Electron microscope examination of fragmented elastic tissue obtained from fish swim bladder, bovine ligamentum nuchae, and aortas of various mammals, including man, reveals characteristically formed fibers and much amorphous material. Boiling in dilute acid destroys the associated collagen but does not obviously alter the elastic tissue.

Digestion in crystalline trypsin of either boiled or unheated tissue from any of the above-mentioned sources causes the release of thin threads ranging in length from 0.1 µ to many microns. A large proportion of these threads are evenly and tightly coiled double helices formed from at least two interlacing filaments and measuring about 120 A in width. The distance between coils ranges from about 470 to 590 A. The individual smooth filaments, many of which are present in parallel pairs, measure approximately 70 A in width.

Raising the pH of a neutral suspension of threads from ligamentum nuchae lowers the ratio of helical threads to uncoiled filaments, whereas lowering the pH with acetic acid results in clumping of threads with complete loss of identity at about pH 3.6.

Threads and filaments obtained from all sources studied were destroyed in the temperature range 75–85°C. at pH 7.

It is concluded that the elastic fiber is a two component system composed of bundles of trypsin-resistant threads of characteristic form and size plus a trypsin-sensitive, heat-resistant "amorphous" binding matrix.

The possible relationship of this structure to the elastic properties of the tissue is discussed.

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