The germinal vesicle (nucleus) of immature starfish oocytes is about 70-90 Ám in diameter and is thus well suited for microscopy studies. The germinal vesicle undergoes breakdown during meiotic maturation induced by 1 methyladenine.
The nuclear pores exclude large molecules. A 70 kD fluorescent dextran was injected into the cytoplasm, where it was excluded from the germinal vesicle. The process by which the 70 kD dextran redistributed during maturation was observed. A massive influx occurred about 25 min after application of the maturation hormone movie (1.1 MB); this influx very probably corresponds to the disruption of the membrane bilayer permeability barrier of the nucleus. The influx usually occurred from one site rather than all around the nucleus, and often occurred on the animal pole side. Preceding the entry due to disruption by about 5 min, there was also a slow increase in fluorescence within the nucleus (not seen too well on the movie). This probably corresponds to a slow leakage of the 70 kD dextran through the nuclear pores.
Other observations were made on how ribosomes and also the ER become redistributed after germinal vesicle breakdown.