The mechanism of sensory transduction in a mechanoreceptor. Functional stages in campaniform sensilla during the molting cycle.

This paper describes the ultrastructural modifications that cockroach campaniform sensilla undergo at three major stages in the molting cycle and finds that the sensilla are physiological functional at all developmental stages leading to ecdysis. Late stage animals on the verge of ecdysis have two completely separate cuticles. The campaniform sensillum sends a 220-mum extension of the sensory process through a hole in its cap in the new (inner) cuticle across a fluid-filled molting space to its functional insertion in the cap in the old (outer) cuticle. Mechanical stimulation of the old cap excites the sensillum. The ultrastructural geometry of late stage sensilla, coupled with the observation they are physiolgically functional, supports the hypotheses (a) that sensory transduction occurs at the tip of the sensory process, and (b) that cap identation causes the cap cuticle to pinch the tip of the sensory process, thereby stimulating the sensillum.

The vomeronasal (Jacobson's) organ in man: ultrastructure and frequency of occurrence.

These investigations address three major questions: (1) What is the frequency of occurrence of the vomeronasal (Jacobson's) organ (VNO) in man? (2) what is the ultrastructure of the human VNO? and (3) does the VNO contain sensory receptor cells? Macroscopic and microscopic intranasal clinical examinations of over 200 persons revealed paired bilateral vomeronasal pits on the anterior 1/3 of the nasal septum in all cases. Biopsies of the vomeronasal pits and surrounding tissues were examined by light and electron microscopy. These studies showed that the vomeronasal pit leads to a closed tube, 2-8 mm long, lined by a unique pseudostratified columnar epithelium unlike any other in the human body. The anterior end of the tube is lined by tall, columnar cells with a sparse population of short microvilli. The posterior end of the VNO is lined by an epithelium that contains three morphologically distinct cell types: (1) basal cells; (2) "dark cells--tall, slender cells with heterochromatic nuclei and electron-dense cytoplasm that often contain mucigen-like granules; and (3) "light" cells--large, clear cells, extending from the basement membrane to the organ's lumen. Each "light" cell has a round, euchromatic nucleus and a clear cytoplasm that often contains many Golgi stacks and membrane-limited vesicles filled with material of modest electron density. The cell apex is tipped by a few short microvilli. Whether these cells subserve any sensory function awaits further investigation.

Ultrastructural histopathology of human olfactory dysfunction.

This paper presents electron-microscopic observations on biopsies of the olfactory mucosae of several classes of patients with smell disorders: 1) patients with loss of smell function following head injury (post-traumatic anosmics or hyposmics); 2) patients with loss of smell function following severe head colds and/or sinus infections (post-viral olfactory dysfunction, or PVOD); and 3) patients that have lacked smell function since birth (congenital anosmics). Of these, the traumatic anosmics' olfactory epithelia were quite disorganized; the orderly arrangement of supporting cells, ciliated olfactory receptor neurons, microvillar cells, and basal cells was disrupted. Although many somata of ciliated olfactory receptors were present, few of their dendrites reached the epithelial surface. The few olfactory vesicles present usually lacked olfactory cilia. The post-viral anosmics, too, had a greatly reduced number of intact ciliated olfactory receptor neurons, and most of those present were aciliate. The post-viral hyposmics had a larger population of intact, ciliated olfactory receptor cells. In the seven cases of congenital anosmia studied, no biopsies of olfactory epithelium were obtained, indicating the olfactory epithelium is either absent--or greatly reduced in area--in these individuals.

Ultrastructural neurobiology of the olfactory mucosa of the brown trout, Salmo trutta.

This paper describes four investigations of the olfactory mucosa of the brown trout: 1) the ultrastructure of the olfactory mucosa as revealed by scanning (SEM), conventional transmission (TEM), and high voltage (HVEM) electron microscopy; 2) light and electron-microscopic investigations of retrograde transport of the tracer macromolecule horseradish peroxidase (HRP) when applied to the cut olfactory nerve; 3) SEM and TEM investigations of the effects of olfactory nerve transection on cell populations within the olfactory epithelium; and 4) ultrastructural investigations of reversible degeneration of olfactory receptors caused by elevated copper concentrations. The trout olfactory epithelium contains five cell types: ciliated epithelial cells, ciliated olfactory receptor cells, microvillar olfactory receptor cells, supporting cells, and basal cells. The ciliated and microvillar olfactory receptor cells and a small number of basal cells are backfilled by HRP when the tracer is applied to the cut olfactory nerve. When the olfactory nerve is cut, both ciliated and microvillar olfactory receptor cells degenerate within 2 days and are morphologically intact again within 8 days. When wild trout are taken from their native stream and placed in tanks with elevated copper concentrations, ciliated and microvillar cells degenerate. Replacement of these trout into their stream of origin is followed by morphologic restoration of both types of olfactory receptor cells. Ciliated and microvillar receptor cells are primary sensory bipolar neurons whose dendrites make contact with the environment; their axons travel directly to the brain. Consequently, substances can be transported directly from the environment into the brain via these "naked neurons." Since fish cannot escape from the water in which they swim, and since that water may occasionally contain brain-toxic substances, the ability to close off--and later reopen--this anatomic gateway to the brain would confer a tremendous selective advantage upon animals that evolved the "brain-sparing" capacity to do so. Consequently, the unique regenerative powers of vertebrate olfactory receptor neurons may have their evolutionary origin in fishes.

Electron microscopy of human olfactory epithelium reveals a new cell type: the microvillar cell.

The olfactory epithelium of mammals is generally considered to consist of 3 cell types: basal cells, supporting (sustentacular) cells, and ciliated olfactory receptors. We have completed a detailed ultrastructural study of the fine structure of the human olfactory mucosa. In our electron microscopic observations of biopsies of human olfactory epithelium taken from normal, consenting volunteers under local anesthesia, we have consistently observed a fourth cell type, the microvillar cell, located near the epithelial surface. The apical end of these flask-shaped, electron-lucent cells gives rise to a tuft of microvilli that project into the mucus layer lining the nasal cavity. The cell body itself contains bundles of microfilaments, mitochondria, a well-developed smooth endoplasmic reticulum, a prominent Golgi complex, electron-dense vesicles that resemble lipofuscin granules, free ribosomes, and occasional cisternae of the rough endoplasmic reticulum. A thin, axon-like cytoplasmic process extends from the basal pole of the cell and travels through the epithelium toward the lamina propria. Although there is no physiological evidence that bears upon the function of the microvillar cell, its ultrastructure suggests it may be a bipolar sensory neuron. Based upon morphological and phylogenetic considerations, the authors speculate the microvillar cell represents a second morphologically distinct class of chemoreceptor in the human olfactory mucosa.

Peroxidase backfills suggest the mammalian olfactory epithelium contains a second morphologically distinct class of bipolar sensory neuron: the microvillar cell.

The olfactory epithelium of man, rat and some other mammals consists of 4 cell types: ciliated olfactory receptors, microvillar cells, supporting (sustentacular) cells, and basal cells. Of these, the microvillar cell is least well understood: its function is unknown. In this study, a hypothesis is put forth: that the microvillar cells in the mammalian olfactory epithelium comprise a morphologically distinct class of sensory receptor. The hypothesis is tested by injecting the cytochemical tracer macromolecule horseradish peroxidase (HRP) into the olfactory bulb of the rat, and observing its pattern of uptake in the olfactory epithelium by light and electron microscopy. In these experiments, ciliated olfactory receptors and microvillar cells backfilled with HRP: supporting and basal cells did not. The data, which support the hypothesis, indicate the microvillar cells, along with the ciliated olfactory receptors, send axons to the olfactory bulb. Consequently, it is concluded that the microvillar cell is a sensory bipolar neuron, with the cell body in the olfactory epithelium, that sends a dendrite to the site of stimulus reception at the free surface of the olfactory epithelium, and an axon to the olfactory bulb in the brain. The similarity of microvillar cells in the olfactory epithelium to 'brush cells' found throughout the respiratory tract is discussed in detail.

A somatotopic organization of groups of afferents in insect peripheral nerves.

Sensory axon projections in the main leg nerves of two orthopteran insects, the cockroach Periplaneta americana and the grasshopper Melanoplus bivittatus, were studied by observing patterns of axonal degeneration after ablation of different leg segments. The patterns of degeneration seen in transverse sections of the leg nerves, close to their entrance to the central nervous system (thoracic ganglion) show that sensory axons occur in constant positions in the leg nerves. When distal leg segments are removed, a discrete area of degeneration is found in the leg nerve along its posterior edge. More proximal ablations produce larger areas of degeneration that progressively extend into the anterior half of the nerve. A comparison of the patterns of degeneration produced by different leg ablations shows a posterior to anterior laminar arrangement of groups of sensory axons that corresponds to a distal to proximal map of the leg. This mapping has been confirmed by localized ablations of small groups of leg sensory receptors.

The effect of human olfactory biopsy on olfaction: a preliminary report.

Normal human olfactory function is subject to a wide variety of factors. Although biopsy of human olfactory neuroepithelium has been reported by several researchers, there are no studies which have evaluated the effect of this procedure on olfactory function. In this retrospective study, we sought to determine if tissue removal from the olfactory cleft has an adverse influence on the sense of smell. Nineteen subjects underwent bilateral olfactory testing and subsequent endoscopic olfactory mucosal biopsy. All subjects were retested 6 weeks to 1 year after olfactory neuroepithelial biopsy. No statistical difference was found between olfactory tests performed before or after biopsy. These data suggest that biopsy of human olfactory neuroepithelium has no discernible adverse effect on the ability to smell.

Steroid-dependent anosmia.

In steroid-dependent anosmia (nasal polyps, inhalant allergy, anosmia), high doses of steroids will temporarily restore the sense of smell, a diagnostic test. Appropriate surgery can then be carried out, followed by low-dose, long-term steroid therapy to maintain the sense of smell. Olfactory biopsy specimens taken during the course of evaluation and treatment show electron-optically normal olfactory receptors, meaning that the probable pathogenesis of the sensory deficit is an obstruction, mechanical and possibly biochemical. Two cases of steroid-dependent anosmia are presented to detail a fully reversible anosmia using state-of-the-art techniques.

A simple procedure for mounting wrinkle-free sections on formvar-coated slot grids.

This paper describes a simple technique for consistent production of clean, unwrinkled, flat thin (500-1000 A) sections for TEM and thick (1/2-1 micron) sections for HVEM mounted on Formvar-covered slot grids for use in conventional and high voltage electron microscopy. The technique centers around use of a Formvar-covered aluminum supporting rack onto which slot grids that contain sections suspended in water are placed.

The fine structure of the cockroach subgenual organ.

This paper describes the fine structure of the cockroach subgenual organ, a complex ciliated mechanoreceptor that detects vibrations in the substrate upon which the animal stands. Located beneath the knee in each walking leg, the cockroach subgenual organ is a thin, fan-shaped flap of tissue slung across the dorsal blood space of the tibia at right angles to the leg's long axis. It is innervated by approximately 50 chordotonal sensilla. The fine structure of the chordotonal sensilla is is described in detail; possible transducer sites are discussed.

Ultrastructure of the grasshopper proximal femoral chordotonal organ.

This paper, the first in a series concerning the neurobiology of sensory cilia, describes the ultrastructure of our chosen model system--the proximal femoral chordotonal organ (FCO) in pro- and mesothoracic grasshopper legs. The FCO is a bundle of 150-200 longitudinally oriented chordotonal sensilla. Each chordotonal sensillum is a mechanoreceptive unit that contains two bipolar neurons whose dendrites bear sensory cilia. The structure of the sensory cilia leads us to suggest that they are motile cilia that respond to the mechanical stimulus with an "active stroke" which excites a transducer membrane at the dendrite tip.

Calcium-binding sites on sensory processes in vertebrate hair cells.

Vertebrate lateral line and vestibular systems center their function on highly mechanosensitive hair cells. Each hair cell is equipped with one kinocilium (which resembles a motile cilium) and 50-100 actin-containing stereocilia (which resemble microvilli) at the site of stimulus reception. This report describes electron-microscopic localization of calcium-binding sites on the sensory processes of vertebrate hair cells. Using the Oschman-Wall technique for calcium localization [Oschman, J. L. & Wall, B. J. (1972) J. Cell Biol. 55, 58-73] together with electron-probe x-ray microanalysis of thin sections, we observed: (i) calcium- and iron-containing deposits in the region of the ciliary necklace in goldfish lateral line hair cells, (ii) calcium deposits upon the surface of stereocilia of hair cells of the bullfrog inner ear, and (iii) calcium deposits upon stereocilia of hair cells of the guinea pig vestibular system.

Evidence for active role of cilia in sensory transduction.

Combined high-voltage electron-microscopic and electrophysiological studies strongly suggest that cilia play an active role in sensory transduction in the grasshopper proximal femoral chordotonal organ (FCO) a ciliated mechanoreceptor. The FCO of pro- and mesothoracic legs of Melanoplus bivittatus contains a group of several hundred chorodontal sensilla arranged in a near-parallel bundle and slung between the proximal femur and the knee joint. Both flexion and extension of the tibia stimulate the FCO, which appears to measure the femoro-tibial angle. The FCO's U-shaped response curve indicates that progressive flexion or extension from the resting joint angle of 90 degrees increases the response frequency of individual receptors and recruits additional units as well. Since the FCO is a purely tonic mechanoreceptor, it is possible to fix FCOs during maximum and minimum states of stimulation and electron-microscopically observed changes in the receptor's fine structure. The most conspicuous change is the production of a pronounced bend at the base of the sensory cilia in chordotonal sensilla of maximally stimulated femoral chordotonal organs.

The fine structure of the olfactory mucosa in man.

This report gives a detailed description of the fine structure of the olfactory mucosa in man. Using a special biopsy instrument and technique, fresh biopsies of olfactory epithelium were taken under local anaesthesia from eight normal volunteers. Transmission electron microscopy reveals that human olfactory epithelium has four major cell types: ciliated olfactory receptors, supporting cells, basal cells and microvillar cells. The ciliated olfactory receptors, as in other mammals, are bipolar neurons; the dendrite tip, modified to form the olfactory vesicle, bears 10-30 cilia that lack dynein arms. The supporting cells, markedly different from the goblet cells of respiratory epithelium, are not specialized for mucus secretion. Instead they are equipped to contribute materials to, and remove materials from, the surface mucus. The basal cells are stem cells that serve to replace epithelial cells and receptors lost during normal turnover or injury. In addition to ciliated olfactory neurons, supporting cells and basal cells, the human olfactory mucosa contains a distinct fourth cell type, the microvillar cell, of unknown function. The apical pole of the cell sends a tuft of short microvilli into the nasal cavity; its basal pole gives rise to a slender cytoplasmic process that resembles an axon. If microvillar cells prove to be sensory cells, the current concept of the human olfactory epithelium will have to be revised to include two morphologically distinct classes of receptors.

Biopsy of human olfactory mucosa. An instrument and a technique.

We have developed an instrument that obtains small biopsy specimens of human olfactory mucosa suitable for electron microscopy. Following local anesthesia of the patient, the instrument is introduced through the naris and advanced 60 to 70 mm between the septum and middle turbinate. The cutting edge is pressed against the olfactory area and then is withdrawn with the mucosal specimen. This procedure has been performed successfully on 12 people with no substantial adverse effects. We have provided the indications for the use of this technique and its possible applications.

Electron microscopy of olfactory epithelia in two patients with anosmia.

Ultrastructural alterations were present in biopsy specimens of olfactory epithelia taken from two patients with anosmia. In both cases, the olfactory epithelia presented a disorganized appearance when viewed by transmission electron microscopy. The number of ciliated olfactory receptors was reduced; few olfactory vesicles were present at the epithelial surface. Where present, the olfactory vesicles usually lacked cilia. Since both patients had a history of head trauma, we speculate that the fila olfactoria may have been severed at the level of the cribriform plate. The histopathologic changes in the olfactory receptors that were revealed by electron microscopy may have resulted from the inability of regenerating axons to reach their normal site of synaptic contact--the second-order neurons (mitral cells) in the olfactory bulb of the brain.