Biomedical subjects

S N Hird

Publications and source records attributed to S N Hird.

5 recordsLinked to original sources

PubMed1996

Segregation of germ granules in living Caenorhabditis elegans embryos: cell-type-specific mechanisms for cytoplasmic localisation.

Germ granules are ribonucleoprotein particles that are thought to function in germline specification in invertebrates and possibly in vertebrates. In Caenorhabditis elegans, these structures, termed P granules, are partitioned to the germline P cells during the early embryonic divisions. By injecting a fluorescently labelled anti-P-granule antibody into the C. elegans germline syncitium, we followed P-granule segregation in live embryos using laser-scanning confocal microscopy. We show that, in early P cells (P0 and P1), P-granule partitioning is achieved primarily by their migration through the cytoplasm towards the site of formation of the germline daughter cell. A different mechanism appears to operate in later P cells (P2 and P3): P granules associate with the nucleus and move with it toward the site of formation of the germline daughter cell, where they are then deposited. At each division, there is also disassembly or degradation of those P granules that remain in the cytoplasm destined for the somatic daughter cell. Microfilaments, microtubules and the product of the gene mes-1 are required for the normal pattern of P-granule segregation in P2.

Actin Cytoskeleton↗

PubMed1996

Specification of the anteroposterior axis in Caenorhabditis elegans.

Anteroposterior asymmetries are apparent in C elegans development before the first cell division. Here we identify the cue that specifies the anteroposterior axis, and investigate how this cue is interpreted to generate initial asymmetry. In C. elegans, the sperm normally enters the egg in an invariant position. We have found that causing fertilisation to occur in the abnormal end of the egg completely reverses the orientation of the anteroposterior axis, but gives otherwise normal development. This result suggests that a component of the sperm normally specifies the anteroposterior axis. We have found that a cytoplasmic rearrangement in the uncleaved zygote is directed by the sperm, suggesting a mechanism by which the sperm may specify the axis. The results additionally reveal that the C elegans oocyte is constructed with no axis prespecified in the form of asymmetrically localised cytoplasmic determinants.

Animals↗

PubMed1995

Pseudocleavage is dispensable for polarity and development in C. elegans embryos.

The first cleavage of the Caenorhabditis elegans embryo is asymmetrical, producing daughters with different cell fates. During the first cell cycle, P granules, cytoplasmic components that are segregated to the germ-line, are localized to the posterior of the embryo. It has been hypothesized that the asymmetrical behavior of the daughters of the first division results from a similar localization of developmental determinants. A process called pseudocleavage also occurs during the first cell cycle: Anterior cortical contractions culminate in a single partial constriction of the embryo called the pseudocleavage furrow. Coincident with pseudocleavage, there is an anteriorly directed flow of cortical cytoplasm and a posteriorly directed flow of internal cytoplasm. Foci of filamentous cortical actin become asymmetrically distributed into an anterior cap. Roles for these various first cell cycle events in cytoplasmic localization and development have been suggested but remain unclear. We have isolated a maternal effect mutation, nop-1(it142), which abolishes the anterior cortical contractions and the pseudocleavage furrow. In addition, cortical actin foci remain uniformly distributed in most embryos. Despite these defects, cytoplasmic and cortical streaming is present and P granules are localized to the posterior of early embryos. In most embryos from mutant mothers, development proceeds normally and the embryos hatch and grow into fertile adults. We conclude that the pseudocleavage contractions and furrow are dispensable for the development of C. elegans.

Animals↗

PubMed1993

Cortical and cytoplasmic flow polarity in early embryonic cells of Caenorhabditis elegans.

We have examined the cortex of Caenorhabditis elegans eggs during pseudocleavage (PC), a period of the first cell cycle which is important for the generation of asymmetry at first cleavage (Strome, S. 1989. Int. Rev. Cytol. 114: 81-123). We have found that directed, actin dependent, cytoplasmic, and cortical flow occurs during this period coincident with a rearrangement of the cortical actin cytoskeleton (Strome, S. 1986. J. Cell Biol. 103: 2241-2252). The flow velocity (4-7 microns/min) is similar to previously determined particle movements driven by cortical actin flows in motile cells. We show that directed flows occur in one of the daughters of the first division that itself divides asymmetrically, but not in its sister that divides symmetrically. The cortical and cytoplasmic events of PC can be mimicked in other cells during cytokinesis by displacing the mitotic apparatus with the microtubule polymerization inhibitor nocodazole. In all cases, the polarity of the resulting cortical and cytoplasmic flows correlates with the position of the attenuated mitotic spindle formed. These cortical flows are also accompanied by a change in the distribution of the cortical actin network. The polarity of this redistribution is similarly correlated with the location of the attenuated spindle. These observations suggest a mechanism for generating polarized flows of cytoplasmic and cortical material during embryonic cleavages. We present a model for the events of PC and suggest how the poles of the mitotic spindle mediate the formation of the contractile ring during cytokinesis in C. elegans.

Actin Cytoskeleton↗

PubMed1993

Cell polarity in early C. elegans development.

The polarization of the embryonic axes is a key event in embryogenesis, being one of the earliest manifestations of the shape and form of the organism. The acquisition of polarity by individual blastomeres is one of the earliest indicators of commitment to a particular pathway of differentiation. These phenomena have been studied in the development of C. elegans both at the cellular and organismal level. This review summarizes what is known about how polarity is established in the blastomeres of this organism, how the division axes of polarized cells are determined, and how the embryonic axes are set up.

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