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.     

Comparative study of the diaphragm (gular membrane) in Terebelliformia (Polychaeta, Annelida)

The structure and location of the diaphragm (gular membrane) was studied in five families of Terebelliformia: Terebellidae, Trichobranchidae, Pectinariidae, Ampharetidae and Alvinellidae, using dissections, histology, and scanning and transmission electron microscopy. Position, shape, and structure of the diaphragm differ in these taxa. In Terebellidae and Pectinariidae the diaphragm is straight. In Trichobranchidae, Ampharetidae and Alvinellidae it is funnel-shaped. Diaphragm possesses two contractile sacs in Terebellidae and Pectinariidae, one in Alvinellidae and none at all in Trichobranchidae. The relative size and form of the sacs varied. Representatives of the family Ampharetidae have one or two sacs or none at all. Four kinds of the diaphragm can be distinguished: strait with two sacs, funnel-shaped with two sacs, funnel-shaped with one sac, funnel-shaped without sacs. In some Alvinellidae, the diaphragm is fenestrated, while in all other taxa it is continuous. The wall of the sacs is more muscular than the wall of the remaining diaphragm. The diaphragm is attached to the body wall at different levels: between the third and fourth segments in pectinariids or between the fourth and fifth in terebellids, ampharetids, alvinellids and trichobranchids. In most cases, the diaphragm contains two coelothelial layers with a well-developed extra-cellular matrix in between, and one or two muscle layers. The maximum development of the muscle fibres occurs in Terebellidae; probably related to the length of buccal tentacles. Significance of morphological and ultrastructural peculiarities of the diaphragm is discussed.     

Early expressed genes showing a dichotomous developing pattern in the lancelet embryo

Lancelets (amphioxus), although showing the most similar anatomical features to vertebrates, never develop a vertebrate-like head but rather several structures specific to this animal. The lancelet anatomical specificity seems to be traceable to early developmental stages, such as the vertebrate dorsal and anterior-posterior determinations. The BMP and Wnt proteins play important roles in establishing the early basis of the dorsal structures and the head in vertebrates. The early behavior of BMP and Wnt may be also related to the specific body structures of lancelets. The expression patterns of a dpp-related gene, Bbbmp2/4, and two wnt-related genes, Bbwnt7 and Bbwnt8, have been studied in comparison with those of brachyury and Hnf-3beta class genes. The temporal expression patterns of these genes are similar to those of vertebrates; Bbbmp2/4 and Bbwnt8 are first expressed in the invaginating primitive gut and the equatorial region, respectively, at the initial gastrula stage. However, spatial expression pattern of Bbbmp2/4 differs significantly from the vertebrate cognates. It is expressed in the mid-dorsal inner layer of gastrulae and widely in the anterior region, in which vertebrates block BMP signaling. The present study suggests that the lancelet embryo may have two distinct developmental domains from the gastrula stage, the domains of which coincide later with the lateral diverticular and the somitocoelomic regions. The embryonic origin of the anterior-specific structures in lancelets corresponds to the anterior domain where Bbbmp2/4 is continuously expressed.     

Microscopic Anatomy and Ultrastructure of Nephridium in the Sipunculan Thysanocardia nigra Ikeda, 1904 from the Sea of Japan

The microscopic anatomy and ultrastructure of nephridium have been studied in the sipunculan Thysanocardia nigra Ikeda, 1904 (Sipuncula, Sipunculidea) from the Sea of Japan using histological and electron microscopic techniques (SEM and TEM). This paper describes ultrastructural features of nephridial epithelium, muscle grid, and coelomic epithelium on the surface of the nephridium, the area of the ciliary funnel, and the tongue. Several types of cells were distinguished in the excretory tube of the nephridium: (1) a columnar epithelium of the excretory bunches; (2) a cubical or flattened epithelium of flask-shaped infoldings; and (3) granulocytes that migrate from the coelom to the extracellular matrix of the nephridial wall. The system of podocytes and multiciliary cells were described in the nephridial coelothelium. Two types of secretion of nephridial epithelium have been discovered: a merocrine secretion of columnar cells and an apocrine secretion of cells of the flask-shaped infoldings. Using ultrastructural data, two zones of filtration through the wall of excretory tube have been found, namely (1) the tips of flask-shaped infoldings (via the extracellular matrix and microvillary canals between the epithelial cells) and (2) areas between the flask-shaped infoldings (via the contacts of podocytes, extracellular matrix, and the basal labyrinth of the columnar cells). Unlike previously studied representatives of the genus Phascolosoma, no coelomic epithelium is present on the tips of the flask-shaped infoldings in Th. nigra. This data on the anatomy and histology allow us to conclude that the funnel only works like a gonoduct.     

Ultrastructure of Microvillar Coelomocytes from the Trunk Coelom of Thysanocardia nigra Ikeda, 1904 (Sipuncula) from the Sea of Japan

This research is part of a study on the ultrastructure of coelomocytes and cellular complexes from the body cavity of sipunculans. New free-swimming elements called microvillar cells in the trunk coelom of Thysanocardia nigra Ikeda, 1904 are examined using transmission electron microscopy. The cell harbors a giant vesicle filled with a fibrous matrix and rosettes of minute osmiophilous granules. The nucleus is peripheral, and a few cell organelles are situated between the cell membrane and the vesicular membrane. The cell membrane bears numerous microvilli with enlarged apical points. Numerous small microvillar vesicles swimming in the coelomic fluid separate from the microvillar cells. The functional morphology of coelomocytes and cellular complexes is discussed.Original Russian Text Copyright © 2005 by Biologiya Morya, Maiorova, Adrianov.     

Ultrastructure of the body cavities in Phylactolaemata (Bryozoa)

Only species belonging to the bryozoan subtaxon Phylactolaemata possess an epistome. To test whether there is a specific coelomic cavity inside the epistome, Fredericella sultana, Plumatella emarginata, and Lophopus crystallinus were studied on the ultrastructural level. In F. sultana and P. emarginata, the epistome contains a coelomic cavity. The cavity is confluent with the trunk coelom and lined by peritoneal and myoepithelial cells. The lophophore coelom extends into the tentacles and is connected to the trunk coelom by two weakly ciliated coelomic ducts on either side of the rectum. The lophophore coelom passes the epistome coelom on its anterior side. This region has traditionally been called the forked canal and hypothesized to represent the site of excretion. L. crystallinus lacks an epistome. It has a simple ciliated field where an epistome is situated in the other species. Underneath this field, the forked canal is situated. Compared with the other species, it is pronounced and exhibits a dense ciliation. Despite the occurrence of podocytes, which are prerequisites for a selected fluid transfer, there is no indication for an excretory function of the forked canal, especially as no excretory porus was found. J. Morphol. 2009. © 2008 Wiley‐Liss, Inc.     

Microscopic Anatomy and Ultrastructure of a Polian Vessel in the Sipunculan Thysanocardia nigra Ikeda, 1904 from the Sea of Japan

The microscopic anatomy and ultrastructure of a Polian vessel have been studied in the sipunculan Thysanocardia nigra Ikeda, 1904 from the Sea of Japan using the methods of histology and electron microscopy. We describe ultrastructural features of the inner and outer coelothelium, which is constructed of podocytes and multiciliary cells. Between the processes of the podocyte cells, we found double diaphragms that are considered characteristic macromolecular filters. We conclude from an analysis of the ultrastructural features of the vessel wall that coelomic fluid may be filtered from the tentacular coelom to the trunk coelom via the wall of the Polian vessel.     

虎耳草属异叶虎耳草复合群的数量分类学研究

山羊臭组（Saxifraga sect. Ciliatae Haworth）隶属于虎耳草属（Saxifraga Tourn. ex L.）,具有极高的物种丰富度,但山羊臭组内部的系统发育关系一直都未能很好解决。唐古拉亚组（S. subsect. Hirculoideae Engl.）是山羊臭组中最大的亚组,主要分布于青藏高原及其周边地区。根据形态特征,可将唐古拉亚组划分为3个复合群,其中叶背边缘有突起叶脉的物种被归入异叶虎耳草复合群（S. diversifolia complex）。本研究通过标本查阅,选取异叶虎耳草复合群24个物种、2变种共657份标本,对其24个定性性状信息进行主成分分析和聚类分析;获取443条地理分布信息,构建复合群的分布式样。特征值大于1的前7个主成分的累计贡献率较低,仅为67.748%。“茎生叶是否具柄”“中下部茎生叶形态”“花序类型”“中下部茎生叶大小”“萼片脉纹于先端汇合”“叶基心形”等性状对前3个主成分的贡献值大,可作为异叶虎耳草复合群物种分类与鉴定的关键性状。基于形态聚类结果和地理分布式样,可将异叶虎耳草复合群划分为3个分支：喜马拉雅山南坡分支、环四川盆地山区分支以及横断山分支（包含一个广布种S. egregia）。主成分分析结果支持将异叶虎耳草复合群划分为3个分支。