intracellularly and is completed in the extracellular space. There, the growth of the
spicules is controlled by an ordered arrangement of cells, which – according to his
illustrations – surround the spicules. The ordered arrangement of the cells can even
be traced after dissolution with hydrofluoric acid (HF) (M€ uller et al. 2007c).
To unravel the organizing principles underlying spicule formation, specific
antibodies were applied. The data revealed in S. domuncula that besides silicatein
and galectin, collagen fibers surround the spicules in an ordered pattern (Schr€ oder
et al. 2006; Eckert et al. 2006). The existence of collagen in extracts from spicules
had been recognized already by NaDodSO 4 /PAGE. Careful time-kinetics recording
of the proteins which are released by controlled HF dissolution revealed that prior
to the appearance of the axial filament, a proteinaceous coat around the axial
filament can be identified. Later, by high-resolution scanning electron microscopic
(SEM) analysis, it could be visualized that in the extra-spicular space an ordered
network of collagen fibrils surround the spicules. At the tips of the spicules, where
the knobs of the tylostyles are formed, a mesh of collagen fibers is seen, suggesting
that during spiculogenesis, the enzyme silicatein (which mediates biosilica deposition) is matrix-guided first by galectin (Schr€ oder et al. 2006) and subsequently by
collagen (Eckert et al. 2006). Future experiments will address the question for the
mechanism and the specificity of interactions of these three molecules and will try
to elucidate the controlling genetic machinery switched on during spicule formation. It should be noted here that on the surfaces of the spicules, very frequently
clusters of cell fragments, surrounded by collagen, are seen; these structures are
very reminiscent of the cell-like structures reported by Maas (1901).
9.5 Morphology and Synthesis of Spicules in Hexactinellids
Among the large hexactinellid sponges, Monorhaphis chuni is a giant (Fig. 9.5g).
Monorhaphis chuni (Schulze 1904) is distributed in the Indo-West Pacific region
and found in depths between 516 and 1,920 m. Monorhaphis inhabits muddy
substrata and is fixed there by a single giant basal spicule. Young specimens are
thought to comprise a continuous body, as has been sketched by Schulze (1904)
(Fig. 9.5d–f). The cylindrical/oval body of Monorhaphis is interspersed with many
atrial openings which are located along one side (Fig. 9.5f); the diameter of the
body can reach in larger specimens up to 12 cm. During growth, the specimens
elongate together with an extension of their giant basal spicules (Figs. 9.5e [a–c]
and 9.7a). This growth regime has been deduced from the differently sized
fragments of Monorhaphis collected during the Valdivia Expedition (Schulze
1904), and during the expeditions organized by the Institute of Oceanography
(Qingdao). Older specimens apparently lose the basal portions of their soft body
and expose the bare giant basal spicule (Fig. 9.5g).
Like all other hexactinellids, Monorhaphis possesses microscleres (<0.1 mm)
and megascleres (0.2–30 mm–3 m). Within the oblong, laterally compressed body
264
W.E.G. M€ uller et al.
spicules is controlled by an ordered arrangement of cells, which – according to his
illustrations – surround the spicules. The ordered arrangement of the cells can even
be traced after dissolution with hydrofluoric acid (HF) (M€ uller et al. 2007c).
To unravel the organizing principles underlying spicule formation, specific
antibodies were applied. The data revealed in S. domuncula that besides silicatein
and galectin, collagen fibers surround the spicules in an ordered pattern (Schr€ oder
et al. 2006; Eckert et al. 2006). The existence of collagen in extracts from spicules
had been recognized already by NaDodSO 4 /PAGE. Careful time-kinetics recording
of the proteins which are released by controlled HF dissolution revealed that prior
to the appearance of the axial filament, a proteinaceous coat around the axial
filament can be identified. Later, by high-resolution scanning electron microscopic
(SEM) analysis, it could be visualized that in the extra-spicular space an ordered
network of collagen fibrils surround the spicules. At the tips of the spicules, where
the knobs of the tylostyles are formed, a mesh of collagen fibers is seen, suggesting
that during spiculogenesis, the enzyme silicatein (which mediates biosilica deposition) is matrix-guided first by galectin (Schr€ oder et al. 2006) and subsequently by
collagen (Eckert et al. 2006). Future experiments will address the question for the
mechanism and the specificity of interactions of these three molecules and will try
to elucidate the controlling genetic machinery switched on during spicule formation. It should be noted here that on the surfaces of the spicules, very frequently
clusters of cell fragments, surrounded by collagen, are seen; these structures are
very reminiscent of the cell-like structures reported by Maas (1901).
9.5 Morphology and Synthesis of Spicules in Hexactinellids
Among the large hexactinellid sponges, Monorhaphis chuni is a giant (Fig. 9.5g).
Monorhaphis chuni (Schulze 1904) is distributed in the Indo-West Pacific region
and found in depths between 516 and 1,920 m. Monorhaphis inhabits muddy
substrata and is fixed there by a single giant basal spicule. Young specimens are
thought to comprise a continuous body, as has been sketched by Schulze (1904)
(Fig. 9.5d–f). The cylindrical/oval body of Monorhaphis is interspersed with many
atrial openings which are located along one side (Fig. 9.5f); the diameter of the
body can reach in larger specimens up to 12 cm. During growth, the specimens
elongate together with an extension of their giant basal spicules (Figs. 9.5e [a–c]
and 9.7a). This growth regime has been deduced from the differently sized
fragments of Monorhaphis collected during the Valdivia Expedition (Schulze
1904), and during the expeditions organized by the Institute of Oceanography
(Qingdao). Older specimens apparently lose the basal portions of their soft body
and expose the bare giant basal spicule (Fig. 9.5g).
Like all other hexactinellids, Monorhaphis possesses microscleres (<0.1 mm)
and megascleres (0.2–30 mm–3 m). Within the oblong, laterally compressed body
264
W.E.G. M€ uller et al.