Reorganization of cytoplasm in the zebrafish oocyte and egg during early steps of ooplasmic segregation

Juan Fernández; Macarena Valladares; Ricardo Fuentes; Andrea Ubilla

doi:10.1002/dvdy.20682

Effect of Colchicine and cytochalasin B on ooplasmic segregation of ascidian eggs

Development Genes and Evolution ◽

10.1007/bf00582094 ◽

1974 ◽

Vol 175 (3) ◽

pp. 243-248 ◽

Cited By ~ 36

Author(s):

M. Zalokar

Keyword(s):

Cytochalasin B ◽

Ooplasmic Segregation

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Localization of egg cytoplasm that promotes differentiation to epidermis in embryos of the ascidian Halocynthia roretzi

Development ◽

10.1242/dev.120.2.235 ◽

1994 ◽

Vol 120 (2) ◽

pp. 235-243 ◽

Cited By ~ 5

Author(s):

H. Nishida

Keyword(s):

Specific Antigen ◽

Equatorial Region ◽

Epidermal Cells ◽

Second Phase ◽

Halocynthia Roretzi ◽

Cytoplasmic Factors ◽

Early Embryos ◽

Ooplasmic Segregation ◽

Anucleate Cell ◽

Cell Fragments

Embryogenesis in ascidians is of the mosaic type. This property suggests the presence of cytoplasmic factors in the egg that are responsible for specification of the developmental fates of early blastomeres. The epidermal cells that surround the entire tadpole larva originate exclusively from blastomeres of the animal hemisphere of early embryos. To obtain direct evidence for cytoplasmic determinants of epidermis fate, we carried out cytoplasmic transfer experiments by fusing blastomeres and anucleate cell fragments from various regions of eggs and embryos. Initially, presumptive non-epidermis blastomeres (blastomeres from the vegetal hemisphere) were fused to cytoplasmic fragments from various regions of blastomeres of 8-cell embryos of Halocynthia roretzi, and development of epidermal cells was monitored by following the expression of an epidermis- specific antigen, as well as by observations of morphology and the secretion of larval tunic materials. Formation of epidermis was observed when vegetal blastomeres were fused with cytoplasmic fragments from the presumptive epidermis blastomeres. The results suggested that cytoplasmic factors that promoted epidermis differentiation (epidermis determinants) were present in epidermis progenitors. Vegetal blastomeres only manifested this change in fate when fused with cytoplasmic fragments of roughly equal or larger size. Next, to examine the presence and localization of epidermis determinants in the uncleaved egg, cytoplasmic fragments from various regions of unfertilized and fertilized eggs were fused with the vegetal blastomeres. The results suggested that epidermis determinants were already present in unfertilized eggs and that they were segregated by movements of the ooplasm after fertilization. After the first phase of ooplasmic segregation, these determinants were widely distributed, with the highest activity being located in the equatorial region. There were no indications of regional differences in the activity within the equatorial region of eggs at this stage. After the second phase of ooplasmic segregation, prior to the first cleavage, the activity moved in the animal direction, namely, to the animal hemisphere, from which future epidermis-lineage blastomeres are normally formed.

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Early Development in the Lancelet (= Amphioxus) Branchiostoma floridae from Sperm Entry through Pronuclear Fusion: Presence of Vegetal Pole Plasm and Lack of Conspicuous Ooplasmic Segregation

Biological Bulletin ◽

10.2307/1542182 ◽

1992 ◽

Vol 182 (1) ◽

pp. 77-96 ◽

Cited By ~ 24

Author(s):

L. Z. Holland ◽

N. D. Holland

Keyword(s):

Early Development ◽

Branchiostoma Floridae ◽

Sperm Entry ◽

Vegetal Pole ◽

Ooplasmic Segregation ◽

Pole Plasm

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Fertilization and ooplasmic movements in the ascidian egg

Development ◽

10.1242/dev.105.2.237 ◽

1989 ◽

Vol 105 (2) ◽

pp. 237-249 ◽

Cited By ~ 7

Author(s):

C. Sardet ◽

J. Speksnijder ◽

S. Inoue ◽

L. Jaffe

Keyword(s):

Polar Body ◽

Surface Velocity ◽

Second Phase ◽

Animal Pole ◽

Male Pronucleus ◽

Vegetal Pole ◽

Female Pronucleus ◽

Ooplasmic Segregation ◽

Second Polar Body ◽

Sperm Aster

Using light microscopy techniques, we have studied the movements that follow fertilization in the denuded egg of the ascidian Phallusia mammillata. In particular, our observations show that, as a result of a series of movements described below, the mitochondria-rich subcortical myoplasm is split in two parts during the second phase of ooplasmic segregation. This offers a potential explanation for the origin of larval muscle cells from both posterior and anterior blastomeres. The first visible event at fertilization is a bulging at the animal pole of the egg, which is immediately followed by a wave of contraction, travelling towards the vegetal pole with a surface velocity of 1.4 microns s-1. This wave accompanies the first phase of ooplasmic segregation of the mitochondria-rich subcortical myoplasm. After this contraction wave has reached the vegetal pole after about 2 min, a transient cytoplasmic lobe remains there until 6 min after fertilization. Several new features of the morphogenetic movements were then observed: between the extrusion of the first and second polar body (at 5 and 24–29 min, respectively), a series of transient animal protrusions form at regular intervals. Each animal protrusion involves a flow of the centrally located cytoplasm in the animal direction. Shortly before the second polar body is extruded, a second transient vegetal lobe (‘the vegetal button’) forms, which, like the first, resembles a protostome polar lobe. Immediately after the second polar body is extruded, three events occur almost simultaneously: first, the sperm aster moves from the vegetal hemisphere to the equator. Second, the bulk of the vegetally located myoplasm moves with the sperm aster towards the future posterior pole, but interestingly about 20% remains behind at the anterior side of the embryo. This second phase of myoplasmic movement shows two distinct subphases: a first, oscillatory subphase with an average velocity of about 6 microns min-1, and a second steady subphase with a velocity of about 26 microns min-1. The myoplasm reaches its final position as the male pronucleus with its surrounding aster moves towards the centre of the egg. Third, the female pronucleus moves towards the centre of the egg to meet with the male pronucleus. Like the myoplasm, the migrations of both the sperm aster and the female pronucleus shows two subphases with distinctly different velocities. Finally, the pronuclear membranes dissolve, a small mitotic spindle is formed with very large asters, and at about 60–65 min after fertilization, the egg cleaves.

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A gastrulation center in the ascidian egg

Development ◽

10.1242/dev.116.supplement.53 ◽

1992 ◽

Vol 116 (Supplement) ◽

pp. 53-63 ◽

Cited By ~ 1

Author(s):

William R. Jeffery

Keyword(s):

Nervous System ◽

Uv Irradiation ◽

Second Phase ◽

Uv Sensitivity ◽

Maternal Mrna ◽

Free Translation ◽

Vegetal Pole ◽

Cell Embryo ◽

Ooplasmic Segregation ◽

Uv Dose

A gastrulation center is described in ascidian eggs. Extensive cytoplasmic rearrangements occur in ascidian eggs between fertilization and first cleavage. During ooplasmic segregation, a specific cytoskeletal domain (the myoplasm) is translocated first to the vegetal pole (VP) and then to the posterior region of the zygote. A few hours later, gastrulation is initiated by invagination of endoderm cells in the VP region of the 110-cell embryo. After the completion of gastrulation, the embryonic axis is formed, which includes induction of the nervous system, morphogenesis of the larval tail and differentiation of tail muscle cells. Microsurgical deletion or ultraviolet (UV) irradiation of the VP region during the first phase of myoplasmic segregation prevents gastrulation, nervous system induction and tail formation, without affecting muscle cell differentiation. Similar manipulations of unfertilized eggs or uncleaved zygotes after the second phase of segregation have no effect on development, suggesting that a gastrulation center is established by transient localization of myoplasm in the VP region. The function of the gastrulation center was investigated by comparing protein synthesis in normal and UV-irradiated embryos. About 5% of 433 labelled polypeptides detected in 2D gels were affected by UV irradiation. The most prominent protein is a 30 kDa cytoskeletal component (p30), whose synthesis is abolished by UV irradiation. p30 synthesis peaks during gastrulation, is affected by the same UV dose and has the same UV-sensitivity period as gastrulation. However, p30 is not a UV-sensitive target because it is absent during ooplasmic segregation, the UV-sensitivity period. Moreover, the UV target has the absorption maximum of a nucleic acid rather than a protein. Cell-free translation studies indicate that p30 is encoded by a maternal mRNA. UV irradiation inhibits the ability of this transcript to direct p30 synthesis, indicating that p30 mRNA is a UV-sensitive target The gastrulation center may function by sequestration or activation of maternal mRNAs encoding proteins that function during embryogenesis.

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