title: Direct evidence for the occurrence of simple microtubule organizing centers (MTOCs) and for end-to-end joining of microtubules with modified ends in the reticulopodia of Allogromia laticollaris (foraminifera).
abstract: The partial disassembly of filopodial microtubules into helical filaments, respectively the reassembly from paracrystals of exactly registered tubulin helices was recognized as a special adaptation to the highly dynamic reticulopodial movements in the foraminifer Allogromia laticollaris [4, 16]. Electron microscopic investigations of partially extracted or completely lysed filopodial strands revealed some additional adaptations to the local tubulin demands within the widely spread, rapidly changing reticulopodial networks. As probable foci for the initiation of additional microtubules from pools of tubulin during the netto growth phase, small (40 × 60 nm) cube-shaped, electron dense bodies were discovered which could act as mobile MTOCs at very distant sites from the metabolically active cell body within the reticulopodial system. The microtubule ends, proximally as well as distally to the filopodial origins, are characterized by a closed appearance because of a terminal cap which is continuous with the microtubule wall, whereas some other microtubules appear as the usual open cylinders. It is suggested that the closed ends, which occur either freely or decorated by the cube-shaped, electron dense bodies, may play a role in the de novo initiation of microtubules as well as in the transformation to the helical state or vice versa. Points of discontinuity within single longitudinally cut microtubules can be observed directly by electron microscopy and correspond to end-to-end contacts of closed MT-terminations. These junctions probably represent initial contact points of an annealing process between adjacent shorter MT-segments. Microtubule annealing occurs rapidly in vitro [14, 15] and could explain the elongation rates observed during filopodial outgrowth.
authors.affiliation: Lehrstuhl für Neurophysiologie des Zoologischen Institutes der Universität Münster, Münster, FRG.
authors.forename: R
authors.initials: R
authors.lastname: Golz
authors.forename: M
authors.initials: M
authors.lastname: Hauser