The results of this study on the structure and composition of the mesogloea of the sea anemone Metridium senile have shown the following about this tissue.

1. Mesogloea can be described as a two-phase system of collagen fibres embedded in a matrix.

2. Polarized-light microscopy revealed two distinct layers in the mesogloea; one characterized by crossed-helices of collagen fibres with an unstressed fibre angle of 40-45°, and the other characterized by densely packed radial and circumferential collagen fibres.

3. X-ray diffraction evidence indicated that the collagen fibres align with the stress axis when the material is stretched.

4. Mesogloea is a highly hydrated material containing roughly 86% water. Salt-free, dry collagen makes up about 6·9% of the wet weight, and salt-free, dry matrix protein and polysaccharide makes up another 2%. The hydrated matrix complex makes up about 84% of the total wet weight and was found to have a concentration of about 2·4% (grams dry matrix per 100 g water).

5. The structural similarities of mesogloeal collagen and vertebrate collagens were demonstrated in a number of experiments. On the basis of the structural similarities mesogloeal collagen was assumed to be mechanically similar to vertebrate collagens.

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