region of the cell under the region of membrane at which a surface pole of microvilli will form subsequently. Thus, the cytoplasmic polarization of clathrin precedes overt polarization at the surface. At mitosis, the clathrin relocates to the spindle and is distributed to both daughter cells. It resumes an apical location beneath the surface pole of microvilli in polar daughter 1/16 cells, but remains dispersed in apolar daughter 1/16 cells. Both the lysosomal and Golgi antigens are distributed throughout the cytoplasm until the early 16-cell stage. In pairs of 16-cell blastomeres both antigens aggregate in a single cluster and do so whether the surface phenotype of the blastomeres is polar or apolar. The position of this cluster is not consistently related to the point of contact with the other cell in the pair but there is a suggestion that in cells with a polar surface phenotype the polar foci of Golgi/lysosomal antigens are located between the nucleus and the surface pole at earlier time points, but shift to a position between the basolateral membrane and the nucleus at the later time point. In intact 16-cell embryos also, the aggregated Golgi/lysosomal antigens of polar cells appear to localize to the basal region. The distributions of these various organelles in embryonic cells reported here show a number of differences from those reported previously for mature, differentiated cells.

Animals↗

PubMed1985

Developmental variability within and between mouse expanding blastocysts and their ICMs.

We have attempted to reduce the developmental heterogeneity amongst populations of mouse blastocysts by synchronizing embryos to the first visible signs of blastocoel formation. Using embryos timed in this way, we have examined the extent of variation of inside and outside cell number and of inside cell size, nuclear DNA content and developmental potential, between and within embryos of a similar age postcavitation. The overall impression gained is one of wide heterogeneity in inside:outside cell number ratios and in cell cycling and its relation to cavitation among embryos of similar age postcavitation. However, the simplest explanation of our results suggests that cavitation generally begins at a time when most outside cells are in their sixth developmental cell cycle and that outside cells, as a population, are a little ahead of inside cells in their cell cycling. Additionally we present evidence that, within at least some individual inner cell masses (ICM), there is intraembryo variation in the time at which inside cell developmental potential becomes restricted.

Animals↗

PubMed1985

Maturation and polarization of the endocytotic system in outside blastomeres during mouse preimplantation development.

The maturation and distribution of the endocytotic apparatus in outside cells of cleavage-stage mouse embryos have been studied to determine the nature and sequence of changes associated with the differentiation of the polarized trophectoderm epithelium of the blastocyst. Various quantitative and qualitative techniques used at the light and electron microscopic levels have revealed an incremental pattern of endocytotic maturation and polarization. Oocytes, eggs and blastomeres within embryos up to the early 8-cell stage contain clusters of prelysosomal endocytotic vesicles (endosomes) distributed randomly in the cortical cytoplasm. During the 8-cell stage and continuing into the early 16-cell stage, endosomes become progressively localized in the apical cytoplasm beneath the microvillous pole. Endosome polarization is initiated prior to overt polarization of the surface membrane. Concomitant with endosome polarization, pinocytotic activity at the cell surface, revealed by horseradish peroxidase labelling, becomes segregated preferentially to the apical rather than the basolateral membrane. The final maturation phase occurs at the late 16-cell stage when secondary lysosomes, characterized by trimetaphosphatase reactivity, form and polarize in the basal cytoplasm.

Animals↗

PubMed1984

Microtubules influence compaction in preimplantation mouse embryos.

The role of microtubules during compaction of the 8-cell-stage mouse embryo was investigated using the drugs Taxol (which leads to a non-controlled polymerization of tubulin) and Nocodazole (which causes depolymerization of microtubules). Taxol inhibits compaction in most non-compacted embryos and reverses it in already compacted embryos. These effects were observed on both cell flattening (as judged by phase-contrast microscopy) and on cell surface polarization (as judged by scanning electron microscopy and the surface binding of fluorescent concanavalin A). In contrast Nocodazole does not inhibit cell flattening, but rather accelerates its completion. Nocodazole influences the detailed organization of the surface pole and appears to reduce the incidence of surface polarization but does not reverse polarity once established to a significant extent. We conclude that microtubules exercise a constraining role during compaction, influencing cell shape, cell organization and the time at which compaction takes place.

Alkaloids↗

PubMed1984

Changes in actin distribution during fertilization of the mouse egg.

Unfertilized mouse oocytes and eggs 1-8 h after fertilization in vitro were examined at the light microscope level for structural changes, distribution of actin (as assessed by both antiactin antibodies and NBD-phallicidin), surface binding of concanavalin A (Con A) and chromosomal distribution and condensation. The influence of cytochalasin D on these events was also assessed. Changes in actin distribution were associated with rotation of the second anaphase spindle, formation of the second polar body, the events following incorporation of the sperm nucleus, and formation and migration of the pronuclei. Cytochalasin D prevented spindle rotation, polar body formation, pronuclear migration and the restoration of Con A binding over the site of sperm entry and the site of polar body formation, but did not affect sperm fusion and entry, the loss of Con A binding at the site of sperm entry and pronuclear formation.

Actins↗

PubMedDate not supplied

The development of visuo-spatial working memory.

Children's performance on tests of visuo-spatial working memory improves with age, although relatively little is known about why this happens. One explanation concerns the development of the ability to recode visually presented information into phonological form. This process appears to be used from around 8 years of age and is a major contributor to tasks in which stimuli can be verbally labelled. However, evidence suggests that phonological recoding cannot account for all of the age-related change in performance on visuo-spatial working memory tasks. In this review, four other mechanisms (knowledge, processing strategies, processing speed, and attentional capacity) are considered in terms of their contribution to children's visuo-spatial working memory development.

Child↗