Imaging the Invisible with No Labels: A Major Opportunity in Biology and Medicine
Utilizing vibrational spectroscopy, multi-color images of different chemical components within the sample (e.g. mouse skin tissue in this case) can be highlighted. In this example, the naturally occuring subcutaneous fat layer (shown in red) is visualized by tuning into the lipid vibrational frequency. This fat layer is about 100 microns deep in the mouse skin and provides insulation to the animal. A similar structure is present in human skin. In addition to the naturally occuring fat layer, the SRS microscope was also used to visualize the distribution of a small molecule drug applied to the skin, dimethyl sulfoxide (shown in green). The drug is seen to penetrate the skin via the hydrophilic path, avoiding lipid droplets. SRS microscopy offers a powerful new tool for visualizing drug distributions in animal tissue without the use of exogenous labeling techniques, offering a new approach to the study of drug/tissue interactions, in particular pharmacokinetics. Visually, the fat layer consists of smooth, round oil droplets (shown in red). Some are close to circular, while others are ovoid or almost square, but with rounded corners. They are irregularly shaped and do not appear in any obvious pattern. There are about 12 such structures across the width of the image and 18 or so up the height of the image. They vary in size by a factor of five or more, and in intensity (brightness of red) by more than a factor of two. These droplets appear to float on a sea of green, which represents the applied drug molecule. The gap between the droplets varies from as much as one third the size of a large droplet down to being only a few hundred nanometers wide. As this is an optical micrograph, the entire image was acquired from an area of tissue about 150 x 250 microns in size inside the mouse skin.
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