POLARIZING MICROSCOPE STUDY OF A CONTRACTILE NANOFILAMENT SYSTEM - THE ACANTHARIAN MYONEME

Birefringence changes have been studied during the contraction-relaxation cycle of the myonemes (contractile organelles consisting of a bundle of nonactin filaments) in Acantharians (Protista, Actinopoda). Myonemes can either contract rapidly or undulate slowly between their anchorage points. In thin sections they appear as a large cross-striated bundle with long clear zones (LZs) and thin transversal dense bands (TBs). The filaments (2-4 nm in diameter) are twisted in pairs in elementary microstrands. The spacing of the LZ depends on the extent of contraction [16]. A negative birefringence has been seen in vivo and in vitro. In vivo, retardation varied with the extent of relaxation of the myoneme (2.5 nm-4.4 nm). When the myoneme was tightened, it appeared to be homogeneously dark or bright in contrast depending on the orientation for the vibrating plane of polarized light. When it was partially relaxed and moved slowly, a series of birefringent bands, 0.8-2.8 μm thick, could be seen. They propagated at the same speed (about 2 μm·s-1) by successive trains, either forward or backward. Each of these birefringent bands may correspond to 4-13 contracted LZs. It is suggested that the negative birefringence of the myoneme is mainly caused by the orientation of the filaments forming the microstrands. Our results strongly suggest that the orientation of the myoneme filaments is altered during the movement and that the orientation of the filaments in the negative bands is caused by perpendicular orientation compared to the other parts of the myoneme. These observations support our previous hypothesis in which we postulated that the length of the myoneme varies in relation to the pitch of the elementary microstrands. © 1990.