Collagenous tissues which have different passive mechanical properties in response to different environmental and mechanical stimuli are called mutable collagenous tissues (MCTs). The rapid changes of their mechanical properties are nervously mediated. It is assumed that the tissue properties are regulated by certain molecules which secretion is controlled by nerve terminals connected to the secretory cells. The secretory cells associated with MCTs are called juxtaligamental cells. They are characterized “by the presence in their cytoplasm of numerous electron-dense, membrane-bound granules.” There is no known MCT that lacks these cells, whereas they are absent from the few definitely non-mutable collagenous structures examined. It has always been suggested that the tube feet of sea urchin and sea star are MCTs, but proof was missing until 2005. In 2005, Romana Santos et al. published a paper in which they showed that the tube feet tissue indeed (I) contains juxtaligamental cells and (II) shows differences in stiffness/elasticity in response to environmental stimuli. The mechanical properties of the tube feet from both species were influenced by the environmental calcium concentration. They both became more flexible in an low calcium environment. However, stiffness response to cell-disrupting treatments was more prominent in the sea urchin compared to the sea star. This may be based on their different function (and structure). The feet of the sea urchin are flexible. Their task is mainly “pulling” the sea urchin to the ground and even movement is based on a pulling process. Therefore, cell stress (e.g. because of waves pushing the sea urchin) induces the secretion of molecules which increase the stiffness of the feet which may be helpful for an energy-sparing maintenance of position. Sea stars have a different method of movement, compared to the sea urchin. Instead of pulling, their feet are like little columns which lift the body up. Therefore, they are already quite stiff and cell stress can not significantly increase this stiffness further. "The tube feet of sea urchins and sea stars contain functionally different mutable collagenous tissues."
Romana Santos, et al. Journal of experimental biology 208.12 (2005): 2277-2288.
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