An ultrastructural study on spicule formation in the pennatulid colony Renilla reniformis

Examination of the coenenchyme tissues of Renilla reniformis revealed two regions of crystal formation: the endoderm containing small oval deposits of unknown composition and the mesoglea containing larger elongated spicules composed of calcite. Spicule formation takes place intracellularly in scleroblasts and may be explained by the following sequential processes: an organic matrix consisting of a homogeneous ground substance and fibers is formed in a large vacuole. Calcite needles 0.4 μ in diameter develop in close association with matrix fibers and vesicles, and grow to form the central core of the spicule. Large electron dense bodies dominate the scleroblast cytoplasm during these early stages of growth. In a later stage, smaller needles 0.2 μ in diameter develop surrounding the core to form the distal lobes of the spicule. ‘Lollipop’-shaped vesicles containing fibers appear in the scleroblast cytoplasm at the onset of this stage. This material is released at the calcification front and presumably is incorporated into the spicule as an organic matrix of crystals in the distal lobes.