60
2. ENVIRONMENTALLY DRIVEN PLASTICITY
seawater, the milieu in which most of the sponges live, is much higher for
calcium ions (1O mM) than for silicon (25JIM). One possible reason why the
Demospongiae use the more energy-consuming pathway, the formation of
spicules from silica, can be seen in the occurrence of a large concentration
of polyphosphate (1O gig wet weight, Lorenz et al. 1995). Polyphosphate is
known to chelate calcium and therefore counteracts the precipitation of calcium deposits. The origin of the polyphosphate in sponges is unclear; since
Demospongiae harbor large amounts of (symbiotic) bacteria (Althoff et al.
1998)while Calcarea usually do not, it appears that the bacteria produce this
polymer.
Based on the molecular biological/biochemical data available, it appears
likely that the formation of the skeleton in sponges proceeds according to the
following steps which are supported experimentally (Fig. 2.37). First, dissociated sponge cells form primmorphs (Fig. 2.37a and b), assemblies composed
of dividing cells (Muller et al. 1999b), via an already highly complex interaction of cell surface receptors and their ligands, which allow a controlled
cell-cell and cell-matrix interaction (Muller 1998).These primmorphs do not
contain spicules. In the second step, the enzyme present in the axial filament, silicatein (Fig. 2.37C), recently also cloned (Shimzu et al. 1998, Krasko
et al. 2000), is activated and promotes the silicification of the sponge spicules
(Fig. 2.37d). In this phase polyphosphate suppresses the formation of calcium
precipitates and likely attaches non -covalently to the silica spicules. Although
sponges are rich in polyphosphate, they also contain large quantities of its
catabolic enzymes; exo-polyphosphatases and alkaline phosphatases. Therefore, it can be hypothesized that the arrangement of spicules in the skeleton
(Fig. 2.37e), i.e. their pattern formation, is (partially) directed by a gradient
of these catabolic enzymes : Dissolution of polyphosphate by these enzymes
removes the attached polyphosphate chains from the silica spicules and antagonizes their growing. This view is partially supported by the finding that
the polyphosphate content in gemrnules , the asexual propagative bodies, and
in the following hatching stages from the gemmules is much higher than in
an "adult" specimen (Imsiecke et al. 1996) which contains the sophisticated
spicule skeleton. No enzymic data on the formation of spicules in Calcarea
are known.
Another interesting result came from molecular biological and modern
cell biological studies. Sponge cells have the capacity for indefinite proliferation due to the presence of high telomerase activity. However, since all
sponge species have a characteristic body plan , it was compelling to postulate a developmental mechanism which is based on a balance between
an (almost) unlimited production of immortal cells, a controlled elimination of cells by programmed cell death or apoptosis, and the existence of
telomerase-negative, differentially developed somatic cells. For protostomian and deuterostomian metazoan animals it has been established that
only immortal cells, those which are present in the reproductive lineage or
which are cancer cells, show high levels of telomerase activity that maintains the telomeres. In contrast to these immortal cells, the somatic, mortal
cells lack telornerase activity resulting in a shortening of telomeres with each
cell division. In consequence, telomerase-negative cells show a limited number of cell divisions that range from 50 to 100, the Hayflick limit. If the cells
reach the Hayflick limit, which is also called mortality phase I, the critical
telomere loss on chromosomes initiates a signaling event which results in
2. ENVIRONMENTALLY DRIVEN PLASTICITY
seawater, the milieu in which most of the sponges live, is much higher for
calcium ions (1O mM) than for silicon (25JIM). One possible reason why the
Demospongiae use the more energy-consuming pathway, the formation of
spicules from silica, can be seen in the occurrence of a large concentration
of polyphosphate (1O gig wet weight, Lorenz et al. 1995). Polyphosphate is
known to chelate calcium and therefore counteracts the precipitation of calcium deposits. The origin of the polyphosphate in sponges is unclear; since
Demospongiae harbor large amounts of (symbiotic) bacteria (Althoff et al.
1998)while Calcarea usually do not, it appears that the bacteria produce this
polymer.
Based on the molecular biological/biochemical data available, it appears
likely that the formation of the skeleton in sponges proceeds according to the
following steps which are supported experimentally (Fig. 2.37). First, dissociated sponge cells form primmorphs (Fig. 2.37a and b), assemblies composed
of dividing cells (Muller et al. 1999b), via an already highly complex interaction of cell surface receptors and their ligands, which allow a controlled
cell-cell and cell-matrix interaction (Muller 1998).These primmorphs do not
contain spicules. In the second step, the enzyme present in the axial filament, silicatein (Fig. 2.37C), recently also cloned (Shimzu et al. 1998, Krasko
et al. 2000), is activated and promotes the silicification of the sponge spicules
(Fig. 2.37d). In this phase polyphosphate suppresses the formation of calcium
precipitates and likely attaches non -covalently to the silica spicules. Although
sponges are rich in polyphosphate, they also contain large quantities of its
catabolic enzymes; exo-polyphosphatases and alkaline phosphatases. Therefore, it can be hypothesized that the arrangement of spicules in the skeleton
(Fig. 2.37e), i.e. their pattern formation, is (partially) directed by a gradient
of these catabolic enzymes : Dissolution of polyphosphate by these enzymes
removes the attached polyphosphate chains from the silica spicules and antagonizes their growing. This view is partially supported by the finding that
the polyphosphate content in gemrnules , the asexual propagative bodies, and
in the following hatching stages from the gemmules is much higher than in
an "adult" specimen (Imsiecke et al. 1996) which contains the sophisticated
spicule skeleton. No enzymic data on the formation of spicules in Calcarea
are known.
Another interesting result came from molecular biological and modern
cell biological studies. Sponge cells have the capacity for indefinite proliferation due to the presence of high telomerase activity. However, since all
sponge species have a characteristic body plan , it was compelling to postulate a developmental mechanism which is based on a balance between
an (almost) unlimited production of immortal cells, a controlled elimination of cells by programmed cell death or apoptosis, and the existence of
telomerase-negative, differentially developed somatic cells. For protostomian and deuterostomian metazoan animals it has been established that
only immortal cells, those which are present in the reproductive lineage or
which are cancer cells, show high levels of telomerase activity that maintains the telomeres. In contrast to these immortal cells, the somatic, mortal
cells lack telornerase activity resulting in a shortening of telomeres with each
cell division. In consequence, telomerase-negative cells show a limited number of cell divisions that range from 50 to 100, the Hayflick limit. If the cells
reach the Hayflick limit, which is also called mortality phase I, the critical
telomere loss on chromosomes initiates a signaling event which results in