STUDIES ON CILIA : II. Examination of the Distal Region of the Ciliary Shaft and the Role of the Filaments in Motility

Termination of peripheral filaments of the axoneme of gill cilia of fresh-water mussels (Elliptio or Anodonta) occurs in characteristic fashion: (a) subfiber b of certain doublets ends leaving a single simplified tubular unit; (b) the wall of the unit becomes thick and may even obliterate the interior; and (c) the filament drops out of the 9 + 2 pattern. The order in which doublets begin simplifying is also characteristic. This may be determined by numbering the filaments, those with the bridge being 5–6, with the direction of numbering determined by the apparent enantiomorphic configuration (I to IV) of the cross-section. Shorter filaments can be identified in simplifying tips with mixed double and single peripheral units. In this material, laterofrontal cirri show a morphological specialization in the region where individual cilia simplify. The cilia studied run frontally from the body of the cirrus and point in the direction of effective stroke. The longest filaments (Nos. 3, 4, 5, 6, 7) appear as the doublets at the bottom of the cross-section, nearest the surface of the cell of origin. Above them, and above the central pair, a dark band (a section of a dense rod) runs through the matrix. The remaining filaments are the single units. Effective-pointing frontal and lateral ciliary tips end in a fashion similar to laterofrontal tips, although no dense band is present. For all effective-pointing tips studied, the order in which the peripheral filaments end appears to be Nos. (9, 1), 8, 2, 7, 6, 3, 4, 5. However, recovery-pointing lateral tips show a different order: Nos. 7, 6, 8, 5, 9, 4, 1 (3, 2), although the longer filaments are still at the bottom of the cross-section. In simple models of ciliary movement involving contraction of the peripheral filaments, filaments at the top of the cross-section should be longer, if any are. Such models are not supported by the evidence here. These results can be interpreted as supporting sliding-filament models of movement where no length change of peripheral filaments occurs.     

Observations on the solitary cilium of rabbit oviductal epithelium: its motility and ultrastructure

Solitary cilia have been observed on rabbit oviductal epithelial cells. In tissue cultures of fimbrial epithelium of 3- and 4-day-old animals observed by phase microscopy, most of these single cilia exhibited a vortical or funnel-type movement while others had the usual to-and-fro motility. Primary cilia are usually considered immotile. Transmission electron microscopy of specifically identified single cilia revealed differences between the ciliary shafts and basal bodies of the single cilia as compared to those of mature oviductal ciliated cells. The basal body of the solitary cilium often had at least two triangular, striated, basal foot processes, lacked electron-dense satellite material around its basal end, and occasionally had striated rootlets. In contrast, the cilia of mature ciliated cells had only one basal foot, exhibited much electron-dense satellite material, and lacked rootlets. Cross sections of the single cilia showed patterns of microtubules different from the usual 9 + 2 axonemal complexes of normal cilia and included 9 + 0, 10 + 2 singlets, 7 + 2 doublets, and 8 + 1 doublet and 2 singlets; one did have the usual 9 + 2 arrangement. We postulate that the presence of more than one basal foot process may be responsible for the vortical motility observed. The primary cilia are shorter than normal cilia; the longest one measured was 1.86 micron in length, 0.28 micron in width at its base, and 0.14 micron at its tip. Based on the light-microscopic, scanning-electron-microscopic and transmission-electron-microscopic observations, such solitary cilia were observed more frequently in the oviductal tissues of the 3- to 4-day postnatal rabbits grown in tissue culture and in ovariectomized and ovariectomized/progesterone-treated adult animals than in estrous, ovulatory, or ovariectomized/estradiol-treated rabbits.     

THE FINE STRUCTURE OF THE CILIA FROM CTENOPHORE SWIMMING-PLATES

The ctenophore swimming-plate has been examined with the electron microscope. It has been recognized as an association of long cilia in tight hexagonal packing. One of the directions of the hexagonal packing is parallel to the long edge of the swimming-plate and is perpendicular to the direction of the ciliary beat. All the cilia in the swimming-plate are identically oriented. The effective beat in the movement of the swimming-plate is directed towards the aboral pole of the animal, and this is also the side of the unpaired peripheral filament in all the cilia. The direction of the ciliary beat is fixed in relation to the position of the filaments of the cilia. The swimming-plate cilium differs from other types of cilia and flagella in having a filament arrangement that can be described as 9 + 3 as opposed to the conventional 9 + 2 pattern. The central filaments appear in a group of two "tubular" filaments and an associated compact filament. The compact filament might have a supporting function. It has been called "midfilament." Two of the peripheral nine filaments (Fig. 1, Nos. 3 and 8) are joined to the ciliary membrane by means of slender lamellae, which divide the cilium into two unequal compartments. These lamellae have been called "compartmenting lamellae." Some observations of the arrangement of the compartmenting lamelae indicate that they function by cementing the cilia together in lateral rows. The cilia of the rows meet at a short distance from each other, leaving a gap of 30 A only. The meeting points are close to the termini of the compartmenting ridges. An electron-dense substance is sometimes seen bridging the gap. Some irregularities are noted with regard to the arrangement of the compartmenting lamellae particularly at the peripheral rows of cilia. In many cilia in these rows there are small vesicles beneath the ciliary membrane.     

Splitting the ciliary axoneme: implications for a "switch-point" model of dynein arm activity in ciliary motion

In the presence of specific inhibitors of beat. 20 microM VO4(3-) or pCa 4, mussel gill lateral (L) cilia can be arrested in two positions--"hands down" or "hands up"--at opposite ends of the stroke cycle. Cilia move to these positions by doublet microtubule sliding. Axonemes of arrested cilia, still tethered to the cell, are intact after demembranation and protease treatment. When reactivated by 4 mM ATP with inhibitors present, about 40% split apart. Splits are not random but occur preferentially between different specific doublets in the two opposite arrest positions. Several different related patterns of splitting are observed; for every pattern in "hands down" axonemes, there is a corresponding complementary split pattern in "hands up" axonemes. In some split patterns two doublets remain firmly attached to the central pair; these also differ depending on axonemal position. Although some of the patterns seen may be artifactual or difficult to explain, the complementary splitting patterns are predictable with simple assumptions by a "switch point" hypothesis of ciliary activity where, during each recovery stroke, doublets 6-8 have active dynein arms, while during each effective stroke, arms on doublets 1-4 become active, and arms 6-8 are turned off. Because of a difference between the patterns seen and the predictions, the status of the arms on doublet 9 is unresolved. The patterns also suggest that a spoke-central sheath attachment cycle may correlate with switching of arm activity during the generation of an asymmetric beat.     

An unusual type of cilium.

The connective tissue cells of the dentinal pulp of unerupted dog teeth possess occasional cilia. Internally there are 4 to 8 peripheral doublets and one central doublet. Nine peripheral doublets are observed only close to or near the basal body.     

Microtubule termination patterns in mammalian sperm flagella

Detailed reconstructions of the flagellar tip (end-piece) in rodent spermatozoa have shown patterns of displacement between the termination points of the axonemal doublets (judging the terminations by the loss of electron density from the A-tubule). The patterns are in good agreement with those derived from sliding microtubule theory. In the hamster at least, the axis of major displacement passes approximately through doublet 1 and between doublets 5 and 6, though there may be some skewness in the clockwise direction. Microtubules derived from the plane at right angles to this (the central pair and presumably one or both of doublets 3 and 8) continue beyond the rest to the extreme tip, where they appear to be linked together at the cell membrane. This arrangement suggests that the tapering form of the end-piece, and of flagellar terminal filaments and ciliary tips in general, may be an adaptation to contain the sliding microtubules and prevent them impinging on the membrane overlying the tip.     

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.     

Alternate patterns of doublet microtubule sliding in ATP-disintegrated macrocilia of the ctenophore Beroe

We have used the unique properties of macrocilia from the lips of the ctenophore Beroe to test whether the ciliary beat cycle is caused by sequential activation of doublet sliding on opposite sides of the axoneme (Satir, P., 1982, Soc. Exp. Biol. Symp., 35: 179-201; Sugino, K., and Y. Naitoh, 1982, Nature (Lond.), 295: 609-611; Wais-Steider, J., and P. Satir, 1979, J. Supramol. Struct., 11:339-347). Macrocilia contain several hundred axonemes linked into rows by lamellae between doublets 3 and 8. These connections provide morphological markers for numbering the doublet microtubules in thin sections. Demembranated, detached macrocilia undergo ATP-induced sliding disintegration by extrusion of thick fragments and finer fibers from the proximal end. Disintegration can easily be followed with low-magnification brightfield or phase-contrast optics. Sliding occurs with or without added elastase, and is reversibly inhibited by vanadate. Thin sections through 16 ATP-disintegrated macrocilia showed two mutually exclusive patterns of doublet extrusion with equal frequency. Doublets 9, 1, and 2 or doublets 5, 6, and 7 were usually extruded, but not both groups. We conclude that both subsets of doublets slide by their own active arms, and that the two extrusion patterns represent alternate activation and inactivation of doublet sliding on opposite halves of the axoneme. These findings provide the first direct experimental support for a switching mechanism regulating microtubule sliding in cilia.     

Distinctive features of cilia in metazoans and their significance for systematics.

A comparative study of epidermal cilia in the Turbellaria and Nemertea has revealed features in these organelles that are specific to certain taxonomic groups. Turbellarians of the order Acoela, in particular, have a characteristic pattern of axonemal filament termination in the distal tips of their cilia and a characteristic ciliary rootlet system that is not seen in other turbellarian orders nor in other metazoans. Each epidermal cilium in acoels has a typical 9 + 2 axonemal pattern through the main part of its length, but near its distal tip there is an abrupt shelf-life narrowing at which filaments 4-7 terminate; filaments 1, 2, 8 and 9 continue into the thinner distal-most part of the shaft along with singlet microtubules from the axonemal center. The rootlet system in acoel cilia involves an interconnecting pattern with lateral connectives. The unique structure of these cilia has systematic and phylogenetic significance for the Acoela, and it is argued that ultrastructural characters in general, including characters of organelles, can be validly applied to the phylogeny and systematics of the Metazoa.     

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.     

FINE STRUCTURAL CHANGES IN THE FLAGELLUM OF THE SPERMATID IN EXPERIMENTAL CRYPTORCHIDISM OF THE RAT

Rat testes were confined to the abdominal cavity by operation. After 1 to 26 days they were excised, fixed with osmium tetroxide, sectioned, and examined with the electron microscope. Changes in the axial filament complex of the spermatid flagellum appeared 2 days after operation, and the arrangement of filaments in the middle- and main pieces of some spermatid tails was disordered as compared to the 9 + 2 filament arrangement in the tails of the control spermatids and in other flagella and cilia. In cross-sections, the filaments in the experimental material were nine or less in number, and each of them was single and dense. Occasionally some were double, and in those instances one filament was dense and the other was light and tubular. The central filaments were obscure. In longitudinal sections,the filaments were not parallel to the main axis of the flagella or to each other. It was assumed that the central filaments were more sensitive to the experimental conditions than the peripheral pairs of filaments. Furthermore, the light filaments of the peripheral pairs were more sensitive than the dense filaments. Besides the axial filament complex, the fibrous sheath which surrounds it in the main piece was also changed. The plasma membrane of the changed flagella disappeared or became fragmented.     

The ciliated epidermis of Xenoturbella bocki (Platyhelminthes, Xenoturbellida) with some phylogenetic considerations

The epidermis of Xenoturbella bocki Westblad was studied by scanning and transmission electron microscopy. Two cell types predominate in the epidermis: multiciliated epidermal cells and non-ciliated or monociliated gland cells. A conspicuous feature is the dense ciliary coverage and the numerous gland cell openings. Xenoturbella has a characteristic pattern of axonemal filament termination in the distal tips of their cilia. Each epidermal cilium has the typical 9 + 2 patten through the major part of its shaft. Near the tip there is a shelf at which doublets 4–7 terminate. Doublets 1, 2, 3, 8 and 9 continue into the thinner distal part of the cilium. A similar shelf in cilia is known only from the turbellarian orders Nemertodermatida and Acoela, and hence may be an apomorphic feature which indicates a close relationship between Xenoturbellida, Nemertoder-matida and Acoela. The basal body is provided with a so-called basal foot which has a cross-striated appearance and an expanded distal plate that seems to act as a microtubule organizing center. Approximately 15–25 microtubuli radiate from the endplate of the basal foot to the basal bodies caudally. The arrangement of basal foot and ciliary rootlets in Xenoturbella differs from that of Acoela and related orders in that there are two striated rootlets only (an anterior and a posterior one), rather than one main rootlet and two lateral rootlets.     

Ultrastructure of paired coniform 9 + 0 sensory cilia: a new type in the organ of Bellonci of the marine amphipod Gammarus setosus

Summary The structure of modified 9 + 0 cilia in the organ of Bellonci was studied in Gammarus setosus from late embryonic development to adult after routine fixation, fixation with lanthanum treatment, and prefixation with ethylene diamine tetraacetic acid and sodium dodecyl sulphate. The cilia are distinct from known sensory cilia in that they occur in pairs and lack centrioles. The basal bodies are at right angles to each other. The basal body cylinders consist of dense microtubule doublets and have 3 regions: the basal cartwheel, the middle pinwheel and the distal transitional. The pinwheel, which has 9 fins of dense material attached to the doublets, is differentiated into a spiral attachment of the ciliary roots whose periodicity is 70 nm. The scanning electron microscope shows the roots as beaded, tapering ribbons. The coniform outer segments give rise to tubules, each with 1 or 2 single or double microtubules in its core. The tubules are in contact with extracellular chains of calcium granules inside the organ. A bend in the axoneme brings the paired outer segments together. Lamellar bodies develop from the ciliary tubules in embryos and juveniles, but not in adults, except after exposure to lanthanum.     

Surface specializations of the olfactory epithelium of rainbow trout, Salmo gairdneri

Receptors for olfactory stimulus molecules appear to be located at the surface of olfactory receptor cells. The ultrastructure of the distal region of rainbow trout (Salmo gairdneri) olfactory epithelium was examined by transmission electron microscopy. On the sensory olfactory epithelium, which occurs in the depressions of secondary folds of the lamellae of the rosettes, five cell types were present. Type I cells have a knob-like apical projection which is unique in this species because it frequently contains cilia axonemes within its cytoplasm in addition to being surrounded by cilia. Type II cells bear many cilia oriented unidirectionally on a wide, flat surface. Type III cells have microvilli on a constricted apical surface and centrioles in the subapical cytoplasm. Type IV cells contain a rod-like apical projection filled with a bundle of filaments, and type V cells are supporting cells. Cilia on the sensory epithelium contain the 9 + 2 microtubule fiber pattern. Dynein arms are clearly present on the outer doublet fibers, which suggests that the cilia in the olfactory region are motile. Their presence in olfactory cilia of vertebrates has been controversial. The cilia membrane in this species is unusual in often showing outfoldings, within which are included small, irregular vesicles or channels. In addition, cilia on type II cells frequently contain dense-staining bodies closely apposed to the membranes, along with a densely stained crown at the cilia tip. Previous biochemical evidence indicates that odorant receptors are associated with the cilia.     

Discocilia—A new type of kinocilia in the larvae of Lanice conchilega (Polychaeta,Terebellomorpha)

A modified type of kinocilia has been found in the Aulophora-Iarva of the sedentarian polychaete Lanice conchilega. For this newly described cilium type the term "discocilium" is proposed. The only structural difference from usual locomotory cilia is the tip, which possesses a discoidal head. The head is formed from the terminal part of the cilium shaft, which is bent to give rise to a loop-like ring covered by the ciliary membrane. Three types of discocilia can be distinguished: a) discocilia having swollen, bulblike heads with a central straight axoneme; b) discocilia having heads with a curved lateral axoneme and c) discocilia in which the axoneme forms a loop. The internal structure shows the usual 9 + 2 arrangement of the filaments. The head shows no sign of secretion; it appears structureless in electron microscopical examination. There are two kinds of discocilia arrangements: 1) isolated bunches of cilia especially at the tentacles and in the frontal region, and 2) segmental dorsal rows of cilia. The possible formation of discocilia is described.     

ULTRASTRUCTURAL ORGANIZATION OF CILIA AND BASAL BODIES OF THE EPITHELIUM OF THE CHOROID PLEXUS IN THE CHICK EMBRYO

Ultrastructural studies were performed on normal and abnormal cilia and basal bodies associated with the choroidal epithelium of the chick embryo. Tissues were prepared in each of several fixatives including: 1% osmium tetroxide, in both phosphate and veronal acetate buffers; 2% glutaraldehyde, followed by postfixation in osmium tetroxide; 1% potassium permanganate in veronal acetate buffer. Normal cilia display the typical pattern of 9 peripheral doublets and 2 central fibers, as well as a system of 9 secondary fibers. The latter show distinct interconnections between peripheral and central fibers. Supernumerary fibers were found to occur in certain abnormal cilia. The basal body is complex, bearing 9 transitional fibers at the distal end and numerous cross-striated rootlets at the proximal end. The distal end of the basal body is delimited by a basal plate of moderate density. The tubular cylinder consists of 9 triple fibers. The C subfibers end at the basal plate, whereas subfibers A and B continue into the shaft of the cilium. The 9 transitional fibers radiate out from the distal end of the basal body, ending in bulblike terminal enlargements which are closely associated with the cell membrane in the area of the basal cup. One or 2 prominent basal feet project laterally from the basal body. These structures characteristically show several dense cross-bands and, on occasion, are found associated with microtubules.     

Fine structure of sensory tubes on the antennule of Conchoecia spinirostris (Ostracoda,Crustacea)

Summary In addition to setae, the first antennae of Conchoecia spinirostris also bear soft sensory tubes (4 tubes + 1 seta; 2 tubes + 3 setae). These tubes were examined electron microscopically. Each tube is divided into 4 regions: the stem, the bulbous region, the main region, and the tip. A tube contains 40–60 multiciliated dendrites, some hypodermal cells, and nonneuronal cells, and it has a specialized cuticle. Each dendrite develops within the tube, on the terminal 5–8 m of its inner dendritic segment, approx. 25 cilia in a 9 × 2 + 0 pattern, whose rootlets are absent or only poorly developed. Each cilium splits up into 9 ramifications which extend into the tip. These ramifications partly take a spirallike course and form a ring in the distal main part beneath the cuticle. Their membranes often dilate into spindleshaped swellings. In the center of the middle and distal parts of the main region approx. 7 dendrites without cilia are located, one of them reaches into the tip. The poreless cuticle is extremely delicate and electron lucid. In contrast to the cuticle of the setae it is elastic and soft. Special substructures are described. The tubes are completely covered by a filamentous surface coat. Because of the structure and the thin walled nature of the cuticle, permeability for dissolved substances is assumed. The ciliary ramifications are likely to represent the receptive apparatus. The sensory tubes are interpreted as chemoreceptors. They can best be compared with the chemoreceptors of certain crustaceans, but differ strongly from the types of sensilla found in insects.Supported by project 3540 of the Fonds zur Förderung der wissenschaftlichen Forschung in Österreich. The author is deeply indebted to Prof. Gamulin (Dubrovnik) for his support and to Prof. R. Riedl and Dr. W. Klepal for helpful discussions     

A comparative study of ciliary differentiations in the branchial stigmata of ascidians

In a correlated thin sectioning and freeze-fracturing study, we have examined species belonging to the orders of the ascidian class: Stolidobranchiata (Botryllus schlosseri, Botrylloides leachi, Molgula socialis, Styela plicata), Phlebobranchiata (Ascidiella aspersa, Phallusia ingeria, Ciona intestinalis) and Aplousobranchiata (Clavelina lepadiformis). Though the branchial basket varies in the complexity and filtration efficiency in the three orders, the ciliated epithelia aroand the stigmata contain a common pattern of organization; seven rows of flattened cells, each bearing a single row of long cilia flanked by a single row of microvilli. All the species examined possess ciliary specializations represented by: (a) bridges connecting doublets number 5 and 6 as well as 9.1 and 2; (b) dense material lying between the above mentioned axonemal doublets (5-6 and 1-2) and the ciliary membrane, sometimes in the shape of longitudinal strands or often as lines of dots; (c) a fuzzy coat protruding from the ciliary membrane, consisting of tufts or scattered filaments; (d) intramembrane particles (IMPs) associated with the P-face of the membrane, often arranged in clusters and orderly alignments related to the anderlying axonemal doublets; these IMPs decorate the opposite sides of each cilium facing the adjacent cilia forming the ciliary rows of adjacent cells and are absent on the lateral sides. The stigmatal cilia propel water through the stigmata and their effective strokes follow a line at right angles to the row of cilia in each cell. The usual direction of the effective stroke is toward doublets 5-6. It is possible, therefore, to refer to structure in relation to the ciliary beat cycle. The importance of these specializations is unknown, but the structures appear to vary in the different species. A correlation between the richness of the specializations and the complexity of the branchial basket was not evidenced. It was suggested that the ciliary specializations relate to the peculiar organization of the stigmatal margin and that all are involved in the regulation of the ciliary activity.     

Structure and motility of primary cilia in the follicular epithelium of the human thyroid

In order to clarify contradictory reports concerning ciliary structure and function, follicular epithelium from macroscopically normal portions of 37 surgical specimens of human thyroid were processed for video-microscopy and/or transmission electron microscopy. The cilia of living cells were immotile. In transverse sections the cilia revealed a 9 + 0 pattern at the base of the shaft, whereas towards the distal end the number of microtubular doublets diminished. Dynein arms, radial spokes and central microtubules were absent. The immotility and structure of these primary cilia implies that their function is not related to motility. The phylogenetic and ontogenetic development of the thyroid suggests that tumor cells of follicular origin displaying abnormal secondary cilia may represent a pathological variant of differentiation.     

Structure and motility of primary cilia in the follicular epithelium of the human thyroid

In order to clarify contradictory reports concerning ciliary structure and function, follicular epithelium from macroscopically normal portions of 37 surgical specimens of human thyroid were processed for video-microscopy and/or transmission electron microscopy. The cilia of living cells were immotile. In transverse sections the cilia revealed a 9 + 0 pattern at the base of the shaft, whereas towards the distal end the number of microtubular doublets diminished. Dynein arms, radial spokes and central microtubules were absent. The immotility and structure of these primary cilia implies that their function is not related to motility. The phylogenetic and ontogenetic development of the thyroid suggests that tumor cells of follicular origin displaying abnormal secondary cilia may represent a pathological variant of differentiation.