Structural variability of collagen fibers in the calcareous axial rod of a sea pen

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Abstract
Previous studies revealed that the organic matrix of the skeletal rod of the sea pen, Veretillum cynomorium, contained about 50% collagenous protein. The present ultrastructural study, based upon conventional staining methods, shows the existence of an abundant, longitudinally arranged nonbanded and fibrillar material separated by a reticular matrix. After incubation with 3H-proline, labeling is specifically localized on the fibrillar material. Some fibers occasionally display a transverse striation with a period of 11 to 14 nm which can be associated with a chevron striation. Infrequently, some other fibers display a more distinct banding of 55 to 70 nm or even yield a checkerboard pattern. However, a majority of fibers remain without a regular structure comparable to the periodic striations observed in the collagen of other animals. After treatment with 1% PTA in 70% ethanol, all the fibers show a clear banding of 14 nm and some of them possess two types of striations. The same result is obtained on fibers mechanically dissociated and negatively stained. As these methods show a periodic banding pattern on all the fibers, it is likely that all the fibers (striated or not) observed after routine electron microscopy correspond to collagen material. This collagen appears to be both polymorphic and completely new in comparison to that which is characteristic of the mesoglea. The polymorphic aspect is compared to that obtained from vertebrate collagens.