High time-resolution analysis of transient bending patterns during ciliary responses following electric stimulation in sea urchin embryos

Time and extent of ciliary response to particles in a non-filtering feeding mechanism

Mechanisms of suspension feeding are usually described by the physics of inanimate filters. High-speed videorecordings in this study demonstrated that sea urchin larvae concentrate particles without filtration. They actively captured individual particles. At most times and places, the effective strokes of the swimming/feeding ciliary band were away from the circumoral field. Cilia of this band responded to particles by a reversal of beat that redirected the particle toward the circumoral field. A change of beat occurred along approximately 80 micro m of ciliary band during particle capture. Cilia responded 0.02 to 0.06 s after the particle was within reach of effective strokes and reversed beat, usually for about 0.1 to 0.2 s. The whole event (disruption of forward beat) generally lasted between 0.13 and 0.5 s. These observations imply reversed movement of a parcel of water much larger than the included captured particle, but particles are nevertheless greatly concentrated because water is directed toward the circumoral field only when and where a particle is sensed. Thus most of the concentration of particles occurs by a temporarily and locally redirected current, without filtration, and size and quality of particles captured depends on sensory capabilities, not the mechanics of filtration.

Progression of Flagellar Stages during Artificially Delayed Motility Initiation in Sea Urchin Sperm

Transition from immotile to motile flagella may involve a series of states, in which some of regulatory mechanisms underlying normal flagellar movement are working with others being still suppressed. To address ourselves to the study of starting transients of flagella, we analyzed flagellar movement of sea urchin sperm whose motility initiation had been retarded in an experimental solution, so that we could capture the instance at which individual spermatozoa began their flagellar beating. Initially straight and immotile flagella began to shiver at low amplitude, then propagated exclusively the principal bend (P bend), and finally started stable flagellar beating. The site of generation of the P bend in the P-bend propagating stage varied in position in the basal region up to 10 microm from the base, indicating that the ability of autonomous bend generation is not exclusively possessed by the very basal region but can be unmasked throughout a wider region when the reverse bend (R bend) is suppressed. The rate of change in the shear angle, the curvature of the R bend and the frequency and regularity of beating substantially increased upon transition from P-bend propagating to full-beating, while the propagation velocity of bends remained unchanged. These findings indicate that artificially delayed motility initiation may accompany sequential modification of the motile system and that mechanisms underlying flagellar motility can be analyzed separately under experimentally retarded conditions.

Expression of tryptophan 5-hydroxylase gene during sea urchin neurogenesis and role of serotonergic nervous system in larval behavior

Tryptophan 5-hydroxylase (TPH) is the rate-limiting enzyme in the biosynthesis of serotonin. cDNA cloning of TPH was carried out, and the occurrence of spatiotemporal transcription of TPH message was examined in larvae of the sea urchin, Hemicentrotus pulcherrimus (HpTPH), with in situ hybridization by using the tyramide signal amplification (TSA) technique and Northern hybridization. Based on deduced amino acids sequence of HpTPH, phylogenetically sea urchin locates at the closest position to vertebrates among invertebrates, and HpTPH had common conserved sequences in a catalytic domain. Initiation of HpTPH transcription occurred at the late gastrula stage exclusively in serotonin cells of apical ganglion (SAG) that was composed of a cluster of HpTPH-positive cells and the negative cells in between. In situ hybridization showed that the mRNA expression pattern was similar to the immunohistochemical localization of serotonin cells reported before (Bisgrove and Burke [1986] Dev. Growth Differ. 28:557-569; Yaguchi et al. [2000] Dev. Growth Differ. 42:479-488). p-Chlorophenylalanine (CPA), an irreversible inhibitor of TPH activity, considerably decreased serotonin content in the serotonin cells, whereas the HpTPH expression pattern and timing, and the extension of neurofibers from SAG cells were apparently unaffected, suggesting CPA exclusively perturbed synthesis of serotonin but not nervous system organization. CPA-treated larvae did not swim, despite the occurrence of ciliary beating in culture chamber, suggesting that proper serotonin synthesis is necessary for normal swimming of the larvae.

The ciliary cycle during hyperpolarization-induced activity: an analysis of axonemal functional parameters

Motor responses of the frontal cirri of the ciliate Stylonychia were recorded at the axial view of the ciliary base with high-speed cinematography. Voltage-clamp applying sustained hyperpolarizing voltage steps was used to explore the properties of the ciliary cycle modulated by the membrane potential. Upon hyperpolarization between -1 and -13 mV, a previously inactive frontal cirrus reoriented from a neutral posture and started beating so that the axis of the beating cone of a proximal cirral segment assumed an orientation near 100 degrees (proceeding counterclockwise from posterior = 0 degrees) and inclination near 60 degrees (0 degrees = perpendicular to the cell surface). The major beating amplitude was limited to about 150 degrees. Increasing hyperpolarization increased the spatial polarity of the cycle (ratio of major over minor amplitude, from 2 to 2.4). Rates of the power stroke increased with hyperpolarizations up to -4 mV but were consistently smaller than those of the return stroke during the ciliary cycle (ratio: 0.4 to 0.6; = temporal polarity). Comparison of different hypothetical beat forms (0-shape, D-shape, and egg-shape) showed that the orientation-time data are the major determinants of the angular velocity and rate of reorientation of the cilium during the cycle. Geometric transformation of these data led to descriptions of the cycle of a proximal ciliary segment in terms of active sliding velocities and rates of unidirectional sliding translocation between identified doublets. Three voltage-sensitive functional parameters of the cilium--the inclination (which is noncyclic) and the rates of active sliding and sliding translocation (both of which are cyclic in nature)--are discussed as generating the spatial and temporal properties of the ciliary beat.

Flagellar quiescence and transience of inactivation induced by rapid pH drop

The effects of rapid pH drop on the flagellar movement of reactivated sea urchin sperm were studied by video microscopy and by a newly developed pH jump method. Triton-demembranated sperm were reactivated in a thin layer of the reactivation medium containing ATP and potassium acetate and supported by a ring-shaped Millipore filter stuck to the lower surface of a supported coverslip. The pH of the medium was lowered rapidly by dissolving acetic acid vapor abruptly introduced into a gap between the cover and slide. Flagellar beating ceased immediately when the pH of the reactivation medium was lowered. At least two types of cessation were distinguished: 1) "instantaneous" cessation in a bent form closely resembling those characteristic of steady-state beating before pH drop (waveform freeze), and 2) flagellar quiescence in a cane-shaped form resembling those characteristic of Ca-induced quiescence (cane-shaped quiescence). The flagellum again began beating if the pH was raised to normal but eventually was disintegrated by tubule sliding if the pH was left lowered. Field-by-field analysis of the transient movement of flagella becoming quiescent upon pH drop demonstrated that the proximal bend of the cane-shaped form corresponded to the principal bend of the steady-state beating in some flagella, but in others, to the reverse bend. These observations indicate that low pHs affect flagellar beating by interfering with sliding-bending conversion by a mechanism different from that previously reported.