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.