Changes in rostral structures and the nuclear events which occur in dividing cells of Trichonympha agilis (obtained from experimentally refaunated termites) were studied by means of electron microscopy of serial sections. It is possible to characterize 5 stages of division:
Stage I. During this earliest recognizable division stage, the bilaterally symmetrical hemirostra have begun to separate and spindle microtubules appear in the intervening space. As in interphase, the kinetochore regions of chromosomes are distinguishable as fibrillar masses underlying the intact nuclear envelope; and, in individual sections, they are often seen to occur in pairs. These pairs are taken to be sister kinetochores.
Stage II. The extranuclear spindle has become established between the posterior ends of well separated hemirostral tubes. Elaboration of daughter rostral structures begins and will continue through the subsequent stages of division. Kinetochores differentiate, becoming bipartite structures consisting of a fibrillar element underlain by a dense disk. The fibrillar kinetochore element is associated with the still-intact nuclear envelope which lies between kinetochores and cytoplasmic spindle microtubules. Reconstruction from serial sections shows all kinetochores to be disposed in pairs which are distributed randomly over the nuclear surface.
Stage III. The fibrillar elements of kinetochores are enclosed in evaginations of the nuclear envelope, while the disk elements have come to lie in the plane of the nuclear surface. Kinetochores remain separated from the extranuclear spindle microtubules by the intact nuclear envelope. The distribution of kinetochores has changed relative to that seen in stage II: kinetochores no longer appear to be paired, and they are confined to that hemisphere of the nuclear surface closest to the spindle.
Stage IV. The nuclear envelope opens at the sites of kinetochores, leaving the dense disk kinetochore element inserted in pore-like discontinuities of the nuclear envelope and the fibrillar element in the cytoplasm. Direct interaction between fibrillar kinetochore element and extranuclear spindle microtubules is, however, not yet established.
Stage V. The cytoplasmically situated fibrillar elements of ‘inserted’ kinetochores are now in direct contact with spindle microtubules. As seen in reconstructions of the nucleus from serial sections, kinetochores have become segregated in 2 groups on the nuclear surface, one near each spindle pole. It is during this stage that final elaboration of rostral structures takes place.
On the basis of the observed changes in kinetochore distribution which occur between stages II and III while the intact nuclear envelope prevents any direct interaction between intra-nuclear kinetochores and extranuclear spindle microtubules, it is suggested that kinetochore-membrane interaction is involved in early chromosome movement in Trichonympha agilis. Only during stage V, when direct contact between kinetochores and spindle microtubules is established, may the microtubules assume their usual role in chromosome movement.