受精卵における中心体の継承 : ヒトデ卵減数分裂にかかわる中心体の複製能

The centrosome, which exists universally in animal cells , is a cell organelle consisting of a pair of centrioles and an amorphous matrix that is known as the pericentriolar material. The zygote inherits one centrosome from the meiosis-II spindle and is introduced a second centrosome by sperm at fertilization. If both parental centrosomes were retained and reproduced, the zygote would enter first mitosis with four centrosomes, resulting in abnormal multipolar cleavage. Hence, only one of the parental centrosomes can be used in development. In most animals it is believed that only the paternal centrosome acts as the mitosis-organizing center. If the paternal centrosome is selectively used, the maternal centrosome is expected to be lost or become quiescent by the onset of first mitosis. This raises a question of how the maternal centrosome is selectively suppressed to ensure the uniparental inheritance of centrosomes. This hoary question, first proposed by Boveri (1887), has interested developmental biologists for a century, but has not been fully answered yet. To approach this problem, I investigated the behavior and duplicating capacity of maternal centrioles and centrosomes using parthenogenetic system produced by suppression of polar bodies in starfish oocytes. (1) The behavior of centrioles and ultrastructural changes of the nucleus were observed in maturing oocytes. Observations were focused on the number and behavior of centrioles during two successive meiotic divisions. Examination of serial sections revealed that in meiosis I each division pole has a pair of centrioles, whereas in meiosis II each has only one centriole. Hence, the first polar body retains two centrioles and the second polar body had only one centrioles. These results indicate that no duplication of centrioles occurs during the two successive meiotic divisions and that the mature egg inherits only one centriole from a primary oocytes. These findings indicate that the meiotic division is a cell division that decreases not only the number of chromosomes but also the number of centrioles. (2) After the completion of meiosis, mature eggs inherit a centrosome that retains only one centriole. When these mature eggs were activated artificially with A23187, cyclic changes in the cell surface and repeated reformation-disappearance of nuclear envelope ensued. But the activated eggs did not divide and repeated a single monaster formation in each cycle. These indicate that the centrosome inherited by a mature egg dose not have duplicating capacity, although it still retains activity to nucleate microtubule. (3) To examine duplicating capacity of other centrosomes involved in meiotic divisions, extrusion of polar bodies was suppressed. Treatment of oocytes with a cytokinesis drug during meiosis I produced eggs lacking any polar bodies, i.e. Opb eggs. Treatment during meiosis II induced eggs bearing the first polar body, i.e., 1pb eggs. Since meiotic separation of chromosomes occurred normally, a pronucleus formed in the Opb eggs and 1pb eggs after the chromosome separation was tetraploid and diploid, respectively. When artificially activated, both types of eggs develop parthenogenetically with strong synchrony of cell division among eggs. Regardless of the difference in ploidy, both types of parthenogenotes developed as tetraploids (2n=44 in A. pectinifera). Results described above indicate meiotic centrosomes are diverted into mitosis-organizing center for cleavages and suggest not all of the meiotic centrosomes loses duplicating capacity after completion of meiosis. (4) In Opb eggs, each centrosome of meiosis-I spindle poles that contained a pair of centrioles separated into two containing a single centriole to form a tetrapolar spindle in meiosis II. Through these two rounds of meiosis, tetrad chromosomes separated into monads, which came together into a cluster then formed a single pronucleus. At this time, two centrosomes were present near the nucleus. After first round of chromosome replication, a bipolar spindle was formed, then cleavage took place. Each of two centrosomes of the spindle retained a pair of centrioles. In 1pb eggs, each division pole of meiosis II had a single centriole, as in normal oocytes. Dyad chromosomes separated into monads, then gathered into one cluster to form a pronucleus. The nucleus was accompanied with a centrosome. After the first round of chromosome replication, a bipolar spindle did not appear, but only a half spindle was formed, resulting in the failure of both karyokinesis and cytokinesis in the first mitosis. After the second round of chromosome replication, a bipolar spindle was formed to divide the eggs into a pair of blastomeres (first cleavage). Each of spindle poles in both a half spindle and a bipolar spindle have a pair of centrioles. Based on these observations, I conclude that centrosomes that are cast off into polar bodies retain duplication capacity. If a half of centrosomes observed in meiosis II of both 0pb and 1pb eggs has duplicating capacity, decrease in the number of centrosomes at the first mitosis can be explained. When this assumption is applied to normal meiotic division, the following interpretation would be acceptable: both of the two division poles in meiosis I would retain duplicating capacity, whereas only one of the two centrosomes in meiosis II, which locates beneath the cell surface and is cast off into second polar body, would have duplicating capacity; only one centriole of each pair in meiosis-I division pole must have duplicating capacity. The present study revealed that in oocyte-maturation division not only the number of chromosomes, but also the number of centrioles decreased by half, resulting in leaving of a centrosome/centriole lacking duplicating capacity in mature eggs. Considering maturation division from the viewpoint of centrosomes, it will safe to say that maturation division is the cell division to establish a mechanism to prevent mature eggs from spontaneous development without fertilization that brings about an intermixture between a maternal and paternal genome.東京都立大学, 1999-04-15, 博士(理学), 乙第1010