Hair is one of the defining characteristics of mammals. It serves a number of purposes, including thermoregulation, sensory input, and protection from allergens and other particles. Hair can also be used to express oneself, as in the case of hairstyles. Hair is a filament that grows from follicles in the skin. It consists of tightly packed cells that are filled with keratin, a protein. Most of the interest in hair is focused on growth, styling, and care, but it is also an important biomaterial with fundamental structural properties.
All hair grows from a hair follicle, a tiny, saclike hole in the skin. Each follicle contains a cluster of special cells that reproduce rapidly to produce new hair cells. The new cells push the old ones upward, creating the shaft of the hair. As the hair grows, it passes through a process called keratinization, in which it becomes filled with the protein keratin. This gives the hair its strength and stiffness.
A thin layer of cells surrounds the hair follicle and the keratinized cells that form the shaft. This layer is known as the cuticle, and it helps to protect the softer, more pliable inner layers from the more rigid outer layers of the hair. A number of different bonds-including disulfide, hydrogen, electrostatic, and amide-bind the proteins together to make a strong, insoluble material.
The sturdiness of hair depends on its consistency, which is determined by the volume of the hair follicle and condition of the hair shaft. The consistency of hair is usually classified as fine, medium, or coarse. Fine hair is the thinnest and least pliable, while coarse hair is the thickest and most pliable.
Each individual hair is a little bit different from the others, and this is because each grows from a hair follicle that has a slightly different shape. The follicles that grow straight hair are roughly oval in cross-section, while the ones that grow curly hair have one main c-shaped curve and a smaller secondary curve at the bottom.
Almost every part of the body has some hair, but the most significant concentration is on the head. Head hair provides insulation and protects the scalp from sun damage. It also serves to control sweat and regulate body temperature. Hair in other parts of the body-such as eyelashes-protects the skin from dust, allergens, and microbes.
Hair can be a useful piece of evidence in the investigation of crimes, as it can indicate the gender and age of the perpetrator, and can be used to determine if multiple people were present at the crime scene. However, forensic investigators need to be careful with this type of evidence. It is often contaminated with chemicals from shampoos, dyes, gels, and other cosmetics, and the hair can be difficult to distinguish from the skin it covers. For these reasons, it is important for forensic investigators to understand how to properly collect, preserve, and analyze hair. Luckily, there are many books that can help them in this endeavor.