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PubMed1997

Root development: signaling down and around.

Because of its elegant simplicity, the Arabidopsis root has become a model for studying plant organogenesis. In this review we focus on recent results indicating the importance of signaling in root development. A role for positional information in root cell specification has been demonstrated by ablation analyses. Through mutational analysis, genes have been identified that play a role in radial pattern formation. The embryonic phenotypes of these mutants raised the possibility that division patterns in post-embryonic roots are dependent on signaling that originates during embryonic development. Analysis of expression of the SCARECROW gene indicates that it may play a role in this 'top-down' signaling process. Characterization of root epidermis development has led to the identification of negative regulators of root-hair formation. These appear to set up a prepattern which is reinforced by signaling by plant hormones.

Arabidopsis↗
PubMed2005

Molecular control of cell polarity and asymmetric cell division in Drosophila neuroblasts.

In the embryonic central nervous system of the fruit fly Drosophila, most neurons and glial cells are generated by asymmetric division of neural stem cells called neuroblasts. Several genes have been identified that are required for the establishment of neuroblast polarity, for the asymmetric segregation of cell fate determinants and for the proper orientation and geometry of the mitotic spindle. However, little was known about the interactions between these genes and their respective gene products. It has emerged that most of the relevant proteins are assembled into three major protein complexes whose molecular interactions are conserved in evolution.

Animals↗
PubMed1971

Primitive erythropoiesis in early chick embryogenesis. I. Cell cycle kinetics and the control of cell division.

The primitive line of embryonic chick blood cells develop as a relatively homogeneous cohort of cells. Using an analysis based on the continuous uptake of thymidine-(3)H, we have established the generation time, G1, S, and G2 for progressively more mature generations of these immature erythroblasts. The data indicate that after the initiation of hemoglobin synthesis, the average cell will yield six generations of hemoglobin producing erythroblasts. The older generations of erythroblasts exhibit a longer generation time, G1, S, and G2 than the earlier generations of erythroblasts. Other methods of analysis corroborated these findings. One of these methods, an estimate of total erythrocyte productivity from the primitive stem cells (hematocytoblasts), led to the conclusion that the erythroblast cell lineage might be initiated as early as the sixth or seventh division following fertilization. In addition, primitive erythroblasts characterized by one set of cell cycle parameters, when grown in serum associated with erythroblasts of different parameters, showed no alteration in mitotic behavior. These results suggest the presence of programmed cell division not immediately cued by extracellular influence.

Age Factors↗
PubMed1998

Differential expression and functions of cortical myosin IIA and IIB isotypes during meiotic maturation, fertilization, and mitosis in mouse oocytes and embryos.

To explore the role of nonmuscle myosin II isoforms during mouse gametogenesis, fertilization, and early development, localization and microinjection studies were performed using monospecific antibodies to myosin IIA and IIB isotypes. Each myosin II antibody recognizes a 205-kDa protein in oocytes, but not mature sperm. Myosin IIA and IIB demonstrate differential expression during meiotic maturation and following fertilization: only the IIA isoform detects metaphase spindles or accumulates in the mitotic cleavage furrow. In the unfertilized oocyte, both myosin isoforms are polarized in the cortex directly overlying the metaphase-arrested second meiotic spindle. Cortical polarization is altered after spindle disassembly with Colcemid: the scattered meiotic chromosomes initiate myosin IIA and microfilament assemble in the vicinity of each chromosome mass. During sperm incorporation, both myosin II isotypes concentrate in the second polar body cleavage furrow and the sperm incorporation cone. In functional experiments, the microinjection of myosin IIA antibody disrupts meiotic maturation to metaphase II arrest, probably through depletion of spindle-associated myosin IIA protein and antibody binding to chromosome surfaces. Conversely, the microinjection of myosin IIB antibody blocks microfilament-directed chromosome scattering in Colcemid-treated mature oocytes, suggesting a role in mediating chromosome-cortical actomyosin interactions. Neither myosin II antibody, alone or coinjected, blocks second polar body formation, in vitro fertilization, or cytokinesis. Finally, microinjection of a nonphosphorylatable 20-kDa regulatory myosin light chain specifically blocks sperm incorporation cone disassembly and impedes cell cycle progression, suggesting that interference with myosin II phosphorylation influences fertilization. Thus, conventional myosins break cortical symmetry in oocytes by participating in eccentric meiotic spindle positioning, sperm incorporation cone dynamics, and cytokinesis. Although murine sperm do not express myosin II, different myosin II isotypes may have distinct roles during early embryonic development.

Animals↗
PubMed1990

Early onset of phenotype and cell patterning in the embryonic zebrafish retina.

The regular arrangement of retinal cone cells in a mosaic pattern is a common feature of teleosts. In the zebrafish, Brachydanio rerio, the retinal cone mosaic comprises parallel rows consisting of a repeating motif of four cone types. In order to elucidate the temporal and spatial aspects of the genesis of the cone mosaic in the developing retina, we generated a monoclonal antibody that specifically binds to the double cone photoreceptor of the adult. We first saw staining in the developing retina with this antibody, FRet 43, at 48 hours postfertilization, the time at which the first photoreceptor cells undergo their final mitotic division. We then injected embryonic fish with the thymidine analog, 5-bromo-2'-deoxyuridine (BrdU), confirming with a double-labeling experiment that the onset of FRet 43 antigenicity occurs within three hours of the cellular division that generates the double cone photoreceptors. Then we stained tangential sections of the 54-hour embryonic retina with FRet 43, further showing that cells devoid of staining alternate with stained pairs of cells in a pattern that is consistent with the arrangement of photoreceptors in the adult cone mosaic. These results indicate that a marker of the double cone phenotype is expressed at approximately the same time as cellular birthday and that the mosaic patterning is present within 6 hours of this expression.

Animals↗
PubMed2006

Estradiol stimulates progenitor cell division in the ventricular and subventricular zones of the embryonic neocortex.

Two distinct populations of cerebral cortical progenitor cells that generate neurons during embryogenesis have been identified: radial glial cells and intermediate progenitor cells. Despite advances in our understanding of progenitor cell populations, we know relatively little about factors that regulate their proliferative behaviour. 17-beta-Estradiol (E2) is present in the adult and developing mammalian brain, and plays an important role in central nervous system processes such as neuronal differentiation, survival and plasticity. E2 also stimulates neurogenesis in the adult dentate gyrus. We examined the role of E2 during embryonic cortical neurogenesis through immunohistochemistry, in situ hybridization, functional enzyme assay, organotypic culture and in utero administration of estradiol-blocking agents in mice. We show that aromatase, the E2 synthesizing enzyme, is present in the embryonic neocortex, that estrogen receptor-alpha is present in progenitor cells during cortical neurogenesis, that in vitro E2 administration rapidly promotes proliferation, and that in utero blockade of estrogen receptors decreases proliferation of embryonic cortical progenitor cells. Furthermore, the E2 inhibitor alpha-fetoprotein is expressed at high levels by radial glial cells but at lower levels by intermediate progenitor cells, suggesting that E2 differentially influences the proliferation of these cortical progenitor cell types. These findings demonstrate a new functional role for E2 as a proliferative agent during critical stages of cerebral cortex development.

Androstenedione↗
PubMed1993

The relationship between cell size and cell fate in Volvox carteri.

In Volvox carteri development, visibly asymmetric cleavage divisions set apart large embryonic cells that will become asexual reproductive cells (gonidia) from smaller cells that will produce terminally differentiated somatic cells. Three mechanisms have been proposed to explain how asymmetric division leads to cell specification in Volvox: (a) by a direct effect of cell size (or a property derived from it) on cell specification, (b) by segregation of a cytoplasmic factor resembling germ plasm into large cells, and (c) by a combined effect of differences in cytoplasmic quality and cytoplasmic quantity. In this study a variety of V. carteri embryos with genetically and experimentally altered patterns of development were examined in an attempt to distinguish among these hypotheses. No evidence was found for regionally specialized cytoplasm that is essential for gonidial specification. In all cases studied, cells with a diameter > approximately 8 microns at the end of cleavage--no matter where or how these cells had been produced in the embryo--developed as gonidia. Instructive observations in this regard were obtained by three different experimental interventions. (a) When heat shock was used to interrupt cleavage prematurely, so that presumptive somatic cells were left much larger than they normally would be at the end of cleavage, most cells differentiated as gonidia. This result was obtained both with wild-type embryos that had already divided asymmetrically (and should have segregated any cytoplasmic determinants involved in cell specification) and with embryos of a mutant that normally produces only somatic cells. (b) When individual wild-type blastomeres were isolated at the 16-cell stage, both the anterior blastomeres that normally produce two gonidia each and the posterior blastomeres that normally produce no gonidia underwent modified cleavage patterns and each produced an average of one large cell that developed as a gonidium. (c) When large cells were created microsurgically in a region of the embryo that normally makes only somatic cells, these large cells became gonidia. These data argue strongly for a central role of cell size in germ/soma specification in Volvox carteri, but leave open the question of how differences in cell size are actually transduced into differences in gene expression.

Animals↗
PubMed2005

Lgl, Pins and aPKC regulate neuroblast self-renewal versus differentiation.

How a cell chooses to proliferate or to differentiate is an important issue in stem cell and cancer biology. Drosophila neuroblasts undergo self-renewal with every cell division, producing another neuroblast and a differentiating daughter cell, but the mechanisms controlling the self-renewal/differentiation decision are poorly understood. Here we tested whether cell polarity genes, known to regulate embryonic neuroblast asymmetric cell division, also regulate neuroblast self-renewal. Clonal analysis in larval brains showed that pins mutant neuroblasts rapidly fail to self-renew, whereas lethal giant larvae (lgl) mutant neuroblasts generate multiple neuroblasts. Notably, lgl pins double mutant neuroblasts all divide symmetrically to self-renew, filling the brain with neuroblasts at the expense of neurons. The lgl pins neuroblasts show ectopic cortical localization of atypical protein kinase C (aPKC), and a decrease in aPKC expression reduces neuroblast numbers, suggesting that aPKC promotes neuroblast self-renewal. In support of this hypothesis, neuroblast-specific overexpression of membrane-targeted aPKC, but not a kinase-dead version, induces ectopic neuroblast self-renewal. We conclude that cortical aPKC kinase activity is a potent inducer of neuroblast self-renewal.

Animals↗
PubMed2006

Chromosome elimination in germ line-soma differentiation of Acricotopus lucidus (Diptera, Chironomidae).

During germ line-soma differentiation in early syncytial embryonic development of the chironomid Acricotopus lucidus, a complement of supernumerary chromosomes, the so-called germ line limited chromosomes (Ks), is excluded from the future somatic nuclei in the course of elimination mitoses. The Ks lag behind in the equatorial plane, while the somatic chromosomes (Ss) segregate equally. After elimination mitoses, the Ks are only present in the pole cells, the primary germ cells. In the divisions before their elimination, the Ks frequently showed delayed separation of sister chromatids with high-frequency formation of anaphasic bridges and lagging in pole movement as detected in 4',6-diamidino-2-phenylindole (DAPI)-stained squash preparations of early embryos. To determine if all of the Ks are eliminated in one step during a single mitosis, a fluorescence in situ hybridization (FISH) analysis of early embryonic divisions was performed using probes of germ line specific repetitive DNA sequences, which specifically label the Ks in their centromeric regions. In most cases, all of the Ks are lost in one mitosis; however, occasionally one or several of the Ks can escape their elimination by segregating and moving poleward together with the Ss. The escaping Ks will then be eliminated in one of the following mitoses. This clearly indicates that the specific conditions to eliminate Ks are not restricted to only one division. Possible mechanisms of elimination of Ks are discussed.

Animals↗
PubMed2004

The cfy mutation disrupts cell divisions in a stage-dependent manner in zebrafish embryos.

The zebrafish curly fry (cfy) mutation leads to embryonic lethality and abnormal cell divisions starting at 12-14 h postfertilization (hpf) during neural tube formation. The mitotic defect is seen in a variety of tissues including the central nervous system (CNS). In homozygous mutant embryos, mitoses are disorganized with an increase in mitotic figures throughout the developing neural tube. One consequence of aberrant mitoses in cfy embryos is an increase in cell death. Despite this, patterning of the early CNS is relatively unperturbed with distribution of the early, primary neurons indistinguishable from that of wild-type embryos. At later stages, however, the number of neurons was dramatically decreased throughout the CNS. The effect on neurons in older cfy embryos but not young ones correlates with the time of birth of neurons: primary neurons are born before the action of the cfy gene and later neurons after. Presumably, death of neuronal progenitors that divide beginning at the neural keel stage or death of their neuronal progeny accounts for the diminution of neurons in older mutant embryos. In addition, oligodendrocytes, which also develop late in the CNS, are greatly reduced in number in cfy embryos due to an apparent decrease in oligodendrocyte precursors. Genetic mosaic analysis demonstrates that the mutant phenotype is cell-autonomous. Furthermore, there are no obvious defects in apical/basal polarity within the neuroepithelium, suggesting that the cfy gene is not critical for epithelial polarity and that polarity defects are unlikely to account for the increased mitotic figures in mutants. These results suggest that the cfy gene regulates mitosis perhaps in a stage-dependent manner in vertebrate embryos.

Animals↗
PubMed1999

Distinct neural stem cells proliferate in response to EGF and FGF in the developing mouse telencephalon.

Multipotent, self-renewing neural stem cells reside in the embryonic mouse telencephalic germinal zone. Using an in vitro neurosphere assay for neural stem cell proliferation, we demonstrate that FGF-responsive neural stem cells are present as early as E8.5 in the anterior neural plate, but EGF-responsive neural stem cells emerge later in development in a temporally and spatially specific manner. By separately blocking EGF and FGF2 signaling, we also show that EGF alone and FGF2 alone can independently elicit neural stem cell proliferation and at relatively high cell densities separate cell nonautonomous effects can substantially enhance the mitogen-induced proliferation. At lower cell densities, neural stem cell proliferation is additive in the presence of EGF and FGF2 combined, revealing two different stem cell populations. However, both FGF-responsive and EGF-responsive neural stem cells retain their self-renewal and multilineage potential, regardless of growth factor conditions. These results support a model in which separate, lineage-related EGF- and FGF-responsive neural stem cells are present in the embryonic telencephalic germinal zone.

Animals↗
PubMed1990

Fine-structure alterations in female Brugia malayi and Litomosoides carinii after in vivo treatment with flubendazole.

Female filariae of the species Brugia malayi and Litomosoides carinii were investigated by means of electron microscopy after in vivo treatment with flubendazole. The earliest fine-structure alteration in both species was the disappearance of microtubuli from the intestinal cells as soon as 6 h after treatment. There was no further disintegration of intestinal cells for several days. Microtubuli disappeared from the outer zone of the hypodermal cytoplasm 24 h after treatment. At this time, marked alterations were also observed in the oogonia and in the embryonic cells. Many of these were swollen; their nuclear envelope was partly resolved and the chromatin was condensed, but no spindle apparatus was formed. The early fine-structure alterations observed after in vivo treatment with flubendazole consisted of the disappearance of microtubuli from various tissues. This led to the interruption of cell division in oogonia and embryonic cells and, and subsequently, to the disintegration of most other filarial tissues. These morphological alterations differed considerably from those observed after treatment with benzothiazole derivatives, which do not affect the microtubuli of the filariae.

Animals↗
PubMed2005

Tetraploid cells of enhanced green fluorescent protein transgenic mice in tetraploid/diploid-chimeric embryos.

We succeeded in noninvasively analyzing the distribution of tetraploid (4n) cells in tetraploid<-->diploid (4n<-->2n) chimeric embryos by using enhanced green fluorescent protein (EGFP) transgenic (Tg) mouse embryos. We also evaluated whether this technique of analyzing 4n-cells in EGFP Tg 4n<-->2n chimeric embryos could be used to determine which characteristics of 4n-cells cause the death of 4n-embryos and restricted distribution of 4n-cells in 4n<-->2n-chimeric embryos after implantation. In our experiments, the distribution of 4n-cells in 4n<-->2n-embryos was normal until an embryonic age of 3.5 days (E3.5). With respect to morphological development, there were no differences between 4n-, diploid (2n), 4n<-->2n-, and diploid/diploid (2n<-->2n) chimeric embryos, but the number of cells in the tetraploid (4n) blastocyst was smaller than expected. This decrease in the number of cells may have caused cell death or reduced the rate of cell division in 4n-cells, and may have restricted the distribution of 4n-cells in 4n<-->2n-chimeric embryos. This study demonstrated the utility of EGFP transgenic mouse embryos for relatively easy and noninvasive study of the sequential distribution of cells in chimeric embryos.

Animals↗
PubMed2002

Stem cells: hype and reality.

This update discusses what is known regarding embryonic and adult tissue-derived pluripotent stem cells, including the mechanisms underlying self-renewal without senescence, differentiation in multiple cell types both in vitro and in vivo, and future potential clinical uses of such stem cells. In Section I, Dr. Lansdorp reviews the structure and function of telomerase, the enzyme that restores telomeric ends of chromosomes upon cell division, highly present in embryonic stem cells but not adult stem cells. He discusses the structure and function of telomerase and signaling pathways activated by the enzyme, with special emphasis on normal and leukemic hematopoietic stem cells. In Section II, Dr. Pera reviews the present understanding of mammalian pluripotent embryonic stem cells. He discusses the concept of pluripotentiality in its embryonic context, derivation of stem cells from embryonic or fetal tissue, the basic properties of the stem cells, and methods to produce specific types of differentiated cell from stem cells. He examines the potential applications of stem cells in research and medicine and some of the barriers that must be crossed to achieve these goals. In Section III, Dr. Verfaillie reviews the present understanding of pluripotency of adult stem cells. She discusses the concept of stem cell plasticity, a term used to describe the greater potency described by several investigators of adult tissue-derived stem cells, critically reviews the published studies demonstrating stem cell plasticity, and possible mechanisms underlying such plasticity, and examines the possible role of pluripotent adult stem cells in research and medicine.

Animals↗
PubMed1993

A second pathway for regeneration of adult exocrine and endocrine pancreas. A possible recapitulation of embryonic development.

Substantial regeneration of both the endocrine and exocrine pancreas occurs after a 90% partial pancreatectomy in the young adult rat. We have reported previously that replication of preexisting islet and exocrine cells is enhanced 3- to 4-fold. Here, we report a second pathway of regeneration, that of proliferation and differentiation of precursor cells in the ductal epithelium. As shown with in vivo pulse labeling using 5-bromo-2'-deoxyuridine, an expansion of the ductal epithelium occurs. Proliferation is seen first in the common pancreatic duct and sequentially in smaller ducts of the ductal tree as focal areas of proliferation small ductules form. By 60 h after pancreatectomy, only these focal areas show heavy 5-bromo-2'-deoxyuridine staining. These proliferating ductules comprise 12.8% of the pancreatic volume at 3 days after pancreatectomy but are uncommon at 7 days after pancreatectomy. Coincident with the appearance and disappearance of these regions was a 3.5-fold increased growth of the pancreatic remnant compared with its equivalent of sham animals. These small ductules differentiate into new pancreatic islets and exocrine tissue, forming new lobules of pancreas that are indistinguishable from the preexisting ones. This second pathway of rapid regeneration recapitulates embryonic development in its pattern of ductal proliferation and subsequent differentiation. Furthermore, these studies provide evidence of the presence of precursor/stem cells in the adult pancreas.

Animals↗
PubMed2002

Cell division and cell enlargement in isolated Cucurbita cotyledons grown in darkness and in light.

The spatial and temporal patterns of post-embryonal cell growth and cell division were characterised in excised cotyledons of vegetable marrow ( Cucurbita pepo L. var. giromontia Alef.) incubated in water. The concurrent roles of these two processes in cotyledon growth were determined using paradermal sections of the first palisade layer of developing cotyledons. Tissue specificity was observed in the pattern of cell division. The daughter cells derived from an initial cell, which had already differentiated before imbibition of the seeds, were tightly packed in a cluster, which enabled us to monitor cell division during early cotyledon development. Heterogeneity of cell size was recognised during the process of cell proliferation in the cluster, suggesting that cell division is uncoupled from control of cell size. There was significantly more cell division in the marginal part of the cotyledons than in other parts, suggesting high activity of the marginal meristem. Light enhanced cell and cotyledon enlargement, but had no effect on the number of divisions. This study elucidated the cellular basis of post-germinative Cucurbita cotyledon morphogenesis and development.

Journal Article↗
PubMed2004

Geminin coordinates cell cycle and developmental control.

Growth and differentiation are two major themes in embryonic development. Numerous cell divisions have to be regulated on the path from a unicellular embryo, the zygote, to the multicellular structures of a mature being. Numerous functions, specializations and cellular identities have to be generated, in order to form a complex and mature animal. Numerous mechanisms have to control the correct assignment and acquisition of cellular fates, as well as the right timing and allocation of cells. Therefore, a strict coordination has to occur between embryonic patterning and the cell cycle. From this point of view, dual roles or mutual interactions of typical proliferation and developmental control genes are likely. Recently, new light was shed on these issues by identifying the nuclear protein Geminin as a molecular coordinator between the cell cycle and axial patterning. We summarize the role of Geminin in cell cycle, in the embryonic patterning controlled by Hox genes, providing insights into cell cycle regulators in embryonic development, and conversely, typical developmental control genes in cell cycle regulation.

Animals↗