Fig. 1

Schematic diagram summarizing the essential cytology known for demosponges, as reported previously (e.g., [18,19,20,21,22,23]). Sponges are bi-epithelial organisms. The external epithelium consists of pavement-like cells (exopinacocytes, xp). The internal epithelium delimits internal aquiferous canals and also consists of pavement-like cells (endopinacocytes, xp). However, at some points, the aquiferous canals expand into chambers, in which the pavement-like cells are replaced by cuboidal (cells provided with distal microvilli and a flagellum), the choanocytes. The beating of the choanocyte flagellum creates an inflow of ambient seawater (dashed arrows), which enters the inhalant aquiferous canals (iac) through the pores (po) at the sponge “skin,” carrying particles (fp) in suspension to the choanocyte chambers (cc). The choanocytes retain and engulf microorganisms or other particles (fp) that arrive at the lumen of the chambers, where digesting food particles into digesting vesicles (dv) begins. The seawater that is cleared of particles flows out through the exhalant aquiferous canals (eac) and the oscule (os). Between the internal and external epithelia, there is a thick region—a sort of internal “tissue”—known as the mesohyl. It consists of a gel-like intercellular medium with abundant collagen fibrils (cf) in which several types of amoeboid cells wander around. Among others, there are the collencytes (which produce the collagen fibrils, cf), sclerocytes (sc; which build the spicules, sp), and the archeocytes (which serve as cellular defense system). Archeocytes are totipotent and can transdifferentiate into almost any other cell type. In the mesohyl, there are also free-living symbiotic microbes (sm) as well as cells hosting microbes. The bacteriocytes (ba) are cells that host symbiotic microbes (sm) in the intracellular environment (ie) of a large intracytoplasmic vesicle. A peculiar type of bacteriocyte is the pocket bacteriocyte (pb). It is believed to derive from an epithelial cell that attracts free-living microbes (am) from the ambient water to the sponge surface. Then, the cell leaves the epithelium to enter the mesohyl (red arrows), folding over itself to form an extracellular sack-like cavity, in which microbes are host in the extracellular environment (ee). Oocytes (oo) and brooded embryos (be) also develop in the mesohyl