Tambutté E., 1996. Calcification process of hermatypic scleractinians,
Stylophora pistillata. In situ growth in Mururoa. PhD from Nice Sophia
The aim of this work was to determine the cellular mechanisms involved
in the calcification processes of hermatypic scleractinians. The new biological
material used is clones of the coral Stylophora pistillata developed into
microcolonies whose skeleton is entirely covered by tissues, thus avoiding
the non specific adsorption of radioisotopes. In the protocol set up for
measuring calcification, we use 45Ca. One characteristics of this protocol
is the presence of an efflux phase at the end of the incubation period
which allows the emptying of an extracellular compartment, the coelenteron.
Thanks to the utilization of both the microcolonies and this new protocol,
we evidence four compartments involved in calcification processes. The
first compartment is extracellular and corresponds to the coelenteron.
It seems that the second compartment (7 nmol Ca2+. mg-1protein) which
corresponds to the whole tissues and has a long half-time (20 min) is
indirectly involved in calcium transport for calcification processes.
The third compartment which is at the origin of the calcium used for the
synthesis of the skeleton has a very short half-time (2 min) and we suggest
that this compartment corresponds to the calicoblastic epithelium. The
cells of this epithelium are sensitive to L-type calcium channels inhibitors.
The molecular characterization of a L-type calcium channel and the utilization
of specific antibodies have revealed the existence of such a channel on
both the oral and the calicoblastic ectoderms. Calcium transport in this
compartment is transcellular and coupled to cellular energy dependant
mechanisms, probably Ca2+ ATPases. The fourth compartment corresponding
to the skeleton is characterized by an unidirectional calcium flux of
975 pmol Ca2+. mg-1 protein. min-1. Thanks to the study of this compartment,
we have evidenced that calcification is linear along time.
We have demonstrated that organic matrix incorporation is coupled with
calcium incorporation into the skeleton. We suggest that the amount of
aspartic acid in the external medium is a limiting factor for calcification.
TBT, an antifouling agent, could inhibit the synthesis of the organic
matrix in the animal tissues and thus affect calcification.
Scanning and transmission electron microscope studies of the calicoblastic
epithelium and the skeleton have revealed the importance of anchoring
structures on the skeleton, the desmoidal processes.
Key-words : Scleractinian - Stylophora pistillata - Calcification -
Calcium - L-type channel - Desmoidal process - Organic matrix - Aerial