Scanning electron microscope studies of some skink papillae basilares

Summary The papilla basilaris of scincid lizards is relatively long, slightly curved or bowed, and characteristically has an apical terminal expansion. A limbus-attached tectorial membrane is present but is apparently not continuous with the tectorial material covering the hair cells of the papilla. The hair cells of the apical expansion are covered by a thick spongy mass of tectorial material, while the hair cells above (dorsal to) the apical region are covered by thickened tectorial material that is in the form of uniquely sculptured, twisted or folded drape-like masses (sallets). The surface of the basal (dorsal) quarter of the papilla is unusual in that it is concave rather than convex. The expanded terminals of the hair cell kinocilia are also unusual in being arrowhead-shaped.Kinocilial orientation of the non-apical papillary hair cells is simply bidirectional; the hair cells on each side of the papillary axial midline are oriented toward the midline. Kinocilial orientation of the hair cells of the apical expansion is more complex with the peripheral neural and abneural rows both being abneurally directed, and the central rows being at first neural in orientation, but becoming abneurally oriented as the apical tip is approached. At the apical tip region, most all hair cells are abneurally oriented.I would like to thank Ms Maria Maglio for her skill in handling the technical aspects of the electron microscope, Mr. David Akers for expert photographic assistance, and Ms. Michiko Kasahara for aid in all aspects of the work. Research sponsored by United States Public Health Service Grant NS-09231.     

Scanning electron microscope studies of the papilla basilaris of some turtles and snakes

The papillae basilares of three species of turtles and four species of snakes were studied by SEM. The papillae of turtle are relatively large among reptiles and are characterized by a long, horizontal middle section resting on wide basilar membrane. Both terminal ends of the papilla extend onto the surrounding limbus in the form of a forked or "T" -shaped end or as a curved, "hook"- like processes. Details vary with the species. In the three species of turtles studied, there were between 1,100 and 1,400 hair cells on a papilla. The tectorial membrane covering the horizontal portion of the papilla is heavy in appearance and tightly attached to the kinocilial bulbs. The terminal ends of the papilla are covered by a thin gelatinous material. In addition, mat-like tectorial network covers the supporting cells and extends from the microvilli of the supporting cells to the overlying tectorial membrane. All hair cells are unidirectionally and abneurally oriented. The supporting cell surfaces form a large part of the papilla and, thus, hair cell density is low. The papillae of the two boid snake species studied are moderately long among snakes and contain a moderate number of hair cells (574 in Epicrates and 710-780 in Constrictor). Papillar form is elongate, avoid, or canoe-shaped. The tectorial membrane may be either highly fenestrated or moderately dense and covers all but a few of the terminal hair cells. A tectorial-like mat covers all but a few of the terminal hair cells. Most hair cells are unidirectionally and abneurally oriented. A few terminal cells in boids may show reverse orientation. Hair cell density is similar to that of turtles.     

Peripheral auditory processing in the bobtail lizard Tiliqua rugosa

Summary We have labelled single physiologically-characterized primary auditory neurones in the bobtail lizard and traced them to their innervation sites within the basilar papilla. The distribution of stained fibre terminals shows that low frequencies (up to a characteristic frequency, CF, of about 0.8 kHz) are processed in the smaller apical segment of the papilla and medium to high frequencies in the much longer basal segment. It is possible that the frequency ranges of these segments partly overlap in individual animals.The tonotopic organization of the basal segment is well described by an exponential relationship; the CF increases towards the basal end. Systematic, peripheral recordings from the auditory nerve very close to the papilla confirm this tonotopicity for the basal segment.The apical segment of the papilla shows an unusual tonotopic organization in that the CF appears to increase across the epithelium, from abneural to neural. A tonotopicity in this direction has not previously been demonstrated in vertebrates.All stained neurones branched within the basilar papilla to innervate, typically, between 4 and 14 hair cells. The branching patterns of fibres innervating in the apical and basal papillar segment, respectively, show characteristic differences. Apical fibres tend to innervate hair cells with the same morphological polarity and often branch extensively along the segment. Basal fibres, in contrast, typically innervate about equal numbers of hair cells of opposing polarity and are more restricted in their longitudinal branching.Abbreviation CF characteristic frequency     

A scanning electron microscope study of the papilla basilaris of Gekko gecko

Summary The sensory hair cells of the ventral 2/3 of the papilla basilaris of Gekko gecko are divided into anterior (pre-axial) and posterior (post-axial) portions by a mid-axial gap or hiatus where there are no hair cells. There is no separation of the hair cells in the dorsal third of the papilla. There are three tectorial membrane modifications: an attached thickened membrane covering the pre-axial hair cells, sallets covering the post-axial hair cells, and an attached filamentous membrane covering the dorsal hair cells. The number of hair cells is greatest ventrally and decreases dorsally. There are approximately 2000 to 2100 hair cells. The kinocilia of the hair cells of the anterior halves of both the pre- and the post-axial vertical hair-cell rows are oriented posteriorly, while the kinocilia of the posterior halves are oriented anteriorly. The kinocilia of the hair cells of the dorsal third of the papilla are mostly oriented posteriorly. Thus, kinocilial orientation of the ventral 2/3 of the papilla is doubly bidirectional, and the dorsal 1/3, largely unidirectional.I would like to thank Ms. Maria Maglio for her skill in handling the technical aspects of the scanning electron microscopy as well as her artistry in achieving photographic excellence on the scope, David Akers for expert photographic assistance, and Wayne Emery for the drawings. Research sponsored by United States Public Health Service Grant NS-09231.     

The cellular organization and nervous supply of the basilar papilla in the lizard,Calotes versicolor

Summary The basilar papilla of the lizard Calotes versicolor contains about 225 sensory cells. These are of two types: the short-haired type A cells in the ventral (apical) part of the organ, and the type B cells with long hair bundles, in the dorsal (basal) part of the organ. The type A cells are unidirectionally oriented and are covered by a tectorial membrane while the type B cells lack a covering structure and their hair bundles are oriented bidirectionally. Apart from those differences, the type A and type B cells are similar. They are columnar, and display the features common to most sensory cells in inner ear epithelia. The sensory cells are separated by supporting cells, which have long slender processes that keep the sensory cells apart. Close to the surface of the basilar papilla a terminal bar of specialized junctions interlocks adjacent cells. Below this, adjacent supporting cells are linked by an occluding junction.The cochlear nerve enters from the medial (neural) aspect. The fibres of the nerve lose their myelin sheaths as they enter the basilar papilla. Each sensory cell is associated with several nerve endings. All the nerves identified were afferent. Marked variations were seen between nerve endings in the basilar papilla, but no morphological equivalents of any functional differences were observed.This work is supported by grant no. B76-12X-00720-11A from the Swedish Medical Research Council, and by funds from the Karolinska Institute, Stockholm, Sweden.     

Hair Cell Polarization in the Inner Ear of a Caecilian,Ichthyophis glutinosus (Amphibia: Gymnophiona)

The inner ear of Ichthyophis glutinosus is described with emphasis on the position of the sensory organs and the polarization of the hair cells. The hair cell polarization patterns of the maculae, cristae and papilla basilaris is similar to previous observations in other tetrapods. The papilla amphibiorum shows a simpler bidirectional polarization than described in other amphibians. The papilla neglecta, a sensory organ in the utriculus shows a unidirectional posteriorly directed polarization. A neglecta has not been found in the utriculus of anurans and urodeles previously.     

Development of the supporting cells and structures derived from them in the inner ear of the grass frog,Rana temporaria (Amphibia,Anura)

Summary The inner ear of Rana t. temporaria comprises sensory structures with various special functions, i.e., the detection of spatial orientation (utricle, saccule, lagena), of rotation (ampullae), and of acoustic signals (amphibian and basilar papillae). In each of these structures, there is a sensory epithelium made up of hair (sensory) cells and supporting cells. As the supporting cells differentiate, they produce the organic matrix of the otoconia in the gravity-sensing organs, the ground substance of the cupulae in the ampullae, and the ground substance of the tectorial membranes in the auditory papillae. The supporting cells associated with these various derivative structures have correspondingly different cytoplasmic properties. The preotoconia are formed by extrusion; the otoconia develop from these filamentous precursors by growth and calcium deposition. The organic material that forms the cupulae and tectorial membranes is released from the supporting cells by exocytosis. The organization of this material into the ground substance is initiated mainly around the distal ends of the hair-cell kinocilia, eventually giving rise to the marked morphological differences that distinguish the cupulae from the tectorial membranes.Abbreviations bb basal body - c cilia - ca crista ampullaris - ch chromosome - cu cupula - d dictyosome - hc hair cell - kc kinocilia - ld lipid droplet - m mitochondrion - ma main axis - mb multilamellated body - mc macula communis - mi mitosis - mv microvillus - n nucleus - on organic net - pa amphibian papilla - pb basilar papilla - pg pigment granule - po preotoconia - rer rough endoplasmic reticulum - s saccule - sc supporting cell - sci stereocilia - sd spot desmosome - t tegmentum - tf tonofilaments - tj tight junction - tm tectorial membrane - yp yolk platelet     

Diversity of form in the amphibian papilla of Puerto Rican frogs

Summary In modern frogs, the amphibian papilla exhibits a caudal extension whose shape, relative length, and proportion of hair cells vary markedly from species to species. Tuning in the caudal extension is organized tonotopically and evidently involves the tectorium. In terms of the proportion of amphibian-papillar hair cells in the caudal extension, we report more diversity among 8 species of a single genus (Eleutherodactylus) on a single island (Puerto Rico) than has been found so far among all of the (more than 50) other modern anurans examined for this feature from around the world. These 8 Puerto Rican species have overlapping habitat and conspicuous diversity in the male advertisement call. For 7 of the 8 species, we report that the call has transient spectral components in the frequency range of the amphibian papilla, and that the proportion of caudal extension hair cells and the frequency distribution of those components are correlated. Thus one might conclude that the selective pressures that led to diversity of calls among the 8 species also led to diversity in form of the amphibian papilla.Abbreviations AP amphibian papilla - BEF best excitatory frequency - PR Puerto Rican - SEM scanning electron microscope - SVL snout-vent length - TC tectorial corner - TM tectorial membrane     

The lizard ear: Gekkonidae

The gecko ear was studied in 36 species belonging to 24 genera. This receptor has attained an advanced level of structure and performance in this group of lizards, but there are many variations among species. To a large extent these variations follow subfamily lines as represented in Kluge's system of classification. Brief consideration is given to features of the outer and middle ear, but chief concern is with inner ear structures and their relations to auditory sensitivity as represented by the cochlear potentials. The auditory papilla is segmented, with a dorsal portion whose hair cells have their ciliary tufts attached to a tectorial membrane, and a ventral portion in which these cells form tow assemblages, one with tectorial connections and the other with connections to a line of sallets. The dorsal segment varies greatly in length and in the form of ciliary orientation. In Eublepharinae and most Gekkoninae the ciliary orientation is unidirectional, and the degree of sensitivity relates to the length of this segment. In Diplodactylinae and Sphaerodactylinae the orientation is bidirectional, and this segment functionally hardly differs from the ventral segment. Auditory sensitivity as measured in terms of the cochlear potentials shows close relations with subfamily groupings, except for the Gekkoninae in which considerable diversity is found. The evidence from structural differentiation, along with that derived from the forms of the cochlear potential functions, leads to the suggestion that these ears possess a high degree of pitch discrimination and capability for the analysis of complex sounds.     

Morphological and fine structural features of the basilar papilla in ambystomatid salamanders (Amphibia; Caudata)

The morphology and fine structure of the basilar recess and basilar papilla were investigated in four species of salamanders from the family Ambystomatidae. The otic relationships of the recess and papilla to the proximal part of the lagena and saccule are described, and new terminology is suggested for the periotic relationships of the basilar recess to a diverticulum of an intracapsular periotic sac. The basilar papilla consists of supporting cells united laterally by gap junctions, capped by microvilli uniformly arranged around a short, central cilium, and hair cells that typically show several synapses with a single afferent nerve fiber, each marked by a rounded synaptic body surrounded by vesicles. In contrast to anuran basilar papillae, efferent nerve terminals were observed in synapse with hair cells and, rarely, upon afferent fibers. The distal half of the ambystomatid papilla contained hair cells capped by tall ciliary bundles, with kinocilia that show swellings near their tips with delicate attachments to adjacent tall stereocilia. A tectorial body covers only this region of the papilla. Hair cells with shorter stereocilia, situated in the proximal half and at the papillar margins, are related only to filamentous extensions of the tectorial body. The ambystomatid basilar recess and papilla are compared to auditory end-organs in other vertebrates, and it is suggested that a basic distinction can be made between aural neuroepithelia in amniotes versus that in nonamniotic vertebrate ears.     

The fine structure of epidermal papillae of Travisia forbesii (Annelida)

The structure of the epidermis of Travisia forbesii was described using light and electron microscopy. The epidermis is a highly modified variant of the normal one-layer polychaete epithelium. It consists of basal epidermal cells and an external layer of closely sited papillae consisting of glandular and supportive epidermal cells, and extensive electron-transparent intercellular spaces. The papillae are embedded in the thick cuticle. Each papilla has a peduncle, which is formed by one cell that penetrates the inner cuticle layer to the basal epidermal cells. A fold of basement membrane forms the core of the peduncle and ends in the base of a papilla. All epidermal cells are connected to each other with apical cell junctions and to the basement membrane with hemidesmosomes, so the epithelium is continuous and uninterrupted. The epidermis has an intra-epidermal neuron plexus. The structure of the papillae is compared with papillae and tubercles of other polychaetes, and the possible functional significance and phylogenetic implications of these structures are discussed.     

THE OCCURRENCE OF WALL PAPILLAE IN ROOT EPIDERMAL CELLS OF AXENICALLY GROWN SEEDLINGS OF ZEA MAYS

Lens-shaped wall papillae, resembling those known to form in response to fungi or mechanical damage, occur in root epidermal cells of axenically grown seedlings of Zea mays. Papillae are most common in the tabular epidermal cells but also occur in younger cells. Not all tabular cells have papillae, and they are more frequent in some seedlings. Where present, there is usually only one papilla per cell and it lies against the outer periclinal wall just proximal to an emerged root hair or near the position where a hair would be expected to form. Electron micrographs show that a papilla is structurally heterogeneous. Papillae fluoresce strongly in the presence of aniline blue even in freeze-substituted material.     

Chiton integument: Ultrastructure of the sensory hairs of Mopalia muscosa (Mollusca: Polyplacophora)

Summary The dorsal integument of the girdle of the chiton Mopalia muscosa is covered by a chitinous cuticle about 0.1 mm in thickness. Within the cuticle are fusiform spicules composed of a central mass of pigment granules surrounded by a layer of calcium carbonate crystals. Tapered, curved chitinous hairs with a groove on the mesial surface pass through the cuticle and protrude above the surface. The spicules are produced by specialized groups of epidermal cells called spiniferous papillae and the hairs are produced by trichogenous papillae. Processes of pigment cells containing green granules are scattered among the cells of each type of papilla and among the common epidermal cells.The wall or cortex of each hair is composed of two layers. The cortex surrounds a central medulla that contains matrix material of low density and from 1 to 20 axial bundles of dendrites. The number of bundles within the medulla varies with the size of the hair. Each bundle contains from 1 to 25 dendrites ensheathed by processes of supporting cells. The dendrites and supporting sheath arise from epidermal cells of the central part of the papilla. At the base of each trichogenous papilla are several nerves that pass into the dermis. Two questions remain unresolved. The function of the hairs is unknown, and we have not determined whether the sensory cells are primary sensory neurons or secondary sensory cells.     

The organization and isolating function of insect rectal sheath cells: a freeze-fracture study.

In termites and roaches the well-defined rectal papillae each comprise a layer of columnar principal cells specialized for active transport and a layer of basal cells. The whole cell group is entirely surrounded by several series of flattened 'sheath cells' (formerly termed 'junctional cells') which abut onto the basal components of the papilla. The sheath cells secrete a specialized sclerified cuticle which forms the framework of the papilla. Their regularly pleated apical membrane is closely apposed to the cuticle and contains parallel and closely spaced rows of intramembranous particles. at this level, no subcuticular space is present and hence the space associated with the apical surface of the principal cells is defined as an isolated compartment. Typical septate junctions are present between the sheath and basal cells; however those linking adjacent sheath cells are structurally unusual: they extend to the basal surface rather than being restricted to the apical zone, are frequently interrupted and in replicas are represented by relatively short and irregularly oriented particle rows. Moreover, lateral sheath cell contacts display two further peculiarities: absence of an apical desmosomal ring and paucity of gap junctions. Structural observations suggest that the sheath cells isolate the principal cells from communication with the hemolymph, consequently enhancing their efficiency in water and ionic regulation. Comparable cells have been described in a number of insects, but the 'isolation' system presents varying degrees of complexity, for which an evolutionary scheme is proposed.     

Ultrastructure of osmoregulatory organs in larvae of the brackish-water mosquito,Culiseta inornata (Williston)

The ultrastructure of the Malpighian tubules, ileum, rectum, anal canal, and anal papillae of larvae of the mosquito Culiseta inornata was examined. The Malpighian tubules, rectum, and anal papillae have many of the ultrastructural features characteristic of ion transport tissues, i.e., elaboration of the basal and apical membranes and a close association of these membranes with mitochondria. The Malpighian tubules possess two cell types, primary and stellate. The larval rectum of C. inornata is composed of a single segment containing a homogenous population of cells. In this respect, the larval rectum of C. inornata is distinct from that of saline-water species of Aedes. The cells in the larval rectum of C. inornata, however, closely resemble those of one cell type, the anterior rectal cells, of the saline-water mosquito Aedes campestris with regard to cell and nuclear size, the percentage of the cell occupied by apical folds, and mitochondrial density and distribution. No similarities can be found between the rectum of C. inornata and the posterior segment of the saline-water Aedes, which functions as a salt gland. On this basis, we have postulated that the rectum of C. inornata does not function as a site of hyperosmotic fluid secretion. The ultrastructure of the anal papillae of C. inornata is consistent with a role in ion transport. The significance of these findings to comparative aspects of osmoregulatory strategies in mosquito larvae is discussed.     

Molecular cloning of chick beta-tectorin, an extracellular matrix molecule of the inner ear

The tectorial membrane is an extracellular matrix lying over the apical surface of the auditory epithelium. Immunofluorescence studies have suggested that some proteins of the avian tectorial membrane, the tectorins, may be unique to the inner ear (Killick, R., C. Malenczak, and G. P. Richardson. 1992. Hearing Res. 64:21-38). The cDNA and deduced amino acid sequences for chick beta-tectorin are presented. The cDNA encodes a protein of 36,902.6 D with a putative signal sequence, four potential N-glycosylation sites, 13 cysteines, and a hydrophobic COOH terminus. Western blots of two-dimensional gels using antibodies to a synthetic peptide confirm the identity of the cDNA. Southern and Northern analysis suggests that beta-tectorin is a single-copy gene only expressed in the inner ear. The predicted COOH terminus is similar to that of glycosylphosphatidylinositol-linked proteins, and antisera raised to this region react with in vitro translation products of the cDNA clone but not with mature beta-tectorin. These data suggest beta- tectorin is synthesized as a glycosylphosphatidyl-inositol-linked precursor, targeted to the apical surface of the sensory epithelium by the lipid moiety, and then further processed. Sequence analysis indicates the predicted protein possesses a zona pellucida domain, a sequence that is common to a limited number of other matrix-forming proteins and may be involved in the formation of filaments. In the cochlear duct, beta-tectorin is expressed in the basilar papilla, in the clear cells and the cuboidal cells, as well as in the striolar region of the lagena macula. The expression of beta-tectorin is associated with hair cells that have an apical cell surface specialization known as the 275-kD hair cell antigen restricted to the basal region of the hair bundle, suggesting that matrices containing beta-tectorin are required to drive this hair cell type.     

Further serial transmission electron microscopy studies of auditory hair cell innervation in lizards and in a snake

Auditory hair cells of three lizard and one snake species were studied by serial transmission electron microscopy (TEM) sections of two unidirectional hair cells (UHC) and two bidirectional hair cells (BHC) and by nonserial section montages of each entire papilla cut at 2-microns intervals across the papillar width. The unidirectional hair cell region of the agamid lizard, Acanthosaura crucigera, lacked efferent innervation. Another agamid lizard, Agama agama, studied by nonserial section only, also lacked efferent innervation to the UHC. Afferent innervation to both the UHC and BHC of Acanthosaura was primarily exclusive (each nerve fiber innervates only one hair cell), although an occasional nerve fiber innervated two hair cells. Both the UHC and the BHC of the anguid, Celestus costatus, were exclusively innervated. Both hair cell types of the varanid, Varanus exanthematicus, were nonexclusively innervated (all afferent nerve fibers innervate two or more hair cells). The auditory papilla of the colubrid snake, Elaphe obsoleta, has only one type of hair cell and each is nonexclusively innervated. The numbers of afferent and efferent nerve fibers and of afferent synapses are presented in tabular form.     

Defective calmodulin‐dependent rapid apical endocytosis in zebrafish sensory hair cell mutants

Vertebrate mechanosensory hair cells contain a narrow “pericuticular” zone which is densely populated with small vesicles between the cuticular plate and cellular junctions near the apical surface. The presence of many cytoplasmic vesicles suggests that the apical surface of hair cells has a high turnover rate. The significance of intense membrane trafficking at the apical surface is not known. Using a marker of endocytosis, the styryl dye FM1‐43, this report shows that rapid apical endocytosis in zebrafish lateral line sensory hair cells is calcium and calmodulin dependent and is partially blocked by the presence of amiloride and dihydrostreptomycin, known inhibitors of mechanotransduction channels. As seen in lateral line hair cells, sensory hair cells within the larval otic capsule also exhibit rapid apical endocytosis. Defects in internalization of the dye in both lateral line and inner ear hair cells were found in five zebrafish auditory/vestibular mutants: sputnik, mariner, orbiter, mercury, and skylab. In addition, lateral line hair cells in these mutants were not sensitive to prolonged exposure to streptomycin, which is toxic to hair cells. The presence of endocytic defects in the majority of zebrafish mechanosensory mutants points to a important role of apical endocytosis in hair cell function. © 1999 John Wiley & Sons, Inc. J Neurobiol 41: 424–434, 1999