Hormone-induced release of intracellular Ca²⁺ triggers meiosis in starfish oocytes

Abstract

THE oocytes of most vertebrates and invertebrates develop to a particular stage of meiosis and then stop¹. Various external stimuli, such as hormones, sperm or specific proteases² cause them to resume maturation, but it is not known by what mechanism these act. Indirect evidence suggests the involvement of Ca²⁺ in triggering this process in various invertebrate oocytes^3–6, and in a few species Ca²⁺ is known to act directly on the oocyte. Such oocytes include those not normally associated with follicular cells, in particular the annelids Pomatoceros⁷, Hydroides⁵ and Chaetopterus⁸ or the lamellibranchs Barnaea^4,6 and Spisula⁹. The reinitiation of meiosis under the influence of the divalent cation ionophore A23187 has been demonstrated with oocytes of Spisula⁹, Nereis¹⁰ and Chaetopterus¹¹ or follicle-free Xenopus oocytes^12–13 in the presence of external Ca²⁺. The assumption that Ca²⁺ can trigger meiosis in starfish^3,14 has been questioned because it seems that follicle cells release a meiosis-inducing substance when isolated in normal or Ca²⁺-enriched artificial seawater (ASW)¹⁵, and because the ionophore also activates germinal vesicle-bearing oocytes without inducing nuclear maturation¹⁶. Here we present evidence that Ca²⁺ can act directly on starfish oocytes and that a transient release of intracellular Ca²⁺ is a major step in the action of 1-methyladenine (1-MeAde), the natural hormone which triggers meiosis¹⁷.