CORTICAL CHANGES IN STARFISH (ASTERINA-PECTINIFERA) OOCYTES DURING 1-METHYLADENINE-INDUCED MATURATION AND FERTILIZATION ACTIVATION

Maturation of the starfish oocyte cortex to produce an effective corti cal granule reaction and fertilisation envelope is believed to develop in three phases: (1) pre-methyladenine (1-MA) stimulation; (2) post-1 -MA stimulation, pregerminal vesicle breakdown; and (3) post-germinal vesicle breakdown. The present study was initiated to identify what ea ch of these phases may encompass, specifically with respect to structu res associated with the oocyte cortex, including cortical granules, mi crovilli and vitelline layer. 1-MA treatment brought about an orientat ion of cortical granules such that they became positioned perpendicula r to the oocyte surface, and an similar to 4-fold decrease in microvil lar length. A-23187 activation of immature oocytes treated with (10 mi n; pregerminal vesicle breakdown) or without 1-MA resulted in a reduct ion in cortical granule number of 21% and 41%, respectively (mature oo cytes underwent a 96% reduction in cortical granules). Elevation of th e fertilisation envelope in both cases was significantly retarded comp ared with activated mature oocytes. In activated mature oocytes, the v itelline layer elevated 20.0 +/- 5.4 mu m from the egg's surface, wher eas in immature oocytes treated with just A-23187 or with 1-MA (10 min ) and A-23187, it lifted 0.35 +/- 0.1 and 0.17 +/- 0.04 mu m, respecti vely. The fertilisation envelopes of activated (or fertilised) immatur e oocytes also differed morphologically from those of mature oocytes. In activated, immature oocytes, the fertilisation envelope was not uni form in its thickness and possessed thick and thin regions as well as fenestrations. Additionally, it lacked a complete electron-dense strat um that characterised the fertilisation envelopes of mature oocytes. T he nascent perivitelline space of immature oocytes was also distinguis hed by the presence of numerous vesicles which appeared to be derived from microvilli. Differences in the morphology of cortices from activa ted (fertilised) and non-activated, immature and mature oocytes substa ntiate previous investigations demonstrating three phases of cortical maturation, and are consistent with physiological changes that occur d uring oocyte maturation, involving ionic conductance of the plasma mem brane, establishment of slow and fast blocks to polyspermy and elevati on of a fertilisation envelope.