Studies on Hexactinellid Sponges. I. Histology of Rhabdocalyptus dawsoni (Lambe, 1873)

This paper presents results from the first comprehensive study of hexactinellid tissue organization by electron microscopy. It is confirmed that the trabecular tissue of Rhabdocalyptus dawsoni, which constitutes the bulk of the cellular material in the animal, is a syncytium. The dermal membrane and other similar membranes are specialized regions of the trabecular syncytium, as are thickened regions provisionally equated with the \`cord syncytia' of Reiswig (Coll. int. Cent. natn. Rech. Scient. no. 291, pp. 173-180 (1979)). Trabecular tissue contributes to the walls of the flagellated chambers and provides the processes that form Reiswig's secondary reticulum. It is confirmed that choanocytes are absent. The sponge has conventional \`collar bodies' (collar, flagellum and basal cytoplasm) but many collar bodies are syncytially interconnected via narrow \`stolons', and there are no nuclei in these complexes in the fully differentiated state. It is suggested that collar bodies are dehiscent, and are periodically replaced. A novel feature is the perforate septum, or junctional \`plug'. Plugs are not specialized portions of the cell membranes of adjacent cells. They are complex, disc-shaped structures, probably Golgi secretion products, which are inserted into syncytial bridges and appear to form a filter or partial barrier limiting translocation of materials between differentially specialized portions of the cytoplasm. In this respect they more closely resemble red algal pit connections than junctions found in animals. Gap junctions are absent in Rhabdocalyptus (and probably in all sponges) but a type of septate junction is described. Plugged junctions occur between elements of the trabecular syncytium and collar bodies, and between the latter and cells termed \`choanoblasts', which are probably derived from archaeocytes. A developmental sequence is proposed wherein the collar bodies and their interconnecting stolons are produced as outgrowths from choanoblasts, which may function singly or in syncytial groups during this phase. The cytoplasm is originally continuous throughout these systems, but plugging occurs progressively, leading to segregation of collar body complexes from their mother cells. Plugged junctions are seen between a variety of cells and the trabecular tissues in which they lie. These cells, whose characteristics are described, are archaeocytes, thesocytes, choanoblasts, granulated cells, spherulous cells and gametes. Sclerocytes, however, appear to lack specialized connections with surrounding tissues. As noted by Okada (1928), spicules are produced intracellularly in hexactinellids. Spiculation has not been studied in this investigation, and no details have been obtained on embryos or development. Nerves are absent. The system responsible for impulse conduction is almost certainly the trabecular syncytium. Impulses can probably cross plugged junctions, as pores with internal diameters of about 7 nm are seen in them. There is no reason to suppose that the tissues lining the openings in the body wall or the internal water passages are contractile. Tests with the dermal membrane show that its pores are not contractile. Regulation of water flow is therefore held to be a property of the sum total of the collar body flagella. Phagosomes occur both in collar bodies and in the trabecular syncytium. It is assumed that food particles can be taken up throughout the internal surfaces. Mucus nets span the internal lacunae in some places, but information is sketchy. Mucus strands interconnect the collar microvilli and may assist in particle capture. It is suggested that food breakdown products pass directly from collar bodies to choanoblasts and trabecular issues, crossing junctional plugs, essentially a \`symplastic' transport mechanism as found in plants. Archaeocytes are probably immobile and do not appear to be involved in digestion. External transport of nutrients via the mesolamella is probably of minor importance, and this densely collagenous material is probably not a pathway for cell migration. However, bacteria, presumably symbionts, do occur widely in the mesolamella. The paper concludes with a review of the phenomenon of syncytialization in plants and animals. Hexactinellids are considered in the same context and the features that set them apart from all other Porifera are listed.