Development of the ciliature of Tetrahymena thermophila: I. Temporal coordination with oral development

The number of basal bodies and cilia along pole-to-pole ciliary rows was enumerated in Tetrahymena thermophila cells sampled during the rapid-exponential phase of culture growth in three different media that supported generation times ranging from 2 to 4 hr. The time required for oral development was nearly constant in the three media, and thus most of the differences in generation time were accounted for by differences in the interval prior to the onset of oral development (stage 0), which ranged from 50% of the generation time in the “poorest” medium to 20% in the “richest.” There was very little increase in number of basal bodies and of cilia along ciliary rows during stage 0, irrespective of the duration of this stage. The bulk of the increase took place during oral development, following a time course suggestive of coordination wth oral development. The same temporal pattern of increase was found in several ciliary rows, although the proportion of basal bodies that were ciliated differed among rows. There is no simple relationship between the number of basal bodies along ciliary rows and cell length, surface area, or volume. However, a large and constant proportion of the total division-to-division cell growth took place during the interval prior to the onset of oral development, suggesting that an ensemble of developmental events, including oral development and an associated activation of the remainder of the cell surface, may be triggered by attainment of a threshold cell size.     

Development of the ciliature of Tetrahymena thermophila: II. Spatial subdivision prior to cytokinesis

The cell surface of Tetrahymena thermophila is made up of an anterior region in which virtually all basal bodies of ciliary rows are ciliated, and the remainder in which ciliated and unciliated basal bodies are fairly irregularly interspersed. This pattern persists through interfission development until the stage of appearance of the equatorial ring of gaps in the ciliary rows that marks the fission zone. The ciliation pattern then becomes subdivided, in large part through the rapid ciliation of contiguous basal bodies located posterior to the fission zone. We interpret this process as a wave of ciliation of preexisting basal bodies that propagates posteriorly from the site of the fission zone. The location, extent, and timing of the ciliation process are the same in inverted as in normally oriented ciliary rows, in spite of the fact that in inverted rows the visible fission zone gap is tardily formed and the local configuration of ciliature around this gap is abnormal. The putative ciliation wave thus does not depend directly upon the local manifestations of the fission zone. However, in a cell-division-arrest mutant, cdaA1, analyzed under conditions in which formation of fission-zone gaps is permanently prevented in some ciliary rows but not in all, it is found that the ciliation pattern becomes subdivided in those ciliary rows that express fission-zone gaps and fails to become subdivided in neighboring rows that fail to manifest gaps. We interpret this combination of findings to indicate that a signal localized at the cell equator initiates a set of polarized developmental events that simultaneously create and demarcate two cellular fields within what was previously one. We further suggest that the characteristic tandem cell division pattern of ciliates is fundamentally a process of segmentation, which might involve mechanisms of gradient subdivision analogous to those taking place during segmentation of insects and other multicellular organisms.     

Scanning Electron Microscopy of Cytoskeletal Elements in the Oral Apparatus of Tetrahymena1

ABSTRACT. Extraction of the ciliated protozoon Tetrahymena with nonionic detergents produces a surface-related cytoskeleton that consists of a basic lamina of whole-cell dimensions together with associated microtubule and microfilament systems, including all ciliary basal bodies. The organization of the isolated cytoskeleton has been studied using scanning electron microscopy, and several new features are described in the oral region. Here the ciliary basal bodies are arranged in a very stable and highly complex pattern. This pattern was found to be identical in the four species of Tetrahymena we examined. In addition, various microtubular bundles and two separate systems of filaments were observed in scanning electron micrographs of isolated oral skeletons. The appearance of the deep fiber bundle in preparations of this type suggests that it arises, at least in part, as an extension of the ribbed wall microtubules. On the basis of its distribution within the oral skeleton, one of the filament systems described is suggested to be a contractile system responsible for pinching off food vacuoles.     

Hereditary variation of ciliary patterns in ciliated protozoa

Summary— In ciliates, the basic pattern of ciliature consists of longitudinal polarized ciliary rows. This basic pattern is expected to be retained through successive binary fissions by means of the so-called cytotactic mechanisms, as a consequence of autogenetic proliferation of basal bodies within each row. This idea is supported by the hereditary transmission of 180° rotated rows in Tetrahymena and Paramecium. These mechanisms should theoretically ensure intraclonal homogeneity for ciliary row number. In fact, some alterations are responsible for either loss or addition of rows. Such alterations are not strictly random processes, since they have been shown to be controlled by nuclear genes. Cytotaxis does not account for exact positioning of primordia for complex structures (buccal ciliature, for instance): the sites of basal body proliferation are determined by certain “positional information” which, in turn, is controlled by chromosomal genetic material more or less independently of pre-existing structures, as illustrated by many spontaneous or induced mutations. All deviations from the wild-type phenotype seem to be associated with very reduced fitness, at least in laboratory conditions. On the other hand, the currently known variants cannot account for intraspecific diversity. Thus, the evolutionary significance of these phenomena remains somewhat obscure.     

Different functions of tight junctions in the ascidian branchial basket

The stigmatal cells in the branchial basket of ascidians from a number of genera have been examined as to the nature and distribution of their intercellular junctions. The branchial wall consists of ciliated and parietal cells; the ciliated cells are arranged in seven rows and are associated by junctions with other cells in the same row as well as with those in adjacent rows. They are also associated by junctions with peripheral parietal cells. Junctions between adjacent ciliated cells in all cases exhibit tight junctions or zonulae occludentes. However, these cell borders also possess fasciae or zonulae adhaerentes if they are in the same row and the ciliary rootlets insert-into these junctions. If the cells are in adjacent rows they exhibit adhaerentes junctions only in species belonging to the orders Phlebobranchiata and Aplousobranchiata. In contrast, if the cells in adjacent rows belong to the order Stolidobranchiata. they never exhibit any adhaerentes junctions and the ciliary rootlets of the basal bodies from the cilia insert instead into the tight junctions and the non-junctional membrane below them. At the homologous junctional borders between adjacent parietal cells and also at heterologous junctional borders between parietal and ciliated cells, tight junctions alone occur, with no co-existing adhaerentes junctions along their lateral borders. Again, fibrils from ciliary rootlets insert into zonulae occludentes. This shows that tight junctions are capable both of forming permeability barriers, in that they can be seen to prevent the entry of exogenous tracers such as lanthanum, and of acting as adhesive devices.     

Gradients of proliferation of ciliary basal bodies and the determination of the position of the oral primordium in Tetrahymena.

The pattern of proliferation of new basal bodies in ciliary rows (somatic proliferation) in Tetrahymena was observed. Starved and refed cells were used, because proliferation in these cells is more pronounced than that under other circumstances. The formation of new basal bodies is locally determined by the position of "old" pre-existing basal body (short range determination). However, the probability of proliferation associated with any given "old" basal body differs very much. This probability is determined by the spatial coordinates of the particular region of the cell (long range determination); however some randomness in this process was also observed. Two different gradients of proliferation were found. The first gradient is circumferential with a maximum number of new basal bodies added in ciliary rows n, 1, 2 and 3 and the minimum number added in ciliary rows 7, 8 and 9. The second is an antero-posterior gradient with the highest number of new basal bodies added in the midbody region. Moreover, at least in some cases, new oral primordia first appear, as a random proliferation of new basal bodies adjacent to a few old cilia of ciliary row No. 1, resembling somatic proliferation. Then 2,3 or even more clumps of basal bodies appear, each having one old cilium posteriorly. These clumps, however, are not linear groups within the ciliary row but instead they form small fields of basal bodies. These findings suggest, that the same two-gradient system for new basal body addition operates during somatic proliferation and also determines the position of the new oral primordium as the site of the highest gradient value at the intersection of two gradients.     

The Positioning of Ciliary Organelles in Hypotrich Ciliates*†

It is commonly observed in hypotrichs that new ciliary rudiments arise directly from or in close juxtaposition to certain pre-existing ciliary elements. Oral primordia often are initiated near specific cirri, cirral rudiments frequently arise as a result of the disaggregation of certain old cirri, and new dorsal ciliature is formed within pre-existing ciliary rows. In the first 2 situations it has been demonstrated experimentally that neither the old ciliature in question nor the specific cortical site marked by that ciliature is essential for the appearance of the new cirral rudiment. The experimental analysis done thus far suggests that the positions of oral and cirral primordia are determined by interacting gradients established in relation to certain reference points. The nature of the reference points is not fully elucidated; in some cases at least these points appear to be more closely related to topographic features of the cell than to specific pre-existing cortical structures. In the dorsal ciliary rows of Euplotes new ciliary units are formed usually and perhaps invariably in close proximity to old ones, and are generally oriented along the axis of the pre-existing row. The result is a tendency to perpetuate the preexisting row number across cell generations. Changes in row number, however, can occur as a result of occasional formation of new units at right angles to the row, a process that is much enhanced in certain homozygous segregants (basal body deficient). The optimal row number (stability range) as well as the number of ciliary units are under genic control. In addition, the spatial pattern of distribution of ciliary units among rows is invariant in all of the material examined. This pattern is presumed to result from an underlying field whose geometry is independent of both the number of units and the number of rows.     

The filaments supporting ciliary primordia and fission furrow in the hypotrich ciliateParaurostyla weissei, revealed by the monoclonal antibody 12G9: Studies on wild-type and ciliary hypertrophy mutant

Summary The unique monoclonal antibody FXXXIX 12G9 obtained againstTetrahymena cortices was used to label cytoskeletal structures related to basal body proliferation inParaurostyla weissei. The antibody binds to an amorphous material interconnecting basal bodies in compound ciliary structures: dorsal units, cirri and membranelles in interfission cells, and filamentous structures supporting the primordia of ciliary structures and fission line in dividing cells. The antibody visualized meridional filaments preceding proliferation of new basal bodies in the oral primordium and structures accompanying all developing ciliary primordia. It congregated in differentiating new procirri and membranelles, whereas another population of transient meridional structures accompanied the final distribution of new structures. A meridional filament connecting transverse cirri with the oral apparatus, marking the future stomatogenic meridian, persisted in both division products until completion of cell elongation. The fission line was found to originate from an anterior extension of the pre-oral filament toward the parental oral structures. It then encircled the cell's midbody demarcating the boundary between daughter cells; two additional circumferential structures bordering the anterior and posterior ends of differentiating division products participate in formation of the new poles. They disappear after separation of daughter cells and completion of resorption of parental ciliature. In the enhanced multi-left-marginal mutant expressing gross hyperduplication of basal bodies, the location of the 12G9 antigen corresponded to that in wild-type cells. The sequence of formation of meridional filaments in the mutant was found to be altered. The filaments in the left lateral domain preceded the formation of the preoral filament, yet the temporal pattern of basal body assembly was not modified. The fission line, as in wild-type cells, originated in connection with the oral primordium. We conclude that the nucleation of the filamentous structures bearing the 12G9 antigen and the basal body assembly occur by independent mechanisms reading the same cell cycle signals. We suggest that the 12G9-antigen-bearing protein might be similar to septins: involved in signaling the position of the oral primordium and the fission line and functioning in establishing and maintaining the asymmetric cortical domain characteristics.Abbrevations AZM zone of adorai membranelles - bb basal bodies - CC caudal cirri - FC frontal cirri - Fmf frontal meridional filament - FTV the primordia of fronto-ventro-transverse cirri - LD, RD dorsal rows of bristle units - LM, RM left or right marginal cirral row - OA oral apparatus - OP primordium of the adoral membranelles - pLM, pRM primordium of the left or right marginal cirri - pLD, pRD primordia of the left or right dorsal bristle rows - pUM primordium of the undulating membranes - TC transverse cirri - UM undulating membranes - VC ventral cirral rows     

Organization of actin microfilaments in the apical border of oviduct ciliated cells

Actin microfilaments were localized in quail oviduct ciliated cells using decoration with myosin subfragment S1 and immunogold labeling. These polarized epithelial cells show a well developed cytoskeleton due to the presence of numerous cilia and microvilli at their apical pole. Most S1-decorated microfilaments extend from the microvilli downward towards the upper part of the ciliary striated rootlets with which they are connected. From the microvillous roots, a few microfilaments connect the proximal part of the basal body or the basal foot associated with the basal body. Microfilament polarity is shown by S1 arrowheads pointing away from the microvillous tip to the cell body. Furthermore, short microfilaments are attached to the plasma membrane at the anchoring sites of basal bodies and run along the basal body. The polarity of these short microfilaments is directed from the basal body anchoring fibers downward to the cytoplasm. At the cell periphery, microfilaments from microvillous roots and ciliary apparatus are connected with those of the circumferential actin belt which is associated with the apical zonula adhaerens. Together with the other cytoskeletal elements, the microfilaments increase ciliary anchorage and could be involved in the coordination of ciliary beating. Moreover, microvilli surrounding the cilia probably modify ciliary beating by offering resistance to cilium bending. The presence of microvilli could explain the fact that mainly the upper part of the cilia appanars to be involved in the axonemal bending in metazoan ciliated cells.     

The Permanence of the Infraciliature in Suctoria: An Electronmicroscopic Study of Pattern Formation in Tokophrya infusionum*

SYNOPSIS. The adult Tokophrya infusionum does not possess cilia, but has 20–30 barren basal bodies arranged in 6 short rows adjacent to the contractile vacuole pore. During reproduction, which is by internal budding, the contractile vacuole sinks into the parent along with the invaginating membranes that form the embryo and the wall of the brood pouch. The 6 rows of basal bodies radiate away from the pore and elongate to form 5 long ciliary rows, that encircle the anterior half of the embryo, and 1 short row at the posterior end. The contractile vacuole pore, along with several barren basal bodies, remains in the parent when the embryo is completed. The pore rises to the surface when the embryo is born. New basal bodies are then formed in the parent to replace those which were incorporated into the embryo, and formation of another embryo may begin. The cilia of the embryo are partially resorbed 10 min after the start of metamorphosis, with depolymerization of the ciliary microtubules. Later, the cilia and most of the basal bodies disappear completely, except for a group of barren basal bodies near the embryo's contractile vacuole pore, which form 6 rows and serve as an anlage for the basal bodies and cilia that arise during embryogenesis. There is, therefore, an organized infraciliature in Suctoria throughout their life cycle, and a distinct continuity of basal bodies across the generations.     

Relationship Between Spatial Pattern of Basal Bodies and Membrane Skeleton (Epiplasm) During the Cell Cycle of Tetrahymena: cdaA Mutant and Anti-Membrane Skeleton Immunostaining

Microtubular basal bodies and epiplasm (membrane skeleton) are the main components of the cortical skeleton of Tetrahymena. The aim of this report was to study functional interactions of basal bodies and epiplasm during the cell cycle. The cortex of Tetrahymena cells was stained with anti-epiplasm antibody. This staining produced a bright epiplasmic layer with a dark pattern of unstained microtubular structures. The fluorescence of the anti-epiplasm antibody disappeared at sites of newly formed microtubular structures, so the new basal body domains and epiplasmic layer could be followed throughout the cell cycle. Different patterns of deployment of new basal bodies were observed in early and advanced dividers. In advanced dividers the fluorescence of the epiplasmic layer diminished locally within the forming fission line where the polymerization of new basal bodies largely extincted. In wild type Tetrahymena, the completion of the micronuclear metaphase/anaphase transition was associated with a transition from the pattern of new basal body deployment and epiplasm staining of the early divider to the pattern of the advanced dividers. The signal for the fission line formation in Tetrahymena (absent in cdaA1 Tetrahymena mutationally arrested in cytokinesis) brings about 1) transition of patterns of deployment of basal bodies and epiplasmic layer on both sides of the fission line; and 2) coordination of cortical divisional morphogenesis with the micronuclear mitotic cycle.     

Cytoskeletal proteins of the cell surface in Tetrahymena : I. Identification and localization of major proteins

Isolated pellicles (cell ‘ghosts’) have been prepared from Tetrahymena thermophila strain B by two different methods. Using differential solubilization in combination with polyacrylamide gel electrophoresis and electron microscopy, we have tentatively identified the major proteins found in the surface-associated cytoskeleton. The ‘epiplasm’, a continuous layer of fibrous material found just beneath the surface membranes, appears to contain two major proteins. The smaller of the two (mol. wt 122 000 D) is believed to be present throughout the layer, whereas the larger protein (mol. wt 145 000 D) appears to be localized in the regions where ciliary basal bodies connect to the epiplasmic layer and to surface membranes. Evidence is presented which suggests that actin may also be present in this structure. Tubulin has been isolated from the cytosol of Tetrahymena and compared with cytoskeletal tubulin and porcine brain tubulin. A major protein of mol. wt 250 000 D which is found in Tetrahymena pellicles appears to be the major component of kinetodesmal fibers (striated elements which attach to the ciliary basal bodies).     

Ultrastructure and cell size-dependent regulation of the adoral zone of membranelles in the heterotrich ciliate Stentor coeruleus

The number and length of oral membranelles were determined for both large and small Stentor from well-fed, growing cultures and nutrient-deprived cultures, respectively. Small cells possess both significantly fewer and shorter membranelles than do large cells. For both large and small cells, each membranelle is composed of three rows of basal bodies. The membranelles closest to the gullet have a third row that is only slightly shorter than the other two. The third row becomes rapidly shorter as membranelles become increasingly distant from the gullet. A short distance from the gullet, and for the remainder of the band, the third row is composed ofonly one to four basal bodies. The first two rows consist of approximately 35 basal bodies each in large cells and approximately 26 basal bodies each in small cells. This indicates that Stentor regulates the number of basal bodies per row, but not the number of rows, in response to changes in cell size. © 1992 Wiley-Liss, Inc.     

Patterns of basal body addition in ciliary rows in Tetrahymena

Most naked basal bodies visualized in protargol stains on the surface of Tetrahymena are new basal bodies which have not yet developed cilia. The rarity of short cilia is explained by the rapid development of the ciliary shaft once it begins to grow. The high frequency of naked basal bodies (about 50 percent) in log cultures indicates that the interval between assembly of the basal body and the initiation of the cilium is long, approximately a full cell cycle. Naked basal bodies are more frequent in the mid and posterior parts of the cell and two or more naked basal bodies may be associated with one ciliated basal body in these regions. Daughter cells produced at division are apparently asymmetric with respect to their endowment of new and old organelles.     

Immuno-electron-microscopic localization of a centriole-related antigen in ciliated cells

Summary An antigen common to purported centriolar and basal body regions of a variety of cell types was previously visualized by immuno-fluorescence microscopy. The present study demonstrates the localization of the antigen relative to the defined basal body structures of ciliated tracheal cells at the electron-microscopic level. After ethyldimethylaminopropyl carbodiimide-glutaraldehyde-saponin (EGS) fixation and permeabilization, immunoferritin labeling is consistently found associated with amorphous electron-opaque material in proximity to basal bodies and their ciliary rootlets, but not with basal body microtubules themselves. This distribution pattern is distinct from that of other proteins found in the apical region of ciliated cells, such as calmodulin. It is proposed that the dense material may be analogous to pericentriolar material of centrosomes.     

The Dynamics of Filamentous Structures in the Apical Band, Oral Crescent, Fission Line and the Postoral Meridional Filament in Tetrahymena thermophila Revealed by Monoclonal Antibody 12G9

The ciliate Tetrahymena thermophila possesses a multitude of cytoskeletal structures whose differentiation is related to the basal bodies the main mediators of the cortical pattern. This investigation deals with immunolocalization using light and electron microscopy of filaments labeled by the monoclonal antibody 12G9, which in other ciliates identifies filaments involved in transmission of cellular polarities and marks cell meridians with the highest morphogenetic potential. In Tetrahymena interphase cells, mAb 12G9 localizes to the sites of basal bodies and to the striated ciliary rootlets, to the apical band of filaments and to the fine fibrillar oral crescent. We followed the sequence of development of these structures during divisional morphogenesis. The labeling of the maternal oral crescent disappears in pre-metaphase cells and reappears during anaphase, concomitantly with differentiation of the new structure in the posterior daughter cell. In the posterior daughter cell, the new apical band originates as small clusters of filaments located at the base of the anterior basal bodies of the apical basal body couplets during early anaphase. The differentiation of the band is completed in the final stages of cytokinesis and in the young post-dividing cell. The maternal band is reorganized earlier, simultaneously with the oral structure.The mAb 12G9 identifies two transient structures present only in dividing cells. One is a medial structure demarcating the two daughter cells during metaphase and anaphase, and defining the new anterior border of the posterior daughter cell. The other is a post-oral meridional filament marking the stomatogenic meridian in postmetaphase cells. Comparative analysis of immunolocalization of transient filaments labeled with mAb12G9 in Tetrahymena and other ciliates indicates that this antibody identifies a protein bound to filamentous structures, which might play a role in relying polarities of cortical domains and could be a part of a mechanism which governs the positioning of cortical organelles in ciliates.     

Structural inheritance in Paramecium: ultrastructural evidence for basal body and associated rootlets polarity transmission through binary fission

Abstract One main difference between basal bodies and centrioles resides in the expression of their polarity: centrioles display a structural nine‐fold radial symmetry, whereas basal bodies express a circumferential polarity, thanks to their asymmetric set of rootlets. The origin of this polarity during organelle duplication still remains under debate: is it intrinsic to the nine‐fold structure itself (i.e. the nine microtubular triplets are not equivalent) or imposed by its immediate environment at time of assembly? We have reinvestigated this problem using the Ciliate Paramecium, in which the pattern of basal body duplication is well known. In this cell, all basal bodies produced within ciliary rows appear immediately anterior to parental ones. Observations on cells fixed with the tannic acid protocol suggest that, to be competent for basal body assembly, parental basal bodies have to be individually associated with a complete set of rootlets (monokinetid structure). During pro‐basal body assembly, full microtubular triplets were detected according to a random circumferential sequence; during the whole process, the new basal body and its associated rootlets maintained structural relations with the parental monokinetid structure by way of specific links. These results strongly suggest that basal body and associated rootlets (kinetid) polarity is driven by its immediate environment and provide a basis for the structural heredity property observed by Sonneborn some decades ago.     

The ciliary rootlet interacts with kinesin light chains and may provide a scaffold for kinesin-1 vesicular cargos

The ciliary rootlet is a large striated fibrous network originating from basal bodies in ciliated cells. To explore its postulated role in intracellular transport, we investigated the interaction between kinesin light chains (KLCs) and rootletin, the structural component of ciliary rootlets. We show here that KLCs directly interact with rootletin and are located along ciliary rootlets. Their interactions are mediated by the heptad repeats of KLCs. Further studies found that these interactions tethered kinesin heavy chains along ciliary rootlets. However, the ciliary rootlet-bound kinesin-1 did not recruit microtubules or move along ciliary rootlets. Additionally, amyloid precursor protein (APP; a kinesin-1 vesicular cargo receptor) and presenilin 1 (a presumed cargo of APP/kinesin-1) were found to be enriched along the rootletin fibers, suggesting that the interaction between ciliary rootlets and kinesin-1 recruits APP and presenilin 1 along ciliary rootlets. These findings indicate that ciliary rootlets may provide a scaffold for kinesin-1 vesicular cargos and, thus, play a role in the intracellular transport in ciliated cells.     

Development of the Ciliary Pattern of the Oral Apparatus of Tetrahymena thermophila1

ABSTRACT. The sequence of formation and ciliation of basal bodies and the subsequent organization of compound ciliary structures of the oral apparatus of Tetrahymena thermophila was reanalyzed with the aid of scanning electron microscopy of cells in which the epiplasmic layer was exposed, as well as by light microscopy of protargol-impregnated specimens. This combination of methods allowed the delineation of numerous steps in the patterning of the oral ciliature, some of which have received little or no previous attention. Highlights include: the initial formation of “strings” of nonciliated new basal bodies in juxtaposition to relatively few basal bodies of the stomatogenic kinety; generation of basal body pairs, roughly oriented along the anteroposterior axis of the cell, that later align side-by-side to assemble promembranelles; condensation and reorientation of promembranelles simultaneous with addition of a third row of basal bodies anterior to the original two rows; production of a very short fourth row of basal bodies at the anterior right end of each developing membranelle; generation of the outer basal body row of the undulating membrane (UM) after alignment of the inner row, with transient ciliation of the inner row preceding permanent ciliation of the outer row; limited basal body resorption at the ends of membranelles; and sculpturing of the right ends of membranelles by a movement of basal bodies associated with formation of the ribbed wall adjacent to the UM. In the old anterior oral apparatus a repetition of the processes of generation of a new outer UM row and sculpturing of right ends of membranelles takes place in synchrony with the corresponding events in the oral primordium, following prior shedding of the old outer UM row and loss of the sculptured pattern in association with temporary regression of the ribbed wall micro-tubules. Oral development is complex, with different processes involved in the assembly of the membranelles and the UM, and with a sequence of distinct events involved in the generation of each of these structures. Speaking comparatively, membranelle development follows the same pathway in many, perhaps all, ciliates in which these structures or their homologues develop from a common stomatogenic field.     

Patterns of comb row development in young and adult stages of the ctenophores Mnemiopsis leidyi and Pleurobrachia pileus

The development of comb rows in larval and adult Mnemiopsis leidyi and adult Pleurobrachia pileus is compared to regeneration of comb plates in these ctenophores. Late gastrula embryos and recently hatched cydippid larvae of Mnemiopsis have five comb plates in subsagittal rows and six comb plates in subtentacular rows. Subsagittal rows develop a new (sixth) comb plate and both types of rows add plates at similar rates until larvae reach the transition to the lobate form at ～5 mm size. New plate formation then accelerates in subsagittal rows that later extend on the growing oral lobes to become twice the length of subtentacular rows. Interplate ciliated grooves (ICGs) develop in an aboral‐oral direction along comb rows, but ICG formation itself proceeds from oral to aboral between plates. New comb plates in Mnemiopsis larvae are added at both aboral and oral ends of rows. At aboral ends, new plates arise as during regeneration: local widening of a ciliated groove followed by formation of a short split plate that grows longer and wider and joins into a common plate. At oral ends, new plates arise as a single tuft of cilia before an ICG appears. Adult Mnemiopsis continue to make new plates at both ends of rows. The frequency of new aboral plate formation varies in the eight rows of an animal and seems unrelated to body size. In Pleurobrachia that lack ICGs, new comb plates at aboral ends arise between the first and second plates as a single small nonsplit plate, located either on the row midline or off‐axis toward the subtentacular plane. As the new (now second) plate grows larger, its distance from the first and third plates increases. Size of the new second plate varies within the eight rows of the same animal, indicating asynchronous formation of plates as in Mnemiopsis. New oral plates arise as in Mnemiopsis. The different modes of comb plate formation in Mnemiopsis versus Pleurobrachia are accounted for by differences in mesogleal firmness and mechanisms of ciliary coordination. In both cases, the body of a growing ctenophore is supplied with additional comb plates centripetally from opposite ends of the comb rows. J. Morphol. 2012. © 2012 Wiley Periodicals, Inc.