Hormone‐cytoplasmic interactions controlling sperm nuclear decondensation and male pronuclear development in starfish oocytes

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Abstract
Numerous highly condensed sperm nuclei were present in the cytoplasm of starfish oocytes fertilized at the germinal vesicle stage. Decondensation of sperm nuclei and formation of male pronuclei and asters followed treatment of such fertilized oocytes with meiosis‐inducing hormone 1‐methyladenine (1‐MA). Transformation of sperm nuclei was asynchronous, occurred only after germinal vesicle breakdown (GVBD), and was associated with characteristic clearing of the adjacent cytoplasmic organelles. GVBD and sperm nuclear changes were both inhibited when dinitrophenol was added to previously fertilized GV‐intact oocytes. Results demonstrate that polyspermy occurs readily in immature oocytes and that 1‐MA plays a key role in establishing the block to polyspermy. Furthermore, the factors necessary for sperm nuclear decondensation and pronuclear development do not exist in the cytoplasm of immature oocytes but arise following dispersal of germinal vesicle contents into the cytoplasm. Thus, absence of embryonic development in fertilized GV‐intact oocytes is linked to a lack of both oocyte and sperm nuclear differentiation rather than to a lack of sperm entry into the cytoplasm.