Hypercalcified sponges, endowed with a calcium carbonate basal skeleton
in addition to their spicules, form one of the most basal metazoan group
engaged in extensive biomineralization. The Mediterranean species
Petrobiona massiliana was used to investigate biological controls
exerted on the biomineralization of its basal skeleton. Scanning and
transmission electron microscopy (SEM, TEM) confirmed that
basopinacocytes form a discontinuous layer of flattened cells covering
the skeleton and display ultrastructural features attesting intense
secretory activity. The production of a highly structured fibrillar
organic matrix framework by basopinacocytes toward the growing skeleton
was highlighted both by potassium pyroantimonate and ruthenium red
protocols, the latter further suggesting the presence of sulfated
glycosaminoglycans in the matrix. Furthermore organic material
incorporated into the basal skeleton was shown by SEM and TEM at
different structural levels while its response to alcian blue and
acridine orange staining might suggest a similar acidic and sulfated
chemical composition in light microscopy. Potassium pyroantimonate
revealed in TEM and energy electron loss spectroscopy (EELS) analysis,
heavy linear precipitates 100-300 nm wide containing Ca2+ and Mg2+ ions,
either along the basal cell membrane of basopinacocytes located toward
the decalcified basal skeleton or around decalcified spicules in the
mesohyl. Based on the results of the previous mineralogical
characterization and the present work, an hypothetical model of
biomineralization is proposed for P. massiliana: basopinacocytes would
produce an extracellular organic framework that might guide the
assemblage of submicronic amorphous Ca- and Mg-bearing grains into
higher structural units. (c) 2012 Elsevier Inc. All rights reserved.