Fine structure of oocyte maturation in a crinoid echinoderm, Oxycomanthus japonicus: A time-lapse study by serial...

Natural maturation of the oocytes of Oxycomanthus japonicus can be predicte in advance, and the multiple ovaries permit unintrusive serial biopsies. Ovaries were fixed for transmission electron micrscopy at 15-min intervals before, during, and after oocyte maturation. The start of maturation of each oocyte is signaled by the breakdown of the germinal vesicle and the disruption of a macula adhaerens associating the oocyte with nongerminal cells of the ovary. This disruption is followed by an ovulation of the oocyte into the ovarian lumen. Ovulation takes about 1 hr, and a continuous vitelline coat is produced around the oocyte during this interval. Within the oocyte cytoplasm, patches of nuage and the annulate lamellae disappear at 30 and 45 min after the start of oocyte maturation, respectively. Micropapillae become transiently abundant at the oocyte surface both at the time of germinal vesicle breakdown and around the time when the first and second polar bodies are produced. Oocyte maturation takes about 2 hr from start to finish, and the emission of the second polar body marks the beginning of the stage of the ovum. Within the cytoplasm of the ovum, the haploid chromosomes develop into chromosome-containing vesicles, which later fuse into a single female pronucleus. Pronuclear ova are retained in the ovarian lumen for about 1 hr and are then spawned into the surrounding seawater, where fertilization takes place.