Cytological Studies of Large Nucleus-Like Structures Formed by Exogenously-Injected Linear and Circular DNAs in...

Relatively large amounts of linear or circular DNAs were injected into the animal or vegetal cytoplasm of fertilized eggs of Xenopus laevis and the nature of large nucleus-like structures formed by the injected DNAs was studied cytologically and electron microscopically. Results of fluorescent microscopic examination combined with immunohistochemical analysis strongly suggested that the injected DNAs were assembled into the nucleus-like structures probably after being complexed with maternal histones. The assemblage of nucleus-like structures preferentially took place in the animal most region of the egg, and the size of the nucleus-like structures formed depended on the amount of the injected DNA. The nucleus-like structures were surrounded by double membranes equipped with nuclear pore complexes, but there were at least two abnormal features in their ultrastructures. First, nucleus-like structures contained cytoplasmic particulate materials, most probably ribosomes and/or glycogen granules. Secondly, many of the nuclear pore complexes on the "nuclear envelope" appeared to be incomplete, with blebs formed from inner leaflet and protruded into the perinuclear space. Injected circular plasmid DNAs were also assembled into large nucleus-like structures in the animal most region, and appeared to be partitioned into descendant cells during the cleavage.