Ultrastructure of the genital organs in interstitial polychaetes

Summary The paired copulatory stylets of the hermaphroditicMicrophthalmus cf.listensis are hard cone-shaped tubes with a syringe-like distal opening and a cuff-like lower edge and surround the external openings of the two ejaculatory ducts. They each lie in a deeply invaginated epidermal fold and are attached basally to an elongated muscle bulb, which is composed of a number of disc-like muscle cells. A prominent gland is situated behind the stylets. Transfer of sperm into the partner occurs probably by mechanical hypodermal injection. Hereby, the epidermal folds are protruded as small sacks, pulling out the stylets. The development of the entire male genital apparatus occurs in autumn when the animals have about 16 setigerous segments. During this differentiation, two elongated papillae arise. They consist of various well defined cells, some of which border a central ciliated lumen. The stylet tubes arise by transformation of the at first normal cuticle of these papillae into a hard electron-dense wall.

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Zusammenfassung Die beiden paarigen Kopulationsstilette des zwittrigenMicrophthalmus cf.listensis sind feste, tütenförmige Röhren mit einer spitzen distalen Öffnung und einer manschettenförmigen unteren Kante. Sie liegen in je einer tief in den Körper hineinziehenden Epidermisfalte und umgeben die Endabschnitte der beiden Ductus ejaculatorii. Basal sind sie an je einem länglichen Muskelkörper befestigt, der aus scheibenförmigen Elementen zusammengesetzt ist. Hinter dem Stilett befindet sich eine Drüse. Das Sperma wird wahrscheinlich mechanisch hypodermal in den Geschlechtspartner injiziert. Die Epidermisfalten sind hierbei sackförmig ausgestülpt und die Stilette dabei nach außen gezogen. Die Entwicklung des gesamten männlichen Geschlechtsapparates erfolgt im Herbst, wenn die Tiere ungefähr 16 Borstensegmente besitzen. Hierbei bilden sich im 3. Borstensegment zwei längliche Papillen. Sie bestehen aus verschiedenen, gut zu unterscheidenden Zellen, von denen einige um ein zentrales bewimpertes Lumen angeordnet sind. Durch Umwandlung der zunächst normalen Kutikula dieser Papille zu einer elektronendichten festen Wand entsteht die Stilettröhre.

     

The nephridia of the interstitial polychaete Hesionides arenaria and their phylogenetic significance (Polychaeta,Hesionidae)

Summary The small hesionid polychaete Hesionides arenaria possesses paired segmental excretory organs that closely resemble solenocytic protonephridia. The nephridium consists of one terminal cell and four tubule cells which form the emission channel. From the terminal cell, up to six flagella arise each surrounded by a weir of ten regularly arranged cytoplasmic rods. The structure of the cytoplasm of three of the following cells suggests that they function in active transport and storage. Because all of the larger, more primitive species of this family are equipped with metanephridia, the possibility is discussed that these organs have been developed out of metanephridia. The Hesionides arenaria nephridium may be a morphological stage in the evolutionary pathway from metanephridia to solenocytes. This would mean that solenocytes can no longer be considered to be homologous in every case with other protonephridial organs in polychaetes and may well be derived several times independently out of metanephridia or true protonephridia.     

Ultrastructure of nuchal organs in some marine polychaetes

Polychaetes normally possess one pair of nuchal organs at the posterior edge of the prostomium or peristomium. They have been regarded as chemosensory organs. The nuchal organs of four marine polychaete species with different habits were investigated by electron microscopy. Although the shapes of nuchal organs can vary greatly from simple ciliary bands (Scolelepis squamata, Spionidae) to retractile tongue-like, piston- or finger-shaped forms (Eteone longa, Anaitides mucosa, Phyllodocidae; Heteromastus filiformis, Capitellidae), the structural components, including the ciliated supporting cells, sensory cells, and nuchal epidermal cells, are essentially similar. The differences basically concern 1) the position of the sensory cells with relation to the ciliated supporting cells, 2) the location and structure of the nuchal nerve, and 3) the structure of the nuchal cuticle. The diverging nature of this modified cuticle is described and discussed in detail. Comparisons are made with the fine structure of nuchal organs of other polychaete species. Similarities of cellular components of nuchal organs are found not only in the four species studied here but also in all nuchal organs investigated so far. This is hypothesized to be due to the fact that the polychaete stem species already possessed nuchal organs with the respective cell types. Differences in the number and distribution of cellular components and in the overall shape of nuchal organs are thought to have evolved in correlation with the equipment of other cephalic appendages and with different habits and modes of nutrition.     

Sinohesione genitaliphora gen. et sp. n. (Polychaeta, Hesionidae), an interstitial annelid with unique dimorphous external genital organs

A new hesionid. Sinohesione genitaliphora gen. et sp. n., is described from intertidal sandy sediments of Hainan Island, China. It differs from hitherto known hesionids by the presence of external genital organs in both sexes. In the males there is one pair of sae-like appendages, each bearing a tube-shaped penis, on chaetiger 10. In the females the paired sae-shaped organs are situated on chaetiger 12. Reconstructions of semi- and ultrathin sections show that a long, heavily coiled sperm duct opens at the tip of each penis. The duct opens with a ciliated funnel into a seminal vesicle in chaetiger 9. Prominent gland cells surround the sperm duct for the most part. The female genital organs each have two openings; one of which leads to a blind ending seminal receptacle. The other is the external pore of a ciliated oviduct that originates as an open funnel in the coelom of chaetiger 10. The functional and phylogenetic significance of these structures is discussed.     

Sense organs in polychaetes (Annelida)

Polychaetes possess a wide range of sensory structures. These form sense organs of several kinds, including the appendages of the head region (palps, antennae, tentacular cirri), the appendages of the trunk region and pygidium (parapodial and pygidial cirri), the nuchal organs, the dorsal organs, the lateral organs, the eyes, the photoreceptor-like sense organs, the statocysts, various kinds of pharyngeal papillae as well as structurally peculiar sensory organs of still unknown function and the apical organs of trochophore larvae. Moreover, isolated or clustered sensory cells not obviously associated with other cell types are distributed all over the body. Whereas nuchal organs are typical for polychaetes and are lacking only in a few species, all other kinds of sensory organs are restricted to certain groups of taxa or species. Some have only been described in single species till now. Sensory cells are generally bipolar sensory cells and their cell bodies are either located peripherally within the epidermis or within the central nervous system. These sensory cells are usually ciliated and different types can be disinguished. Structure, function and phylogenetic importance of the sensory structures observed in polychaetes so far are reviewed. For evaluation of the relationships of the higher taxa in Annelida palps, nuchal organs and pigmented ocelli appear to be of special importance.     

Ultrastructure of genital system ducts of Diphyllobothrium latum (Cestoda:Pseudophyllidae): the ducts of the female reproductive system

The components of the female reproductive system of Diphyllobothrium latum, including ovary, ovicapt, oviduct, vitelline ducts, vitelline reservoir, vagina, seminal receptacle, ootype with unicellular Mehlis's gland, ootype-uterine duct and uterus were observed with the electron microscope. The epithelium of the female reproductive system ducts consists of a nucleate syncytial layer. Structural differences in apical surface of the ducts, the number of nuclei and organoids in syncytial layer as well as the number of underlaid muscles were revealed. The regional differentiations of the uterus wall were registered. The middle and distal region of uterus was covered with microtriches. The filamentous microtriches were observed in apical surface of vagina. The epithelium of seminal receptacle and distal region of uterus were underlaided by the powerful muscle layers. The fertilization canal was revealed. It was shown that the formation of egg shells implemented by the deposit of vitelline globules in their surface in the ootype-uterine duct. Structural and functional differences of different parts of female apparatus in various groups of cestodes are conditioned by species biology.     

Developmental changes in interstitial collagens of fetal rat genital ducts

The distribution of interstitial collagen types I and III was studied by immunocytochemistry in the mesenchyme of progressing and regressing mesonephric and paramesonephric ducts of male and female rat fetuses from the age of 15 days until birth. Immunocytochemistry revealed a collagen-poor mesenchymal area around the genital ducts and in continuation with the coelomic epithelium on the lateral edge of the mesonephric ridge of 15-day-old fetuses. Ultrastructurally, collagen fibrils were accumulated along the continuous lamina densa of the mesonephric ducts, whereas they were absent on the medial side of the male and female paramesonephric ducts. In males, the amount of collagen fibrils increased with the histological maturation of the mesenchyme around the mesonephric duct, whereas around the regressing paramesonephric duct collagens disappeared from the basement membrane region and the surrounding mesenchyme of the 16-day-old male duct. After the completion of the paramesonephric regression, the mesenchyme acquired a uniformly collagen containing interstitial matrix. In females, the collagens increased in the mesenchyme around the progressing paramesonephric duct, and the original site of the regressing mesonephric duct became occupied with a collagen-containing mesenchyme by the age of 19 days. The results suggest a close structural linkage between the mesonephric duct and the established early paramesonephric duct. The differences in the developmental maturation of the periductal mesenchyme and the observed changes in the composition of the interstitial matrix probably reflect the functional differences in the regulatory factors acting on the progression and regression of the male and female genital ducts.     

Lateral organs in sedentary polychaetes (Annelida) – Ultrastructure and phylogenetic significance of an insufficiently known sense organ

Lateral organs are sense organs visible as densely ciliated pits or papillae between the noto‐ and the neuropodia in certain taxa of sedentary polychaetes. Ultrastructural studies in about 10 species of the following taxa Maldanidae, Opheliidae, Orbiniidae, Paraonidae, Magelonidae, Spionidae, Poecilochaetidae and Terebellidae have been designed to evaluate whether these organs are homologous among polychaetes. In spite of great external diversity, the investigations revealed an overall ultrastructural similarity. Differences between species investigated mainly concern the size of the organs as well as the number and arrangement of cells. The organs comprise supportive cells and uniciliated penetrative sensory cells. Their dendrites are closely arranged and thus their cilia may resemble multiciliated cells. There are two types of sensory cells: one type possesses no or mainly thin microvilli of which usually only a few reach the cuticular surface, and in the other type the cilium is consistently surrounded by 10 strong microvilli, which form a pore‐like opening in the cuticle. Further differences occur in the structure of the rootlet system. Basally, a retractor muscle attaches to the organ. The systematic significance of these organs within Annelida is discussed with respect to the conflicting phylogenetic hypotheses explaining the relationships of annelid taxa.     

Ultrastructure of the male genital ducts of the clearnose skate Raja eglanteria

Tissues from the male genital ducts of six specimens of the clearnose skate Raja eglanteria, comprising the Leydig gland, upper and lower epididymis, ductus deferens and seminal vesicle, were fixed and embedded for ultrastructural examination. In the Leydig gland, two types of columnar cells were identified, one bearing microvilli, a basal nucleus and evidence of active secretion with plentiful endoplasmic reticulum and numerous secretory droplets, and the other pyriform with cilia, and swathes of cytofilaments emanating from prominent desmosomes. Occasional crystalloid intramitochondrial inclusions were seen in the first type, with a periodicity of 24 nm. The upper epididymis was composed of cuboidal cells with microvilli and cilia and irregular electron dense granules, some of which were basally situated and extremely large, often within cells resembling intraepithelial leucocytes; such cells were also seen in the stroma underlying the epithelium. The lower epididymis cells also bore microvilli and cilia and were heavily vacuolated with fatty inclusions as well as the granule-laden leucocytes seen previously. In the ductus deferens, cells had masses of long cilia with occasional microvilli; endoplasmic reticulum was well developed, forming complex arrays with sparse secretory droplets and basal mitochondria. In the seminal vesicle there were two cell types, the most common having long cilia and short microvilli and an occasional, paler cell with supranuclear accumulations of small, round mitochondria. These ultrastructural appearances have been related to cell glycosylation and functions including protein secretion, water absorption and waste removal, and illustrate how structure and function vary down the length of the genital tract.     

Ultrastructure and functional morphology of male genital organs and spermatophore formation inProtodrilus (Polychaeta,Annelida)

Summary The structure and functional morphology of lateral organs and sperm ducts, as well as the mechanisms of spermatophore formation and transfer, are investigated by means of light and electron microscopy in the genusProtodrilus. The sperm ducts are simple, ciliated, intercellular gonoducts with a funnel section surrounded by a thin muscle layer and a tube section opening externally in the anterior region of the lateral organs. No glands are present in the sperm ducts. The lateral organs are formed by long epidermal invaginations enclosing an elongate lumen into which numerous cilia project and a large number of glands open. Five to ten different gland types with strikingly distinctive secretory granules are found in the different species. In addition, special supporting cells, the so-called sponge cells, sensory cells and an underlying nervous tissue are developed in the lateral organs. It is stated that apart from some similarities to the ventral atrium ofNerilla antennata no corresponding organs are known within the Annelida. It is argued that inProtodrilus the spermatophores are formed by the lateral organs as there are a high number of glands opening into the lumen of the organ. The possible origin and genesis of the male gonoducts as well as the mode of spermatophore transfer inProtodrilus is discussed.Abbreviations used in the figures bl basal lamina - cc coelomic cell - ci ciliated cell - cir ciliary root - cr ciliary ring - cu cuticle - cv bs contractile ventral blood sinus - d dissepiment/septum - dbs dorsal blood sinus - es euspermatozoa - f funnel - fi filament - g gut - glo gland openings - lgl lateral organ gland - lm longitudinal muscle - lo lateral organ - lu lumen - mi mitochondrion - mt microtubules - mu muscle - mv microvilli - mvc microvillar crown - n nucleus - ne nervous tissue - o opening - ps paraspermatozoa - rer rough endoplasmatic reticulum - s spermatozoa - sc sponge cell - sg salivary gland - spd sperm duct - spdo sperm duct opening - t tube - tm transverse muscle - vc ventral ciliary band     

Development of the genital ducts in Telmatodrilinae (Tubificidae, Clitellata)

In Tubificidae, the male genital duct comprises a funnel in the testes segment, followed by a vas deferens, an atrium, and, frequently, a copulatory structure in the adjacent ovarian segment. There may also be a diffuse or compact prostate gland in association with the duct. The morphology and position of the genital ducts are important for the classification of the oligochaetous Clitellata. Different parts of the male duct, however, have been named without regard to whether they are homologous or not. One way to establish better hypotheses of homology is to study the detailed morphology and/or the development of the genital ducts. The morphogenesis of the genital ducts in Alexandrovia onegensis (Telmatodrilinae) is described. The male funnel originates by multiplication of peritoneal (mesodermal) cells in the posterior septum in the testes segment. A cord of these cells breaks through the septum and grows backwards into the next segment, where it connects to the epidermis. This cord gives rise to the vas deferens, and is therefore mesodermal in origin. The atrium in A. onegensis develops from a primary epidermal (ectodermal) invagination. The vas deferens and atrium connect and a continuous duct from the testes segment to the exterior is formed. Several compact prostate glands develop along the atrium, each being formed from cells in the atrial epithelium. The spermatheca develops from an invagination of the epidermis in the testes segment. The female duct is formed from peritoneal (mesodermal) cells in the posterior septum in the ovarian segment. These developmental findings strengthen the hypothesis about a closer relationship between the Telmatodrilinae and Tubificinae (both Tubificidae).     

Ultrastructure of the male nephridium and its role in spermatophore formation in spionid polychaetes (Annelida)

Summary The mature male nephridia ofPolydora ligni andP. websteri (Polychaeta: Spionidae) are segmental organs composed of a ciliated nephrostome connected to a nephridial canal that crosses the intersegmental septum, expands into a large modified part extending dorsally through the coelom and subsequently narrows into a canal terminating in a dorsal nephridiopore. The nephridial canal is ciliated throughout and is composed of several cell types. Cells in the expanded region of the nephridia of both species contain large urn-shaped depressions filled with long microvilli. InP. ligni, one section of a nephridium contains cells packed with electron-dense granules that are not observed inP. websteri.The spermatophores ofPolydora ligni are composed of a central sperm mass surrounded by a layer of randomly oriented tubules that form a capsule around the sperm and taper into a long thin tail. These tubules are identical in dimensions to the microvilli present in parts of a nephridium and apparently are derived from these microvilli. The spermatophore capsule ofP. websteri is composed of similar tubules also presumed to originate from nephridial microvilli.The microvilli in nephridia of both species are surrounded with a glycocalyx that may function as an adhesive to hold the spermatophore capsule together. This glycocalyx may also function as a species specific message when encountered by a receptive female.Contribution Number 179 from Harbor Branch Foundation, Inc.     

Ultrastructure and histochemistry of secretory ducts in Artemisia campestris ssp. maritima (Compositae)

The histochemical characterization of the oleoresin produced by secretory ducts of Artemkiu campestris ssp. maritima revealed the presence of terpenoids (essential oils, resiniferous acids and probably steroids), alkaloids and fatty acids, eventually polyacetylenes.
The ultrastructural study of A. campestris ducts enabled us to consider that the secretory activity begins very early during the duct development. The oleoresin deposited in the duct cavity is produced by epithelial and sub-epithelial cells that contain, at the secretory phase, a great amount of smooth endoplasmic reticulum (SER) surrounding plastids with few thylakoids. These organelles may play an important role in the oleoresin production, the SER probably being responsible for some steps in the biosynthesis of oleoresin components and for the transport of the secreted material towards the plasmalemma. Endoplasmic reticulum and mitochondria probably have a role in the synthesis of steroids.
After secretion, the duct glandular cells degenerate progressively. The extrusion process of secretion may be considered mero-holocrine.     

Ultrastructure of the body cavity lining in a secondary acoelomate,Microphthalmus cf. Listensis westheide (Polychaeta: Hesionidae)

The organization of the body cavity lining in selected regions of the juvenile and adult of the interstitial hesionid polychaete Microphthalmus cf. listensis is described. Tissues comprising the body cavity lining in the juvenile consist of somatic and splanchnic circular and longitudinal muscles and undifferentiated cells. Somatic and splanchnic cell layers exhibit epithelial ( = eucoelomate) organization in the pharyngeal region. In the midbody, some undifferentiated cells exhibiting mesenchymal organization persist among the epithelially organized somatic and splanchnic cells, forming a gradation between eucoelomate and acoelomate tissue organizations. A coelomic cavity is absent. Tissues comprising the body cavity lining of the adult consist of somatic and splanchnic circular and longitudinal myocytes and coelenchymal cells. Coelenchymal cells are shown from serial section analysis to be mesenchymal in organization and derived from the somatic peritoneum. A 30–65-nm coelomic cavity lies between the apices of somatic and splanchnic cell layers in the pharyngeal region. In the anterior setigerous segments, the coelom is reduced to a narrow cavity surrounded by coelenchymal cells lying midventrally between the paired ejaculatory ducts. There is a regional obliteration of the splanchnic musculature in the posterior segments so that apices of the coelenchymal cells lie in direct apposition to the basal extracellular matrix of the gut. The coeom is only present middorsally as a 0.7-μm-wide cavity. Although the coelomic cavity is highly reduced in the adult, the body cavity lining still reveals its origin from the epithelial ( = eucoelomate) organization. The findings of this study illustrate possible organizational intermediates in the evolution of the acoelomate from the eucoelomate condition in annelids.     

Ultrastructure of sperm storage and male genital ducts in a male holocephalan, the elephant fish, Callorhynchus milii.

In chondrichthyes, the process of spermatogenesis produces a spermatocyst composed of Sertoli cells and their cohort of associated spermatozoa linearly arrayed and embedded in the apical end of the Sertoli cell. The extratesticular ducts consist of paired epididymis, ductus deferens, isthmus, and seminal vesicles. In transit through the ducts, spermatozoa undergo modification by secretions of the extratesticular ducts and associated glands, i.e., Leydig gland. In mature animals, the anterior portion of the mesonephros is specialized as the Leydig gland that connects to both the epididymis and ductus deferens and elaborates seminal fluid and matrix that contribute to the spermatophore or spermatozeugmata, depending on the species. Leydig gland epithelium is simple columnar with secretory and ciliated cells. Secretory cells have periodic acid-Schiff positive (PAS+) apical secretory granules. In the holocephalan elephant fish, Callorhynchus milii, sperm and Sertoli cell fragments enter the first major extratesticular duct, the epididymis. In the epididymis, spermatozoa are initially present as individual sperm but soon begin to laterally associate so that they are aligned head-to-head. The epididymis is a highly convoluted tubule with a small bore lumen and an epithelium consisting of scant ciliated and relatively more secretory cells. Secretory activity of both the Leydig gland and epididymis contribute to the nascent spermatophores, which begin as gel-like aggregations of secretory product in which sperm are embedded. Fully formed spermatophores occur in the ductus. The simple columnar epithelium has both ciliated and secretory cells. The spermatophore is regionalized into a PAS+ and Alcian-blue-positive (AB+) cortex and a distinctively PAS+, and less AB+ medulla. Laterally aligned sperm occupy the medulla and are surrounded by a clear zone separate from the spermatophore matrix. Grossly, the seminal vesicles are characterized by spiral partitions of the epithelium that project into the lumen, much like a spiral staircase. Each partition is staggered with respect to adjacent partitions while the aperture is eccentric. The generally nonsecretory epithelium of the seminal vesicle is simple columnar with both microvillar and ciliated cells.     

Frontal organs in the Acoelomorpha (Turbellaria): Ultrastructure and phylogenetic significance

Using characters discernible through electron microscopy, we redefine the organ traditionally identified as the frontal organ in acoelomorph turbellarians as being a collection of two to several large mucus-secreting glands whose necks emerge together through a frontal pore at the exact apical pole of the body, i.e. at the point where the pattern of epidermal ciliary rootlets converges. Representatives that we have studied of each of the acoel families Paratomellidae, Diopisthoporidae, Solenofilomorphidae, Convolutidae, Otocelidae, and Mecynostomidae, as well as a representative of the Nemertodermatida, have such glands. Up to five additional types of glands that open anteriorly outside of the frontal pore, some of which are indistinguishable from glands of the general body wall, could be seen in the nemertodermatid, in Hesiolicium inops (Paratomellidae), and in representatives of the latter four acoel families. In Paratomella, three different types of glands open in diffuse fashion in a frontal glandular complex reminiscent of that in the Macrostomida.Sensory elements near the frontal pore appear to be independent of the gland necks, and so the organ cannot be considered a sensory organ.The frontal organ, as described above, appears very likely to be homologous within the Acoelomorpha, and represents another strong (although unrooted) autapomorphy for this line of turbellarian evolution.     

Ultrastructure of pigmented adult eyes in errant polychaetes (Annelida): implications for annelid evolution

The evolution of photoreceptor cells and eyes in Metazoa is far from being resolved, although recent developmental and structural studies have provided strong evidence for a common origin of photoreceptor cells and existence of sister cell types already in early metazoans. These sister cell types are ciliary and rhabdomeric photoreceptor cells, depending on which part of each cell is involved in photoreception proper. However, a crucial point in eye evolution is how the enormous structural diversity of photoreceptor cells and visual systems developed, given the general molecular conservation of the photoreceptor cells. One example of this diversity can be observed in Annelida. Within the polychaetes the errant forms, taxon Aciculata, constitute the only group possessing true multicellular eyes in the adult stage. Thus far these organs have been investigated only in taxa of Phyllodocida, a subgroup of Aciculata. Data on Eunicida and Amphinomida as well as certain phyllodocidan taxa had been lacking. The ultrastructure of these adult eyes was investigated in various species of errant polychaetes, belonging to Amphinomidae, Eunicidae and Hesionidae, to elucidate whether they provide any phylogenetic clues regarding either the evolution of visual systems in Annelida or lophotrochozoan phylogeny in general. These eyes are composed of numerous supportive pigment cells and rhabdomeric photoreceptor cells and sometimes additional cell types. As a rule the pigment and rhabdomeric cell types form a continuous epithelium in which the two types intermingle. Presence of granules with shading pigment in sensory cells is a common feature but is apparently restricted to a taxon comprising Phyllodocida and Eunicida s. str. Very likely a lens-like structure does not belong to the ground pattern of annelid eyes, despite its presence in Phyllodocida. These lens-like structures are formed by secretions or cellular processes of the pigment cells. In many species the eye cup communicates with the exterior via a small cuticularized canal. This canal is interpreted as a rudiment due to the mode of formation in the epidermis. With respect to current phylogenetic hypotheses, these multicellular eyes have either been developed in the stem species of a taxon Aciculata nested within the polychaetes or have been evolved in the stem lineage of Annelida. Similarities to gastropod eyes are interpreted as convergent and not as indication of common origin. Except for the photoreceptor cells proper, the structure of the adult eyes in polychaetes most likely does not help to resolve lophotrochozoan phylogeny.