Spicule formation in the calcareous sponge Sycon ciliatum

Summary The spicule primordium is formed in an intercellular cavity within a group of sclerocytes. This cavity contains organic material which ensheaths the growing spicule but does not appear to determine the nature of the mineral morph (magnesian calcite) or the crystallographic orientation of the spicule. The tip of each growing spicule ray is seated in a dense cup in the cytoplasm of the sclerocyte concerned. Both ends of monaxons are initially inserted each into a dense cup. As rays elongate the sclerocyte membrane around the tip becomes invaginated and forms a system of converging spaces that possibly indicate high secretory activity in that region. Spicule growth involves the displacement and expansion of the organic sheath by the enlarging spicule. Fully formed spicules which are exposed to the mesohyl become surrounded by collagen fibrils. However, these fibrils are in no way concerned with the process of mineral deposition and are never found within the spicule calcite.     

Homeobox gene diversification in the calcareous sponge, Sycon raphanus

Knowledge of the developmental mechanisms in living basal metazoan phyla is crucial for understanding the genetic bases of morphological evolution in early animal history. We looked for homeobox genes in the calcareous sponge, Sycon raphanus, using the polymerase chain reaction. Partial sequences of eight homeoboxes were recovered, five of which are assignable to the NK-2 class of homeoboxes. The three remaining sequences are related members of a new class of homeoboxes, the Sycox class, showing limited similarity to bilaterian Lbx, Hlx, HEX, En, and Cad classes. Among the five NK-2 class homeoboxes are four closely related sequences occupying a divergent position within the class, the remaining one on the contrary showing high sequence similarity with members of the NK-2 family, a particular subgroup within the NK-2 class, previously known only from the Bilateria. This suggests that diversification of the NK-2 class occurred early in metazoan history. Altogether, the results reveal an unexpected diversification of homeobox genes in S. raphanus.     

Types of collagen fibres in the calcareous sponges Sycon and Leucandra

Collagen from the calcareous sponges Sycon and Leucandra has been studied in thin sections and in sheaths from decalcified spicules. Sections show abundant thin fibrils 8–15 mμ in diameter and, in Sycon juveniles, 20 mμ fibrils with a clear 50 mμ period. The sheaths are also mainly composed of the thin fibrils but giant fibres up to 130 mμ in diameter and many μ in length are also found.     

Septate junctions between digestive vacuoles in human malacoplakia

Typical septate junctions between digestive vacuoles in phagocytic cells of human malacoplakia are described in this paper. Evidence for a honeycomb pattern of hexagonal subunits is presented for their cleft material. Junctions were not observed between other organelles or in cells other than phagocytes. It is assumed that the septate junctions described here may reflect a pathological change in the organization of the membrane components of digestive organelles.     

Septate and paranodal junctions: kissing cousins

A series of recent publications significantly advances our knowledge about the evolution and composition of septate junctions in arthropod and chordate species. These papers indicate that insect septate junctions share several structural and functional components with paranodal junctions, which join myelinating glial cells to axons in the vertebrate nervous system, and that both probably evolved from a common ancestral precursor.     

Septate junctions in the cephalic epidermis of turbellarians (Bipalium)

Summary In the epidermis of turbellarians septate junctions of the pleated sheet type have been demonstrated in conventional thin sections and freeze fractured preparations. The structure of these junctions entirely agrees with that found in molluscs and arthropods.Financially supported by DFG (Sto 75/3,4; We 380/5)     

Septate junctions mediate the barrier to paracellular permeability in sea urchin embryos

In sea urchin embryos, blastula formation occurs between the seventh and tenth cleavage and is associated with changes in the permeability properties of the epithelium although the structures responsible for mediating these changes are not known. Tight junctions regulate the barrier to paracellular permeability in chordate epithelia; however, the sea urchin blastula epithelium lacks tight junctions and instead possesses septate junctions. Septate junctions are unique to non-chordate invertebrate cell layers and have a characteristic ladder-like appearance whereby adjacent cells are connected by septa. To determine the function of septate junctions in sea urchin embryos, the permeability characteristics of the embryonic sea urchin epithelia were assessed. First, the developmental stage at which a barrier to paracellular permeability arises was examined and found to be in place after the eighth cleavage division. The mature blastula epithelium is impermeable to macromolecules; however, brief depletion of divalent cations renders the epithelium permeable. The ability of the blastula epithelium to recover from depletion of divalent cations and re-establish a barrier to paracellular permeability using fluorescently labelled lectins was also examined. Finally, septate junction structure was examined in embryos in which the permeability status of the epithelium was known. The results provide evidence that septate junctions mediate the barrier to paracellular permeability in sea urchin embryos.     

Arthropod immune system. III. Septate junctions in the hemocytic capsule of the german cockroach, Blattella germanica

Granulocytes (GRs) and/or plasmatocytes (PLs), the two major immunocytes in arthropods, participate in cellular encapsulation of foreign tissue. Although gap and desmosome junctions have been reported in insect capsules, smooth septate junctions are being reported for the first time by both thin section and freeze-fracture techniques in Blattella germanica. In 7-day-old capsules, the septa are 23 nm thick, faintly 'scalloped' and slightly curved in appearance; the interseptal space has a periodicity of about 5 nm. In freeze-fractured capsules, the septa are associated on both sides with the corresponding intramembranous structures, belonging to the plasma membranes of the two junction-forming GRs. The intercellular space is 27 nm wide. There are 36-40 septa/1 mum junctional length. The junctions show furrows on the extracellular fracture face (E) and the complementary regular rows of intramembranous particles on the cytoplasmic face (P). The septate junctions often occur in the region of the capsule that also shows the presence of gap junctions, but only rarely that of desmosomes. The septate junctions are in close proximity with mitochondria. It is suggested that the function of these junctions is to produce compact capsules.     

Locomotion and contraction in an asconoid calcareous sponge

Various large‐scale behaviors (e.g., locomotion, shape changes, contractions) have been documented numerous times in intact sponges of the class Demospongiae. However, little is known about such motile events in calcareous sponges (Class Calcarea). Here, we report on whole‐sponge behaviors of the calcareous asconoid sponge Leucosolenia botryoides, as revealed by time‐lapse videos. These behaviors included locomotion and contraction. Locomotion in these sponges appeared as an outward movement (25–130 μm h^?1) of the asconoid tubes away from the sponge's center; such translocations were always accompanied by extensive movements of protruding spicules, which appear to act as anchoring hooks for the sponge's translocations. This is the first report of whole‐sponge locomotion in the Calcarea. Contractile waves also were propagated in these sponges at speeds of 50–150 μm h^?1, and they involved systemic contraction, then re‐extension of the asconoid tubes. The observations suggest that, like the more complex demosponges, these simple calcareous sponges are capable of adaptive whole‐animal behaviors (changes in flow, shape, and location), which occur in response to environmental stimuli such as crawling intruders.     

Hieracium ciliatum n. sp.

Ohne Zusammenfassung     

Septate junctions in insects: comparison between intercellular and intramembranous structures

Septate junctions have been studied in the hind-gut of Periplaneta americana, Incisitermes schwartzi and Thermobia domestica. The topographical disposition of intercellular septa revealed by lanthanum impregnation corresponds well with that of particle rows seen in freeze fracture preparations. However, there is no precise correspondence between the undulations of septa and the disposition of particles within a single row. The spacing of particles is variable and generally less than that of the undulation periodicity of septa. On the other hand, the disposition of septa, and of the rows of particles that correspond to them, appears to be variable: sometimes rectilinear and in close parallel, these may curve or form series of 'finger-print' loops. Moreover, the septa are evidently not deployed as continuous ribbons around the cell since intrruptions are freuqently observed. In addition to their mechanical role in intercellular cohesion, septate junctions apparently ensure a more or less complete closure of the intercellular space (i.e. provide a permeability barrier) and thus play a role comparable with that of tight junctions in epithelia of vertebrates.     

Septate junctions are required for ommatidial integrity and blood-eye barrier function in Drosophila

The anatomical organization of the Drosophila ommatidia is achieved by specification and contextual placement of photoreceptors, cone and pigment cells. The photoreceptors must be sealed from high ionic concentrations of the hemolymph by a barrier to allow phototransduction. In vertebrates, a blood-retinal barrier (BRB) is established by tight junctions (TJs) present in the retinal pigment epithelium and endothelial membrane of the retinal vessels. In Drosophila ommatidia, the junctional organization and barrier formation is poorly understood. Here we report that septate junctions (SJs), the vertebrate analogs of TJs, are present in the adult ommatidia and are formed between and among the cone and pigment cells. We show that the localization of Neurexin IV (Nrx IV), a SJ-specific protein, coincides with the location of SJs in the cone and pigment cells. Somatic mosaic analysis of nrx IV null mutants shows that loss of Nrx IV leads to defects in ommatidial morphology and integrity. nrx IV hypomorphic allelic combinations generated viable adults with defective SJs and displayed a compromised blood-eye barrier (BEB) function. These findings establish that SJs are essential for ommatidial integrity and in creating a BEB around the ion and light sensitive photoreceptors. Our studies may provide clues towards understanding the vertebrate BEB formation and function.     

Feeding in a calcareous sponge: particle uptake by pseudopodia

Sponges are considered to be filter feeders like their nearest protistan relatives, the choanoflagellates. Specialized "sieve" cells (choanocytes) have an apical collar of tightly spaced, rodlike microvilli that surround a long flagellum. The beat of the flagellum is believed to draw water through this collar, but how particles caught on the collar are brought to the cell surface is unknown. We have studied the interactions that occur between choanocytes and introduced particles in the large feeding chambers of a syconoid calcareous sponge. Of all particles, only 0.1-microm latex microspheres adhered to the collar microvilli in large numbers, but these were even more numerous on the choanocyte surface. Few large particles (0.5- and 1.0-microm beads and bacteria) contacted the collar microvilli; most were phagocytosed by lamellipodia at the lateral or apical cell surface, and clumps of particles were engulfed by pseudopodial extensions several micrometers from the cell surface. Although extensions of the choanocyte apical surface up to 16 microm long were found, most were 4 microm long, twice the height of the collar microvilli. These observations offer a different view of particle uptake in sponges, and suggest that, at least in syconoid sponges, uptake of particles is less dependent on the strictly sieving function of the collar microvilli.     

Septate junctions in the gastrodermal epithelium of Phialidium: A fine structural study utilizing ruthenium red

The septate junctions of the gastrodermis of the hydromedusa, Phialidium gregarium, are composed of septa (80 A thick) which bridge the gap (130 A) between the outer leaflets of the plasma membranes of adjacent cells. The septa are parallel walls, presumably continuous around the cells, and en face show a periodicity of 110 A. Examination of material fixed in a ruthenium red-containing mixture shows that this dye penetrates the interseptal compartments and illucidates the finer structure of the septa. A model of an interpretation of the three-dimensional structure of the junction is presented and relevance of the results to current theories of cell communication is discussed.     

Ultrastructure and embryonic development of a syconoid calcareous sponge

Abstract. Recent molecular data suggest that the Porifera is paraphyletic (Calcarea+Silicea) and that the Calcarea is more closely related to the Metazoa than to other sponge groups, thereby implying that a sponge‐like animal gave rise to other metazoans. One ramification of these data is that calcareous sponges could provide clues as to what features are shared among this ancestral metazoan and higher animals. Recent studies describing detailed morphology in the Calcarea are lacking. We have used a combination of microscopy techniques to study the fine structure of Syconcoactum Urban 1905, a cosmopolitan calcareous sponge. The sponge has a distinct polarity, consisting of a single tube with an apically opening osculum. Finger‐like chambers, several hundred micrometers in length, form the sides of the tube. The inner and outer layers of the chamber wall are formed by epithelia characterized by apical–basal polarity and occluding junctions between cells. The outer layer—the pinacoderm—and atrial cavity are lined by plate‐like cells (pinacocytes), and the inner choanoderm is lined by a continuous sheet of choanocytes. Incurrent openings of the sponge are formed by porocytes, tubular cells that join the pinacoderm to the choanoderm. Between these two layers lies a collagenous mesohyl that houses sclerocytes, spicules, amoeboid cells, and a progression of embryonic stages. The morphology of choanocytes and porocytes is plastic. Ostia were closed in sponges that were vigorously shaken and in sponges left in still water for over 30 min. Choanocytes, and in particular collar microvilli, varied in size and shape, depending on their location in the choanocyte chamber. Although some of the odd shapes of choanocytes and their collars can be explained by the development of large embryos first beneath and later on top of the choanocytes, the presence of many fused collar microvilli on choanocytes may reflect peculiarities of the hydrodynamics in large syconoid choanocyte chambers. The unusual formation of a hollow blastula larva and its inversion through the choanocyte epithelium are suggestive of epithelial rather than mesenchymal cell movements. These details illustrate that calcareous sponges have characteristics that allow comparison with other metazoans—one of the reasons they have long been the focus of studies of evolution and development.     

Septate junctions in imaginal disks of Drosophila: a model for the redistribution of septa during cell rearrangement

The organization of septate junctions during morphogenesis of imaginal disks is described from freeze-fracture replicas and thin sections with a view to understanding junction modulation during rearrangements of cells in epithelia. The septate junctions of each epithelial cell of the disk are distributed in a number of discrete domains equal to the number of neighboring cells. Individual septa traverse domains of contact between pairs of adjacent cells, turn downwards at the lateral boundary of the domain and run parallel to the intersection with a third cell. This arrangement leaves small channels at three-cell intersections that are occupied by specialized structures termed "tricellular plugs." Cell rearrangement involves a progressive change in the width of contact domains between adjacent cells, until old contacts are broken and new ones established. It is proposed that the septate junction adjusts to the changing width of domains by the compaction or extension of existing septa. This redistribution of septa theoretically allows a transepithelial barrier to be maintained during cell rearrangements. The applicability of this model to other epithelial tissues is discussed.     

Metamorphosis of coeloblastula performed by multipotential larval flagellated cells in the calcareous sponge Leucosolenia laxa

The calcareous sponge Leucosolenia laxa releases free-swimming hollow larvae called coeloblastulae that are the characteristic larvae of the subclass Calcinea. Although the coeloblastula is a major type of sponge larva, our knowledge about its development is scanty. Detailed electron microscopic studies on the metamorphosis of the coeloblastula revealed that the larva consists of four types of cells: flagellated cells, bottle cells, vesicular cells, and free cells in a central cavity. The flagellated cells, the principal cell type of the larva, are arranged in a pseudostratified layer around a large central cavity. The larval flagellated cells characteristically have glutinous granules that are used as internal markers during metamorphosis. After a free-swimming period the larva settles on the substratum, and settlement apparently triggers the initiation of metamorphosis. The larval flagellated cells soon lose their flagellum and begin the process of dedifferentiation. Then the larva becomes a mass of dedifferentiated cells in which many autophagosomes are found. Within 18 h after settlement, the cells at the surface of the cell mass differentiate to pinacocytes. The cells beneath the pinacoderm differentiate to scleroblasts that form triradiate spicules. Finally, the cells of the inner cell mass differentiate to choanocytes and are arranged in a choanoderm that surrounds a newly formed large gastral cavity. We found glutinous granules in these three principal cell types of juvenile sponges, thus indicating the multipotency of the flagellated cells of the coeloblastula.     

Polyacelylenic alcohols and alkaloid derivatives from a calcareous marine sponge,Leucetta chagosensis

A chemical investigation of the CH₂Cl₂ extract of the marine sponge Leucetta chagosensis afforded 8 secondary metabolites, namely, pellynol A (1), pellynol B (2), pellynol I (3), pellynol D (4), plakohypaphorine B (5), plakohypaphorine E (6), 4-bromo-1H-pyrrole-3-carboxamide (7), and 2-phenylacetamide (8). The structures of these secondary metabolites were elucidated via NMR spectroscopic analysis, MS experiment and compared with those reported in the literature. This is the first report of compounds 1–8 isolated from the marine sponge genus Leucetta and from the class Calcarea. This work contributes expands the knowledge of the chemical diversity of calcareous marine sponges, and the chemotaxonomic significance of the isolates is also discussed.