Marked Decrease in the Rigidity of Starfish Oocytes Induced by 1-Methyladenine1

Abstract

Extreme rigidity of immature starfish oocytes as measured by compression method was found to decline during the early phase of their maturation when induced by 1-methyladenine (I-MeAde). The onset of this decrease in stiffness occurred within 5 to 9 min of 1-MeAde treatment, well before the breakdown of the germinal vesicle, progressively declining to reach a minimum stiffness after 20 min. Dithiothreitol, known as an artificial maturation-inducing agent, caused a similar change. The stiffness is thus expected to serve as a quantitative indicator of the early process of cytoplasmic events, which would induce the breakdown of the germinal vesicle. Cytochalasin B (3 pg/ml) also reduced the stiffness, but unlike the former two agents, the effect was reversible, and did not interfere with the process of maturation. Due to the effect of cytochalasin B, it became possible to enucleate immature oocytes by centrifugal force. Non-nucleate fragments thus obtained still maintained their marked stiffness, which was decreased by the action of 1-MeAde, with a time-course similar to that of intact oocytes.It is known that I-methyladenine, the hormone which initiates maturation of starfish oocytes (9, 1 I ) by acting on the cell surface (10, 18), induces the production of a 'maturationpromoting factor' in the cytoplasm. This in turn triggers the breakdown of the germinal vesicle (15, 16). It is most likely that there are some other accompanying cellular processes initiated also by 1-methyladenine, which interract and result in cytoplasmic maturation of the oocytes.HIRAMOTO and his colleagues (1 9, 22) noticed marked changes in the mechanical properties of the cytoplasm, which occur prior to the breakdown of the germinal vesicle (GVBD). In the present study, detailed investigation of oocyte rigidity, which we postulate as serving as a quantitative indicator of the process of maturation, was carried out. MATERIALS AND METHODS Materialswith cold sea water (15-20°C) until use. Astropecten scoparius and Asterias amurensis were also employed in some experiments. Solutions(DTT) in 0.25 M NaCI, and 2 mg cytochalasin B in ml dimethylsulfoxide, were prepared.Starfishes were collected during their respective breeding seasons, and stored in aquaria supplied Materials mainly used were the oocytes of Asterina pectinifera.As stock solutions, I mM 1-methyladenine (I-MeAde) in de-ionized water, 0.5 M dithiothreitol These were diluted * This paper is dedicated to the memory of the late Professor Jean Clark Dan, 315

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

“…2, a). A rapid decrease in cytoplasmic rigidity in 1-MeAde-treated oocytes has been demonstrated (10,11). In fusions between the same types of oocyte.…”

Centrosome destined to decay in starfish oocytes

“…To obtain follicle-free immature oocytes arrested at prophase of meiosis I, isolated ovaries were treated with Ca 2+ -free artificial seawater and then transferred into filtered natural seawater to induce spawning of oocytes (Nemoto et al, 1980).…”

V-2 Micromanipulation

“…MPF is amplified in the cytoplasm under the participation of germinal vesicle materials (Kishimoto et al, 1981). MPF is species non-specific in a wide range of invertebrates and vertebrates (Kishimoto and Kanatani, 1977 ;Kishimoto et al, 1982 (Hiramoto, 1976a(Hiramoto, , b, 1979Ikeda et al, 1976;Nemoto et al, 1980;Yamamoto and Yoneda, 1983;Yoneda, 1976Yoneda, , 1980Yoneda et al, 1978 ; for reviews, see Hiramoto, , 1982 (Nakamura andHiramoto, 1978, Hiramoto, 1979) and in amphibian eggs (Sawai, 1979 ;Sawai and Yoneda, 1974).…”