/. Embryol. exp. Morph. Vol. 43, pp. 157-166, 197S 1 57 Printed in Great Britain © Company of Biologists Limited 1978 Parthenogenetic activation of mouse oocytes induced by inhibitors of protein synthesis By G. SIRACUSA, 1 D. G. WHITTINGHAM, 2 M. MOLINARO 1 AND E. VIVARELLP From the University of Rome and the MRC Mammalian Development Unit, University College, London SUMMARY Recently ovulated mouse oocytes at the Metaphase II stage undergo parthenogenetic activation (as indicated by the formation of pronuclei) when incubated for 6 h in the presence of cycloheximide or puromycin; the activation response increases progressively with the concentration of inhibitor. Activation is induced with concentrations of cycloheximide that depress protein synthesis by more than 70 %. Pronoculear formation occurs when protein synthesis is almost totally inhibited. Incubation of oocyte in Actinomycin D failed to initiate activation. The results show that the Metaphase II oocyte of the mouse synthesizes protein factor(s) which are necessary for the maintenance of the meiotic block. Other protein(s) having opposite effects and a different rate of turnover may also participate in activation since when the oocytes are treated with a high concentration of cycloheximide (10 fig ml -1 ) for varying periods of time, or with varying concentrations for a short period of time (1 h), a more complex activation response curve is obtained. Oocytes activated with cycloheximide are capable of further development, following transfer to the oviducts of pseudopregnant recipients, in a proportion similar to that of oocytes activated in other ways. INTRODUCTION The mechanisms responsible for the pre-fertilization meiotic block, occurring in the oocyte of most mammals at the second metaphase stage, are unknown. Studies with frog oocytes suggest that Metaphase II-arrest results from the production of a cytostatic factor, as yet unidentified, in the cytoplasm of the secondary oocyte, and this factor disappears following fertilization or artificial activation (Masui & Markert, 1971). When the oocytes of the marine inver- tebrate Chaetopterus are treated with cycloheximide, meiosis is resumed and the oocytes proceed from Metaphase I to the emission of the second polar body (Zampetti-Bosseler, Huez & Brächet, 1973). Artemia salina eggs remain at the Metaphase I stage of meiosis following sperm penetration when they are removed prematurely from the uterus but if treated on removal with low concentrations 1 Author's address: Université di Roma, Istituto di Istologia ed Embriologia Generale, Via Alfonso Borelli 50, 00161 Roma, Italy. 2 Author's address: MRC Mammalian Development Unit, Wolfson House, University College London, 4 Stephenson Way, London, NW1 2HE, England. H E M B 4 3