Possible sites of ultrafiltration in Tubifex tubifex Müller (annelida, oligochaeta)

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.

Electron microscopic study of possible sites of ultrafiltration in Lumbricus terrestris (Annelida, Oligochaeta)

Electron microscopic investigations of blood vessels of Lumbricus terrestris (L. terrestris) were conducted to show sites of filtration such as podocytes or irregular fenestrations. The endothelia of the blood vessels consist of myoendothelial cells, chloragocytes and podocytes. The podocytes form large archs over a considerable area of the vessels. Numerous small pedicels are presented on the lumen side and the gaps between adjacent pedicels are bridged by slit membranes. The podocytes are restricted to the front part of the ventral vessel. They are regarded as the structural basis for ultrafiltration. Additionally to the filtration site between ventral vessel and coelomic cavity, a second putative filtration site was found in the front part of the body between intestinal blood sinus and coelomic cavity. The endothelium of the sinus is formed by myoendothelial cells, pedicels and chloragocytes with footlike processes. In such areas the basement membrane is the only continous layer between blood vessel and coelomic cavity. The basement membrane of the blood vessels of L. terrestris is a characteristic association of three layers of different composition and thickness. Possible filtration sites in form of podocytes and irregular fenestrations could be localized at the border between the blood compartment and the coelomic compartment. Presumably the primary urine is formed by ultrafiltration of blood.

The syncytial nature and phagocytic activity of the branchial podocytes in the grass shrimp, Palaemonetes pugio

The morphology of the branchial podocytes in the grass shrimp, Palaemonetes pugio, was investigated in relation to the molt cycle. The podocytes are located in the efferent hemolymph channels in the gill axis, and possess a specialized plasmalemma consisting of interdigitating pedicel processes which are bridged by thin diaphragms. The topography of the plasmalemmal surface suggests that these cells, like similar cells in other arthropods, function in the ultrafiltration of micro- and macromolecular substances from the hemolymph. Additionally, the branchial podocytes exhibit phagocytic activity. This activity, though evident during the pre-molt period, is most prominent during the early post-molt period. Among the cell types subjected to phagocytosis by podocytes are the secretory cells of the tricellular and rosette-type dermal glands and the epithelial cells of the gill axis. During the late pre-molt and early post-molt periods, the podocytes often appear as syncytia, containing as many as four nuclei. The exact interrelationships between phagocytosis and syncytial formation remain to be ascertained. These aspects and the possible ambulatory abilities of the branchial podocytes are discussed.