The Hair Shaft
For the purpose of hair coloring, the part of the hair in which we are interested is the hair shaft, since this is the only portion to which coloring is applied.
Under the microscope the hair shaft reveals three different cell layers. The outermost is called the cuticle. The innermost part is called the medulla and may be present only intermittently, and is often com pletely absent from the hair. The part between the cuticle and medulla represents the bulk of the hair and is called the cortex. In addition membrane-like structures are present in the cuticle and between cuticle and cortex.
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Hair shaft is covered with a cuticle layer of horny, flat cells.
The cuticle of the hair shaft is composed of horny, flat cells which lie on the surface of the hair in overlapping formation, much like fish scales or the shingles on a roof. These cells contain no pigment, and are translucent. The tooth-like edges of the cuticle are projected upward and are affected by combing and brushing. When hair is combed or brushed away from the scalp these edges are left smooth; combing or brushing in the opposite direction causes damage. To allow tint to penetrate to the cortex, these scales must be opened up or "softened."
In tinting, in order to keep the hair color natural looking, the coloring must penetrate inside the hair shaft. No coloring should be deposited on the cuticle layer. When the cuticle layer becomes colored, and this only happens when coating types of dyes are used, the hair no longer looks natural.
The cuticle and the membrane are sometimes resistant to penetration of the dye inside the hair shaft and it becomes difficult for tints to penetrate inside the hair shaft. If this happens, hair must be pre-softened, pre-bleached or other techniques used, to make the hair receptive to tints.
The cortex is composed of longitudinally arranged spindle-like hornified cells firmly adhering to each other. The natural pigment which gives hair its color is found in this layer. It is in the cortex layer that color changes are made during bleaching and tinting.
The medulla is often absent and is unimportant in hair coloring.