Background

The cuticle is made up of overlapping plates or scales of keratin arrayed in characteristic patterns. Although these scale patterns may be visible on a wet-mounted specimen at high magnification, it is often difficult or impossible to discern the scale pattern if the refractive index of the scales is very close to that of the mounting fluid. One way around this problem is to make a cast of the exterior surface of the hair and examine that cast under high magnification.

Figure 1 shows the three major types of scale patterns. The imbricate scale pattern is a flattened wavy pattern that is commonly found on human hair and many types of animal hair. The coronal scale pattern is a crown-like pattern that resembles a stack of paper cups, and is normally found only on very fine hair. Coronal scales are found on many types of animal hair and are very rarely present on human hair. The spinous scale pattern is a petal-like pattern made up of triangular scales that protrude from the cuticle. Spinous scales are found in the proximal (root) region of the fur hair of some animals, including bobcat, chinchilla, fox, lynx, mink, mouse, otter, raccoon, rat, sable, sable, seal, and sea lion. Spinous scales are never found in human hair.

Although scale patterns are seldom useful for characterizing human hair specimens, they are important for discriminating human hair from animal hair and for determining the type of animal from which a specimen originated. In this lab session, we’ll make scale casts using ordinary colorless nail polish as a casting medium, the same method used by professional forensics labs.