Nature and action of the mediators inducing maturation of the starfish oocyte

The process of oocyte maturation and ovulation in starfish is triggered by a gonad-stimulating substance (GSS) present in the granules contained in the supporting cells of the nervous system. The GSS of Asterias amurensis is a polypeptide with a relative molecular mass (Mr) of about 2100, consisting of 22 amino acid residues. This peptide hormone is secreted from the nervous system and acts on the follicular cells around the oocyte to stimulate the production of the second mediator, maturation-inducing substance (MIS). MIS has been identified as 1-methyladenine. 1-Methyladenine acts on the surface of the oocyte, probably on the oocyte-surface factor, to induce the production of a cytoplasmic factor called maturation-promoting factor (MPF) in the ooplasm. This third mediator induces germinal vesicle breakdown and the subsequent processes of oocyte maturation up to the formation of the female pronucleus. MPF appears to be a phosphoprotein and is known in other animals. MPF obtained from any source appears to bring about nuclear membrane breakdown in both meiosis and mitosis, and the nature of MPF is very similar in vertebrates and invertebrates.