Manipulative methods for analyzing embryogenesis

The effect of taxol microinjection on the microtubular structure in polar body formation of starfish oocytes

In starfish oocytes, microtubules (MTs) form a spindle, which plays an important role in contributing to the selective loss of chromosomes and centrosomes to the polar bodies (PBs) during meiosis. When Taxol was locally injected near the germinal vesicle (GV) or the mitotic apparatus during meiosis I, PB formation was inhibited as mentioned below. In the oocytes, which were injected with Taxol after spindle formation, the spindle became large, and then the volume of the first PB also increased more than that of the control. In contrast, in the oocytes injected with Taxol before the spindle formation, chromosome capture and alignment were inhibited. These oocytes did not form PB, but only a bulge at the cell cortex was occasionally observed. Moreover, in the oocytes injected with Taxol before GV breakdown, the chromosomes did not gather in one place, and then two asters were observed at distant positions from the cell cortex. These results suggested that MTs lost not only the ability to obtain the bipolar attachment of chromosomes by Taxol injection but also the aster closer to the cell cortex lost its interaction with the cell cortex of the animal pole.

Reproductive maternal centrosomes are cast off into polar bodies during maturation division in starfish oocytes

Animal egg inherits a maternal centrosome from the meiosis-II spindle and sperm can introduce another centrosome at fertilization. It has been believed that in most animals only the sperm centrosome provides the division poles for mitosis in zygotes. This uniparental (paternal) inheritance of the centrosome must depend on the loss of the maternal centrosome. In starfish, suppression of polar body (PB) extrusion is a prerequisite for induction of parthenogenesis (Washitani-Nemoto et al. (1994) Dev. Biol. 163, 293-301), suggesting that the centrosomes cast off into PBs have reproducing capacity. Due to the absence of centriole duplication in meiosis II of starfish oocytes, each centrosome of a meiosis-II spindle has only one single centriole, whereas in meiosis I each has a pair of centrioles (Sluder et al. (1989) Dev. Biol. 131, 567-579; Kato et al. (1990) Dev. Growth Differ. 32, 41-49). Hence, the first PB (PB1) has two centrioles, whereas the second PB (PB2) and the mature egg have only one centriole, respectively. The present study examined the reproductive capacity of PB centrosomes by transplanting them into artificially activated eggs, and then the recipient egg nucleus with the surrounding cytoplasm was removed. A transplanted PB2 centrosome with a single centriole formed a monopolar spindle at the first mitosis, followed by formation of a bipolar spindle in the next mitosis, leading to actual cleavage and subsequent development. This proves the reproducing capacity of the single centriole in the PB2 centrosome. The behavior of the transplanted PB1 centrosome was exactly the same as in the PB2 centrosome, in spite of the difference in the number of centrioles. These results clearly show that four maternal centrioles are heterogeneous in duplicating capacity, during meiosis only one centriole in each of the centrosomes of a meiosis-I spindle pole retains duplicating capacity, the reproductive centrioles are successively cast off into PBs, and finally a mature egg inheriting a nonreproductive centriole alone is formed, and the presence of a single reproductive centriole is sufficient condition for embryonic development in starfish.

A metaphase pause: hormone-induced maturation progresses through a long pause at the first meiotic metaphase in oocytes of the starfish, Pisaster ochraceus

In several species of starfish, it has been reported that the meiotic divisions in fertilized oocytes occur precociously compared to those in unfertilized oocytes. The nature of the 'acceleration' of meiosis was studied using Pisaster ochraceus oocytes. The extent of the acceleration of first polar body formation was found to be completely dependent on the time of fertilization (or artificial activation); fertilization at about 100 min after 1-methyladenine application accelerated meiosis I the most, while earlier or later fertilization resulted in a smaller extent of accelerations of meiosis I. Observation of isolated meiotic spindles and fluorescent visualization of meiotic spindles in whole oocytes showed that progression of meiosis I in Pisasteroocytes pauses transiently at metaphase I for more than 40 min unless they are activated. The activation shortened the duration of metaphase I, which resulted in the acceleration of first polar body formation. A new term 'metaphase pause' is proposed to define this long duration of metaphase I in starfish oocytes.

Activation at the germinal vesicle stage of starfish oocytes produces parthenogenetic development through the failure of polar body extrusion

Starfish oocytes can be fertilized after germinal vesicle breakdown (GVBD) and artificial parthenogenesis can be induced by activating the oocytes after GVBD (post-GVBD activation). In the present study, parthenogenotes were obtained by the activation of immature oocytes with caffeine before treatment with 1-methyladenine (1-MeAde) to induce oocyte maturation. Most of the caffeine-treated eggs developed as tetraploids, as parthenogenotes produced by the post-GVBD activation. The parthengenotes were derived only from eggs that failed to extrude polar bodies, mostly from eggs failing to extrude a second polar body. Eggs derived from immature oocytes activated by A23187, treated with 1-MeAde and post-treated with cytochalasin B failed to extrude polar bodies, and eventually developed into parthenogenetic embryos. These results indicate that the present parthenogenesis mechanism shares the same characteristics as that achieved by post-GVBD activation in the suppression of polar body formation as a key means for successful starfish parthenogenesis.

Determination of dorso-ventral axis in early embryos of the sea urchin, Hemicentrotus pulcherrimus

To elucidate a relationship between early cleavage planes and dorso-ventral (DV)-axis of sea urchin embryos, a fluorescent dye, Lucifer Yellow CH, was iontophoretically introduced into one blastomere at the 2-cell stage, and the location of the progeny cells was determined in the half-labeled prism larvae by examining the embryos from the animal pole. The boundary plane which divides the embryonic tissue into the labeled and nonlabeled parts was (1) coincident with, (2) perpendicular to, or (3) obliquely crossing the larval plane of bilateral symmetry. The oblique boundaries took only two angles mutually symmetrical with regard to the DV-axis of embryos. Combining these labeling patterns, the tissue of prism larvae could be divided into 8 sectors around the animal-vegetal axis. When the 2-cell stage embryos with different diameters of sister blastomeres were labeled with the dye, one end of the boundary plane was again found at one of the 8 boundary points noticed in equally cleaved embryos, while the other was observed to fall in the middle of a sector. These results indicate that the DV-axis of the embryo is established according to the spatial arrangement of blastomeres during the 5-6th cleavage stages when blastomeres align in 8 rows in meridional direction. It was also suggested that intercellular communication takes part in the determination of the fate of individual founder blastomeres during the two subsequent cleavages, i.e., 7-8th cleavage stages.