chapter 11

polypeptide chain

A long chain of amino acids linked by peptide bonds is called a polypeptide chain

keratinization

As these newly formed cells mature, they fill up with a fibrous protein called keratin. After they have filled with keratin, the cells move upward, lose their nucleus, and die. By the time the hair shaft emerges from the scalp, the cells of the hair are completely keratinized and are no longer living. The hair shaft that emerges is a nonliving fiber composed of keratinized protein.

side bonds

Polypeptide chains are cross-linked like the rungs on a ladder by three different types of side bonds that link the polypeptide chains together and are responsible for the extreme strength and elasticity of human hair

cysteine

The disulfide bond joins the sulfur atoms of two neighboring

amino acids

The protein is made up of long chains of amino acids, which, in turn, are made up of elements. The major elements that make up human hair are carbon, oxygen, hydrogen, nitrogen, and sulfur and are often referred to as the amino acids units that are joined together end-to-end like pop beads

helix

The spiral shape of a coiled protein is called a helix which is created when the polypeptide chains intertwine with each other

peptide bond

The strong, chemical bond that joins amino acids is a peptide bond also known as end bond.

cystine

amino acids to create one cystine

Proteins

are long, coiled complex polypeptides made of amino acids

Sebaceous glands

are the oil glands in the skin that are connected to the hair follicles. The sebaceous glands secrete a fatty or an oily substance called sebum. Sebum lubricates the skin.

dermal papilla

is a small, cone-shaped elevation located at the base of the hair follicle that fits into the hair bulb. The dermal papilla contains the blood and nerve supply that provides the nutrients needed for hair growth. Some people refer to the dermal papilla as the "mother" of the hair because it contains the blood and nerve supply that provides the nutrients needed for hair growth.

disulfide bond

is a strong, chemical, side bond that is very different from the physical side bond of a hydrogen bond or salt bond.

hydrogen bond

is a weak, physical, cross-link side bond that is easily broken by water or heat. Although individual hydrogen bonds are very weak, there are so many of them that they account for about one-third of the hair's overall strength. Hydrogen bonds are broken by wetting the hair with water That allows the hair to be stretched and wrapped around rollers. The hydrogen bonds reform when the hair dries.

salt bond

is also a weak, physical, cross-link side bond between adjacent polypeptide chains. Salt bonds depend on pH, so they are easily broken by strong alkaline or acidic solutions Even though they are weak bonds, there are so many of them that they account for about one-third of the hair's overall strength.

medulla

is the innermost layer of the hair and is composed of round cells. It is quite common for very fine and naturally blond hair to entirely lack a medulla. Generally, only thick, coarse hair contains a medulla. All male beard hair contains a medulla. The medulla is not involved in salon services.

hair bulb

is the lowest part of a hair strand. It is the thickened, club-shaped structure that forms the lower part of the hair root. The lower part of the hair bulb fits over and covers the dermal papilla.

cortex

is the middle layer of the hair. It is a fibrous protein core formed by elongated cells containing melanin pigment. About 90 percent of the total weight of hair comes from the cortex. The elasticity of the hair and its natural color are the result of the unique protein structures located within the cortex. The changes involved in oxidation haircoloring, wet setting, thermal styling, permanent waving, and chemical hair relaxing take place within the cortex

hair cuticle

is the outermost layer of the hair. It consists of a single overlapping layer of transparent, scale-like cells that look like shingles on a roof. The cuticle layer provides a barrier that protects the inner structure of the hair as it lies tightly against the cortex. It is responsible for creating the shine and the smooth, silky feel of healthy hair.

arrector pili muscle

is the small, involuntary muscle in the base of the hair follicle. Strong emotions or a cold sensation cause it to contract, which makes the hair stand up straight and results in what we call goose bumps.

hair follicle

is the tube-like depression or pocket in the skin or scalp that contains the hair root. Hair follicles are distributed all over the body, with the exceptions of the palms of the hands and the soles of the feet. The follicle extends downward from the epidermis into the dermis (the inner layer of skin), where it surrounds the dermal papilla. Sometimes more than one hair will grow from a single follicle.