Scanning electron microscopical studies of the arrangements and numbers of hair cells in the statocysts of Octopus vulgaris, Sepia officinalis and Loligo vulgaris

Saccadic Movement Strategy in Common Cuttlefish (Sepia officinalis)

Most moving animals segregate their locomotion trajectories in short burst like rotations and prolonged translations, to enhance distance information from optic flow, as only translational, but not rotational optic flow holds distance information. Underwater, optic flow is a valuable source of information as it is in the terrestrial habitat, however, so far, it has gained only little attention. To extend the knowledge on underwater optic flow perception and use, we filmed the movement pattern of six common cuttlefish (Sepia officinalis) with a high speed camera in this study. In the subsequent analysis, the center of mass of the cuttlefish body was manually traced to gain thrust, slip, and yaw of the cuttlefish movements over time. Cuttlefish indeed performed short rotations, saccades, with rotational velocities up to 343°/s. They clearly separated rotations from translations in line with the saccadic movement strategy documented for animals inhabiting the terrestrial habitat as well as for the semiaquatic harbor seals before. However, this separation only occurred during fin motion. In contrast, during jet propelled swimming, the separation between rotational and translational movements and thus probably distance estimation on the basis of the optic flow field is abolished in favor of high movement velocities. In conclusion, this study provides first evidence that an aquatic invertebrate, the cuttlefish, adopts a saccadic movement strategy depending on the behavioral context that could enhance the information gained from optic flow.

Contribution to the Understanding of Particle Motion Perception in Marine Invertebrates

Marine invertebrates potentially represent a group of species whose ecology may be influenced by artificial noise. Exposure to anthropogenic sound sources could have a direct consequence on the functionality and sensitivity of their sensory organs, the statocysts, which are responsible for their equilibrium and movements in the water column. The availability of novel laser Doppler vibrometer techniques has recently opened the possibility of measuring whole body (distance, velocity, and acceleration) vibration as a direct stimulus eliciting statocyst response, offering the scientific community a new level of understanding of the marine invertebrate hearing mechanism.

Aminoglycoside-induced damage in the statocyst of the longfin inshore squid, Doryteuthis pealeii

Squid are a significant component of the marine biomass and are a long-established model organism in experimental neurophysiology. The squid statocyst senses linear and angular acceleration and is the best candidate for mediating squid auditory responses, but its physiology and morphology are rarely studied. The statocyst contains mechano-sensitive hair cells that resemble hair cells in the vestibular and auditory systems of other animals. We examined whether squid statocyst hair cells are sensitive to aminoglycosides, a group of antibiotics that are ototoxic in fish, birds, and mammals. To assess aminoglycoside-induced damage, we used immunofluorescent methods to image the major cell types in the statocyst of longfin squid (Doryteuthis pealeii). Statocysts of live, anesthetized squid were injected with either a buffered saline solution or neomycin at concentrations ranging from 0.05 to 3.0 mmol l(-1). The statocyst hair cells of the macula statica princeps were examined 5 h post-treatment. Anti-acetylated tubulin staining showed no morphological differences between the hair cells of saline-injected and non-injected statocysts. The hair cell bundles of the macula statica princeps in aminoglycoside-injected statocysts were either missing or damaged, with the amount of damage being dose-dependent. The proportion of missing hair cells did not increase at the same rate as damaged cells, suggesting that neomycin treatment affects hair cells in a nonlethal manner. These experiments provide a reliable method for imaging squid hair cells. Further, aminoglycosides can be used to induce hair cell damage in a primary sensory area of the statocyst of squid. Such results support further studies on loss of hearing and balance in squid.

Synaptic interactions between crista hair cells in the statocyst of the squid Alloteuthis subulata

Intracellular injections of the fluorescent dye Lucifer yellow into the various cell types within the anterior transverse crista segment of the statocyst of squid revealed that the primary sensory hair cells and both large and small first-order afferent neurons have relatively simple morphologies, each cell having a single, unbranched axon that passes directly into the small crista nerve that innervates the anterior transverse crista. However, the small first-order neurons have short dendritic processes occurring in the region of the sensory hair cells. The secondary sensory hair cells have no centripetal axons, but some have long processes extending from their bases along the segment. Simultaneous intracellular recordings from pairs of the different cell types in the anterior transverse crista segment demonstrated that electrical coupling is widespread; secondary sensory hair cells are coupled electrically along a hair cell row, as are groups of primary sensory hair cells. Secondary sensory hair cell also are coupled to neighboring small first-order afferent neurons. However, this coupling is rectifying in that it only occurs from secondary sensory hair cells to first-order afferent neurons. Direct electrical stimulation of the small crista nerve to excite the efferent axons revealed efferent connections to both the primary sensory hair cells and the small first-order afferent neurons. These efferent responses were of three types: excitatory or inhibitory postsynaptic potentials and excitatory postsynaptic potentials followed by inhibitory postsynaptic potentials. The functional significance of the cell interactions within the crista epithelium of the statocyst of squid is discussed and comparisons drawn with the balance organs of other animals.

Structure and function of the Nautilus statocyst

The two equilibrium receptor organs (statocysts) of Nautilus are avoid sacks, half-filled with numerous small, free-moving statoconia and half with endolymph. The inner surface of each statocyst is lined with 130,000-150,000 primary sensory hair cells. The hair cells are of two morphological types. Type A hair cells carry 10-15 kinocilia arranged in a single ciliary row; they are present in the ventral half of the statocyst. Type B hair cells carry 8-10 irregularly arranged kinocilia; they are present in the dorsal half of the statocyst. Both type of hair cells are morphologically polarized. To test whether these features allow the Nautilus statocyst to sense angular accelerations, behavioural experiments were performed to measure statocyst-dependent funnel movements during sinusoidal oscillations of restrained Nautilus around a vertical body axis. Such dynamic rotatory stimulation caused horizontal phase-locked movements of the funnel. The funnel movements were either in the same direction (compensatory funnel response), or in the opposite direction (funnel follow response) to that of the applied rotation. Compensatory funnel movements were also seen during optokinetic stimulation (with a black and white stripe pattern) and during stimulations in which optokinetic and statocyst stimulations were combined. These morphological and behavioural findings show that the statocysts of Nautilus, in addition to their function as gravity receptor organs, are able to detect rotatory movements (angular accelerations) without the specialized receptor systems (crista/cupula systems) that are found in the statocysts of coleoid cephalopods. The findings further indicate that both statocyst and visual inputs control compensatory funnel movements.

The structure of the statocyst of the freshwater snail Biomphalaria glabrata (Pulmonata, Basommatophora)

The structure of the statocyst of the freshwater snail Biomphalaria glabrata has been examined by light and electron microscopy. The two statocysts are located on the dorsal-lateral side of the left and right pedal ganglion. The statocysts are spherical, fluid-filled capsules with a diameter of approximately 60 microm for young and 110 microm for adult snails. The wall of the cyst is composed of large receptor cells and many smaller supporting cells. The receptor cells bear cilia which are evenly distributed on the apical surface. The cilia have the typical 9+2 internal tubule configuration. Striate rootlets originate from the base of the basal body and run downward into the cytoplasm. Side-roots arise from one side of the basal body and a basal foot from the other. For each receptor cell, the basal foot always points to the periphery of the surface, indicating that the receptor cell is non-polarized. The receptor cells contain cytoplasmic organelles such as mitochondria, ribosomes, rough and smooth endoplasmic reticulum, compact Golgi bodies and multivesicular bodies. Supporting cells bearing microvilli are interposed between the receptor cells. The junction complex between the supporting cells and the receptor cells is composed of adherens and septate junctions, while between supporting cells only the adherens junctions are present. The static nerve arises from the lateral side of the cyst and contains axons in which parallel neurotubules and mitochondria are found. The axons arise directly from the base of the receptor cells without synapse. In the cyst lumen there are unattached statoconia. The statoconia have a plate-like or concentric membranous ring structure. Based on the morphology, the function of the statocyst in Biomphalaria is discussed.

The angular acceleration receptor system of the statocyst of Octopus vulgaris : morphometry, ultrastructure, and neuronal and synaptic organization

The angular acceleration receptor system (crista/cupula system) of the statocyst of Octopus vulgaris has been thoroughly reinvestigated, and detailed information is presented regarding its morphometry, ultrastructure, and neuronal and synaptic organization. In each of the nine crista sections, some receptor hair cells are primary sensory cells with an axon extending from their base. Also, there are large and small secondary sensory hair cells without axons, which make afferent synapses with large and small first-order afferent neurons. The afferent synapses are of two morphologically distinct types, having either a finger-like or a flat postsynaptic process; both can be seen in the same hair cell. In addition to the afferents, there is a dense plexus of efferent fibres in each crista section, and efferent synapses can be seen at the level of the hair cells and of the neurons. The morphometric analysis of the nine crista sections shows obvious differences between the odd-numbered (C1, C3, C5, C7, C9) and the even-numbered (C2, C4, C6, C8) crista sections: they differ in length, in the number of the small primary sensory cells and in the number of the small first-order afferent neurons. Centrifugal cobalt filling of the three crista nerves revealed a disproportionate innervation of the nine crista sections: the anterior crista nerve innervates section C1 and the first half of section C2, the medial crista nerve innervates the second half of section C2, sections C3, C4, C5, and the first half of section C6, and the posterior crista nerve innervates the second half of section C6, and sections C7, C8 and C9. In each of the three crista nerves, only 25% of the total number of axons are afferent fibres, the remaining 75 % are efferent. To each of the nine crista sections a cupula is attached. In the form and size of the cupulae there is again a conspicuous difference between the odd and the even crista sections: a small widebased cupula is attached to each of the odd crista sections, whereas the even crista sections each have a large narrow-based cupula with a small area of attachment. The results are discussed with reference to their functional consequences.

Proprioceptive hair cells on the neck of the squid Lolliguncula brevis: a sense organ in cephalopods for the control of head-to-body position

Decapod cephalopods, such as cuttlefishes and squids, have a distinct neck region that allows movements (roll, pitch and yaw) of the head relative to the body. This paper describes the structure, innervation and central pathways of proprioceptive hair cells on the neck of the squid Lolliguncula brevis that sense such movements and control head-to-body position. These hair cells exist on the dorsal side of the neck underneath the nuchal cartilage, close to the animal's midline on either side of the nuchal crest. On each side, the hair cells can be divided into an anterior and a posterior group of 25-35 and 70-80 cells, respectively. An individual hair cell carries up to 300 kinocilia of equal length (about 30 microns), arranged in up to seven rows. The hair cells of the left and right anterior group are morphologically polarized in the medial direction, whereas the hair cells of the left and right posterior group are polarized in the anterior direction. The hair cells are primary sensory cells. They are innervated by a branch of the postorbital nerve and project ipsilaterally into the ventral part of the ventral magnocellular lobe. Efferent synaptic contacts are present at the base of the hair cells. In behavioural tests the influence of the neck hair cells on head position control was investigated. During imposed body rolls, a unilateral deafferentation of the cells caused an asymmetric change of the compensatory head roll response and elicited a head roll offset to the operated side. Bilateral deafferentation of the cells elicited a downward head pitch offset. This offset was superimposed on the compensatory head pitch response during imposed body pitch. These morphological and behavioural findings show that the neck hair cells and the associated nuchal cartilage structures of Lolliguncula brevis form a neck receptor organ that, together with statocyst and visual inputs, controls the position of the animal's head and body.

Electrical coupling between primary hair cells in the statocyst of the squid, Alloteuthis subulata

Intracellular recordings were made from primary sensory hair cells located on the dorsal side of the anterior crista segment of the squid statocyst. These hair cells were electrophysiologically identified by the occurrence of an antidromic action potential after electrical stimulation of the crista nerve. Two types of subthreshold, depolarising potentials were observed in the primary sensory hair cells. Firstly, those due to efferent inputs onto the primary hair cells and secondly those correlated one-to-one with action potentials in neighbouring primary hair cells. The former depolarising potentials could be blocked by bath applied cobalt, indicating chemical transmission, while the latter could not. Injection of a depolarising or hyperpolarising current into a primary hair cell depolarised or hyperpolarised, respectively, a neighbouring primary hair cell implying that the hair cells are electrically coupled with an electrical coupling coefficient of up to 0.4.

The central afferent and efferent organization of the gravity receptor system of the statocyst of Octopus vulgaris

In Octopus vulgaris, the projections of the afferent fibers from, and the locations of cell bodies of the efferent fibers of, the nerve innervating and macula (statocyst gravity receptor epithelium) were studied within the CNS using iontophoretic whole nerve injection of either cobalt chloride or Lucifer Yellow CH.13 Afferent fibers from the macula nerve project to subesophageal, periesophageal and supraesophageal areas of the brain. Large numbers of such fibers were found in the subeosophageal lateral pedal and posterior lateral pedal lobes, palliovisceral lobe, and the magnocellular commissure. Afferent fibers were also found in the periesophageal ventral and dorsal magnocellular lobes. Supraesophageal macular nerve afferent projections were seen to the peduncle lobe and the ipsilateral median basal lobe. There is evidence for two different macular nerve afferent projections to the contralateral median basal lobe, via and suprapedal commissure and the macula-to-contralateral-median-basal-lobe tract. Evidence is presented for the location of at least some of the macula's efferent fiber's cell bodies in the lateral and posterior lateral pedal lobes, and magnocellular lobes. The differences between the results obtained here with the two different staining methods are discussed. The results imply that processing of static information occurs in many areas of the Octopus CNS, and is much more complex than previously thought. Some of the possible physiological consequences are considered.

Afferent synaptic connections between hair cells and the somata of intramacular neurons in the gravity receptor system of the statocyst of Octopus vulgaris

In the sensory epithelium (macula) of the gravity receptor system of the statocyst of Octopus vulgaris, there are two types of afferent neurons, distinguished according to their position in the epithelium. The somata of one type lie accumulated in a ring peripheral to the hair cell layer of the epithelium; these are designated as perimacular neurons. The somata of the other type lie among the hair cells, below the level of their nuclei; these are designated as intramacular neurons. Axons of afferent neurons which touch the hair cells are postsynaptic to some of the hair cells touched. As the somata of the intramacular neurons also touch the hair cells, they were investigated by serial electron microscopic reconstruction to determine if afferent synaptic contacts between hair cells and these somata occur. An average of about 600 intramacular neurons was counted in two maculae. Afferent synapses were seen to occur between hair cells and the somata of 76% of the intramacular neurons investigated. The postsynaptic processes of the intramacular neurons' somata were of two morphological types; one with a finger-like and one with a flat postsynaptic process (average of one synapse of each type per soma). The soma of an intramacular neurons can be postsynaptic to more than one hair cell simultaneously (average of 1--2 hair cells per soma). In addition to being presynaptic to only one neuron's soma, a hair cell could be simultaneously presynaptic to the axons of one or more afferent neurons. The morphological findings are discussed as to their possible physiological consequences.

Fine structural study of the statocysts in the veliger larva of the nudibranch, Rostanga pulchra

The two statocysts of the veliger larva of Rostanga pulchra are positioned within the base of the foot. They are spherical, fluid-filled capsules that contain a large, calcareous statolith and several smaller concretions. The epithelium of the statocyst is composed of 10 ciliated sensory cells (hair cells) and 11 accessory cells. The latter group stains darkly and includes 2 microvillous cells, 7 supporting cells, and 2 glial cells. The hair cells stain lightly and each gives rise to an axon; two types can be distinguished. The first type, in which a minimum of 3 cilia are randomly positioned on the apical cell membrane, is restricted to the upper portion of the statocyst. The second type, in which 9 to 11 cilia are arranged in a slightly curved row, is found exclusively around the base of the statocyst. Each statocyst is connected dorso-laterally to the ipsilateral cerebral ganglion by a short static nerve, formed by axons arising from the hair cells. Ganglionic neurons synapse with these axons as the static nerve enters the cerebral ganglion. The lumen of the statocyst is continuous with a blind, constricted canal located beneath the static nerve. A diagram showing the structure of the statocyst and its association with the nervous system is presented. Possible functions of the statocyst in relation to larval behavior are discussed.

Hair cell polarization in the gravity receptor systems of the statocysts of the cephalopods Sepia officinalis and Loligo vulgaris

The complete patterns of polarization of the sensory epithelia of the various gravity receptor systems of the decapods Sepia and Loligo have been described (Fig. 6). Each individual receptor cell (hair cell) bears up to 150 kinocilia, but is polarized unidirectionally by 3 morphological features: (I) by the orientation of the internal 9X2+2 tubuli structure of each kinocilium, (II) by the location of their basal feet. Each hair cell is additionally polarized (III), in that its kinociliary group is inclined toward the plane of the macula surface, forming an angle of 40-60 degrees with it (Figs. 1-3); the direction of polarization, as given by the ultrastructural features (I and II), is always opposite to this acute angle (Fig. 4). The results are discussed with reference to their physiological consequences.

Ultrastructural studies on the ciliated receptors of the long tentacles of the giant scallop, Placopecten magellanicus (gmelin)

The long tentacles of the Giant scallop Placopecten magellanicus (Gmelin) have been examined with light, scanning, and transmission electron microscopy. Three types of ciliated cells have been observed, one of which is located in specialised papillae born on the distal third of the tentacle. There are two separate cell types within the papillae. Type I cells are non-ciliated supporting cells, which form a capsule within which are found the Type II cells. These cells bear up to five cilia at their apices, and it is suggested that these are the receptor cells of the organ. No function has yet been determined for the receptors, but is suggested that they might be mechanoreceptors. A third cell type, Type III cells, occur at the base of the papillae. These cells bear many cilia and also macrocilia. Another ciliated cell type occurs on the proximal two thirds of the tentacle. These cells bear many cilia that are thought to be motile and not sensory.

Hair cell distribution and orientation in goldfish otolith organs

Structurally diverse sensory regions occur in the otolith organs of the goldfish inner ear. Scanning electron microscopy reveals regional distinctions based on three criteria. (1) Hair cells have different sizes of apical bundles, based on thickness. In all three maculae, two central regions have hair cells with bundles significantly thicker than those in surrounding regions. (2) Hair cell population density varies, with regional aggregations present. The central regions with thick bundles have two to three times the density of surrounding regions with thin bundles, and contain 40-80% of the total hair cell number in each macula. (3) Hair cell orientation maps show that each macula has two oppositely oriented cell populations that can be separated completely, not by a zone of interspersion, but apparently by a single unbroken line. The lagena is like the utricle in having hair cells with the kinocilium on the side of the cell toward the opposition line, but in the saccule the kinocilia face away from the line, and the small macula neglecta consists of two completely separate, oppositely oriented patches. The opposition line does not divide each macula simply down its midline; instead, the line divides the regions with thich bundles into nearly equal opposing areas, except for a remarkably abrupt large loop in the line in the anterior part of the saccule. The regional structural diversity in these organs may relate to localized functional diversity of responses to tilt, vibration and sound.

A scanning electron microscopic study of the morphology and geometry of neural surfaces and structures associated with the vestibular apparatus of the pigeon

The scanning electron microscope (SEM) was used to investigate the morphology of the neuroepithelial regions of the vestibular ampullary structures in 47 White King pigeons. The specific neural surfaces studied were (1) the cristae ampullares of the vertical and lateral membranous ampullae, (2) the hair cells lining the cristae, (3) the ampullary nerve fibers, and (4) the bipolar cells of the vestibular (Scarpa's) ganglion. Additionally, some observations of the gross anatomical structures of the bony labyrinth are given. Arguments are advanced which show that if the surface area of a given semicircular canal can be projected onto one of the three normal head planes, then that canal can be made to respond to motion in the appropriate plane, provided that the projected area is sufficiently large to achieve a threshold pressure as determined by a generalized form of Groen's equation ('57). With regard to the cristae ampullares, it is hypothesized that their surface areas can be described by means of a revolved catenary, i.e., a catenoid of revolution. (The catenary is found in nature as the approximate shape taken by a flexible cable when it is suspended at two points). The surface area of a catenoid provides a minimum surface of revolution. In the context of a crista, this implies that the given number of hair cells could not be fitted onto a smaller surface area. One advantage of this is that nature is able to utilize a thinner cupula than would be possible with other configurations and therefore an increased sensitivity to cupular motion can be realized. A second important factor is that all hair cells must revolve (by way of cupular motion) about the same centre of rotation in response to angular acceleration. Thus, all of the orthogonally-positioned hair cell tufts on the cristae surface may be stimulated simultaneously by way of a tangential shear. Other arguments show that the classical "swing door" type of cupular motion is not consistent with SEM and other recent observations. Two alternate modes of cupular motion are presented, each of which requires far less energy expenditure than does the "swing door" cupula. The suggestion is then made that, during normal head movements, the cupula behaves as a drum much like the tympanic membrane and that only for large, non-physiological motions does the "swinging door" mode of cupular motion take place. It must be remembered, however, that cupular motions during normal physiological head movements are infinitesimally small (Oman and Young, '72).