Modifications of size and pattern of microtubular organelles in overfed cells of a ciliate Dileptus.

The size of mouthparts and their constituent organelles was studied in cells of Dileptus anser, enlarged by overfeeding. The oral structures of large cells were either of normal dimensions and appearance, or they were enlarged and deformed in shape. The increase in size of mouthparts was accompanied by an increase in number of their microtubular organelles. There was, however, no net increase in size of the organelles, as defined by length and number of microtubules they contained. It is concluded that there exists an upward regulation in the size of the mouthparts as a whole, whilst the size of their constituent organelles is probably not so regulated. It is supposed that the patterning of normal-size organelles into large oral structure may lead to the observed deformations in the shape of the mouthparts.

A model for chromosome movement based on lateral interaction of spindle microtubules.

Quantitative morphological studies of meiotic spindles in the crane fly Pales ferruginea (Fuge, 1980, 1984, 1985) were the basis for the development of a model explaining anaphase chromosome transport in higher eukaryotes. Two main features of chromosome fibres were important for the model: (1) the existence of microtubules oriented obliquely with respect to kinetochore microtubules, and (2) a higher degree of disorder in fibres exerting a pulling force. It is postulated that microtubules of the same polarity being inclined to each other at a certain angle are able to slide past each other by means of mechano-chemically active side-arms working in alternate succession. Sliding is suggested to lead to a displacement of microtubules and chromosomes in direction towards the poles. Furthermore, it is suggested that the chromosome fibre in anaphase becomes progressively disintegrated by fragmentation and disassembly of microtubules. Fragmentation may be induced by bending stress within the dynamic system.