Oocyte Competence to Maturation‐inducing Hormone. I. Breakdown of Germinal Vesicles of Small Oocytes in Starfish, Asterina pectinifera*

Abstract

1-Methyladenine (1-MeAde) is the endogenous maturation-inducing substance (MIS) in starfish. However. small oocytes have no competence to 1-MeAde even at the concentration of 10V'M. Furthermore, when they were injected with cytoplasm of fully-grown (large) and maturing (1-MeAdetreated) oocytes, known to contain maturation-promoting factor (MPF), they did not undergo germinal vesicle breakdown (GVBD). On the other hand, germinal vesicles (GV) of the small oocytes underwent nuclear breakdown when the small oocytes were fused with the large maturing oocytes. Therefore it is concluded that the GV of the small oocytes are capable of undergoing nuclear breakdown in the presence of the sufficient MPF, but that the small oocytes can not amplify the injected MPF. Fused cells displayed particular shape changes during the course of nuclear breakdown of both the large and the small oocytes.In starfish it is well established that fully-grown oocytes (150-180pm in diameter, designated as large) are induced to resume maturation division by 1-methyladenine (1-MeAde), the endogenous maturation-inducing hormone secreted by follicle cells under the influence of gonad-stimulating substance (GSS) (1-5). Generally, 1-MeAde at concentrations around 10-8M can trigger germinal vesicle breakdown (GVBD). It has also been estabished that maturation-promoting factor (MPF) is generated in the large 1-MeAde-treated oocytes, and that the quantity increases during the process of GVBD (6). The quantity of MPF can be measured by transferring the cytoplasm of maturing oocytes into the large non-1-MeAdetreated oocytes, and by observing the occurrence of GVBD. Using this microinjection bioassay system, MPF can generally be detected about 10 min after the commencement of 1-MeAde treatment. The level increases gradually, reaching a plateau about 20 to 40 min after 1-MeAde treatment (20°C), after which the activity gradually decreases.However, in ma\\ oocytes (about three-quarters of the diameter of the large oocytes), GVBD does not occur even when they are treated with I-MeAde at a concentration of M. It was thought that a better understanding of the conditions necessary for meiosis would be reached through a comparison of the large (competent to 1-MeAde) and the small (non-competent to 1-MeAde) oocyte. Therefore we first tried to clarify why the small oocytes can not undergo GVBD, checking their ability (1) to generate MPF, and (2) to undergo * This paper is dedicated to our late Prof. H. Kanatani with deep respects and love.

Cell‐cycle aspects of growth and maturation of mammalian oocytes

“…I t could be thus concluded that a proportion of about 40% of growing cytoplasm in the combined cells is sufficient to counteract MPF activity. These results are in full agreement with observations in growing starfish oocytes (Bulet et al, 1985). GVBD in growing oocytes of this species is not induced by a n injection of maturing cytoplasm, known to contain MPF, but only by fusion with maturing oocytes.…”

“…These oocytes have a very limited ability to initiate nuclear maturation in vitro. Similarly, growing Xenopus or starfish oocytes do not respond to progesterone or to l-methyl adenine (Wasserman et al, 1984;Bulet et al, 1985) by resuming meiosis. Since the lack of meiotic response to progesterone by late stage IV Xenopus oocytes is not due to the absence of steroid receptors (Sadler and Maller, 1983), the inability of growing oocytes to resume meiotic maturation may be attributed to the absence of messenger RNAs coding for a cytoplasmic factor, e.g., as the maturation promoting factor (MPF), which was originally described as enabling cytoplasm from progesterone-treated Xenopus oocytes to cause a rapid germinal vesicle breakdown (GVBD) upon microinjection into recipient oocytes (Dettlaff et al, 1964;Masui and Markert, 1971;Smith and Ecker, 1971).…”

Dynamics of Mitotic Apparatus Formation and Tubulin Content during Oocyte Maturation in Starfish

“…Maturation-promoting factor (MPF) is generally retained during the metaphase and its disappearance by cyclin degradation triggers the promotion to anaphase (12,13). On the other hand, the polar body formation in a fused oocyte, where an oocyte undergoing GVBD is fused with an immature but fully grown oocyte, takes place one by one (altogether four polar bodies) (2). Since the cytoplasm is common in this case, the timing of appearance and disappearance of MPF must be controlled only around each MA.…”

Reconstruction of Starfish Eggs by Electric Cell Fusion: A New Method of Detect the Cytoplasmic Determinant for Archenteron Formation

“…Some studies using the cell fusion technique in echinoderm development have been reported (1,2,16,17,22,23). In these studies, fused oocytes or eggs displayed only oocyte maturation (germinal vesicle breakdown) or several divisions.…”

“…These naked AEFs were kept until cell fusion on 1% agar-coated plates filled with ASW. : GV-oocytes were exposed to ASW containing nile blue (1 or 2 drops of 1 % nile blue to about 2 ml ASW) for a few min to stain the cortices, in particular the nile blue-positive granules (2,12). Alternatively the entire cytoplasm of other GV-oocytes was stained by keeping them in ASW containing rhodamine B isothiocyanate (RITC, 10 pglml; Sigma) for 1 hr (4).…”