Fine Structure of Conjugation of the Ciliate Blepharisma japonicum I. Changes of the Old Macronucleus

Abstract

The macronucleus of vegetative cells of Blepharisma japonicum contains numerous discrete chromatin bodies and nucleoli of the compact type. The nucleoli contain very dense inclusions likely to be the dense fibrillar component of the nucleoli themselves. The macronuclear envelope displays tightly packed pore complexes. Until the 10th hour of conjugation, the old macronucleus shows the same fine structure as in vegetative cells. At 12 hours (approximately during metaphase I), the fibrillar and granular parts of the nucleoli segregate and the latter gradually disappear. At 14 hours, all nucleoli are reduced to their fibrillar parts that have the aspect of small dense bodies scattered in the nucleus. Simultaneously, the chromatin bodies begin to unite into a common network. This process continues at 16 hours (stage of pronuclei). The chromatin network is connected to the nuclear envelope via submembrane bodies of condensed chromatin tightly applied to the inner nuclear membrane. The macronuclear envelope has no coating of dense material in these areas. Thereafter (24 and 26 hours), areas of nuclear matrix free of the chromatin network appear in the macronucleus. At 28 and 34 hours, the chromatin network condenses and the residual nucleoli are caught in its meshes, being incorporated into the chromatin. The points of contact of the network with the nuclear envelope become less numerous.