Statocysts of medusae and evolution of stereocilia

Ectodermal sensory cells of the statolith each bear one nonmotile kinocilium, and in some hydromedusae bear stereocilia which are defined as distinct from microvilli by having fibrillar rootlets. The progressive elaboration of stereocilia can be traced. A hypothesis in which jellyfish statocysts evolve from vibration receptors avoids the difficulty of the uselessness of incipient gravity organs. Comparative study suggests that the kinocilium or its basal body is the transducer.     

The ultrastructure of the statocysts in the pediveliger larvae of Pecten maximus (L.) (Bivalvia)

The pediveliger of Pecten maximus (L.) has a pair of statocysts situated at the base of the foot on either side of a bilobed pedal ganglion. The statocysts consist of a spherical sac connected to the mantle cavity by a cylindrical ciliated canal. Within the sac there are statoconia which are variable both in shape and structure. The cells of the sac are joined by septate desmosomes. There is a non-ciliated cell in each sac containing a variety of granules some of which resemble certain of the statoconia. The remainder of the sac is composed of hair cells, which bear a circular array of radiating cilia. The basal bodies and horizontal striated roots of these cilia are directed radially. The hair cells give rise to thin processes which probably join together to form the static nerve. This nerve runs from the static canal to the pleural ganglion.     

Statocysts of hydromedusae

The statocysts of Leptomedusae are formed as a depression in the velum. They are lined on the inside towards the distal part of the velum by thin epithelium and towards the proximal part by ciliated sensory cells. Lithocytes are present in the centre. The concretion contains calcium sulphate and in some cases, calcium phosphate is also present in addition to some membranous material. The statocysts of Narcomedusae arise from the exumbrellar nerve ring as free sensory clubs. They have a proximal basal cushion of sensory cells from the centre of which arises a sensory club (Aegina) or a sensory papilla carrying a sensory club (Solmissus). The sensory club has an axial strand of endodermal cells covered by ciliated sensory cells. Some of the endodermal cells have a concretion. While the statocysts of Leptomedusae are totally ectodermal, those of Narcomedusae are ecto-endodermal in origin. The sensory cilia of Leptomedusae, especially those present on the sensory cells adjacent to the lithocyte, run close and parallel to the lithocyte membrane. In Narcomedusae the sensory cilia of the basal cusion and sensory papilla are tall and strong. Ciliary rootlets are missing in the sensory cilia of Leptomedusae and in the sensory club of Narcomedusae but they are strongly developed in the cilia of basal cusion and sensory papilla. The cilia have 9+2 filament content. A ring of stereocilia surrounds the kinocilium of the sensory club cells. Mechanism of statocyst function is discussed.     

ETUDE ONTOGENETIQUE DE L'ORGANE SENSORIEL DU MUSCLE CRUCIFORME DES TELLINACEA

The closed cruciform muscle sense organ of Donax trunculus hasa similar origin to the open one of Scrobicularia plana. Bothdifferentiate as ectodermal pits before the cruciform muscleitself, which is issued from their wall. The sense organ ofDonax is closed immediately after the invagination of the pitand its closure is independent of the differentiation of theventral expansion which characterizes the closed sense organand of the canal which characterizes the open one. (Received 8 August 1983;     

Ultrastructural aspects of nervous-system and statocyst morphogenesis during embryonic development of Convoluta psammophila (Turbellaria,Acoela)

The notion that statocysts originated from an infolding of ectoderm lined by ciliated sensory cells has been challenged with evidence of capsule-limited, non-ciliary statocysts in several independent phyla. Statocysts in turbellarians primitively lack cilia and are embedded within or closely adjoined to the cerebral ganglion; they are likely to be derived from nervous tissue. We investigated the development of the simple statocyst in an acoel turbellarian, a statocyst consisting of three cells. Observations of serial TEM sections of embryos at different stages of development support the hypothesis of an inner (non-epithelial) origin of the statocyst. First, a three-cell complex is delimited by a basal lamina; it then undergoes cavitation by swelling, autophagy, and fluid secretion. The statocyst becomes discernible within the precursor ganglion cells while they still contain yolk inclusions. The two outer (parietal) cells, enclosed together by a 10-nm-thick basal lamina, arrange themselves in an ovoid of about 10 µm diameter and surround the inner statolith-forming cell. The statolith is formed later within vacuoles of the statolith-forming cell.     

Ultrastuctural and cytochemical study of spermatogenesis in Scrobicularia plana (Mollusca,Bivalvia)

The ultrastructure of the mature spermatozoa and spermatogenesis of the bivalve Scrobicularia plana are described. Support cells extend from the basal lamina to the lumen of the testis and are laterally connected to the germinal epithelium. Germ cells present intercellular bridges and flagella since the spermatogonial stage. While spermatogonia and spermatocytes appear connected to support cells by desmosome-like junctions, elongated spermatids are held at the acrosomal region by support cell finger-like processes. During spermiogenesis, the acrosomal vesicle differentiates from a golgian saccule and then migrates to the nuclear apex. A microtubular manchette arising from centrioles surrounds the acrosomal vesicle, the nucleus, and the mitochondria at the time these three organelles start their elongation, disappearing after that. The mature spermatozoon of S. plana lacks a distinct midpiece because the mitochondria extend from the region of the pericentriolar complex along the nucleus anteriorly for approximately 1.4 μm. The features of this bivalve type of modified spermatozoon are compared with those of other animal groups having similar modifications.     

Ionic currents in secondary sensory hair cells isolated from the statocysts of squid and cuttlefish

Mechanosensitive hair cells in the statocysts of cephalopods underlie a sophisticated detection system for linear and angular accelerations. To investigate the operation of this system, secondary sensory hair cells were dissociated from the sensory epithelia of these statocysts and their voltage sensitive ionic conductances identified and characterized under whole cell voltage clamp.All secondary hair cells showed two outward potassium conductances; first, a current similar to the previously described delayed rectifier, I_K and second, a current similar to the molluscan A current, I_A. A small number of hair cells (15%) also showed an inward sodium current; the presence of this current was correlated with the presence of small membrane extensions at the base of the cell. The sodium current could be blocked by TTX and was abolished by substituting choline for sodium in the external medium. An inward L-type, calcium current was also identified. This current showed rapid activation, with little inactivation, could be carried by barium ions, and was blocked by Nifedipine in the external solution.These data provide the first information on the ionic conductances in the basolateral membranes of invertebrate secondary sensory hair cells and form a basis for comparison with analogous vertebrate hair cells.     

The ultrastructure of the eyes in the veliger-larvae of Aporrhais sp. and Bittium reticulatum (Mollusca,Caenogastropoda)

Summary The cerebrally innervated larval eyes of Aporrhais sp. and Bittium reticulatum are investigated by means of transmission electron microscopy. Each organ consists of a pigmented cup containing an acellular lens. The cornea overlaps the anterior portion of the eye. The retina is composed of sensory cells and supportive cells. The sensory cells of Aporrhais sp. bear one cilium and in Bittium reticulatum two cilia, the ciliary membrane being folded into numerous finger-shaped evaginations. The supportive cells contain the pigment granules and most of them bear one or two cilia, the plasmalemma of which is likewise folded. It is supposed that: (a) these cilia have a transportive function for lens material and (b) that the ciliary photoreceptor of Aporrhais sp. and Bittium reticulatum is a functional adaptation to a relatively long larval period.Abbreviations bb basal body - bp basal plate - c cilium - cc corneal cell - cm ciliary membranes - cw ciliary whorl - gd Golgi dictyosomes - gm granular material - l lens - m mitochondrion - mt microtubules - mv microvilli - mvb multivesicular body - n nucleus - pb pigment border - pg pigment granule - rer rough endoplasmic reticulum - sc sensory cell - sj septate junctions - spc supportive cell     

Evolution of sensory structures in basal metazoa

Cnidaria have traditionally been viewed as the most basal animalswith complex, organ-like multicellular structures dedicatedto sensory perception. However, sponges also have a surprisingrange of the genes required for sensory and neural functionsin Bilateria. Here, we: (1) discuss "sense organ" regulatorygenes, including; sine oculis, Brain 3, and eyes absent, thatare expressed in cnidarian sense organs; (2) assess the sensoryfeatures of the planula, polyp, and medusa life-history stagesof Cnidaria; and (3) discuss physiological and molecular datathat suggest sensory and "neural" processes in sponges. We thendevelop arguments explaining the shared aspects of developmentalregulation across sense organs and between sense organs andother structures. We focus on explanations involving divergentevolution from a common ancestral condition. In Bilateria, distinctsense-organ types share components of developmental-gene regulation.These regulators are also present in basal metazoans, suggestingevolution of multiple bilaterian organs from fewer antecedentsensory structures in a metazoan ancestor. More broadly, wehypothesize that developmental genetic similarities betweensense organs and appendages may reflect descent from closelyassociated structures, or a composite organ, in the common ancestorof Cnidaria and Bilateria, and we argue that such similaritiesbetween bilaterian sense organs and kidneys may derive froma multifunctional aggregations of choanocyte-like cells in ametazoan ancestor. We hope these speculative arguments presentedhere will stimulate further discussion of these and relatedquestions.     

MECHANICAL AND MORPHOLOGICAL PROPERTIES OF THE SHELL OF SCROBICULARIA PLANA (DA COSTA) UNDER NORMAL AND STRESS CONDITIONS

Scrobicularia plana reacts to salinity stress by closing itsvalves and respiring anaerobically. The acidic products of respirationare buffered by calcium carbonate removed from the shell. Shellmass decreases by 15% over a period of three weeks, and thereis a similar reduction in shell strength. Erosion of the shell surface, indicating buffering, takes placequickly, pits can be seen in the valves of shells at the endof six hours of normal intertidal emersion. (Received 10 February 1982;     

Effects of osmotic swelling on voltage-gated calcium channel currents in rat anterior pituitary cells

Decrease in extracellular osmolarity ([Os]_e) results in stimulation of hormone secretion from pituitary cells. Different mechanisms can account for this stimulation of hormone secretion. In this study we examined the possibility that hyposmolarity directly modulates voltage-gated calcium influx in pituitary cells. The effects of hyposmolarity on L-type (I_L) and T-type (I_T) calcium currents in pituitary cells were investigated by using two hyposmotic stimuli, moderate (18-22% decrease in [Os]_e) and strong (31-32% decrease in [Os]_e). Exposure to moderate hyposmotic stimuli resulted in three response types in I_L (a decrease, a biphasic effect, and an increase in I_L) and in increase in I_T. Exposure to strong hyposmotic stimuli resulted only in increases in both I_L and I_T. Similarly, in intact pituitary cells (perforated patch method), exposure to either moderate or strong hyposmotic stimuli resulted only in increases in both I_L and I_T. Thus it appears that the main effect of decrease in [Os]_e is increase in calcium channel currents. This increase was differential (I_L were more sensitive than I_T) and voltage independent. In addition, we show that these hyposmotic effects cannot be explained by activation of an anionic conductance or by an increase in cell membrane surface area. In conclusion, this study shows that hyposmotic swelling of pituitary cells can directly modulate voltage-gated calcium influx. This hyposmotic modulation of I_L and I_T may contribute to the previously reported hyposmotic stimulation of hormone secretion. The mechanisms underlying these hyposmotic effects and their possible physiological relevance are discussed. L-type channels; mechanosensitivity; somatotrophs; lactotrophs     

Organogenesis in cultured petiole explants of Begonia erythrophylla: the timing and specificity of the inductive stimuli

Caulogenesis and rhizogenesis were studied in cultured petioleexplants of Begonia erythrophylla in order to link the developmentalstages of primordia initiation with the physiological requirementsof the explant. Petiole sections excised from B. etythrophyllaplants grown in vitro, were highly organogenic, with shootsand roots arising directly from cells of epidermal origin. Epidermalcells associated with glandular hairs appeared to be most responsiveto organogenic stimuli. The point of explant determination foreach form of organogenesis was ascertained by media transferexperiments. Explants became determined for caulogenesis after7 d exposure to shoot-inducing medium (SIM), while requiring3 d on root-inducing medium (RIM) for determination. Explantswere strongly canalized for caulogenesis once determined, but5 d on RIM were required before becoming strongly canalizedfor rhizogenesis. No organ specific differentiation was observedat the point of determination for explants exposed to eithershoot- or root-inducing conditions. Preculture on a basal mediumcontaining no growth regulators resulted in a gradual loss ofcompetence with time, but preculture for up to 2 d on SIM orRIM resulted in a reduction in the time for determination forboth forms of organogenesis. Key words: Organogenesis, Begonia erythrophylla, tissue culture, epidermis, determination     

Anatomy of Bathyacmaea secunda Okutani, Fujikura & Sasaki, 1993 (Patellogastropoda: Acmaeidae)

Anatomy of a vent-endemic patellogastropod limpet, Bathyacmaeasecunda, was examined by gross dissection and serial sections.It was revealed that B. secunda is anatomically distinct fromother patellogastropod taxa in that (1) the intestine runs throughthe ventricle; (2) the ventral approximator muscle of odontophoralcartilages is ventrally single-layered; (3) there is a pairof radular teeth with long shafts; and (4) the statocysts areisolated from pleural and pedal ganglia. In addition, the speciesis characterized by short pallial margin papillae, lack of pallialstreaks, presence of a ctenidium, obliquely tubular salivaryglands, simple gut configuration and acmaeoidean type of buccalmass musculature and odontophoral cartilages. The comparisonin anatomical and shell microstructural characters suggeststhat B. secunda shares no unique similarities with the Patelloideaor Neolepetopsidae and, therefore, is not closely related tothese groups. In contrast, a number of similarities were foundbetween B. secunda and acmaeid species. These results supportthe current systematic position of Bathyacmaea within the Acmaeidae,although anatomical data from some related genera are stillinsufficient. (Received 3 September 2005; accepted 25 January 2006)     

Some Observations on Infection of Arachis hypogaea L. by Rhizobium

The infection process in Arachis hypogaea by rhizobia differsfrom that normally found in Trifolium spp. in that no infectionthreads are formed. The root hairs, which are long (up to 4mm), septate, and often with large basal cells, occur only atthe sites of emerging lateral roots. Infection occurs only wherethe root hairs have large basal cells. Rhizobia cause curlingand deformation of the root hairs (as in Trifolium spp.) butenter the root at the junction of the root hair and the epidermaland cortical cells. The bacteria are distributed intercellularlyvia the middle lamellae and enter the cortical cells throughthe structurally altered cell wall, often close to the hostcell nucleus. The root hairs and large basal cells become infectedin the same way. Within the cortical cells of the emerging lateralroot the rhizobia multiply rapidly and the invaded cells dividerepeatedly to form the nodule tissue. Bacteriod formation occursonly when the host cell ceases to divide.     

THE FUNCTIONAL MORPHOLOGY OF THE ANTARCTIC BIVALVE THRACIA MERIDIONALIS SMITH, 1885 (ANOMALODESMATA: THRACIIDAE)

The functional morphology of the Thraciidae is poorly understood.Although some morphological aspects of several members havebeen described, only Trigonothracia jinxingae from Chinese watersis known in detail. Thracia meridionalis is the only representativeof the family in Antarctic waters, and is common in AdmiraltyBay, King George Island, where it inhabits muddy sediments.Thracia meridionalis shares many features with Tr. jinxingaethat are typical of most Anomalodesmata, i.e. a secondary ‘ligament’of thickened periostracum, extensively fused mantle margins,ctenidia of type E, a ctenidial-labial palp junction of categoryIII, a stomach of type IV and simultaneous hermaphroditism.Thracia meridionalis is, however, strikingly different fromTr. jinxingae in a number of ways, such as the presence of afourth pallial aperture, statocysts of type B₃, heterorhabdicctenidia, direct communication between the mantle chambers,a deep-burrowing habit (individuals lying on the left shellvalve), siphons that retract into mucus-lined burrows, a stomachwith extensive sorting areas, a rectum which passes over thekidneys and separate male and female gonadial apertures. Thereis, therefore, a greater range of morphological diversity withinthe Thraciidae than previously suspected. (Received 27 April 2004; accepted 30 November 2004)     

Basal Plate Tissue in Narcissus Bulbs and in Shoot Clump Cultures: Its Structure and Role in Organogenic Potential of Single Leaf Cultures

Light microscopy demonstrated that the apparently amorphous,achlorophyllous tissue at the base of in vitro shoot clump cultureof Narcissus was comparable in structure to the basal plateof Narcissus bulbs. Both had very complex vascularisation andsmall, densely packed parenchymatous cells. In shoot clump cultures, primordia were produced by meristematiczones at the surface of this achlorophyllous tissue, very closeto the base of leaves. Single leaf units excised from the invitro shoot clump cultures with a wedge of basal achlorophylloustissue were highly organogenic when used as secondary explantsfor in vitro culture of Narcissus. No organogenesis occurredin the absence of the leaf base and achlorophyllous (basal plate)tissue and little organogenesis occurred unless the leaf baseand basal plate tissue were immersed in the culture medium (i.e.explants inoculated into liquid medium or upright in agar-solidifiedmedium). After two 5-week culture passages in liquid medium, more thanfive leaves were produced per leaf base inoculated. Thus rapidmicropropagation of Narcissus can be achieved using only thebase of single leaf units excised from shoot clump cultures.Copyright1993, 1999 Academic Press Anatomy, basal plate, bulb, in vitro, leaf culture, Narcissus, organogenesis     

The ultrastructure of the vomero-nasal organ in reptilia

Summary There are three types of cells in the vomero-nasal organ of Lacerta sicula and Natrix natrix: receptor cells, supporting cells and basal cells. The receptor cells bear microvilli and no cilia. In Lacerta centrioles are lacking, indicating that the ciliary apparatus can have no essential significance in the transducer process. In Natrix centrioles occur in the deeper dendritic region. The structural constituents of the dendrites are mitochondria, microtubules and characteristic vesicles the properties of which are described. The perikarya which have uniform structure send off axons of about 0.2 diameter. The supporting cells show signs of a very moderate secretory activity, which is different among the species investigated. The microvilli of the supporting cells are not distinguishable from those of the receptor cells. The dendrites of the latter are completely isolated by the apical parts of the supporting cells. The sheet-like processes of the supporting cells contain strands of tonofilaments and do not cover the perikarya of the receptor cells completely. Thus adjacent sensory cells or dendrites and sensory cells are separated among themselves only by the normal intercellular space. The ratio of sensory cells to supporting cells is about 71. The basal cells resemble the supporting cells and replace these in the lower portion of the epithelium. The typical cellular junctions between sensory cells and supporting cells are described. There are no true tight junctions in the vomero-nasal sensory epithelium, and they are most probably absent from the nasal mucosa too. This absence would seem to indicate special conditions for cellular communication and the accessibility of the intercellular space for certain molecules. There is no sign of regeneration of sensory cells. Both immature blastema cells and degenerating receptor cells are not discernible.     

Histochemical localization of NADPH-diaphorase-positive elements in the enteric nervous system of bivalve molluscs

Using a nicotinamide adenine dinucleotide phosphate-diaphorase(NADPH-d) histochemistry, localization and morphology of putativenitric oxide synthase-containing elements were studied in theintestine of the following bivalves: Ruditapes philippinarum,Callithaca adamsi, Mercenaria stimpsoni (Veneridae), Corbiculajaponica (Corbiculidae), Nodularia vladivostokensis and Cristariatuberculata (Unionidae). NADPH-d-positive nerve cells and plexuseswere found in the intestine of all species studied. Labelleddiformazan bipolar nerve cells were present in the epitheliumof the intestinal groove and typhlosole of the anterior midgut,the midgut proper, and the hindgut. Their apical process extendedtowards the gut lumen, whereas the basal one was connected withthe basiepithelial NADPH-d-positive nerve plexus. In the typhlosoleof N. vladivostokensis, these cells constituted up to 2.68%of the total number of epithelial cells. The bivalve speciesstudied exhibit a similar distribution pattern of NADPH-d-positivecells, which lie separately or form small groups of two to threein the basal part of the epithelium. Basiepithelial NADPH-d-positiveplexus was connected by separate fibres with subepithelial NADPH-d-positivenerve plexus. Both the plexuses were moderately developed inall intestinal regions in the majority of the species examined. (Received 27 March 2007; accepted 4 September 2007)     

On the Embryogeny of Three Taxa of Paepalanthoideae (Eriocaulaceae)

Embryo development in Leiothrix nubigena, Paepalanthus bifidusand Syngonanthus nitens is described. Distinct differentiationof cotyledonary and epicotyledonary sectors in the three taxais demonstrated. The cotyledonary and the epicotylary loci shareequal amounts of the embryonic shoot apex as in most monocotyledons.The cotyledon and its adjacent epicotyl are both terminal onthe embryonic axis. The basal cell of the two-celled proembryocontributes two basal rows of cells at the radicular pole inthe ripe seed. A typical radicle is not organized. Leiothrix nubigena, Paepalanthus bifidus, Syngonanthus nitens Paepalanthoideae, Eriocaulaceae, embryogeny     

腔肠动物平衡感觉器官结构的研究进展

本文概述了腔肠动物的平衡感觉器官结构：刺胞动物掌状伞形螅（Corymorpha palma）固着器末端的无纤毛平衡囊、软水母（Leptomedusae）钟形伞缘的开放型和封闭型平衡囊、筐水母（Narcomedusae）外伞表面外伞神经环上方边缘有感觉棍的间囊水母（Aegina）和有感觉乳突及感觉棍的嗜阳水母（Solmissus）的平衡囊、硬水母（Trachymedusae）外伞神经环上方边缘的平衡囊、钵水母（Scyphozoa）伞缘的感觉棍和立方水母（Cubozoa）伞缘稍上方的感觉棍,栉水母（Ctenophore）反口面中央的平衡囊或顶器官。本文内容对理解其他水生无脊椎动物的平衡感觉器官的结构及功能有重要意义,同时也可能作为对现行动物学相关教材内容的有益补充。