Invertebrate Biology 133(1): 1-46.

Scanning electron microscopic study of tile pattern of ciliary coordination and the form of the ciliary beat is now possible. Rapid fixation stops tile ciliary activity instantaneously, and critical point drying avoids distortion of the cilia by surface tension forces. Such stuidies have been made on the ciliate Opalina with this new technique.

We have used a newly discovered reversal response of ctenophore comb plates to investigate the structural mechanisms controlling the direction of ciliary bending. High K+ concentrations cause cydippid larvae of the ctenophore Pleurobrachia to swim backward . High-speed cine films of backward-swimming animals show a 180°reversal in beat direction of the comb plates . Ion substitution and blocking experiments with artificial seawaters demonstrate that ciliary reversal is a Ca'-dependent response . Comb plate cilia possess unique morphological markers for numbering specific outer-doublet microtubules and identifying the sidedness of the central pair . Comb plates of forward-and backward-swimming ctenophores were frozen in different stages of the beat cycle by an "instantaneous fixation" method . Analysis of transverse and longitudinal sections of instantaneously fixed cilia showed that the assembly of outer doublets does not twist during ciliary reversal . This directly confirms the existence of a radial switching mechanism regulating the sequence of active sliding on opposite sides of the axoneme.We also found that the axis of the central pair always remains perpendicular to the plane of bending; more importantly, the ultrastructural marker showed that the central pair does not rotate during a 180°reversal in beat direction . Thus, the orientation of the central pair does not control the direction of ciliary bending (i .e ., the pattern of active sliding around the axoneme) . We discuss the validity of this finding for three-dimensional as well as two-dimensional ciliary beat cycles and conclude that models of central-pair function based on correlative data alone must now be re-examined in light of these new findings on causal relations.Various modifications in the pattern of ciliary and flagellar beating are known to be caused by transient increases in free Ca" acting directly on the 9 + 2 axoneme itself (3-6, 15, 17, 20, 24, 34, 54, 56). These Ca'-dependent motor responses include changes in the direction of ciliary beating in protozoa (8,10,26,29,34), reversal of the direction of flagellar bend propagation in trypanosomes (20), alterations in he symmetry of flagellar wave forms in sperm and algae (3-6, 24, 45), regulation of ciliary beat frequency (8,9,30,31), and arrest of ciliary beating in various animals (12,17,32,54,56) .Despite their common ionic basis, little is known about the molecular mechanisms and axonemal components responsible for these motor responses . We investigate here the ultrastructural basis of one aspect of the Ca"-regulatory system in cilia-the control of bend direction . This problem is closely related to the basic mechanism of how cilia and flagella beat . Bending is the result of active sliding between adjacent doublet microtubules, driven by dynein ATPase arms (16,43,46,57), coupled with resistance to sliding, which converts translational

Regeneration of missing body parts in model organisms provides information on the mechanisms underlying the regeneration process. The aim here is to use ctenophores to investigate regeneration of their giant ciliary swimming plates. When part of a row of comb plates on Mnemiopsis is excised, the wound closes and heals, greatly increasing the distance between comb plates near the former cut edges. Video differential interference contrast (DIC) microscopy of the regeneration of new comb plates between widely separated plates shows localized widenings of the interplate ciliated groove (ICG) first, followed by growth of two opposing groups of comb plate cilia on either side. The split parts of a new plate elongate as their bases extend laterally away from the ICG widening and continue ciliogenesis at both ends. The split parts of a new plate grow longer and move closer together into the ICG widening until they merge into a single plate that interrupts the ICG in a normal manner. Video DIC snapshots of dissected gap preparations 1.5-3-day postoperation show that ICG widenings and/or new plates do not all appear at the same time or with uniform spacing within a gap: the lengths and distances between young plates in a gap are quite variable. Video stereo microscopy of intact animals 3-4 days after the operation show that all the new plates that will form in a gap are present, fairly evenly spaced and similar in length, but smaller and closer together than normal. Normal development of comb plates in embryos and growing animals is compared to the pattern of comb plate regeneration in adults. Comb plate regeneration differs in the cydippid Pleurobrachia that lacks ICGs and has a firmer mesoglea than Mnemiopsis. This study provides a morphological foundation for histological, cellular, and molecular analysis of ciliary regeneration in ctenophores.