Oocyte Maturation in Starfish: Sequential Changes of Oocyte Shape Induced by 1-Methyladenine Associated with Germinal Vesicle Breakdown

Observation of the early events of starfish oocyte maturation revealed a sequential change of the oocyte shape induced by 1-methyladenine (1-MeAde). The lengths of two diameters of the whole oocyte, one parallel to the egg axis (a–v diameter) and the other which is perpendicular to a–v diameter (e diameter) were measured by taking photographs successively. About 10 min after 1–MeAde administration, a sudden decrease of the a–v diameter (shortening of oocyte) occurred followed by a sudden increase (elongation of oocyte). Germinal vesicle breakdown (GVBD) occurred when the a–v diameter suddenly increased. The change of the oocyte shape occurred differentially in the animal and vegital halves i.e. when the egg-axis diameter shortened the surface of the vegital pole side shrank and the surface of the animal pole side expanded. GVBD was suppressed under hypertonic conditions and facilitated under hypotonic conditions. Cytological examination of the process of GVBD revealed (1) separation of the nuclear membrane from the granular cytoplasmic mass, (2) depression of several parts of the nuclear membrane and the increase of amorphous matrix on the outside of the nuclear membrane, and (3) disappearance (fragmentation) of nuclear membrane. The morphological changes were not the same along the egg-axis temporally and spacially. These observations suggest firstly that the cytoplasm, which has been in a certain quiescent state with a certain rigidity before 1–MeAde administration, not only undergoes a decrease in stiffness but also acquires a contractile property after 1–MeAde administration, and secondly that contractile or cytoskeletal components do not evenly function or are not evenly arranged in the animal and vegital halves.