This is a question for which there is no satisfactory answer, because we are only beginning to understand the physical and anatomical basis of transparency in living tissue.
Although the physical and anatomical bases for some transparent tissue (e.g. the cornea and lens in the eye) are better understood than others, the situation in the eye is unique in the sense that the tissues are highly modified for transparency and these modifications (e.g. complete absence of a circulatory system) are not applicable to muscular tissue.
Furthermore, many of the primary modifications for transparency are ultrastructural and can only be seen with an electron microscope. For biological tissue to become transparent, the primary mechanism is to reduce the amount of light being scattered as it passes through it: the less light scattered by the tissue, the more light will be transmitted through it and the more it becomes transparent.
While we do not yet fully understand how biological tissues (particularly muscles) can be transparent, there are several possible mechanisms which might contribute. The first is that transparent fishes such as glass catfishes and glassfish have very thin bodies. The flatter the body, the less the potential scattering of light (and hence the easier it is to make the tissue transparent).
Another possible mechanism is the ordered packing of small molecules within the cytoplasm of the cells to reduce the scattering of light.
Lastly, theoretical models also predict that the many subcellular components of transparent tissues (e.g. mitochondria, ribosomes) should be small and highly dispersed.
As a recent review paper on biological transparency states, this field of study has more questions than answers, and we await future studies to fully understand this phenomenon.
This item was first published in the September 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.