Oviduct development inDrosophila

Summary The lateral oviducts ofDrosophila virilis were investigated in normal unoperated adult females, after unilateral and bilateral ovariectomy, and following the transplantation of genital discs. Subsequent to unilateral ovariectomy at larval stages of development, mature adult females exhibited reproductive systems with a free lateral oviduct which appeared somewhat shorter and less distended than a lateral oviduct normally attached to an ovary. Histological examination revealed that such free lateral oviducts have undergone considerable growth and differentiation in the absence of direct ovarian attachment, but exhibited a smaller lumen and more highly folded epithelium. They may be distinguished from attached lateral oviducts by conformational differences and by a possibly lesser size of the epithelial cells. Free lateral oviducts observed among bilaterally ovariectomized and sham-ovariectomized specimens were indistinguishable from those obtained after unilateral ovariectomy. The results are at variance with the previously accepted conclusion that oviduct growth inDrosophila is dependent upon inductive influences emanating from the ovary and directing the outgrowth of the oviducts. Differences in the developmental performance of the oviducts as a function of age at the time of ovariectomy were not evident in the study which included larvae ranging from the second instar to late in the third instar. Transplants of larval female genital discs to other larvae revealed a lesser development of the lateral oviducts than that exhibited by a genital disc developingin situ. A range of oviducal growth which lacked any relation to ovarian influences or to other internal organs of the hosts was obtained. In general, decreased amounts of oviducal muscle were found associated with the transplants.Presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the University of Virginia. This investigation was supported in part by: a National Science Foundation Co-operative Fellowship, by the United States Public Health Service Predoctoral Grant 1F1-GM 19,262-1, and by a Summer Research Grant, University of Missouri-St. Louis.     

Genesis of the reproductive system of mosquitoes. I. Female of Aedes stimulans (Walker)

The feminine dimorph has unique structures that produce eggs, select salubrious sites for the offspring, store sperm, and void the eggs. This paper provides a time table for development of these parts in Aedes stimulans based on preparations examined at 5-hour intervals when reared at 21°C. All growths of imaginal parts proceeds independent of activities in the larval tissues. Ovaries produce the eggs in terminal follicles of the ovarioles. Besides ovarioles each ovary contains sheaths for the ovarioles, pedicels attaching them to a central canal, the calyx, ovarian sheath and muscles. Ovaries are recognizable in newly hatched larvae as caps of cells on larger masses which become part of the delivery system for eggs. Each ovary grows forward from its attachment first as a column of cells that differentiates into the several tissues by the time the insect enters pupal life. Prior accounts have considered the ovary as the whole mass of cells on each side of the hemocoel of segment 6. Only the most anterior cells recognizably distinct at the end of embryogeny are generative. The delivery system for eggs is composed of the lateral oviducts and median or common oviduct. Primordia from which the former are derived are present from the end of embryogeny and throughout larval life as two distinct parts. Two ovoid masses occur in the hemocoel of segment 6. To each of these is attached a filament extending backward to an attachment ventrally and caudally in segment 7. They are rapidly changed into definitive lateral oviducts late in pupal life. The single primordium for generating the median genital tract appears during instar 3 as a caudal ventral plate of cells in segment 8 between a pair of bilateral buds and invaginates during instar 4 to form (1) the common oviduct from a midventral pouch, (2) three spermathecae from two lateral invaginations and (3) the elaborate vaginal area. The bilateral buds form no parts of the female. The post-vaginal area or atrium with its accessory organs is derived in part from the ventral plate of segment 8 and that of segment 9. The imaginal disc in segment 9 is present at the end of embryogeny as primordial buds and ventral plate and development is delayed until early pupal life when it projects inward to form part of the atrium and pouches once to form the common opening for the duct of the accessory gland and the canal to the bursa copulatrix. The buds of this disc produce no feminine parts. During the second larval instar lateral primordia appear as a pair of shields in the anal segment. They develop slowly until pupation when they extend caudally as two flaps called “cerci” in culicid literature and this paper.     

Structure of the ovaries in larvae and mature females of euholognathan stoneflies (Plecoptera)

The morphology of ovaries, oviducts and egg capsules in four species of euholognathan stoneflies was investigated. The characteristic features found were as follows: (i) numerous, long ovarioles, that open individually to the extensively folded, lateral oviducts; (ii) a thin, morphologically undifferentiated chorion; (iii) a thick gelatinous layer (extrachorion) which acts as an adhesive layer fixing the eggs to the substrate. Additionally, in the larval ovariole of Leuctra sp. the terminal filament anlage and clusters of germ cells have been found. These observations are in agreement with the classification of stonefly ovaries as primary (true) panoistic.     

Regression of the lateral oviducts during the larval-adult transformation of the reproductive system of Melipona quadrifasciata and Frieseomelitta varia

Toward the end of the larval phase (prepupa), the reproductive systems of Melipona quadrifasciata and Frieseomelitta varia workers are anatomically similar. Scanning electron microscopy showed that during this developmental phase the right and left ovaries are fused and form a heart-shaped structure located above the midgut. Each ovary is connected to the genital chamber by a long and slender lateral oviduct. During pupal development, the lateral oviducts of workers from both species become extremely reduced due to a drastic process of cell death, as shown by transmission electron microscopy. During the lateral oviduct shortening, their simple columnar epithelial cells show some signs of apoptosis in addition to necrosis. Cell death was characterized by cytoplasmic vesiculation, peculiar accumulation of glycogen, and dilation of cytoplasmic organelles such as mitochondria and rough endoplasmic reticulum. The nuclei, at first irregularly contoured, became swollen, with chromatin flocculation and various areas of condensed chromatin next to the nuclear envelope. At the end of the pupal phase, deep recesses marked the nuclei. At emergence, worker and queen reproductive systems showed marked differences, although reduction in the lateral oviducts was an event occurring in both castes. However, in queens the ovarioles increased in length and the spermatheca was larger than that of workers. At the external anatomical level, the reproductive system of workers and queens could be distinguished in the white- and pink-eyed pupal phase. The metamorphic function of the death of lateral oviduct cells, with consequent oviduct shortening, is discussed in terms of the anatomical reorganization of the reproductive system and of the ventrolateral positioning of adult worker bee ovaries.     

Ultrastructure and resistance to water loss in eggs of Acanthoscelides obtectus Say (Coleoptera: Bruchidae)

The mature oöcyte of Acanthoscelides obtectus is surrounded by three envelopes: an external layer, a chorion and a vitelline membrane. The external layer is secreted by the walls of the lateral oviducts. The chorion and vitelline membrane are secreted by the follicular cells. The vitelline membrane becomes very compact during the hour following fertilization and laying. The chorion is composed of three layers, one of which has a paracrystalline ultrastructure.Mature, unfertilized, chorion-containing oöcytes, whose vitelline membranes are loose, dehydrate rapidly in a dry atmosphere after laying or after removal from the lateral oviducts. Fertilized eggs are quite resistant to desiccation: after 12 days at 25°C and 5% relative humidity, viable larvae are obtained.The compact vitelline membrane is the most effective protection against dehydration. The chorion and the external layer are much less effective in preventing water loss from the egg.The retention of eggs in the lateral oviducts does not seem to lead to any modification of the structure of their envelopes.     

Cytoskeletal F-actin patterns in whole-mounted larval and adult salivary glands of the fleshfly, Sarcophaga bullata.

The patterns of filamentous actin were analysed in different larval, pupal and adult stages in the salivary glands of the fleshfly Sarcophaga bullata. Using the rhodamine labelled phalloidin staining method in combination with detergent extraction specific actin filament distribution was detected. The salivary glands which are histolysed during the process of metamorphosis show distinct cellular morphology and actin filament patterns in larvae and adults. The large third instar larval salivary gland cells contain a well developed apicolateral microvillar zone. In third instar larvae this microvillar zone invaginates and expands in the basal part of the lateral membranes. Larval salivary gland cells also contain numerous parallel basal actin bundles. The larval glands are histolysed during metamorphosis and adult glands are formed out of the imaginal cell group. At the onset of metamorphosis these basal actin bundles form a network of crossing bundles. The filamentous actin patterns of the proximal part of adult gland cells is confined to the apicolateral microvillar membranes. The cells in the distal, tubular part of the adult salivary glands show intense staining of their folded lateral membranes.     

Metamorphosis of the marine gastropod Phestilla Sibogae Bergh (nudibranchia: aeolidacea). I. Light and electron microscopic analysis of larval and metamorphic stages

The structure and fate of transitory larval organs (velum, shell, operculum, retractor muscles, part of the epidermis) of Phestilla sibogae Bergh were studied before, during, and after metamorphosis with both light and electron microscopy to elucidate the morphology of these organs and the mechanisms by which they are lost.Loss of the velar lobes is the first morphological sign of metamorphosis, and involves selective dissociation and subsequent ingestion of the ciliated velar cells; the remaining aggregate of supportive cells is apparently incorporated into cephalic epidermis. Attachment of the larval body to shell and operculum is primarily at sites of retractor muscle insertions; once the velum is gone, the attachment between shell and larval body is lost and the shell is cast off as the visceral organs exit through the shell aperture. Merger of visceral and cephalopedal elements results in flattening of the postlarval body and reorientation of internal organs. Simultaneously, a rapid spreading of epipodial epidermis over the lateral, dorsal, and posterior sides of the body produces the definitive integument. The squamous cells which comprise the larval perivisceral epidermis are pushed ahead of the definitive epidermis and are seen shortly after the shell is cast as a constricted aggregate of cells on the posterior end of the body. Autolysis of the left and right retractor muscles begins during metamorphosis and no trace of them is left after 24 to 48 h. The metapodial mucous glands which hypertrophy before metamorphosis are also lost within 48 h following exit of the post larva from the shell. Metamorphosis produces a detorsion caused in part by muscular action and in part by continuing growth and development.     

Ultrastructure of the protocerebral neurosecretory cells of larval Galleria mellonella, in situ and after culture of the brain in vitro

Three major groups of neurosecretory cells are described in the larval brain of Galleria mellonella at two different times during the last larval instar and in larval brains after 72 hr of culture in vitro. The medial group in vivo consists of four distinct neurosecretory cell types, based on characteristic size and morphology, while the posterior and lateral groups each contain a single distinct type of neurosecretory cell. Morphological differences between the same neurosecretory cells at the different times during the last instar are most apparent in the lateral L-1 cells and in the medial M-2 cells, where pleiomorphism is particularly evident in the size, density and accumulations of neurosecretory granules. The only neurosecretory cells in which apparent synthesis of neurosecretory granules is still observed after culture of the brain in vitro are the medial M-2 cells. The other neurosecretory cell types show no accumulation of neurosecretory granules nor new synthesis of neurosecretory material, but are similar to neurosecretory cells in the brain in vivo in all other respects. The morphology of the neurosecretory cells in the larval brain in vivo and in vitro is discussed in relation to their appearance at the light microscopic level and to a known neurohormonal function of the brain which is maintained during 72 hr in vitro.     

Functional morphology of the female reproductive apparatus of Stephanitis pyrioides (Heteroptera,Tingidae): A novel role for the pseudospermathecae

At mating, female insects generally receive and store sperm in specific organs of their reproductive tract called spermathecae. Some Heteroptera, such as Cimicomorpha, lack a true spermatheca; some have receptacles of novel formation where sperm cells can transit or be stored. In Tingidae, there are two sac‐like diverticula, the “pseudospermathecae,” each at the base of a lateral oviduct, which previously were considered to function as spermathecae. However, this role has never been documented, either by ultrastructural studies or by observations of sperm transit in the female reproductive tract. In this article, we investigate the morphology and the ultrastructure of the female reproductive apparatus in the economically important tingid species Stephanitis pyrioides, focusing our attention on the functional role of the pseudospermathecae in an evolutionary perspective. Each ovary consists of seven telotrophic meroistic ovarioles, the long pedicels of which enlarge into a bulb‐like structure near the terminal oocyte. The ovarioles flow into two long lateral oviducts, which join to form a very short common oviduct. Basally, each lateral oviduct is connected through a short duct to one of two pseudospermathecae. The ultrastructure of the ectodermal epithelium of the pseudospermathecae is dramatically different in sexually immature or mated females. In virgin females, cells delimit a very irregular lumen, filled with a moderately electron‐dense granular material. The large nucleus adapts to their irregular shape, which can have long projections in some regions and be flattened in others. After mating, epithelial cells generally elongate and display an apical layer of microvilli extending beneath the cuticle, often containing mitochondria. In the lumen of the pseudospermathecae there is a dense brownish secretion. No sperm cells were ever found inside this organ. After mating, sperm move upward along the lateral oviducts and the ovarioles, accumulating in the bulb‐like structure of the pedicels, and proceeding into the distal region between the follicle cells surrounding the oocyte and the ovariole wall. The egg, most likely fertilized in the bulb‐like region of the ovariole, moves through the lateral oviduct, entirely enters the pseudospermatheca and is smeared with its secretion just before oviposition. We exclude a function of sperm storage for the pseudospermathecae, and instead suggest a novel role for these organs as reproductive accessory glands. J. Morphol., 2010. © 2009 Wiley‐Liss, Inc.     

Ontogenese de la fixation visuelle au cours de la marche orientee chez le grillon des bois Nemobius sylvestris

Wood-crickets $\text{Nemobius sylvestris}$ of various ages, usually orientate towards black targets on a white background. During this walking movement, the strategy they use to maintain the image of the target edges on the retina is studied through a frame by frame analysis. For larvae and adults, the visually orientated locomotion is jerky with successive runs and pauses but the characteristics of the rotations and translations of the body long axis are different according to the larval instar considered.Many runs and pauses of long duration occur during the first larval instar paths. The young insects make greater body long axis rotations and their changes of direction also have a larger amplitude. The images of the edges of the target are not recorded by exactly the same part of the retina during ontogeny.For the early larval instars, a wide lateral ocular area has this function, but for the adults a well differentiated visual fixation area occurs. The results for the older larval instars are intermediate.     

Structure of the ovary and the differentiation of follicular epithelium in the pig louse, Haematopinus suis (Insecta: Phthiraptera)

The female reproductive system of the pig louse, Haematopinus suis (Insecta: Phthiraptera) is composed of paired ovaries, lateral oviducts, and a common oviduct that leads into a vagina. Clusters of mycetocytes (= cells filled with symbiotic organisms) are associated with lateral oviducts. Each ovary is composed of five loosely arranged ovarioles of the polytrophic-meroistic type. An individual ovariole is covered by a basal lamina and is composed of a terminal filament, germarium, and vitellarium. The terminal filament is composed of large, disc-shaped cells that are orientated perpendicularly to the long axis ofthe ovariole. The basal part of the terminal filament is separated from the germarium by a well-developed transverse septum. The germarium is short and filled with clusters of oogonial cells. In each cluster the cells arejoined by intercellular bridges, filled with fusomal material. Within the cluster, only one cell, the future oocyte, enters the prophase of the first meiotic division; the other cells differentiate into nurse cells. The basal part ofthe germarium is filled with the somatic prefollicular cells. The boundary between the germarium and the vitellarium is not distinct. The vitellarium contains linearly arranged ovarian follicles in subsequent stages of oogenesis (previtellogenesis, vitellogenesis and choriogenesis). Each follicle consists of an oocyte and 7 nurse cells and is surrounded by follicular cells. During oogenesis the follicular cells diversify, so that ultimately, five morphologically distinct subpopulations of these cells can be distinguished: (1) cells in contact with the nurse cells, (2) anterior cells, (3) mainbody cells, (4) posterior cells, and (5) interfollicular cells. Interestingly, the follicular cells associated with the anterior part of the oocyte, i.e. located in space at the oocyte/nurse cell border (fold cells) are mitotically active throughout previtellogenesis. It might be suggested, in this context, that the separation of the oocyte from the nurse cell compartment is brought about by mitotic divisions, consequent multiplication and centripetal migration of these cells.     

Transformation of the intestinal epithelium of the larval anadromous sea lamprey Petromyzon marinus L. during metamorphosis

The events in the transformation of the intestine of the larval lamprey into the adult intestine were followed through the seven (1–7) stages of metamorphosis in anadromous Petromyzon marinus L. Light and electron-microscope observations demonstrated that the processes of degeneration, differentiation, and proliferation are involved in the transformation. In the anterior intestine, degeneration of cells and the extrusion of others into the lumen results in the disappearance of secretory (zymogen) cells and the decline in numbers of endocrine and ciliated cells. Larval absorptive cells, with a prominent brush border, are believed to dedifferentiate into unspecialized columnar cells with few microvilli. Degeneration and removal of cells occurs by both autophagy and heterography and cells extruded into the lumen in the anterior intestine are phagocytosed by epithelial cells of the posterior intestine. The loss of epithelial cells during transformation results in the folding and degradation of parts of the basal lamina and in an extensive widening of the lateral intercellular spaces in all parts of the intestine. As metamorphosis is a nontrophic period of the lamprey life cycle, the possible morphological effects of starvation on the intestinal epithelium are discussed. The development of longitudinal folds is a consequence of the events of metamorphic transformation of the intestinal mucosa. Although an interaction between the epithelium and the underlying tissues is believed to be importent, the actual mechanism of fold development is unknown. The intestinal epithelium of adult lampreys develops from surviving cells of the larval (primary) epithelium. Unlike the situation in amphibians, there does not appear to be a group (nest) of undifferentiated larval cells which differentiate into the adult (secondary) epithelium. Instead, in lampreys, columnar cells that persist through the degradative processes seem to be the source of absorptive and ciliated cells and probably are responsible for mucous and secretory cells. Preliminary observations indicate that the intestinal epithelium of feeding adults is specialized into an anterior region which liberates a secretion, absorbs lipid, and possesses the machinery for ion transport. A posterior region absorbs lipid, secretes mucus, and likely is involved in some protein absorption.     

Electron microscopy of extracellular materials during the development of a sea star,Patiria miniata (Echinodermata: Asteroidea)

The fine structure of conspicuous extracellular materials during the life history of a sea star (Patiria miniata) is described. The outer surface of the developing sea star is covered by two morphologically different cuticles that appear sequentially during ontogeny. The primary cuticle, which is about 120 nm thick and two-layered, is present from mid-blastula through the end of the larval stage. The secondary cuticle, which is about 1 micron thick and three-layered, first appears on the epidermis of the rudiment region of the larva and, after metamorphosis, covers the entire epidermis of the juvenile and adult stages. During ontogeny, there are only two conspicuous gut cuticles: the first lines the newly invaginated archenteron at the start of the gastrula stage, and the second lines the esophagus during the larval stage. A blastocoelic basal lamina first appears at mid-blastula and persists as subectodermal and subendodermal basal laminae. Ruthenium red-positive granules are detectable between the lateral surfaces of adjacent ectodermal cells during part of the gastrula stage; this transient intercellular material may possibly aid in lateral adhesion between cells.     

Ultrastructure of the mushroom body: digestion during metamorphosis of Utterbackia imbecillis (Bivalvia: Unionidae)

Abstract. Larvae of the freshwater mussel Utterbackia imbecillis metamorphose to juveniles either during their attachment to a host fish, or in vitro in a culture medium. This transformation includes degeneration of larval structures and development of the juvenile morphology. Early in metamorphosis the cells comprising the larval mantle enlarge and project into the mantle cavity, forming a structure referred to as the mushroom body. Its cells, which are ultrastructurally very similar to digestive cells of adult bivalves, are involved in pinocytosis or phagocytosis of the larval adductor muscle and of tissue from the host fish that is enclosed between the larval shells. Ingested material is passed from pinosomes to heterophagosomes which in turn fuse with heterolysosomes, where final degradation of ingested material occurs. Acid phosphatase activity was detected in heterophagosomes and heterolysosomes of all animals examined. In larvae that metamorphosed in vitro , the apical cytoplasm of the cells of the mushroom body, and the extracellular spaces among them, also exhibited acid phosphatase activity. Larvae reared on a host fish accumulated substantial deposits of lipids and glycogen within larval mantle cells during metamorphosis, whereas larvae reared in vitro did not. The larval mantle cells which constitute the mushroom body appear to be the primary sites of intracellular digestion of the larval adductor muscle and host tissue during metamorphosis.     

The ontogeny of the lateral line system in Telmatobius atacamensis (Anura,Telmatobiidae)

The lateral line system in anurans is functional during aquatic stages and therefore could provide characters related to larval morphological variation. However, few studies have addressed its components in an integrated overview, and little is known about its ontogenetic variation. This study describes the postembryonic trajectory of the lateral system in Telmatobius atacamensis up to its metamorphic regression. This includes structure, number, topography, and innervation of neuromasts, to contribute new and complete information about its larval organization and its temporal sequence of regression. The arrangement and innervation of lateral lines in T. atacamensis resembles those described for other Type IV tadpoles. Its distinctive features are the orientation of the neuromast stitches in the lateral lines, the presence of supraotic neuromasts, and the first-described case of asymmetry of the ventral trunk line. The temporal sequence of regression during metamorphosis differs between the lateral lines and the lateral line nerves, which remain myelinated into postmetamorphic stages. This asynchronous pattern between different components of the system has also been described for Pseudis paradoxa, which shares with T. atacamensis a remarkably long larval period. This long larval period and gradual metamorphosis could also be related to the constitutive metamorphic regression of the system, in spite of the aquatic lifestyle of these frogs.     

Genesis of the reproductive system of mosquitoes II. Male of Aedes stimulans (walker)

The imaginal male of mosquitoes bears a combination of organs and appendages that make it morphologically distinctive. Its reproductive organs produce sperm cells, convey and extrude them, provide accessory fluids, and insure copulation and insemination. In Aedes stimulans (Walker) these organs are derived from one of the two sets of primordia provided by the embryo. The second set of primordia is capable of producing the feminine reproductive system under unusual circumstances. Testes are derived from two compact ovoid masses of cells suspended in the hemocoel of abdominal segment 6. Each enlarges slowly throughout larval instars 1–3 and elongates very rapidly late in instar 4. Specialization of the cellular mass into sperm cells proceeds forward from the caudal end early in pupal life. From the beginning, a sheath of nutritive cells or fatbody encases each gonad, and no tracheation of the mass is evident although one small trachea sends branches to the encasing fatbody late in larval life. The efferent canal from each testis is derived from a tenuous filament extending caudally from each gonad to the venter of segment 9 and a small cluster of cells in the wall of the hemocoel on the ental surface of imaginal disc 9. Early in pupal life the filaments become the tubular vasa efferentia. The caudal clusters are primordial terminal parts of the lateral tract that become vasa deferentia, seminal vesicles and associated accessory glands. The ejaculatory canal comes from a short pouch derived from the median genital plate of disc 9. All external parts except the paraprocts are products of disc 9. The bilateral buds begin to proliferate in larval instar 4 and become the basistyles, dististyles and claspettes of the gonapophyses during pupal life. The phallosome is derived from the median genital plate. Primordia of a possible feminine reproductive system and cerci remain undifferentiated and disappear early in pupal life in the normal course of events. Primordia that were recognizable include those of ovaries, parts of lateral oviducts, median genital tract and cerci.     

Sequential oogenesis is controlled by an oviduct factor in the locusts Locusta migratoria and Schistocerca gregaria: Overcoming the doctrine that patency in follicle cells is induced by juvenile hormone

In insects that lay eggs in large clutches, yolk accumulation in each of the many ovarioles is restricted to the basal (terminal) oocyte, the one closest to the lateral oviduct. All succeeding (subterminal) oocytes remain small until the terminal oocytes finished their development and were ovulated into the oviduct. The major step regulating yolk uptake by terminal oocytes is the formation of gaps between cells of the follicle layer, a process termed patency. In the migratory as well as in the desert locust, patency is induced by a Patency Inducing Factor (PIF) produced by the lateral oviducts. PIF is secreted in all regions of the lateral oviducts and interacts with the basal follicle cells via the pedicel, a fine duct that connects an ovariole with the oviduct. By this mechanism, patency is triggered in the follicle cells of the terminal oocyte only, restricting yolk accumulation to the oocytes next to ovulation. In contrast to the previous hypothesis, juvenile hormone (JH) is not necessary to induce patency, rather JH amplifies the effect of PIF.     

Morphofunctional changes in the midgut of tick nymphs of the genus Ixodes (Acarina: Ixodidae) during and after feeding

The midgut epithelium of feeding nymph is represented by the digestive cells of larval phase. Digestion of the main part of feed is performed by the one generation of digestive cells of nymphal phase after detachment, during moult. This period precedes the apolysis. The generation of secretory cells is absent on the nymphal phase. Secretory vacuoles are formed in the digestive cells of larval phase. All functioning cells form a peritrophic matrix on their apical surface. The replacement of the digestive cells of larval phase by the digestive cells of nymphal phase proceeds gradually, during the first 5-10 days after detachment. The beginning of the accumulation of digestive inclusions in the young digestive cells of nymphal phase takes place in the 10-15 days after detachment.     

Development of lateral line organs in leptocephali of the freshwater eel Anguilla japonica (Teleostei,Anguilliformes)

A study of the ontogeny of the lateral line system in leptocephali of the Japanese eel Anguilla japonica reveals the existence of three morphologically different types of lateral line organs. Type I is a novel sensory organ with hair cells bearing a single kinocilium, lacking stereocilia, distributed mainly on the head of larvae, and morphologically different from typical superficial neuromasts of the lateral line system. Its developmental sequence suggests that it may be a presumptive canal neuromast. Type II is an ordinary superficial neuromast, common in other teleost larvae, which includes presumptive canal neuromasts that first appear on the trunk and accessory superficial neuromasts that later appear on the head and trunk. Type III is a very unusual neuromast located just behind the orbit, close to the otic vesicle, with radially oriented hair cells, suggesting that these serve as multiple axes of sensitivity for mechanical stimuli. The behavior of larval eels suggests that the radially oriented neuromasts may act as the sole mechanosensory organ until the ordinary superficial neuromasts develop. The finding that larval eels possess a well-developed mechanosensory system suggests the possibility that they are also capable of perceiving weak environmental mechanical stimuli, like other teleost larvae.