In contrast to the somatic cell cycle, duplication of the centrioles does not occur in the second meiotic cycle. Previous studies have revealed that in starfish each of the two centrosomes in fully-grown immature oocytes consists of two centrioles with different destinies: one survives and retains its reproductive capacity, and the other is lost after completion of meiosis. In this study, we investigated whether this heterogeneity of the meiotic centrioles is already determined before the re-initiation of meiosis. We prepared a small fragment of immature oocyte containing the four centrioles and fused it electrically with a mature egg in order to transfer two sets of the premeiotic centrioles into the mature cytoplasm. Two asters were present in this conjugate, and in each of them only a single centriole was detected by electron microscopy. In the first mitosis of the conjugate artificially activated without sperm, two division poles formed, each of which doubled in each subsequent round of mitosis. These results indicate that only two of the four premeiotic centrioles survived in the mature cytoplasm and that they retained their reproductive capacity, which suggests that the heterogeneity of the maternal centrioles is determined well before re-initiation of meiosis,and that some factor in the mature cytoplasm is responsible for suppressing the reproductive capacity of the centrioles destined to decay.
In most animals, fertilized eggs inherit one centrosome from a meiosis-II spindle of oocytes and another centrosome from the sperm. However, since first proposed by Boveri [Sitzungsber. Ges. Morph. Phys. Münch. 3 (1887) 151-164] at the turn of the last century, it has been believed that only the paternal (sperm) centrosome provides the division poles for mitosis in animal zygotes. This uniparental (paternal) inheritance of centrosomes is logically based on the premise that the maternal (egg) centrosome is lost before the onset of the first mitosis. For the processes of the selective loss of the maternal centrosome, three models have been proposed: One stresses the intrinsic factors within the centrosome itself; the other two emphasize external factors such as cytoplasmic conditions or the sperm centrosome. In the present study, we have examined the validity of one of the models in which the sperm centrosome overwhelms the maternal centrosomes. Because centrosomes cast off into both the first and the second polar bodies (PB) are known to retain the capacity for reproduction and cell-division pole formation, we observed the behavior of those PB centrosomes with reproductive capacity and the sperm centrosome in the same zygotic cytoplasm. We prepared two kinds of fertilized eggs that contain reproductive maternal centrosomes, (1) by micromanipulative transplantation of the PB centrosomes into fertilized eggs, and (2) by suppression of the PB extrusions of fertilized eggs with cytochalasin B. In both types of eggs, the PB centrosomes could double and form cell-division poles, indicating that they are not suppressed by the sperm centrosome, which in turn indicates that selective loss of the maternal centrosome is due to intrinsic factors within the centrosomes themselves.
The morphogenetic processes of coelomic pouch (CP) formation in starfish embryos that were experimentally dissociated and induced to undergo reconstruction were studied. An analysis of these embryos randomly chosen from several cultures showed that CP always form on either side of the esophagus, even though the CP formation can differ in timing of initiation and duration, and can vary in number and size from embryo to embryo. Successive observations of CP formation in living embryos revealed two distinct sequences of CP development that were accompanied by different appearances of the blastocoele. These processes were named 'enterocoelic-like' and 'schizocoelic-like' CP formation. The former resembled normal development and occurred in embryos with a transparent blastocoele. The latter was characterized by the aggregation and epithelialization of mesenchyme-like cells on either side of the esophagus and was observed in embryos possessing a cloudy blastocoele. In a few embryos, both types of CP formation were seen in the same individual ('mosaic type' CP formation). Thick sections of embryos possessing a cloudy blastocoele revealed that aggregates of mesenchyme-like cells undergoing CP formation directly contact the developing esophagus. Together, these data demonstrate flexibility in the morphogenetic processes that regulate CP formation, and suggest that positional cues in the esophagus regulate the placement of CP.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.