Polypide anatomy of hornerid bryozoans (Stenolaemata: Cyclostomatida)

To be a transit link: Similarity in the structure of colonial system of integration and communication pores in autozooids and avicularia of Terminoflustra membranaceotruncata (Bryozoa: Cheilostomata)

Bryozoan colonies consist of zooids, which can differ in structure and function. Most heteromorphic zooids are unable to feed and autozooids supply them with nutrients. The structure of the tissues providing nutrient transfer is poorly investigated. Here, I present a detailed description of the colonial system of integration (CSI) and communication pores in autozooids and avicularia of the cheilosome bryozoan Terminoflustra membranaceotruncata. The CSI is the nutrient transport and distribution system in the colony. In both autozooids and avicularia it consists of a single cell type, that is, elongated cells, and has a variable branching pattern, except for the presence of a peripheral cord. The general similarity in the CSI structure in avicularia and autozooids is probably due to the interzooidal type of the avicularium. Interzooidal avicularia are likely to consume only a part of the nutrients delivered to them by the CSI, and they transit the rest of the nutrients further. The variability and irregularity of branching pattern of the CSI may be explained by the presence of single communication pores and their varying number. The structure of communication pores is similar regardless of their location (in the transverse or lateral wall) and the type of zooid in contact. Rosette complexes include a cincture cell, a few special cells, and a few limiting cells. Along each zooidal wall, there are communication pores with both unidirectional and bidirectional polarity of special cells. However, the total number of nucleus-containing lobes of special cells is approximately the same on each side of any zooidal wall. Supposing the polarity of special cells reflects the direction of nutrient transport, the pattern of special cells polarity is probably related to the need for bidirectional transport through each zooidal wall. The possibility for such transport is important in large perennial colonies with wide zones of autozooids undergoing polypide degeneration.

Colonial system of integration and communication pores in a polymorphic bryozoan Dendrobeania fruticosa (Bryozoa: Cheilostomata)

Bryozoan colonies are composed of zooids, which can differ in structure and function. Autozooids supply heteromorphic zooids with nutrients, which are usually unable to feed. To date, the ultrastructure of the tissues providing nutrient transfer is almost unexplored. Here, we present a detailed description of the colonial system of integration (CSI) and the different types of pore plates in Dendrobeania fruticosa. All cells of the CSI are joined by tight junctions that isolate its lumen. The lumen of the CSI is not a single structure, but a dense network of small interstices filled with a heterogeneous matrix. In autozooids, the CSI is composed of two types of cells: elongated and stellate. Elongated cells form the central part of the CSI, including two main longitudinal cords and several main branches to the gut and pore plates. Stellate cells compose the peripheral part of the CSI, which is a delicate mesh starting from the central part and reaching various structures of autozooids. Autozooids have two tiny muscular funiculi, which start from the caecum apex and run to the basal wall. Each funiculus includes a central cord of extracellular matrix and two longitudinal muscle cells; together they are enveloped with a layer of cells. The rosette complexes of all types of pore plates in D. fruticosa display a similar cellular composition: a cincture cell and a few special cells; limiting cells are absent. Special cells have bidirectional polarity in interautozooidal and avicularian pore plates. This is probably due to the need for bidirectional transport of nutrients during degeneration-regeneration cycles. Cincture cells and epidermal cells of pore plates contain microtubules and inclusions resembling dense-cored vesicles, which are typical of neurons. Probably, cincture cells are involved in the signal transduction from one zooid to another and can be a part of the colony-wide nervous system.

The epithelial layers of the body wall in hornerid bryozoans (Stenolaemata: Cyclostomatida)

Bryozoans are small colonial coelomates. They can be conceptualised as “origami‐like” animals, composed of three complexly folded epithelial layers: epidermis of the zooidal/colonial body wall, gut epithelium and coelothelium. We investigated the general microanatomy and ultrastructure of the hornerid (Cyclostomatatida) body wall and polypide in four taxa, including three species of Hornera and one species belonging to an undescribed genus. We describe epithelia and their associated structures (e.g., ECM, cuticle) across all portions of the hornerid body wall, including the terminal membrane, vestibular wall, atrial sphincter, membranous sac and polypide–skeletal attachments. The classic coelomate body wall composition (epidermis–ECM–coelothelium) is only present in an unmodified form in the tentacle sheath. Deeper within a zooid it is retained exclusively in the attachment zones of the membranous sac: [skeleton]–tendon cell–ECM–coelothelium. A typical invertebrate pattern of epithelial organisation is a single, continuous sheet of polarised cells, connected by belt desmosomes and septate junctions, and resting on a collagenous extracellular matrix. Although previous studies demonstrated that polypide‐specific epithelia of Horneridae follow this model, here we show that the body wall may show significant deviations. Cell layers can lose the basement membrane and/or continuity of cell cover and cell contacts. Moreover, in portions of the body wall, the cell layer appears to be missing altogether; the zooidal orifice is covered by a thin naked cuticle largely devoid of underlying cells. Since epithelium is a two‐way barrier against entry and loss of materials, it is unclear how hornerids avoid substance loss, while maintaining intracolonial metabolite transport with imperfect, sometimes incomplete, cell layers along large portions of their outer body surface.