Localization of determinants for formation of the anterior-posterior axis in eggs of the ascidian Halocynthia roretzi

Development ◽  
1994 ◽  
Vol 120 (11) ◽  
pp. 3093-3104 ◽  
Author(s):  
H. Nishida
Keyword(s):  
Muscle Cells ◽  
Bilateral Symmetry ◽  
Radial Symmetry ◽  
Second Phase ◽  
Cleavage Pattern ◽  
Halocynthia Roretzi ◽  
Ooplasmic Segregation ◽  
Anterior Posterior ◽  
Anterior Posterior Axis

Unfertilized eggs of the ascidian Halocynthia roretzi are radially symmetrical along the animal- vegetal axis. After fertilization, ooplasmic segregation results in formation of an anterior-posterior axis horizontally, and eggs become bilaterally symmetrical. When 8–15% of the cytoplasm of the posterior- vegetal region of the egg was removed after the second phase of ooplasmic segregation, most of the embryos completed gastrulation but developed into radialized larvae along the animal-vegetal axis with no apparent anterior-posterior axis. Removal of cytoplasm from other regions did not affect formation of this latter axis. The cleavage pattern of the embryos that were deficient in posterior- vegetal cytoplasm (PVC) exhibited radial symmetry instead of the complicated bilateral symmetry of normal embryos. Detailed comparisons of cleavage patterns revealed the duplication of the anterior cleavage pattern in the originally posterior halves of the PVC-deficient embryos. The PVC-deficent larvae lacked muscle cells, which are normally derived from the posterior blastomeres. Examination of the developmental fates of the early blastomeres of the PVC-deficient embryos revealed that all of the vegetal blastomeres had assumed anterior fates. These results suggest that the PVC-deficient embryos are totally anteriorized. When posterior-vegetal cytoplasm was transplanted to the anterior-vegetal position of PVC-deficient eggs, the axial deficiency was overcome, and reversal of the anterior-posterior axis was observed. The results of transplantation of posterior-vegetal cytoplasm to the anterior-vegetal position in normal eggs demonstrated that formation of the anterior structure is suppressed by posterior-vegetal cytoplasm. These results suggest that posterior fate is specified by the presence of posterior-vegetal cytoplasm, while anterior fate is specified by the absence of posterior-vegetal cytoplasm. Thus, posterior-vegetal cytoplasm determines the anterior-posterior axis by generating the posterior cleavage pattern and conferring posterior fates on cells, as well as by inhibiting anterior fates that would otherwise occur by default.

Regionality of egg cytoplasm that promotes muscle differentiation in embryo of the ascidian, Halocynthia roretzi

Development ◽  
1992 ◽  
Vol 116 (3) ◽  
pp. 521-529 ◽  
Author(s):  
H. Nishida
Keyword(s):  
Muscle Cells ◽  
Muscle Differentiation ◽  
Posterior Region ◽  
Second Phase ◽  
Halocynthia Roretzi ◽  
Early Embryos ◽  
Vegetal Pole ◽  
Restricted Region ◽  
Ooplasmic Segregation ◽  
Cytoplasmic Determinants

Development of ascidians occurs in typical mosaic fashion: blastomeres isolated from early embryos differentiate into tissues according to their normal fates, an indication that cytoplasmic determinants exist in early blastomeres. To provide direct evidence for such cytoplasmic determinants, we have devised methods for fusing blastomeres and cytoplasmic fragments from various regions. (1) Presumptive-epidermis blastomeres were fused to cytoplasmic fragments from various regions of blastomeres of 8-cell embryos of Halocynthia roretzi and development of muscle cells was monitored by an antibody to ascidian myosin. Muscle differentiation was observed only when presumptive-epidermis blastomeres were fused with fragments from the posterior region of B4.1 (posterior-vegetal) blastomeres, the normal progenitor of muscle cells. The results indicate that muscle determinants are present and localized in the cytoplasm that enters muscle-lineage cells. (2) To investigate the presence and localization of muscle determinants in the egg, cytoplasmic fragments from various regions of unfertilized and fertilized eggs were fused with the presumptive- epidermis blastomeres, and formation of muscle cells was assessed by monitoring myosin, actin and acetylcholinesterase expression. These proteins were expressed only when cytoplasm from a restricted region of the eggs, i.e. the vegetal region, after the first phase of ooplasmic segregation, and posterior region, after the second phase of segregation, were fused. Based on these experiments, it is suggested that muscle determinants are segregated by ooplasmic movements after fertilization. They move initially to the vegetal pole of the egg and, prior to first cleavage, to the posterior region from whence future muscle-lineage blastomeres are formed. The inferred movements of muscle determinants correspond to those of the myoplasm, a microscopically visible portion of the egg cytoplasm.

Vegetal egg cytoplasm promotes gastrulation and is responsible for specification of vegetal blastomeres in embryos of the ascidian Halocynthia roretzi

Development ◽  
1996 ◽  
Vol 122 (4) ◽  
pp. 1271-1279 ◽  
Author(s):  
H. Nishida
Keyword(s):  
Radial Symmetry ◽  
Second Phase ◽  
Equatorial Position ◽  
Animal Cells ◽  
Halocynthia Roretzi ◽  
Animal Pole ◽  
Vegetal Pole ◽  
Ooplasmic Segregation ◽  
Derivatives Of ◽  
Egg Cortex

An animal-vegetal axis exists in the unfertilized eggs of the ascidian Halocynthia roretzi. The first phase of ooplasmic segregation brings the egg cortex to the vegetal pole very soon after fertilization. In the present study, when 5–8% of the egg cytoplasm in the vegetal pole region was removed between the first and second phase of segregation, most embryos exhibited failure of gastrulation, as reported previously in Styela by Bates and Jeffery (Dev. Biol, 124, 65–76, 1987). The embryos that were deficient in vegetal pole cytoplasm (VC-deficient embryos) developed into permanent blastulae. They consisted for the most part of epidermal cells and most lacked the derivatives of vegetal blastomeres, such as endoderm, muscle and notochord. Removal of cytoplasm from other regions did not affect embryogenesis. The cleavage of the VC-deficient embryos not only exhibited radial symmetry along the animal-vegetal axis but the pattern of the cleavage was also identical in the animal and vegetal hemispheres. Examination of the developmental fates of early blastomeres of VC-deficient embryos revealed that the vegetal blastomeres had assumed the fate of animal cells. These results suggested that the VC-deficient embryos had been totally animalized. When vegetal pole cytoplasm was transplanted to the animal pole or equatorial position of VC-deficient eggs, gastrulation occurred, starting at the site of the transplantation and tissues derived from vegetal blastomeres formed. Therefore, it appears that vegetal pole cytoplasm specifies the site of gastrulation and the cytoplasm is responsible for the specification of vegetal blastomeres. It is suggested that during the second phase of ooplasmic segregation, cytoplasmic factors responsible for gastrulation spread throughout the entire vegetal hemisphere.

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

Development ◽  
1994 ◽  
Vol 120 (2) ◽  
pp. 235-243 ◽  
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.

Sign in / Sign up

Export Citation Format

Share Document