Coelomic Lining Research Articles

A morphogenetic wave in the chick embryo lateral mesoderm generates mesenchymal-epithelial transition through a 3D-rosette intermediate.

Formation of epithelia through mesenchymal-epithelial transition (MET) is essential for embryonic development and for many physiological and pathological processes. This study investigates MET invivo in the chick embryo lateral mesoderm, where a multilayered mesenchyme transforms into two parallel epithelial sheets that constitute the coelomic lining of the embryonic body cavity. Prior to MET initiation, mesenchymal cells exhibit non-polarized distribution of multiple polarity markers, albeit not aPKC. We identified an epithelializing wave that sweeps across the lateral mesoderm, the wavefront of which is characterized by the accumulation of basal fibronectin and a network of 3D rosettes composed of polarized, wedge-shaped cells surrounding a central focus of apical markers, now including aPKC. Initiation of the MET process is dependent on extracellular matrix-integrin signaling acting through focal adhesion kinase and talin, whereas progression through the rosette phase requires aPKC function. We present a stepwise model for MET, comprising polarization, 3D-rosette, and epithelialization stages.

Ultrastructure of proboscis blood vessels in females of Bonellia viridis (Annelida: Bonellinae): New reconstructions.

Many data on echiurid anatomy and ultrastructure are obtained for Bonellia viridis and extrapolated to other species. The ultrastructure of the axial blood vessels, which has been described as an "osmotic pump,"is regarded as one of the unusual features of echiurids. In this study, the ultrastructure of the proboscis blood vessels in females of B. viridis is described, illustrated by accurate schemes, and a new reconstruction of the axial blood vessel is suggested. The walls of the axial and lateral vessels of the proboscis are formed by myoepithelial cells, which are connected to each other via adherence junctions, underlined by basal lamina, and therefore form a true epithelium. Apical, middle, and basal parts of the myoepithelial cells form long, thin projections, which extend to the connective tissue (in axial vessel) or coelomic canals (in lateral vessels) and to the lumen of the vessels. The presence of such projections may evidence active cellular transport. Similarity in the fine structure of the myoepithelial cells of axial and lateral blood vessels evidence their common origin from myoepithelial cells of the coelomic lining. However, in evolution, the coelomic canals were retained around the lateral vessels and disappeared around the axial vessel. The reduction of a hypothetical ancestral axial coelom may be caused by the extensive development of the connective tissue and muscles in the central part of the proboscis, where the axial vessel extends.

Small, but smart: Fine structure of an avicularium in Dendrobeania fruticosa (Bryozoa: Cheilostomata).

Bryozoans are small benthic suspension-feeding colonial animals. Among this phylum, there are representatives showing a lesser or greater degree of polymorphism, and the most common type of polymorphic zooids is the avicularium. Here we present a detailed description of the bird's-head shaped avicularium in Dendrobeania fruticosa. The body cavity of the avicularium demonstrates an acoelomate condition: along the cystid walls, there is neither the layer of extracellular matrix toward the epidermis, nor coelomic lining. However, a layer of extracellular matrix and epithelialized cells lie under the epidermis of the tentacle sheath. Probably, such construction helps the tentacle sheath to acquire some rigidity-it is the only region of the body wall without an ectocyst. We did not find typical funicular strands in the avicularium, but there is a delicate mesh composed of stellate cells with thin and long projections, which sometimes isolate the spaces filled with a heterogeneous matrix. The proximal ends of the adductors, abductors, and polypide retractors are attached to the body wall via typical epidermal tendon cells, which possess numerous bundles of tonofilaments. The distal ends of the abductors and adductors attach to the frontal membrane or upper vestibular membrane, respectively. The inner organic layer of the ectocyst in these regions forms large protrusions, from which numerous thin outgrowths branch off. We suggest them to be a functional analogue of apodemes and apodemal filaments in arthropods. "Apodemal" tendon cells have long and thin projections that line the outgrowths of the ectocyst and surround the distal ends of the muscle cells. At these sites, "apodemal" tendon cells possess numerous tonofilaments. The vestigial polypide includes the tentacle sheath, rudimentary lophophore, cerebral ganglion, and polypide retractors. The sensory part of 5HT-positive cells of the frontal membrane is dendrite-shaped and embedded in the inner organic layer of the ectocyst.

Body cavities in bryozoans: Functional and phylogenetic implications.

Based on morphological evidence, Bryozoa together with Phoronida and Brachiopoda are traditionally combined in the group Lophophorata, although this view has been recently challenged by molecular studies. The core of the concept lies in the presence of the lophophore as well as the nature and arrangement of the body cavities. Bryozoa are the least known in this respect. Here, we focused on the fine structure of the body cavity in 12 bryozoan species: 6 gymnolaemates, 3 stenolaemates and 3 phylactolaemates. In gymnolaemates, the complete epithelial lining of the body cavity is restricted to the lophophore, gut walls, and tentacle sheath. By contrast, the cystid walls are composed only of the ectocyst-producing epidermis without a coelothelium, or an underlying extracellular matrix; only the storage cells and cells of the funicular system contact the epidermis. The nature of the main body cavity in gymnolaemates is unique and may be considered as a secondarily modified coelom. In cyclostomes, both the lophophoral and endosaccal cavities are completely lined with coelothelium, while the exosaccal cavity only has the epidermis along the cystid wall. In gymnolaemates, the lophophore and trunk cavities are divided by an incomplete septum and communicate through two pores. In cyclostomes, the septum has a similar location, but no openings. In Phylactolaemata, the body cavity is undivided: the lophophore and trunk coeloms merge at the bases of the lophophore arms, the epistome cavity joins the trunk, and the forked canal opens into the arm coelom. The coelomic lining of the body is complete except for the epistome, lophophoral arms, and the basal portions of the tentacles, where the cells do not interlock perfectly (this design probably facilitates the ammonia excretion). The observed partitioning of the body cavity in bryozoans differs from that in phoronids and brachiopods, and contradicts the Lophophorata concept.

Trophosome in vestimentifera Ridgeia piscesae Jones, 1985 (Annelida, Siboglinidae) develops from cells of the coelomic lining

The structure of early juvenile specimens of vestimentifera Ridgeia piscesae (Annelida, Siboglinidae) was studied. As adults, vestimentiphers are deprived of the intestine and have a trophosome, in which cells chemoautotrophic bacteria are living. In early juvenile individuals of 280-300 microns in size, it was found that trophosome develops from cells of the coelomic lining on the surface of the intestine and on the side walls of the body. This structure of the rudiment of the trofosome suggests that the bacteria are first captured by the cell wall of the body and then transferred to the cell on the surface of the intestine.

Trophosome in the Vestimentiferan Tubeworm Ridgeia piscesae Jones 1985 (Annelida, Siboglinidae) Develops from Cells of the Coelomic Lining.

The paper reports the study of the anatomy of early juvenile individuals of the vestimentiferan tubeworm Ridgeia piscesae (Annelida, Siboglinidae). Adult vestimentiferans lack the digestive tract but have the trophosome, whose cells are inhabited by chemoautotrophic bacteria. It has been shown, in 280- to 300-µm early juvenile individuals, that the trophosome develops from cells of the coelomic lining on the gut surface and on the lateral body walls. The observed proto-trophosome structure suggests that the bacteria are first captured by the coelomic cells of the body wall and then transferred to the coelomic cells located on the gut surface.

PY489‐β‐Catenin Phosphorylation Involved in the Intestinal Regeneration of the Sea Cucumber H. glaberrima

Organ regeneration is a rare phenomenon in higher vertebrates. However, members of closely related groups such as echinoderms have striking regeneration capacities. In studies of intestinal regeneration using the sea cucumber Holothuria glaberrima, recent results have shown the involvement of the Wnt signaling pathway. One important component in this intracellular cascade is β‐catenin, that, depending on its phosphorylation state, activates transcription of Wnt target genes. In one case, phosphorylation of the tyrosine residue 489 (Y489) increases transcriptional activity of the β‐catenin/Tcf complex. To determine if Y489‐β‐catenin phosphorylation is involved in intestinal regeneration, normal and regenerating intestines of H. glaberrima specimens were dissected at different post‐evisceration stages. Tissues were processed for immunohistochemistry using anti‐PY489‐β‐catenin antibody and assessed by counting the temporal and spatial appearance of labeled cells in the distal, medial and proximal ends of the mesentery and gut rudiment. In normal non‐eviscerated animals, the antibody labeled about the 14% of cells in the coelomic lining and 31.2% of cells in the luminal epithelium. In regenerating specimens, PY489‐β‐catenin was expressed at all stages of regeneration from 3‐dpe up to 14‐dpe, with a peak increment of 40.6% of cells labeled in the mesothelium of the 7‐dpe intestinal rudiment. To determine the role of activating or inhibiting the WNT/β‐catenin pathway on the expression of the PY489 labeling, we treated tissue explants with the Wnt pathway activator LiCl and inhibitor iCRT14. When compared to controls there was no significant difference in the number of PY489‐β‐catenin immunoreactive cells observed in the LiCl‐treated explants, while there was a significant increase in immunoreactive cells in the iCRT14‐treated explants. Our results suggest that the Wnt/β‐catenin signaling is activated during intestinal regeneration in H. glaberrima and that phosphorylation in the Y489 residue of β‐catenin is important for the Wnt role in intestinal regeneration. In particular, the results suggest a possible role for Wnt pathway in the dedifferentiation status of the mesothelium once the initial intestinal blastema is formed. Our study might provide the basis for the role of the Wnt signaling pathway during organogenesis in holothurians and thus provide a better understanding of general regenerative processes.Support or Funding InformationFunded by NIH R15NS081686, NSF IOS‐1252679 and the University of Puerto Rico.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Ultrastructure of the lophophoral coelomic lining in the brachiopod Hemithiris psittacea: functional and evolutionary significance

The ultrastructure of the lophophoral coelomic lining in the articulate brachiopod Hemithiris psittacea was studied using electron microscopy and confocal laser scanning microscopy. The coelomic system of the lophophore consists of large and small canals; both extend along each brachium. A small coelomic canal gives rise to a blind coelomic canal into each tentacle. The lophophoral coelothelium consists of two types of cells, epithelio-muscle and peritoneal cells, and exhibits different types of organization that is known in the Bilateria: from an epithelium consisting only of epithelio-muscle cells to a pseudostratified myoepithelium. The lophophoral coelothelium forms the musculature and blood vessel wall of the brachia and tentacles. The lophophoral blood vessel runs in the extracellular matrix of the septum that separates the large and small canals. In the lophophoral vessel, the upper wall consists of epithelio-muscle cells, whereas the lower wall is formed by peritoneal cells. The lophophoral vessel gives rise to blind branches into each tentacle. The lophophoral wall is highly muscular and contains both longitudinal and transverse muscles. Because the spirolophe lophophore of H. psittacea is supported only by short crura, the coelomic system of the lophophore is assumed to stabilize the lophophore by regulating the hydrostatic pressure of the coelomic fluid.

Body wall structure in the starfish Asterias rubens.

The body wall of starfish is composed of magnesium calcite ossicles connected by collagenous tissue and muscles and it exhibits remarkable variability in stiffness, which is attributed to the mechanical mutability of the collagenous component. Using the common European starfish Asterias rubens as an experimental animal, here we have employed a variety of techniques to gain new insights into the structure of the starfish body wall. The structure and organisation of muscular and collagenous components of the body wall were analysed using trichrome staining. The muscle system comprises interossicular muscles as well as muscle strands that connect ossicles with the circular muscle layer of the coelomic lining. The collagenous tissue surrounding the ossicle network contains collagen fibres that form loop‐shaped straps that wrap around calcite struts near to the surface of ossicles. The 3D architecture of the calcareous endoskeleton was visualised for the first time using X‐ray microtomography, revealing the shapes and interactions of different ossicle types. Furthermore, analysis of the anatomical organisation of the ossicles indicates how changes in body shape may be achieved by local contraction/relaxation of interossicular muscles. Scanning synchrotron small‐angle X‐ray diffraction (SAXD) scans of the starfish aboral body wall and ambulacrum were used to study the collagenous tissue component at the fibrillar level. Collagen fibrils in aboral body wall were found to exhibit variable degrees of alignment, with high levels of alignment probably corresponding to regions where collagenous tissue is under tension. Collagen fibrils in the ambulacrum had a uniformly low degree of orientation, attributed to macrocrimp of the fibrils and the presence of slanted as well as horizontal fibrils connecting antimeric ambulacral ossicles. Body wall collagen fibril D‐period lengths were similar to previously reported mammalian D‐periods, but were significantly different between the aboral and ambulacral samples. The overlap/D‐period length ratio within fibrils was higher than reported for mammalian tissues. Collectively, the data reported here provide new insights into the anatomy of the body wall in A. rubens and a foundation for further studies investigating the structural basis of the mechanical properties of echinoderm body wall tissue composites.

Ultrastructure of the coelom in the brachiopod Lingula anatina.

The organization of the coelomic system and the ultrastructure of the coelomic lining are used in phylogenetic analysis to establish the relationships between major taxa. Investigation of the anatomy and ultrastructure of the coelomic system in brachiopods, which are poorly studied, can provide answers to fundamental questions about the evolution of the coelom in coelomic bilaterians. In the current study, the organization of the coelom of the lophophore in the brachiopod Lingula anatina was investigated using semithin sectioning, 3D reconstruction, and transmission electron microscopy. The lophophore of L. anatina contains two main compartments: the preoral coelom and the lophophoral coelom. The lining of the preoral coelom consists of ciliated cells. The lophophoral coelom is subdivided into paired coelomic sacs: the large and small sinuses (= canals). The lining of the lophophoral coelom varies in structure and includes monociliate myoepithelium, alternating epithelial and myoepithelial cells, specialized peritoneum and muscle cells, and podocyte-like cells. Connections between cells of the coelomic lining are provided by adherens junctions, tight-like junctions, septate junctions, adhesive junctions, and direct cytoplasmic bridges. The structure of the coelomic lining varies greatly in both of the main stems of the Bilateria, that is, in the Protostomia and Deuterostomia. Because of this great variety, the structure of the coelomic lining cannot by itself be used in phylogenetic analysis. At the same time, the ciliated myoepithelium can be considered as the ancestral type of coelomic lining. The many different kinds of junctions between cells of the coelomic lining may help coordinate the functioning of epithelial cells and muscle cells.

Establishment of the Visceral Embryonic Midline Is a Dynamic Process that Requires Bilaterally Symmetric BMP Signaling

The vertebrate body plan contains both dorsal and ventral midline structures. While dorsal midline structures have been extensively studied, formation of ventral midline structures, and how they become aligned with the dorsal midline, is a fundamental aspect of vertebrate development that is poorly understood. This study uses the chick dorsal mesentery (DM) as a model for investigating the formation of ventral midline structures. We document formation of the DM by epithelial-to-mesenchymal transition (EMT) and medial ingression of the lateral plate coelomic lining and show that DM positioning is afundamentally dynamic process regulated by relative levels of bone morphogenetic protein (BMP) signaling in the two sides of the ingressing lateral plate. Disruption of this process causes misalignment of the DM and disturbances during initial stages of lung morphogenesis. Since the dorsal midline is a source of BMP antagonists, these results suggest a mechanism for aligning the dorsal and ventral embryonic midlines.

Structure of the Proboscis Endothelium in Nemertea.

We studied the ultrastructure of the proboscis endothelium of 14 nemertean species. In all nemerteans examined, the endothelium is organized as a pseudostratified myoepithelium consisting of two types of cells resting on the basal extracellular matrix: apically situated supportive cells and subapical myocytes covered by cytoplasmic sheets of the supportive cells. Myocytes form the inner circular musculature of the proboscis; the endothelium in the bulb of monostiliferous nemerteans lacks myocytes. The endothelium of the studied species differs in the number of rows of muscle fibres (one vs. several rows), the number of myofibrils in myocytes (one vs. two to five), the number of processes of myocytes covered by one supportive cell (one vs. two to 23), and in the number of processes in supportive cells (one vs. two to five). In some of the species, rudimentary cilia of supportive cells were revealed by using cLSM and an antibody against tubulin. The data obtained indicate that the proboscis endothelium in nemerteans is in fact a coelothelium, but recognition of the ancestral state of the coelomic lining in Nemertea is problematic, as the rhynchocoel peritoneum lacks myocytes.

Organization of the coelomic system in Phoronis australis (Lophotrochozoa: Phoronida) and consideration of the coelom in the lophophorates

AbstractThe organization of the phoronid coelomic system has traditionally been regarded as similar to that of the deuterostomians, because both have three coelomic compartments: the proto‐, meso‐ and metacoel. Recent investigations, however, have revealed the presence of two coelomic compartments in specimens from the genus Phoronis and three in specimens from the genus Phoronopsis. The current study aimed to determine which type of coelomic system organization is plesiomorphic for phoronids. The organization of the coelomic system of Phoronis australis, which is the largest species of Phoronis, was investigated by transmission electron microscopy and three‐dimensional reconstruction. P. australis has three coelomic compartments. The protocoel is the closed cavity, which is located at the base of the epistome, consists of two lateral branches and a central part, and has its own epithelial lining that is formed by non‐muscular epithelial and myoepithelial cells. According to these features, the protocoel must be regarded as a true coelom. The mesocoel base surrounds the esophagus and gives rise to channels that extend into each tentacle. The mesocoel lining consists of two cell types, non‐muscular epithelial and myoepithelial cells, which alternate and interconnect via desmosomes. The metacoel is subdivided into four chambers by five mesenteries. In the anterior part of the body, the metacoel lining is formed by epithelial cells, which cover the surface of the longitudinal muscles. Coelomic organization was compared in phoronids, brachiopods and bryozoans. Both phoronids and brachiopods exhibit two types of coelomic organization – bipartite and tripartite – while bryozoans have a coelomic organization that is completely different from that in phoronids and brachiopods. The tripartite coelom appears to be the plesiomorphy for the Brachiozoa. In phoronids, the coelomic lining can be organized as a primitive monociliate myoepithelium, as an advanced type with alternation of epithelial and myoepithelial cells, or as a specialized coelothelium.

Wnt signaling orients the proximal-distal axis of chick kidney nephrons.

The nephron is the fundamental structural and functional unit of the kidney. Each mature nephron is patterned along a proximal-distal axis, with blood filtered at the proximal end and urine emerging from the distal end. In order to filter the blood and produce urine, specialized structures are formed at specific proximal-distal locations along the nephron, including the glomerulus at the proximal end, the tubule in the middle and the collecting duct at the distal end. The developmental processes that specify these different nephron segments are not fully understood. Wnt ligands, which are expressed in the nephric duct and later in the nascent nephron itself, are well-characterized inducers of nephrons, and are both required and sufficient for initiation of nephron formation from nephrogenic mesenchyme. Here, we present evidence that Wnt signaling also patterns the proximal-distal nephron axis. Using the chick mesonephros as a model system, a Wnt ligand was ectopically expressed in the coelomic lining, thereby introducing a source of Wnt signaling that is at right angles to the endogenous Wnt signal of the nephric duct. Under these conditions, the nephron axis was re-oriented, such that the glomerulus was always located at a position farthest from the Wnt sources. This re-orientation occurred within hours of exposure to ectopic Wnt signaling, and was accompanied initially by a repression of the early glomerular podocyte markers Wt1 and Pod1, followed by their re-emergence at a position distant from the Wnt signals. Activation of the Wnt signaling pathway in mesonephric explant cultures resulted in strong and specific repression of early and late glomerular markers. Finally, cytoplasmic β-catenin, indicative of active canonical Wnt signaling, was found to be enriched in the distal as compared with the proximal region of the forming nephron. Together, these data indicate that Wnt signaling patterns the proximal-distal axis of the nephron, with glomeruli differentiating in regions of lowest Wnt signaling.

Generation of dorsal aorta hemogenic endothelium involves replacement of the ventral aortic lining with cells derived from the coelomic epithelium

BackgroundThe ventral wall of the dorsal aorta is the earliest known embryonic location of definitive hematopoiesis. The developmental processes that generate this hemogenic endothelium are not well understood. The Coelomic Epithelium (CE) is an epithelial layer that lines the embryonic body cavity and invests the external surface of the viscera. Here we present evidence that the CE can give rise to hemogenic endothelium of the dorsal aortaMethodsWe used two cell tracking techniques in order to trace the fate of coelomic epithelial cells in the chick embryo. In the first approach, the fluorescent vital dye CM-DiI was microinjected into the coelomic cavity. In the second approach, a plasmid expressing eGFP was electroporated into the cells of the coelomic lining. The fate of labeled cells was analyzed after up to 48 hours of incubation by immunofluorescence for the cell label and for cell-specific markers, including the blood cell marker cd45ResultsCoelomic epithelium-derived cells were found to incorporate into the ventral wall of the dorsal aorta, some of which were observed to differentiate into clusters of cd-45-expressing hematopoietic cells. These results indicate that the aortic hemogenic endothelium is generated by a process that involves replacement of the ventral aortic endothelium by coelomic epithelium-derived cells that have hemogenic potential. BackgroundThe ventral wall of the dorsal aorta is the earliest known embryonic location of definitive hematopoiesis. The developmental processes that generate this hemogenic endothelium are not well understood. The Coelomic Epithelium (CE) is an epithelial layer that lines the embryonic body cavity and invests the external surface of the viscera. Here we present evidence that the CE can give rise to hemogenic endothelium of the dorsal aorta The ventral wall of the dorsal aorta is the earliest known embryonic location of definitive hematopoiesis. The developmental processes that generate this hemogenic endothelium are not well understood. The Coelomic Epithelium (CE) is an epithelial layer that lines the embryonic body cavity and invests the external surface of the viscera. Here we present evidence that the CE can give rise to hemogenic endothelium of the dorsal aorta MethodsWe used two cell tracking techniques in order to trace the fate of coelomic epithelial cells in the chick embryo. In the first approach, the fluorescent vital dye CM-DiI was microinjected into the coelomic cavity. In the second approach, a plasmid expressing eGFP was electroporated into the cells of the coelomic lining. The fate of labeled cells was analyzed after up to 48 hours of incubation by immunofluorescence for the cell label and for cell-specific markers, including the blood cell marker cd45 We used two cell tracking techniques in order to trace the fate of coelomic epithelial cells in the chick embryo. In the first approach, the fluorescent vital dye CM-DiI was microinjected into the coelomic cavity. In the second approach, a plasmid expressing eGFP was electroporated into the cells of the coelomic lining. The fate of labeled cells was analyzed after up to 48 hours of incubation by immunofluorescence for the cell label and for cell-specific markers, including the blood cell marker cd45 ResultsCoelomic epithelium-derived cells were found to incorporate into the ventral wall of the dorsal aorta, some of which were observed to differentiate into clusters of cd-45-expressing hematopoietic cells. These results indicate that the aortic hemogenic endothelium is generated by a process that involves replacement of the ventral aortic endothelium by coelomic epithelium-derived cells that have hemogenic potential. Coelomic epithelium-derived cells were found to incorporate into the ventral wall of the dorsal aorta, some of which were observed to differentiate into clusters of cd-45-expressing hematopoietic cells. These results indicate that the aortic hemogenic endothelium is generated by a process that involves replacement of the ventral aortic endothelium by coelomic epithelium-derived cells that have hemogenic potential.

Wt1 and Retinoic Acid Signaling in the Subcoelomic Mesenchyme Control the Development of the Pleuropericardial Membranes and the Sinus Horns

The cardiac venous pole is a common focus of congenital malformations and atrial arrhythmias, yet little is known about the cellular and molecular mechanisms that regulate its development. The systemic venous return myocardium (sinus node and sinus horns) forms only late in cardiogenesis from a pool of pericardial mesenchymal precursor cells. To analyze the cellular and molecular mechanisms directing the formation of the fetal sinus horns. We analyzed embryos deficient for the Wt1 (Wilms tumor 1) gene and observed a failure to form myocardialized sinus horns. Instead, the cardinal veins become embedded laterally in the pleuropericardial membranes that remain tethered to the lateral body wall by the persisting subcoelomic mesenchyme, a finding that correlates with decreased apoptosis in this region. We show by expression analysis and lineage tracing studies that Wt1 is expressed in the subcoelomic mesenchyme surrounding the cardinal veins, but that this Wt1-positive mesenchyme does not contribute cells to the sinus horn myocardium. Expression of the Raldh2 (aldehyde dehydrogenase family 1, subfamily A2) gene was lost from this mesenchyme in Wt1(-/-) embryos. Phenotypic analysis of Raldh2 mutant mice rescued from early cardiac defects by retinoic acid food supply revealed defects of the venous pole and pericardium highly similar to those of Wt1(-/-) mice. Pericardium and sinus horn formation are coupled and depend on the expansion and correct temporal release of pleuropericardial membranes from the underlying subcoelomic mesenchyme. Wt1 and downstream Raldh2/retinoic acid signaling are crucial regulators of this process. Thus, our results provide novel insight into the genetic and cellular pathways regulating the posterior extension of the mammalian heart and the formation of its coelomic lining.

Histological examination of the chaetognathan morphological structure in the region of a supposed trunk-tail septum

A histological examination was conducted based on sections done in series in transverse and longitudinal directions of Aidanosagitta macilenta (Sagittidae, Chaetognatha) in a region which includes the anterior parts of the testes and the posterior part of the ovaries. We found that, in mature individuals (the 4th stage of maturation), the anterior parts of the testes are located in front of the posterior part of the ovaries. The anterior parts of the testes have a ventral position adjoining the ventral muscular bands. The ovaries are located more dorsally than the testes at the level of lateral fi elds. A transverse trunk-tail septum between testes and ovaries was not revealed. These data support an idea that a structure which was earlier treated as a tail-trunk septum is actually the anterior edges of the testes. Both longitudinal coelomic cavities continue into the tail region without any transverse septa. The paired tail cavities as can be seen from their composition and functions represent the testes consisting of a peripheral compact portion with undifferentiated germ cells and a space where the maturation of cells occurs. The tail is distinguished anatomically as a post-anal portion of the body but it is not a true segment as it is not separated from a trunk part of the body by the trunk-tail septum. The body of Chaetognatha consists of two true parts (segments). A mesenterial septum divides the body longitudinally-symmetrically from a trunk-head septum down only to the caudal end of the gut. In the tail region there is a medial septum formed by the medial walls of testes. Gonads in both females and males do not have any duct connecting them with a gut cavity. The ovaries and the testes are covered by a coelomic lining and a duct between the ovaries and the testes is absent.

Metamorphosis in larvae of the temperate sea cucumber Australositchopus mollis

Summary Holothurian larvae that feed on phytoplankton during development exhibit a dramatic metamorphosis involving extensive reorganisation of all larval tissue before adopting a benthic existence and assuming the juvenile form. During this process in the temperate sea cucumber Australostichopus mollis the combined axohydrocoel differentiated into the coelomic lining of the buccal podia and radii, both appearing early. Although podia primordia appeared shortly after, the podia themselves appeared later. This process appeared associated with differentiation of oral cavity tissue and formation of the water vascular ring canal. Oral cavity tissue was displaced temporarily but was not evidence of axial torsion. Tissue differentiation of the reorganised alimentary canal occurred post settlement and the mouth of the benthic juvenile opened well after settlement to initiate feeding. These larvae exhibit a variable and protracted transition to the benthic juvenile form rather than an abbreviated sequence of progressive anatomical events as in indirect non-feeding developers.

Fine structure of the epistome in Phoronis ovalis: significance for the coelomic organization in Phoronida

Abstract. The hypothesis of a common ancestry of the lophophorate taxa Brachiopoda, Bryozoa, Phoronida, and the Deuterostomia can be traced back to the late 19th century when Masterman recognized a tripartite organization of the body consisting of pro‐, meso‐, and metasome, along with coelomic body cavities in each compartment, as characteristic for Echinodermata, Pterobranchia, Phoronida, and Brachiopoda. This idea became quite popular under the name “archicoelomate” concept. The organization of the phoronids, and especially of their transparent actinotroch larva, has for a long time been used as a touchstone for the validity of this concept. As a coelomic lining can reliably be recognized only on the ultrastructural level, this technique has been applied for adults of Phoronis ovalis, which is assumed to be a sister species to all other phoronids. Phoronis ovalis contains only two coelomic compartments, a posterior coelom inside the trunk (metasoma), occupying the space between the trunk epidermis and the digestive epithelium, and an anterior lophophoral coelom inside and basal to the tentacular crown (mesosoma). There is no coelomic cavity inside the epistome (prosoma). This part of the body is filled with myoepithelial cells, which are continuous with the epithelial lining of the lophophore cavity. These cells form a lumenless bilayer and possess long, tiny myofilamentous processes, which are completely embedded in an extracellular matrix. A comparison with data on P. muelleri shows that there is no need to assume three different coelomic cavities in Phoronida, in contrast to the predictions of the archicoelomate concept. At least for this taxon, a correspondence to the situation in deuterostomes can hardly be found.

Organization of the coelomic lining and a juxtaposed nerve plexus in the suckered tube feet of Parastichopus californicus (Echinodermata: Holothuroida)

The coelomic lining of the water-vascular canal in a suckered tube foot from the sea cucumber, Parastichopus californicus, is a pseudostratified myoepithelium consisting of flagellated adluminal cells and myofilament-bearing retractor cells. The bodies of adluminal cells flank the water-vascular canal and send basal processes between the underlying retractor cells to confront the podial connective tissue. Retractor cells have a contractile apparatus of unregistered thick and thin myofilaments. The contractile apparatus is confined to the medullary sarcoplasm and oriented parallel to the primary axis of a tube foot. The bodies and processes of retractor cells intermingle with the basal processes of adluminal cells at the basal lamina of the coelomic lining. A ganglionated nerve plexus in the podial connective tissue approximates the basal lamina. Neuronal connectives link the ganglia to one another and to the nerve plexus in deep sectors of the podial epidermis. External laminae enveloping the ganglia and connectives in the podial connective tissue are continuous with the basal lamina of the epidermis. The adventitial nerve plexus, since it merges with the epidermal nerve plexus, is a component of the ectoneural division of the echinoderm nervous system.