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Scanning Electron Microscopic Observation of the Statocyst in the Crayfish Procambarus clarkii Girard
Auris·Nasus·Larynx (Tokyo) 23,133-139 (1996)
Scanning Electron Microscopic Observation of the Statocyst in the Crayfish Procambarus clarkii Girard Masaya
TAKUMIDA,
M.D. and Koji YAJIN, M.D.
Department of Otolaryngology, Hiroshima University School of Medicine, Hiroshima, Japan
The statocyst in the crayfish Procambarus clarkii was investigated using scanning electron microscopy. The statocyst contains static hairs arranged in four group, i.e. lateral, medial, proximal, and distal. All the hairs are the same in basic structure. They differ in length and diameter and in their position with respect to the other hairs in the group and to the statolith. In terms of morphological polarization, the hairs in the lateral and medial groups are polarized toward the center of the crescent, while the hairs in the proximal group are polarized away from the center of the crescent. In the distal group, most hairs are polarized toward the center of the crescent, while some hairs are at random. This morphological polarization may be consistent with that of functional polarization. The statolith overlays the lateral and medial hair groups. The hairs in the distal group do not touch the statolith, whereas the hairs in the proximal group make contact with it from the side. It has been therefore indicated that the lateral, medial and proximal groups could detect gravity and linear acceleration like macular end organs and distal group could detect angular acceleration like semicircular canal in mammals. The statocysts of decapod crustaceans have been extensively studied both morphologically and physiologically since they were first discovered, in crayfish and lobsters, by Rosenthal. 1 Their morphology was soon investigated and comprehensive monograph was published by Hensen, 2 who classified them as either closed statocyst or open statocysts with sand grains. The mechanoreceptors of arthropods have also attracted attention more recently, now that technological advances have made possible studies in fine detail. 3·9 Contributions to the morphology and cytology of these organs have demonstrated that among the macurans, the fundamental structure is similar, with minor variations. A group of hairs which constitute the majority of the static hairs found in the statocyst form a crescent on the statocyst floor (lateral hair set of Prentiss 3; Sichelgruppe of Panning4 ; lateral group of Hertwig et al. 5 In the crayfish Astacus jluviatilis, Orconectes limosus, and Procambarus clarkii these hairs are all polarized morphologically toward the center of the crescent. 5. 7 In spite of these recent morphological investigations, some questions still remain unsolved. This study was designed in order to find a morphological basis for the recent physiological findings which may be a great help to elucidate the mechanisms of the vestibular sensory transduction system. MATERIALS AND METHODS
Experiments were carried out on American crayfish, P. clarkii Girard of either sex measuring 8 to 15 cm in length, They were obtained commercially and maintained in laboratory tanks until used. After the crayfish had been decapitated, the basal segments of the first antennae were removed and fixed with 2.5% glutaraldehyde in 0.1 M phosphate buffer solution (pH 7.4) for 2 hr. After microdissection in the same buffer solution, the statocysts were postfixed with 1% Received 14 September 1995; accepted 6 November 1995. Correspondence should be addressed to: Masaya Takumida, Department of Otolaryngology, Hiroshima University School of Medicine, 1-2-3 Kasumicho, Minamiku, Hiroshima. 734 Japan.
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OS04 (0.1 M phosphate buffer, pH 7.4) for 1 hr. The statolith, which lies on the statocyst floor, was washed out with a fine buffer jet directed into the lumen in some specimens. The statocysts were then conductive stained with 2% tannic acid for 2hr and 1% OS04 for 1 hr. Following dehydration with graded ethanols, the specimens were transferred to t-butyl alcohol for three changes, freeze-dried (EJKO ID-2 freeze drier) and sputter coated with platinum to about 50 A thickness for scanning electron microscopic observation. A Hitachi 5-800 scanning electron microscope with an accelerating voltage of 15 kV was used for observation and photography. RESULTS
I. General structure of the statocyst The statocyst of crayfish (P. clarkii) is a paired organ located in the dorsal region of the basal segment of the first antenna. Each is an open cup, formed by epidermal invagination and hence lined with cuticle. The opening is approximately triangular. It is covered by hairs that originate laterally, on the upper edge of the cup. The structure of an individual hair in this covering mat resembles that of a feather (Fig. 1). The shafts range in length from 300 to 1,500 pm. Each is seated on a bulbous base embedded in the cuticle that forms the lateral margin of the statocyst. The shaft sends out side branches with a maximal length of about 90 pm, usually arranged in two rows along opposite sides of the shaft. The statocyst cup itself is oval, with its long axis parallel to that of the antenna and its proximal end is pointed. In an antennal basal segment, the length of the cup is about 3.0 mm and its width is about 1.8 mm. The static hairs stand on the ventral wall of the cup, projecting dorsally into the lumen. They can be subdivided into four groups, i.e. lateral, medial, proximal and distal group. The largest of these (lateral group) is situated on the lateral side of the cup and consists of ca. 70 hairs, mostly arranged in two transverse rows of crescent-like shape. On the medial side, opposite this lateral group, about 30 hairs (medial group) are also oriented in the long direction of the statocyst, arranged in 2 or 3 rows. Proximal to the pointed end of the statocyst, there is a single row of 8-9 hairs (proximal group) perpendicular to the long axis. The fourth group of hairs (distal group) in the cup occupies a nearly circular field adjoining the lateral group at its distal end (Figs. 2, 3, and 4). All the hairs in the statocyst cup are uniform in structure, though they differ in length and in the spatial relations of the hairs within each group. In the lateral group the hairs consist of two types of hairs, i.e. short and long. The hairs of the inner row are shorter with average 120 pm in length and have a smaller basal diameter of 6 pm, whereas those of the outer (peripheral) row are longer with 19G-260pm in length and a diameter of 15-25 pm (Figs. 3 and 5). The hairs in the distal group consist of a large number of long hairs up to 260 pm in length and a small number of short hairs 130 pm in length at the periphery (Figs. 3 and 6). In the medial group, the peripheral row contains a few short hairs 90 pm in length and the rest is occupied with long hairs 180 pm in length (Figs, 3 and 7). The hairs of the proximal group are all long with 3OG-400pm in length and a diameter of 15-20pm (Figs. 3 and 8). In a functional statocyst the statolith, made of foreign material, fills the center of the cup, overlying the two hair groups (lateral and medial groups) oriented in the longitudinal direction of the antenna. The hairs in the distal group do not touch the statolith, whereas the hairs in the proximal group make contact with it from the side (Figs. 2 and 8). On the floor of the statocyst, between the groups of hairs, the pores of the large tegmental gland open into the cup (Fig. 5). II. The static hairs Each static hair is subdivided into an expanded basal part and a shaft arising from it. The 134 Auris'Nasus'Larynx (Tokyo) Vol. 23 (1996)
Anterior
L
Medial
•
. short holr
o :long
hair
Respective d irection. 0' morphological polarization 0' statocyst hairs
Fig. 1. The opening of the statocyst is covered by hairs on the upper edge of the cup. The structure of an individual hair resembles that of a feather. Fig. 2. Dorsally opened statocyst cup shows the proximal group of hairs (p), distal group (d) and statolith (s). The statolith, made of foreign material, fills the center of the cup, overlying the two hair groups (lateral and medial groups). The hairs in the distal group do not touch the statolith, whereas the hairs in the proximal group make contact with it from the side. Fig. 3. The static hairs can be subdivided into four groups, i.e. lateral (I), medial (m), proximal (p), and distal group (d). Fig. 4. Schematic illustration of the statocyst. Each hair group consist of long and short hairs. In the lateral and medial groups, the hairs are all polarized morphologically toward the center of the crescent, whereas the hairs in the proximal group are polarized away from the center of the crescent. In the distal group, most hairs are polarized morphologically toward the center of the crescent, while some hairs are polarized at random.
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Fig. 5. Lateral group of hairs is situated on the lateral side of the cup, mostly arranged in two transverse rows of crescent-like shape. The hairs of the inner row (i) are shorter and have a smaller basal diameter, whereas those of the outer (peripheral) row (0) are longer. The teeth of hairs (arrowheads) are all polarized morphologically toward the center of the crescent. On the floor of the statocyst, the pores of the large tegmental gland open into the cup (arrows). Fig. 6. The distal group of hairs (asterisk) occupies a nearly circular field adjoining the lateral group (arrow) at its distal end. The hairs consist of a large number of long hairs and a small number of short hairs at the periphery. Fig. 7. Medial group of hairs are oriented in the long direction of the statocyst, arranged in 2 or 3 rows. Fig. 8. Proximal to the pointed end of the statocyst, there is a single row of 8-9 long hairs (proximal group) perpendicular to the long axis. The hairs make contact with statolith (s) from the side. The teeth of hairs (arrowheads) are all polarized away from the center of the crescent.
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shaft is covered with side branches 6-20/Lm in length, which are usually arranged in two opposing rows (Fig. 9). In a statocyst containing statolith (sand grains), it is only in the branched region that the statolith adhere to one another and to the side branches of the static hair, by way of organic material. The shaft itself does not touch the statolith (Fig. 10).
Fig. 9. The shaft of static hairs is covered with side branches (arrowheads), which are usually arranged in two opposing rows. Fig. 10. In a statocyst containing statolith (s), it is only in the branched region that the statolith adhere to one another and to the side branches of the static hair by way of organic material. The shafts (asterisks) themselves do not touch the statolith. Fig. 11. Around the base of each hair the cuticle forms a ring (arrowhead). The proximal part of the hair is expanded into a bulb (asterisk) which is covered by delicate longitudinal folds. This bulb has an asymmetrically thickened portion called the tooth (arrow) with the countertooth above it. Fig. 12. On the surface of the hair on the opposite side of the tooth is the fulcrum, a sharp fold transverse to the shaft (arrow). Auris·Nasus ·Larynx (Tokyo) Vol. 23 (1996)
137
The static hairs are outgrowths of the epidermis and, as such, are covered by cuticle. Around the base of each hair the cuticle forms a ring. The proximal part of the hair is expanded into a bulb which is distinctly set off from the hair base by a sharp notch. The surface of the bulb is covered by delicate longitudinal folds. This bulb has an asymmetrically thickened portion called the tooth, the only smooth part of the bulb surface. The extension of the shaft above this part of the bulb is called the counter tooth (Fig. 11). On the surface of the hair on the opposite side of the tooth is the fulcrum, a sharp fold transverse to the shaft that continues into the bulb as the lingula (Fig. 12). There is no such fold on the tooth side of the hair, thus being morphologically polarized.
III.
Morphological polarization In the lateral and medial groups, the teeth of hairs are all polarized morphologically toward the center of the crescent (Fig. 5), whereas the hairs in the proximal group are polarized away from the center of the crescent (Fig. 8). In the distal group, most hairs are polarized morphologically toward the center of the crescent, while some hairs are polarized at random such as towards lateral, medial and proximal (Fig. 4). DISCUSSION
The statocysts enable decapod crustaceans to detect changes in the spatial orientation of their bodies and to respond to them in typical ways. 10, 11 The stimulus is mediated by a deflection of the static hairs within the statocyst cup, either by the statolith or, in the case of rotation, by movement of the fluid. 12 Recent physiological investigation revealed that hairs in the lateral group deflection toward the center of the crescent elicited maximal excitatory responses in the majority of receptors. Hair deflection in the opposite direction elicited maximal suppressive responses. 7 The present findings revealed that the hairs in the lateral group are all polarized morphologically toward the center of the crescent as has been demonstrated in many species. 5-7 This plane of morphological polarization is consistent with that of functional polarization. 7 In contrast, the hairs in the proximal group are polarized away from the center of the crescent and the some hairs in the distal group are polarized at random. These findings indicate that in the proximal group, hair deflection away from the center of the crescent elicited maximal excitatory responses and hair deflection in the opposite direction elicited maximal suppressive responses. In the distal group it may be suggested that some hairs showed random polarization. Actually physiological investigation showed transient responses to deflection away from center in the distal group. 7 The present study also revealed two types of hairs i.e. long and short typically in the lateral group. The functional differences between these long and short hairs are still remain obscure. The recent physiological investigations revealed that the statocyst receptors could be classified into two types; tonic (42%) and phasic (50%), according to the time course of the excitatory responses. The remainder (8%) could not be classified as either of these, showing intermediate responses. Some (ca. 40%) of the phasic type receptors responded transiently regardless of the direction of hair deflection. 7 It is suggested that the role of those receptors without any plane of functional polarization, is different from that of other receptors in producing equilibrium responses. 7 The relationship between morphological and physiological findings is not quite clear. It may be suggested that each hairs have different functional properties. Because all the static hairs in the statocyst of P. clarkii are identical in their basic structure, the detection of different spatial orientations of the body must involve anatomical factors such as differences in length and diameter and in the positions of the hairs relative to one another in four groups. The static hairs in Astacus astacus were described by Panning4 and Balss 13 as arranged in three groups, but Hertwig et at5 subdivided their "sickle group" into a lateral 138 Auris'Nasus'Larynx (Tokyo) Vol. 23 (1996)
longitudinal group (lateral group in this study) of hairs and a hair field (distal group in this study). In the present investigation, the static hairs in P. clarkii has been divided in four groups. The reason is that the statolith fills the center of the statocyst cup overlying hairs in the lateral and medial groups, while the hairs in the distal group do not touch the statolith. These findings may indicate that the lateral, medial and proximal groups can detect gravities and linear acceleration, due to their contacts with the statolith, while the free hairs without their contacts with the statolith in the distal group are eminently suited to detect angular accelerations. 5 Filiform hairs 13 are not present in the statocyst of P. clarkii. These findings suggested that the lateral, medial and proximal group might correspond to the macular endorgans and the distal group to the semicircular canal in the mammals. REFERENCES
1. Rosenthal E: Uber den Geruchssinn der Insekten. ReiI's Arch Physiol 10:427, 1811. 2. Hensen V: Studien tiber das GehOrorgan der Decapoden. Z Wiss Zool 13:319-412, 1863. 3. Prentiss CW: The otocyst of decapod crustacea. Bull Mus Comp Zool Harvard 36:167-254, 1901. 4. Panning A: Die statozyste von Astacusfluviatilis (Potamobius astacus Leach) und ihre Beziehung zu dem sie umgebenden Gewebe. Z Wiss Zool 123:305-358, 1924. 5. Hertwig I, Schneider H, Hentschel J: Light- and electron-microscopic analysis of the statocyst of the american crayfish Orconectes limosus (Crustacea, Decapoda). Zoomorphology 110:189-202, 1991. 6. Stein A: Attainment of positional information in the crayfish statocyst. Fortschr Zool 23:100-119, 1975. 7. Takahata M, Hisada M: Functional polarization of statocyst receptors in the craifish Procambarus clarkii Girard. J Comp Physiol 130:201-207, 1979. 8. Kinzig CH: Untersuchungen tiber den Bau der Statocysten einiger decapoden Crustaceen. Verh Naturf-Med Ver Heidelderg, N,F, 14:1-90, 1919. 9. Kouyama N, Shimozawa T: The structure of a hair mechanoreceptor in the antennule of crayfish (Crustacea). Cell Tissue Res 226:565-578, 1982. 10. SchOne H: Statocystenfunktion und statische Lageorientierung bei dekapoden Krebsen. Z Vergl Physiol 36:241-260, 1954. 11. SchOne H: Kurssteuerung mittels der Statocysten (messungen an Krebsen). Z Vergl PhysioI39:235240, 1957. 12. Cohen MJ: The function of receptors in the statocyst of the lobster Homarus americanus. J Physiol 130:9-34, 1955. 13. Balss H: Decapoda. IV. Statocyste. In Die Klassen und Ordnungen des Tierreiches 51 (7). 3. (Bronns HG, ed): Lieferung. Akad Veri Ges Becker und Erler Kom-Ges Leipzig, pp 369-385, 1944.
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Scanning Electron Microscopic Observation of the Statocyst in the Crayfish Procambarus clarkii Girard Masaya
TAKUMIDA,
M.D. and Koji YAJIN, M.D.
Department of Otolaryngology, Hiroshima University School of Medicine, Hiroshima, Japan
The statocyst in the crayfish Procambarus clarkii was investigated using scanning electron microscopy. The statocyst contains static hairs arranged in four group, i.e. lateral, medial, proximal, and distal. All the hairs are the same in basic structure. They differ in length and diameter and in their position with respect to the other hairs in the group and to the statolith. In terms of morphological polarization, the hairs in the lateral and medial groups are polarized toward the center of the crescent, while the hairs in the proximal group are polarized away from the center of the crescent. In the distal group, most hairs are polarized toward the center of the crescent, while some hairs are at random. This morphological polarization may be consistent with that of functional polarization. The statolith overlays the lateral and medial hair groups. The hairs in the distal group do not touch the statolith, whereas the hairs in the proximal group make contact with it from the side. It has been therefore indicated that the lateral, medial and proximal groups could detect gravity and linear acceleration like macular end organs and distal group could detect angular acceleration like semicircular canal in mammals. The statocysts of decapod crustaceans have been extensively studied both morphologically and physiologically since they were first discovered, in crayfish and lobsters, by Rosenthal. 1 Their morphology was soon investigated and comprehensive monograph was published by Hensen, 2 who classified them as either closed statocyst or open statocysts with sand grains. The mechanoreceptors of arthropods have also attracted attention more recently, now that technological advances have made possible studies in fine detail. 3·9 Contributions to the morphology and cytology of these organs have demonstrated that among the macurans, the fundamental structure is similar, with minor variations. A group of hairs which constitute the majority of the static hairs found in the statocyst form a crescent on the statocyst floor (lateral hair set of Prentiss 3; Sichelgruppe of Panning4 ; lateral group of Hertwig et al. 5 In the crayfish Astacus jluviatilis, Orconectes limosus, and Procambarus clarkii these hairs are all polarized morphologically toward the center of the crescent. 5. 7 In spite of these recent morphological investigations, some questions still remain unsolved. This study was designed in order to find a morphological basis for the recent physiological findings which may be a great help to elucidate the mechanisms of the vestibular sensory transduction system. MATERIALS AND METHODS
Experiments were carried out on American crayfish, P. clarkii Girard of either sex measuring 8 to 15 cm in length, They were obtained commercially and maintained in laboratory tanks until used. After the crayfish had been decapitated, the basal segments of the first antennae were removed and fixed with 2.5% glutaraldehyde in 0.1 M phosphate buffer solution (pH 7.4) for 2 hr. After microdissection in the same buffer solution, the statocysts were postfixed with 1% Received 14 September 1995; accepted 6 November 1995. Correspondence should be addressed to: Masaya Takumida, Department of Otolaryngology, Hiroshima University School of Medicine, 1-2-3 Kasumicho, Minamiku, Hiroshima. 734 Japan.
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OS04 (0.1 M phosphate buffer, pH 7.4) for 1 hr. The statolith, which lies on the statocyst floor, was washed out with a fine buffer jet directed into the lumen in some specimens. The statocysts were then conductive stained with 2% tannic acid for 2hr and 1% OS04 for 1 hr. Following dehydration with graded ethanols, the specimens were transferred to t-butyl alcohol for three changes, freeze-dried (EJKO ID-2 freeze drier) and sputter coated with platinum to about 50 A thickness for scanning electron microscopic observation. A Hitachi 5-800 scanning electron microscope with an accelerating voltage of 15 kV was used for observation and photography. RESULTS
I. General structure of the statocyst The statocyst of crayfish (P. clarkii) is a paired organ located in the dorsal region of the basal segment of the first antenna. Each is an open cup, formed by epidermal invagination and hence lined with cuticle. The opening is approximately triangular. It is covered by hairs that originate laterally, on the upper edge of the cup. The structure of an individual hair in this covering mat resembles that of a feather (Fig. 1). The shafts range in length from 300 to 1,500 pm. Each is seated on a bulbous base embedded in the cuticle that forms the lateral margin of the statocyst. The shaft sends out side branches with a maximal length of about 90 pm, usually arranged in two rows along opposite sides of the shaft. The statocyst cup itself is oval, with its long axis parallel to that of the antenna and its proximal end is pointed. In an antennal basal segment, the length of the cup is about 3.0 mm and its width is about 1.8 mm. The static hairs stand on the ventral wall of the cup, projecting dorsally into the lumen. They can be subdivided into four groups, i.e. lateral, medial, proximal and distal group. The largest of these (lateral group) is situated on the lateral side of the cup and consists of ca. 70 hairs, mostly arranged in two transverse rows of crescent-like shape. On the medial side, opposite this lateral group, about 30 hairs (medial group) are also oriented in the long direction of the statocyst, arranged in 2 or 3 rows. Proximal to the pointed end of the statocyst, there is a single row of 8-9 hairs (proximal group) perpendicular to the long axis. The fourth group of hairs (distal group) in the cup occupies a nearly circular field adjoining the lateral group at its distal end (Figs. 2, 3, and 4). All the hairs in the statocyst cup are uniform in structure, though they differ in length and in the spatial relations of the hairs within each group. In the lateral group the hairs consist of two types of hairs, i.e. short and long. The hairs of the inner row are shorter with average 120 pm in length and have a smaller basal diameter of 6 pm, whereas those of the outer (peripheral) row are longer with 19G-260pm in length and a diameter of 15-25 pm (Figs. 3 and 5). The hairs in the distal group consist of a large number of long hairs up to 260 pm in length and a small number of short hairs 130 pm in length at the periphery (Figs. 3 and 6). In the medial group, the peripheral row contains a few short hairs 90 pm in length and the rest is occupied with long hairs 180 pm in length (Figs, 3 and 7). The hairs of the proximal group are all long with 3OG-400pm in length and a diameter of 15-20pm (Figs. 3 and 8). In a functional statocyst the statolith, made of foreign material, fills the center of the cup, overlying the two hair groups (lateral and medial groups) oriented in the longitudinal direction of the antenna. The hairs in the distal group do not touch the statolith, whereas the hairs in the proximal group make contact with it from the side (Figs. 2 and 8). On the floor of the statocyst, between the groups of hairs, the pores of the large tegmental gland open into the cup (Fig. 5). II. The static hairs Each static hair is subdivided into an expanded basal part and a shaft arising from it. The 134 Auris'Nasus'Larynx (Tokyo) Vol. 23 (1996)
Anterior
L
Medial
•
. short holr
o :long
hair
Respective d irection. 0' morphological polarization 0' statocyst hairs
Fig. 1. The opening of the statocyst is covered by hairs on the upper edge of the cup. The structure of an individual hair resembles that of a feather. Fig. 2. Dorsally opened statocyst cup shows the proximal group of hairs (p), distal group (d) and statolith (s). The statolith, made of foreign material, fills the center of the cup, overlying the two hair groups (lateral and medial groups). The hairs in the distal group do not touch the statolith, whereas the hairs in the proximal group make contact with it from the side. Fig. 3. The static hairs can be subdivided into four groups, i.e. lateral (I), medial (m), proximal (p), and distal group (d). Fig. 4. Schematic illustration of the statocyst. Each hair group consist of long and short hairs. In the lateral and medial groups, the hairs are all polarized morphologically toward the center of the crescent, whereas the hairs in the proximal group are polarized away from the center of the crescent. In the distal group, most hairs are polarized morphologically toward the center of the crescent, while some hairs are polarized at random.
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Fig. 5. Lateral group of hairs is situated on the lateral side of the cup, mostly arranged in two transverse rows of crescent-like shape. The hairs of the inner row (i) are shorter and have a smaller basal diameter, whereas those of the outer (peripheral) row (0) are longer. The teeth of hairs (arrowheads) are all polarized morphologically toward the center of the crescent. On the floor of the statocyst, the pores of the large tegmental gland open into the cup (arrows). Fig. 6. The distal group of hairs (asterisk) occupies a nearly circular field adjoining the lateral group (arrow) at its distal end. The hairs consist of a large number of long hairs and a small number of short hairs at the periphery. Fig. 7. Medial group of hairs are oriented in the long direction of the statocyst, arranged in 2 or 3 rows. Fig. 8. Proximal to the pointed end of the statocyst, there is a single row of 8-9 long hairs (proximal group) perpendicular to the long axis. The hairs make contact with statolith (s) from the side. The teeth of hairs (arrowheads) are all polarized away from the center of the crescent.
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shaft is covered with side branches 6-20/Lm in length, which are usually arranged in two opposing rows (Fig. 9). In a statocyst containing statolith (sand grains), it is only in the branched region that the statolith adhere to one another and to the side branches of the static hair, by way of organic material. The shaft itself does not touch the statolith (Fig. 10).
Fig. 9. The shaft of static hairs is covered with side branches (arrowheads), which are usually arranged in two opposing rows. Fig. 10. In a statocyst containing statolith (s), it is only in the branched region that the statolith adhere to one another and to the side branches of the static hair by way of organic material. The shafts (asterisks) themselves do not touch the statolith. Fig. 11. Around the base of each hair the cuticle forms a ring (arrowhead). The proximal part of the hair is expanded into a bulb (asterisk) which is covered by delicate longitudinal folds. This bulb has an asymmetrically thickened portion called the tooth (arrow) with the countertooth above it. Fig. 12. On the surface of the hair on the opposite side of the tooth is the fulcrum, a sharp fold transverse to the shaft (arrow). Auris·Nasus ·Larynx (Tokyo) Vol. 23 (1996)
137
The static hairs are outgrowths of the epidermis and, as such, are covered by cuticle. Around the base of each hair the cuticle forms a ring. The proximal part of the hair is expanded into a bulb which is distinctly set off from the hair base by a sharp notch. The surface of the bulb is covered by delicate longitudinal folds. This bulb has an asymmetrically thickened portion called the tooth, the only smooth part of the bulb surface. The extension of the shaft above this part of the bulb is called the counter tooth (Fig. 11). On the surface of the hair on the opposite side of the tooth is the fulcrum, a sharp fold transverse to the shaft that continues into the bulb as the lingula (Fig. 12). There is no such fold on the tooth side of the hair, thus being morphologically polarized.
III.
Morphological polarization In the lateral and medial groups, the teeth of hairs are all polarized morphologically toward the center of the crescent (Fig. 5), whereas the hairs in the proximal group are polarized away from the center of the crescent (Fig. 8). In the distal group, most hairs are polarized morphologically toward the center of the crescent, while some hairs are polarized at random such as towards lateral, medial and proximal (Fig. 4). DISCUSSION
The statocysts enable decapod crustaceans to detect changes in the spatial orientation of their bodies and to respond to them in typical ways. 10, 11 The stimulus is mediated by a deflection of the static hairs within the statocyst cup, either by the statolith or, in the case of rotation, by movement of the fluid. 12 Recent physiological investigation revealed that hairs in the lateral group deflection toward the center of the crescent elicited maximal excitatory responses in the majority of receptors. Hair deflection in the opposite direction elicited maximal suppressive responses. 7 The present findings revealed that the hairs in the lateral group are all polarized morphologically toward the center of the crescent as has been demonstrated in many species. 5-7 This plane of morphological polarization is consistent with that of functional polarization. 7 In contrast, the hairs in the proximal group are polarized away from the center of the crescent and the some hairs in the distal group are polarized at random. These findings indicate that in the proximal group, hair deflection away from the center of the crescent elicited maximal excitatory responses and hair deflection in the opposite direction elicited maximal suppressive responses. In the distal group it may be suggested that some hairs showed random polarization. Actually physiological investigation showed transient responses to deflection away from center in the distal group. 7 The present study also revealed two types of hairs i.e. long and short typically in the lateral group. The functional differences between these long and short hairs are still remain obscure. The recent physiological investigations revealed that the statocyst receptors could be classified into two types; tonic (42%) and phasic (50%), according to the time course of the excitatory responses. The remainder (8%) could not be classified as either of these, showing intermediate responses. Some (ca. 40%) of the phasic type receptors responded transiently regardless of the direction of hair deflection. 7 It is suggested that the role of those receptors without any plane of functional polarization, is different from that of other receptors in producing equilibrium responses. 7 The relationship between morphological and physiological findings is not quite clear. It may be suggested that each hairs have different functional properties. Because all the static hairs in the statocyst of P. clarkii are identical in their basic structure, the detection of different spatial orientations of the body must involve anatomical factors such as differences in length and diameter and in the positions of the hairs relative to one another in four groups. The static hairs in Astacus astacus were described by Panning4 and Balss 13 as arranged in three groups, but Hertwig et at5 subdivided their "sickle group" into a lateral 138 Auris'Nasus'Larynx (Tokyo) Vol. 23 (1996)
longitudinal group (lateral group in this study) of hairs and a hair field (distal group in this study). In the present investigation, the static hairs in P. clarkii has been divided in four groups. The reason is that the statolith fills the center of the statocyst cup overlying hairs in the lateral and medial groups, while the hairs in the distal group do not touch the statolith. These findings may indicate that the lateral, medial and proximal groups can detect gravities and linear acceleration, due to their contacts with the statolith, while the free hairs without their contacts with the statolith in the distal group are eminently suited to detect angular accelerations. 5 Filiform hairs 13 are not present in the statocyst of P. clarkii. These findings suggested that the lateral, medial and proximal group might correspond to the macular endorgans and the distal group to the semicircular canal in the mammals. REFERENCES
1. Rosenthal E: Uber den Geruchssinn der Insekten. ReiI's Arch Physiol 10:427, 1811. 2. Hensen V: Studien tiber das GehOrorgan der Decapoden. Z Wiss Zool 13:319-412, 1863. 3. Prentiss CW: The otocyst of decapod crustacea. Bull Mus Comp Zool Harvard 36:167-254, 1901. 4. Panning A: Die statozyste von Astacusfluviatilis (Potamobius astacus Leach) und ihre Beziehung zu dem sie umgebenden Gewebe. Z Wiss Zool 123:305-358, 1924. 5. Hertwig I, Schneider H, Hentschel J: Light- and electron-microscopic analysis of the statocyst of the american crayfish Orconectes limosus (Crustacea, Decapoda). Zoomorphology 110:189-202, 1991. 6. Stein A: Attainment of positional information in the crayfish statocyst. Fortschr Zool 23:100-119, 1975. 7. Takahata M, Hisada M: Functional polarization of statocyst receptors in the craifish Procambarus clarkii Girard. J Comp Physiol 130:201-207, 1979. 8. Kinzig CH: Untersuchungen tiber den Bau der Statocysten einiger decapoden Crustaceen. Verh Naturf-Med Ver Heidelderg, N,F, 14:1-90, 1919. 9. Kouyama N, Shimozawa T: The structure of a hair mechanoreceptor in the antennule of crayfish (Crustacea). Cell Tissue Res 226:565-578, 1982. 10. SchOne H: Statocystenfunktion und statische Lageorientierung bei dekapoden Krebsen. Z Vergl Physiol 36:241-260, 1954. 11. SchOne H: Kurssteuerung mittels der Statocysten (messungen an Krebsen). Z Vergl PhysioI39:235240, 1957. 12. Cohen MJ: The function of receptors in the statocyst of the lobster Homarus americanus. J Physiol 130:9-34, 1955. 13. Balss H: Decapoda. IV. Statocyste. In Die Klassen und Ordnungen des Tierreiches 51 (7). 3. (Bronns HG, ed): Lieferung. Akad Veri Ges Becker und Erler Kom-Ges Leipzig, pp 369-385, 1944.
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