Schematic drawing of the major proteins identified within calicoblastic cells. Schematic drawing of the major proteins identified within calicoblastic cells. Calicoblastic cells form an epithelium (calicodermis) whose apical membrane (AM) is attached to the coral skeleton by desmocytes via a hemidesmosome adhesion complex. Their basal pole is close to the mesoglea, a sheet of extracellular matrix proteins. Various transport proteins within calicoblastic cells have been identified or suggested on the basis of pharmacological evidence: ATPases (Na+ pump and Ca2+ pump), antiporters (Na+/Ca2+ antiporter and bicarbonate carrier) and channel proteins (Ca2+ channel). The presence of numerous mitochondria within calicoblastic cells both energizes ion transport and supplies metabolic CO2 as a source of carbon for calcification. The presence of carbonic anhydrase within the extracellular calcifying medium (between the calicodermis and skeleton) facilitates the chemical equilibrium between the different carbon species. In addition, calicoblastic cells synthesize and secrete via vesicles a mixture of macromolecules, called the organic matrix (OM), which acts as an organic framework. Calcium ions may reach this medium both by a paracellular pathway through septate junctions and by transcellular transport, aided by the large surface area of the basal lateral membrane (BLM) of the calicoblastic cells. The symbiont cell may enhance calcification either by altering the physicochemical composition of extracellular fluids by either absorbing CO2 or releasing O2 (1) or by supplying organic compounds to the calicoblastic cells, such as precursors for skeletal organic matrix synthesis or high-energy molecules (2) (see text for more details). Simon K. Davy et al. Microbiol. Mol. Biol. Rev. 2012; doi:10.1128/MMBR.05014-11