Conjugation in Tetrahymena: Ultrastructure of the meiotic prophase of the micronucleus

The early conjugation process of Tetrahymena thermopbila was studied with particular attention to the fine structure of the "crescent" micronucleus. The stationary macro- and micronuclei are tied together by the fused outer nuclear membrane. Inside the micronucleus, thick chromatin threads form a dense network. The micronucleus becomes detached from the concavity of the macronucleus and swells and loosened chromatin threads form fine threads. Most threads are gathered together in the center to form a large chromatin body, but some threads remain partly dispersed along the periphery. The micronucleus is elongated by extended microtubules below the nuclear membrane. Chromatin threads that are anchored to structures considered to be precursors of kinetochores (PKI) are present in the area that is proximal to the joint region of a conjugant as well as to the inner nuclear membrane at the distal end. These extend together with the microtubules and become dispersed. The PKIs extend their own microtubles to the proximal end, pulling the chromatin threads back into the distal area. In the fully extended U-shaped micronucleus, PKI-microtubules have disappeared, and kinetochores (KI) are located in the vicinity of the inner nuclear membrane. The chromatin threads have become detached from the KI and have reassembled to form a dense network in the distal area.