Ultrastructural evidence for the occurrence of three types of mechanosensitive cells in the tentacles of the cubozoan polypCarybdea marsupialis

Summary The ectodermal cell layer in the tentacles of the cubozoan polypCarybdea marsupialis contains four types of cells (types 1–4) bearing specialized cilia. Epitheliomuscular cells (type 1) are characterized by motile cilia with dynein-decorated axonemes. 200 nm long extramembranous filaments of unknown function are restricted to a belt-like region distal to the transition zone. Up to 40 rn long rigid cilia formed by a slender epithelial cell type (type 2) are surrounded by rings of short microvilli. The axonemes of these cilia are composed of incomplete microtubules and lack dynein. Microvilli and cilia are linked by intermembrane connectors. Microtubuledoublets and ciliary membrane are interconnected by microtubule-associated cross-bridges only within this contact region. At the tip of each tentacle a single nematocyte (type 3) is surrounded by 7–10 accessory cells (type 4). These both cell types are equipped with similar cilium-stereovilli-complexes consisting of a cone-like arrangement of stereovilli and a modified cilium. The axonemal modifications of the cilium, its interconnections with the surrounding stereovilli and the linkages between ciliary axoneme and ciliary membrane are similar to those known from the cnidocil-complexes of hydrozoons and other epithelial mechanosensitive cells of the collar-receptor type. Our data indicate that besides the nematocyte two other types of mechanosensory cells (types 2 and 4) are integrated in the ectodermal cell layer ofCarybdea which possibly affect the triggering mechanism of nematocyst discharge.     

Ultrastructural study of sensory cells of the proboscidial glandular epithelium of Riseriellus occultus (Nemertea,Heteronemertea)

Only one sensory cell type has been observed within the glandular epithelium of the proboscis in the heteronemertine Riseriellus occultus. These bipolar cells are abundant and scattered singly throughout the proboscis length. The apical surface of each dendrite bears a single cilium enclosed by a ring of six to eight prominent microvilli. The cilium has the typical 9×2 + 2 axoneme arrangement and is equipped with a cross-striated vertical rootlet extending from the basal body. No accessory centriole or horizontal rootlet was observed. Large, modified microvilli (stereovilli) surrounding the cilium are joined together by a system of fine filaments derived from the glycocalyx. Each microvillus contains a bundle of actin-like filaments which anchor on the indented inner surface of a dense, apical ring situated beneath the level of the ciliary basal body. The tip of the cilium is expanded and modified to form a bulb-like structure which lies above the level where the surrounding microvilli terminate. In the region where the cilium emerges from the microvillar cone, the membrane of the microvillar apices makes contact with a corresponding portion of the ciliary membrane. At this level microvilli and cilium are apparently firmly linked by junctional systems resembling adherens junctions. The results suggest that these sensory cells may be mechanoreceptors. © 1996 Wiley-Liss, Inc.     

High voltage electron stereomicroscopy of the cilium-stereociliary complex of perioral sensory cells in Hydra

The cilium-stereociliary complex in perioral neurons of Hydra was examined by electron microscopy, with emphasis on stereomicrographs of serial, 0.5 micron thick, longitudinal and transverse sections. Longitudinal sections revealed (1) flat-topped cones in which the cilium was bent and the ciliary chamber appeared heart-shaped, and (2) pointed cones in which the cilium was straight and the ciliary chamber appeared triangular. Transverse sections revealed 10-12 stereocilia forming a cone over a central cilium with nine peripheral doublets of microtubules but with often more than two central microtubules. The ciliary membrane was fluted; fine filaments connected the outfoldings of membrane with the center of the microtubule doublets. Thin sections revealed 7 nm microfilaments in the stereocilia cores which branched basally into thick and thin roots; the thick roots surrounded the base of the central cilium. The cilium-stereociliary complex was enveloped by an epitheliomuscular cell sheath with a free margin distally and a septate junction proximally. In flat-topped cones the free margin of the enveloping epitheliomuscular cell was closely applied to the top of the cilium-stereociliary complex, whereas in pointed cones the cilium-stereociliary complex projected above the free margin of the sheath. Thus, the 7 nm actin-like filaments in the stereocilia might function to contract and open the complex in response to favorable stimuli so that the cilium is in contact with the aqueous environment.     

CILIARY MEMBRANE DIFFERENTIATIONS IN TETRAHYMENA PYRIFORMIS : Tetrahymena Has Four Types of Cilia

We have examined thin sections and replicas of freeze-fractured cilia of Tetrahymena pyriformis. The ciliary necklace located at the base of all freeze-fractured oral and somatic cilia has been studied in thin sections. Since electron-dense linkers have been found to connect both microtubule doublets and triplets to the ciliary membrane at the level of the necklace, the linkers and the associated necklace seem to be related to the transition region between the doublets and triplets of a cilium. Plaque structures, consisting of small rectangular patches of particles located distal to the ciliary necklace, are found in strain GL, but are absent in other strains examined in this study. In freeze-cleaved material, additional structural differentiations are observed in the distal region of the ciliary membranes of somatic and oral cilia. Somatic cilia contain many randomly distributed particles within their membrane. Oral cilia can be divided into three categories on the basis of the morphology of their freeze-fractured membranes: (a) undifferentiated cilia with very few randomly distributed particles: (b) cilia with particles arranged in parallel longitudinal rows spaced at intervals of 810–1080 Å that are located on one side of the cilium; and (c) cilia with patches of particles arranged in short rows oriented obliquely to the main axis of the cilium. The latter particles, found on one side of the cilium, seem to serve as attachment sites for bristles 375–750 Å long and 100 Å wide which extend into the surrounding medium. The particles with bristles are located at the tips of cilia in the outermost membranelle and may be used to detect food particles and/or to modify currents in the oral region so that food particles are propelled more efficiently into the buccal cavity. Examination of thin-sectioned material indicates that the particles in oral cilia which form the longitudinal rows could be linked to microtubule doublets. Linkage between microtubule doublets and adjacent membrane areas on one side of the cilium could modify the form of ciliary beat by restricting the sliding of the microtubules. It is suggested that membrane-microtubule interactions may form the basis for the various forms of ciliary beat observed in different organisms.     

Structure and differentiation of the sensilla of the ventral sensory field on the maxillary palps ofPeriplaneta americana (Insecta,Blattodea), paying special attention to the ciliogenesis of the sensory cells

Summary The embryonic development of palpal contact chemosensitive sensilla was studied from 42% of development up to the hatching of the larvae. Ciliogenesis of the sensory cells can be observed at the earliest stages investigated. A complex consisting of two basal bodies and a cap-like ciliary vesicle is localized in the dendritic inner segment. It migrates apically and fuses with the cytoplasmic membrane. At the same time, microtubule doublets of the distal basal body elongate, thus generating the dendritic outer segment. Furthermore, the typical accessory structures of a motile cilium are formed. Although the central pair of microtubules is lacking, the dendritic outer segment can be considered as a modified motile cilium. At about 84% of development the hair structure starts to be formed. Whereas the socket is generated by the tormogen cell, the trichogen cell produces the hair shaft and terminal porus. The dendrite sheath, which rises above the newly formed hair, is attached apically to the embryonic cuticle forming an irregular pore. In larvae and imagines, the inner surface of the dendrite sheath is highly differentiated. A range of circular ledges and filamentous structures wrapping around the dendritic outer segments can be distinguished. These may have a stabilizing function. Furthermore, in cryofixed specimens, the dendritic outer segments possess regularly spaced swellings which are about 1 m in length and about 0.5 m in diameter. Their functional significance is still unclear.     

Fine structure of the ocellus of Sarsia tubulosa (Hydrozoa,Anthomedusae)

Summary The fine structure of the ocellus of Sarsia tubulosa is described. The ocellar cup is formed of pigment cells and receptor cells. The receptor cells outnumber the pigment cells in almost a 2:1 ratio. Lateral extensions of neighbouring pigment cells enclose a distal region of 2 to 10 receptor cells. The receptor cell body is 5–7 m in diameter with an apical extension (20–60 m long) that reaches the ocellar cavity. A cilium (9+2 microtubules) arises from the distal part of the receptor cell. The ciliary membrane forms lateral microvilli. The tips of a number of cilia are swollen into large vesicles forming a cornea. The central region of the ocellar cavity contains extracellular electron dense homogeneous material surrounded by swollen ciliary tips and small vesicles. The close apposition between the plasma membrane covering the distal part of adjacent receptor cells as well as the adjacent ciliary shafts suggests the presence of gap junctions. The basal part of each receptor cell forms an axon. The axons of receptor cells form 3 to 4 nerve bundles that join to form the optic nerve. Synapses occur between receptor cell bodies, between axons and receptor cell bodies and among axons.     

Ultrastructure of the abdominal sense organ of the scallop <Emphasis Type="Italic">Mizuchopecten yessoensis</Emphasis> (Jay)

The sensory epithelium of the abdominal sense organ (ASO) of the scallop Mizuchopecten yessoensis is composed of three cell types, sensory cells, mucous cells, and multiciliated cells. Sensory cells bear a single long (up to 250 microm) cilium surrounded by an inner ring of nine modified microvilli and an outer ring of ordinary microvilli paired with modified microvilli. Sensory cells make up about 90% of the total number of cells in the sensory epithelium. Mucous cells, which are much wider than sensory cells, bear only ordinary microvilli on their apical surface. Rare multiciliated cells with short (4-6 microm) cilia are scattered in the periphery of the sensory epithelium sheet. All hairs, cilium, and microvilli of each sensory cell are interconnected by a fibrous network. Nine modified microvilli of a single cell are interconnected by prominent laterally running fibrous links. Membrane-associated electron-dense material of modified microvilli is connected to the ciliary membrane-associated electron-dense material by fine string-like links. These links mechanically bridge the space between the cilium and modified microvilli, as do mechanical links, described for the stereocilia and kinocilium of vertebrate vestibular and cochlear hair cells. The proximal portion of a sensory cilium is about 100 microm long and has a typical 9 x 2+2 axoneme arrangement. The distal portion of a cilium is approximately 2 times thinner than the proximal one and is filled with homogeneous electron-dense material. Along the distal portion, diffuse material associated with the external surface of the membrane is found. The rigidity of distal portion of a cilium is much less than that of the proximal one.     

Splayed Tetrahymena cilia. A system for analyzing sliding and axonemal spoke arrangements

This study makes use of a procedure designed to illustrate, without serial section analysis, the three-dimensional changes in the ciliary axoneme produced by microtubule sliding, and to confirm essential features of the sliding microtubule hypothesis of ciliary movement. Cilia, isolated from Tetrahymena pyriformis by the dibucaine procedure, are attached to polylysine substratum, and treated with Triton X-100. Critical point drying maintains three-dimensional structure without embedding. The detergent removes the membrane and many axonemes unroll, always in an organized fashion so that doublets follow one another in sequence, according to the enantiomorphic form of the cilium. The central pair of microtubules fall to the side as a unit. The parallel doublet microtubules retain relative longitudinal positions in part by interdoublet or nexin links. Spoke organization and tip patterns are preserved in the opened axonemes. We generalize the work of Warner and Satir (Warner, F. D., and P. Satir, 1976. J. Cell Biol. 63:35-63) to show that spoke group arrangements are maintained for all doublets in straight regions, while systematic displacements occur in bent regions. The conclusion that local contraction of microtubles is absent in the axoneme is strengthened, and direct graphic demonstrations of sliding at the ciliary tip are shown. A morphogenetic numbering scheme is presented which results in a quantitative fit of the tip images to the images predicated by the equation for doublet sliding, and which makes possible new comparisons of structural parameters between axonemes and with cilia of other organisms.     

Ultrastructural studies of the commensal suctorian,Choanophrya infundibulifera hartog

Summary The feeding tentacles of Choanophrya contain a central canal lined by microtubules. Only one tentacle develops during metamorphosis of the embryo into the adult, but others develop at intervals throughout adult life. Each tentacle forms adjacent to a solitary, subcortical kinetosome which lies parallel to the body surface, lacks accessory elements and never develops a cilium. Small condensations of electron-dense material and short bundles of microtubules form adjacent to the cartwheel region of the kinetosome. Initially these bundles are orientated randomly but later they become radially arranged and curved into prolamellae around a disc-shaped condensation centre, to form a paddlewheel-like tentacle primordium 0.8–1.1 m in diameter. The condensation centre consists of alternating concentric electron-dense and electron-transparent zones, and lies with its axis perpendicular to both the kinetosome and the cortex. The microtubules in each prolamella increase in number and pairs of short tip microtubules develop between adjacent prolamellae. Subsequently the developing lamellae become enclosed by a cylinder of ring microtubules. Once all the microtubule components of the tentacle primordium are established it increases in length by addition of material to the basal ends of the microtubules to form a short microtubule canal. As the canal elongates the epiplasm above it disappears and the pellicle membranes become uplifted around the protruding tentacle. An epiplasmic collar differentiates around the growing tentacle whilst spheroid vesicles and solenocysts begin to accumulate in the surrounding cytoplasm.This investigation was supported by the J.S. Dunkerley Fellowship in Protozoology, awarded by the University of Manchester.     

Ciliogenesis and centriole formation in the mouse embryonic nervous system. An ultrastructural analysis

Serial ultrathin sections were used to study the formation of the primary cilium and the centriolar apparatus, basal body, and centriole in the neuroepithelial primordial cell of the embryonic nervous system in the mouse. At the end of mitosis, the centrioles seem to migrate toward the ventricular process of the neuroepithelial cell, near the ventricular surface. One of these centrioles, the nearest to the ventricular surface, begins to mature to form a basal body, since its tip is capped by a vesicle probably originating in the cytoplasm. This vesicle fuses with the plasmalemma and the cilium growth by the centrifugal extension of the 9 sets of microtubule doublets. These 9 sets invade the thick base of the cilium which is initially capped by a ball-shaped tip with the appearance of a mushroom cilium. The secondary extension of 7, then 5, and finally 2 sets of microtubule doublets contribute to form the tip of the mature cilium, which is associated with a mature centriolar apparatus formed by a basal body and a centriole. Centriologenesis occurs before mitosis and is concomitant with the progressive resorption of the cilium. The daughter centriole, or procentriole, begins to take form near the tips of fibrils that extend perpendicularly and at a short distance from the wall of the parent centriole. Osmiophilic material accumulates around these fibrils, and gives rise to the microtubules of the mature daughter centriole. These centrioles formed by a centriolar process are further engaged in mitosis, after the total resorption of the cilium. This pattern of development suggests that in the primordial cells of the embryonic nervous system, centriologenesis and ciliogenesis are 2 independent phenomena.     

Comparative ultrastructure of the epidermal ciliary rootlets and associated structures in species of the Nemertodermatida and Acoela (Plathelminthes)

 The fine morphology of epidermal ciliary structures in four species of the Nemertodermatida and four species of the Acoela was studied, with emphasis on Meara stichopi (Nemertodermatida). The cilium of M. stichopi has a distal shelf and is proximally separated from the basal body by a cup-shaped structure. The bottom of the cup consists of a bilayered dense plate, or basal plate. The basal body consists of peripheral microtubule doublets continuous with those of the cilium. In the upper part of the basal body, the doublets are set at an angle and are anchored to the enclosing cell membrane by Y-shaped structures. The lower part of the basal body tapers eventually. The striated main rootlet arises on the anterior face of the basal body, initially like a flattened strap, and continues along the basal body shaped as a tube which further down becomes solid. The hour-glass-shaped posterior rootlet arises on the posterior face of the basal body. Contrary to the main rootlet, the striations in the proximal part of the posterior rootlet run parallel to the microtubule doublets of the basal body. A pair of microtubule bundles lead from the posterior rootlet to the two main rootlets in the hind ciliary row, and follow these to their lower tip. In the other species of the Nemertodermatida studied, the structure of the ciliary basal body and the ciliary rootlets is similar to that of M. stichopi. Structural differences in the species of the Acoela are that the lowermost end of the basal body is narrow and bent forwards, the proximal part of the main rootlet is trough-shaped, the main rootlet is accompanied by a pair of lateral rootlets and the posterior rootlet with associated microtubule bundles is thin. The epidermal ciliary structures in species of the Nemertodermatida and Acoela have a number of shared characters which are unique within the Plathelminthes. However, almost all of these characters are found in Xenoturbella bocki (Xenoturbellida), and some even in species of other ”phyla” of the ”lower” Metazoa. Hence, these characters cannot be considered apomorphic for the Acoelomorpha. A character seemingly present only in species of the Nemertodermatida and Acoela is the bilayered dense plate. This feature might represent an autapomorphic character state for the Acoelomorpha. Accepted: 7 March 1997     

Ultrastructure of the sensory palps ofTetranchyroderma papii (Gastrotricha,Macrodasyida)

Summary The sensory palps of the macrodasyoid gastrotrichTetranchyroderma papii contain processes from two types of cell: 22–23 bipolar primary sensory cells and two to three support cells. In the proximal region of the palp each sensory cell contains a short ciliary segment with a basal body and from this ciliary segment a longer distal segment lacking axonemal microtubules extends through the major part of the length of the palp. Each support cell process bears microvilli and contains a conspicuous bundle of microtubules running the entire length of the process. The cell bodies of both cell types are situated in the epidermis of the head region. The palps are interpreted as having a chemosensory function. They are considered to be homologous to the posterior cephalic sensory organ ofTurbanella cornuta, but not the head tentacles ofChordodasys antennatus or nematode amphids.     

Cytoskeleton-membrane interactions in the cnidocil complex of hydrozoan nematocytes

Summary Each cnidocil complex of the hydrozoans Tubularia larynx and Hydra vulgaris consists of 9 or 7–10 large stereovilli (=stereocilia), respectively, and a modified cilium, the cnidocil. The cnidocils comprise the regular 9 microtubule doublets, up to 30 additional microtubules, as well as a central filament body. Adjacent stereovilli are linked together by intermembrane connectors forming the stereovillar cone. The distal tips of the stereovilli surround the cnidocil in a closed tubular arrangement measuring up to 0.7 m in length. Within this contact region the cnidocil is linked to the stereovillar tube by another set of intermembrane connectors, which seem to hold the cnidocil in a central position within the stereovillar cone. Stereovillar membrane and actin core are linked by 16-nm long cross bridges, which display a periodicity of 16 nm and emerge from the actin core. Within the cnidocils periodically arranged membrane-cytoskeleton bridges are uniformly restricted to the contact region. Here, 24-nm long cross bridges, which are spaced by a regular distance of 20 nm, interconnect the A-tubules of the microtubule doublets and the membrane. The cnidociliary membrane is differentiated into distinct domains as revealed by freeze-fracturing. Within the contact region of the nematocytes of Tubularia larynx, intramembrane particles are arranged in 9 rows of 700 nm length and 50 nm width, separated by particlefree areas. Intramembrane particles are irregularly distributed distal to the contact region. Considering recent physiological results we presume that the latter represent chemoreceptor units, while mechanical stimuli are transmitted via the intermembrane connectors and the microtubule-membrane bridges to mechanosensitive channels within the domain of the cnidociliary membrane in the contact region.     

Ultrastructural studies of epidermis, sense receptors and sperm of Craspedella sp. and Didymorchis sp. (Platyhelminthes, Rhabdocoela)

Craspedella has a non-ciliated epidermis with nuclei located in the epidermis and with short microvilli. There is a thin basal lamina and thick underlying fibrous matrix. Rhabdites are secreted through ducts lined by microtubules. Multiciliate sense receptors consist of bundles of dendrites in a depression of the epidermis. Each dendrite has a cilium with a cross-striated rootlet; there are no electron-dense collars. Spermatozoa have peripheral microtubules which in cross-section are arranged in a ring-like or spiral fashion, numerous electron-dense granules, mitochondria and a nucleus; axonemes of the 9 +'1'type are free for most of their length. Centrioles occur in some nerve fibres. In Didymorchis parts of the epidermis are ciliated and epidermal perikarya are 'insunk', connected to the surface part of the epidermis by a single cytoplasmic process. Epidermal cilia have cross-striated vertical and horizontal rootlets. In the ciliary tips a short electron-dense rod along the central pair of tubules extends to the tip, where it widens to become a terminal plate; peripheral doublets gradually disappear by losing their microtubules. Receptors observed are uniciliate. Spermatozoa are as in Craspedella . Ultrastructural evidence indicates that Craspedella and Didymorchis arc closely related and belong to the Rhabdocoela.     

Orientation of cortical microtubules correlates with cell shape and division direction

Summary Cortical microtubules in the epidermis of regeneratingGraptopetalum plants were examined by in situ immunofluorescence. Paradermal slices of tissue were prepared by a method that preserves microtubule arrays and also maintains cell junctions. To test the hypothesis that cortical microtubule arrays align perpendicular to the direction of organ growth, arrays were visualized and their orientation quantified. A majority of microtubules are in transverse orientation with respect to the organ axis early in shoot development when the growth habit is uniform. Later in development, when growth habit is non-uniform and the tissue is contoured, cortical microtubules are increasingly longitudinal and oblique in orientation. Microtubules show only a minor change in orientation at the site of greatest curvature, the transition zone of a developing leaf. To assess the role of the division plane on orientation of arrays, the pattern of microtubules was examined in individual cells of common shape. Cells derived from transverse divisions have predominately transverse cortical arrays, whereas cells derived from oblique and longitudinal divisions have non-transverse arrays. The results show that, regardless of the stage of development, microtubules orient with respect to cell shape and plane of division. The results suggest that cytoskeletal function is best considered in small domains of growth within an organ.Abbrevations DMSO dimethylsulfoxide - EGTA ethylene glycol-bis-(ß-aminoethyl ether)-N, N, N, N-tetra acetic acid - FITC fluorescein isothiocyanate - MTSB microtubule stabilizing buffer - PBS phosphate buffered saline     

The ultrastructure of primary cilia in the endocrine and excretory duct cells of the pancreas of mice and rats

A primary cilium was frequently observed in the endocrine alpha, beta and delta cells, as well as in the excretory duct cells of the pancreas of normal mice and rats. The characteristic components of the cilium including the basal body, axoneme (shaft), and terminal part were clearly recognizable. The basal body or distal centriole surrounded by Golgi vesicles was perpendicularly oriented to the proximal centriole, and a dense striated band was seen filling the gap between them. The microtubules of the basal body consisted of nine peripheral triplets exhibiting a 9 + 0 pattern, an appearance similar to that of the proximal centriole. Rootlets, basal feet and alar sheets associated with the basal body were occasionally seen. The axoneme usually consisted of a 9 + 0 pattern of microtubule doublets, but other irregular patterns of 7 + 2, 7 + 3, and 8 + 1 were also seen. The microtubules in the terminal part of the cilium became fewer in number and had no peculiar arrangement. The cilium of the endocrine cells always projected into the intercellular canaliculus and was covered by the ciliary sheath, and occasionally, double cilia were visualized in the vicinity of beta cells. In the excretory duct cells, the cilium showed similar features, but it was slightly longer and always projected into the dense secretory content of duct lumen. On the other hand, no primary cilium was ever observed in the acinar cells of mouse and rat pancreas. In conclusion, the present study describes the morphology of primary cilia and its associated components in the endocrine and excretory duct cells of the pancreas of mice and rats. The findings suggest that the primary cilium should be considered as a constant intracellular organelle though its function and significance remain speculative.     

Modifications ultrastructurales des glandes vésiculaires des Machilidae (Insecta,Thysanura) au cours des cycles de mue

Summary During the period between apolysis and ecdysis, the vesicular glands show many important transformations which affect not only the cuticular ductules, but all the cells. The cytoplasm of the glandular cells undergoes a partial autolysis, whereas other parts of the cells present a high secretory activity. Immediately after the apolysis the cellular reservoir empties and disappears almost completely; soon after, refills with secretion. The most interesting transformations concern each ciliary cell, always associated with a glandular cell. In the first phase of the moulting cycle, the dendrite of the ciliary cell grows a ciliumlike extension (= distal region of the dendrite), which penetrates into the corresponding ductule; the new intima of this ductule is laid around the cilium. At the same time, the proximal region of the dendrite forms a circular fold around the base of the cilium and begins to secrete a material which will form the end apparatus. This latter is finished during the second phase of the cycle. The third phase is characterized by the degeneration of the distal region of the dendrite and the circular fold. Thus, the end apparatus is not a secretion of the ductule-carrying cell, but of the ciliary cell. At the end of the moulting period, just before ecdysis, the vesicular gland again takes the structure characteristic of the intermoult: the reservoir of the glandular cell is very large; the cuticular apparatus is almost formed; the dendrite of the ciliary cells shows, at its apex, a short cilium (= ciliary region s. str. + short distal region) surrounded by microvilli, free in the secretion of the reservoir.     

Comparative ultrastructural studies of the epidermis, sperm and nerve fibres of Cleistogamia longicirrus and Seritia stichopi (RhabdocoeIa, UmagiIIidae)

The epidermis of Cleistogamia longicirrus consists of columnar, interdigitated cells with apical mitochondria, short microvilli and cilia which have a single horizontal rootlet anchored in the adjacent cytoplasm by transverse bars and a thin side branch. Cells are held together by desmosomes and intermediary junctions. Epidermal cilia have a terminal, electron dense rod between the central microtubules and doublet 1 and a terminal plate; doublets 1 and 6–9 lose one of their microtubules and gradually all doublets lose one microtubule and peripheral doublets disappear. Spermatozoa have a single closed peripheral row of microtubules, numerous electron dense granules and mitochondria and axonemes are of the 9 +"1" type and free for most of their length. Nerve fibres have microtubules and some nerve fibres are surrounded by lamellae and have invaginations of the fibre wall. The epidermis of Seritia stichopi resembles that of Cleistogamia , but cilia have a single horizontal rootlet anchored by transverse bars in the cytoplasm, without a side branch. Spermatozoa also are similar to those of Cleistogamia . Certain ultrastructural similarities between some Umagillidae and Neodermata are apparently due to convergent evolution and do not allow the conclusion that Umagillidae and Neodermata are particularly closely related.     

The multi‐scale architecture of mammalian sperm flagella and implications for ciliary motility

Motile cilia are molecular machines used by a myriad of eukaryotic cells to swim through fluid environments. However, available molecular structures represent only a handful of cell types, limiting our understanding of how cilia are modified to support motility in diverse media. Here, we use cryo‐focused ion beam milling‐enabled cryo‐electron tomography to image sperm flagella from three mammalian species. We resolve in‐cell structures of centrioles, axonemal doublets, central pair apparatus, and endpiece singlets, revealing novel protofilament‐bridging microtubule inner proteins throughout the flagellum. We present native structures of the flagellar base, which is crucial for shaping the flagellar beat. We show that outer dense fibers are directly coupled to microtubule doublets in the principal piece but not in the midpiece. Thus, mammalian sperm flagella are ornamented across scales, from protofilament‐bracing structures reinforcing microtubules at the nano‐scale to accessory structures that impose micron‐scale asymmetries on the entire assembly. Our structures provide vital foundations for linking molecular structure to ciliary motility and evolution.