Hair Chemistry. Chapter 1. Hair Relaxers Science, Design, and Application

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Camron Alfred King
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1 Hair Relaxers Science, Design, and Application Chapter 1 Hair Chemistry We all know that the hair on our head is dead, but underneath the scalp, within the hair follicle, is a surprisingly busy and complex minifactory that produces hair fiber after hair fiber to give us a full head of hair. Whether one s ethnicity is Caucasian, Asian, Latino, or African, hair is basically the same it is composed of the same substances and goes through the same growth stages. But there are distinct differences namely, the spatial shape of the hair fiber and its cross section, which determine how the hair grows and how much curl it has. These characteristics affect the hair s assembly properties or more simply put, how the hair behaves and reacts to being styled. Later in Chapter 2, we will focus on African and Latino hair and discuss the inherent chemistry, genetic factors, and environmental/climatic elements that differentiate it from other types of hair. We are all born with approximately 100,000 hair follicles on our scalp. About 1,100 of these follicles occupy each square centimeter of our head at birth; by age 25, this number decreases to about 600, and by ages 30 50, it drops to 300 or less before tapering off even further. Typically, hair grows about 1 cm (0.39 inch) every 28 days or 13 cm (5 inches) per calendar year, growing at a slightly lower rate at the temples than at the crown area of the scalp. Depending on the stage of life, hair follicles produce different types of hair fibers lanugo hair (fine, unpigmented fuzz) observed during fetal life and infancy; vellus hair (fine pigmented strands) grown until puberty; and club hair (long, thick terminal hair shafts) lasting 1

after hair fiber to give us a full head of hair.

2 Chapter 1 throughout adulthood. Once at maturity, hair continually undergoes periods of growth, rest, and decay, whereby the hair follicle produces a new hair fiber that grows for several years, then spontaneously falls out and is replaced by a new one. Hair was once thought to be a rather unassuming and rudimentary organ. H.B. Chase, a noted hair biologist of the 1950s, was the first to refute this by defining the development, structure, and cycling of the hair follicle. Since then, researchers have come to recognize that the hair follicle is a regenerative structure representing a complex biological continuum of cell and organ growth cycles, involving cell differentiation, epithelial/mesenchymal interactions, stem cell biology, pattern formation, apoptosis, and pigmentation. 1 Growth Stages Although the hair we see is biologically dead, its growth originates in the living part of the fiber located well beneath the skin surface where cells at the base of the hair bulb divide and grow inside a sac called the hair follicle. As they move upwards toward the skin surface, the cells dehydrate and die, giving rise to the emerging fiber structure that forms the hair shaft. 2 Anatomically, the hair follicle is divided into three regions: the upper section, or infundibulum; the middle section, or isthmus; and the lower section, or inferior segment. The infundibulum contains the fully formed hair shaft as it exits the epidermis. Within the isthmus is a bulge area containing follicular stem cells, as well as a vast network of nerve endings and Merkel cells, the presence of which has led researchers to believe it could be involved in signaling mechanisms for hair growth and cycling. 3 The section that actually undergoes cyclical regeneration, however, is the inferior segment, the bottom-most portion of which is the hair bulb. The bulb, together with the dermal papilla surrounding it and its associated nerves and blood vessels, constitutes the hair root. Hair follicle growth is a cycling process that repeats itself about twenty times throughout one s lifetime and consists of four major 2

replaced by a new one. Hair was once thought to be a rather unassuming and rudimentary organ. H.B.

3 Hair Chemistry phases: anagen, the growth phase; catagen, the involuting or regression phase; telogen, the resting or quiescent phase; and exogen, the shedding phase. Although molecular signals specific to each of these phases are known to control the transition of one growth cycle to the next, what these particular signals are has yet to be elucidated. Anagen: The anagen phase begins at the moment a new hair starts to grow and can last as long as seven years. As much as 90% of the hair follicles may be in anagen phase at any one time. Anagen is a regenerative period of complete regrowth that originates in the dermal papilla, a mound-like cluster of fibroblasts derived from mesodermal cells, and lies at the bottom of the hair follicle surrounded by the hair bulb. The dermal papilla is needed to initiate regeneration of the hair follicle and to maintain normal hair growth, and according to some studies, has been associated with the production of growth factors and growth factor receptors. 3 However, the hair bulb itself is the site of actual hair growth. It is believed that the bulb s regenerating cells receive a signal to proliferate, grow down into the dermis, and form the cellular lineages of the mature follicle that eventually differentiate into the hair shaft visible on the scalp. What triggers the spontaneous initiation of a new anagen phase is not well understood, but the interplay between epithelialmesenchymal interactions and the receptive follicular epithelium and follicular connective tissues is thought to be critical to the regeneration process. 1 In short, anagen is that time when growth stimulatory pathways are activated, cells in the root of the hair are dividing rapidly, and fibers are beginning to accumulate on the hair shaft a time akin to the aforementioned mini-factory running at full production. Catagen: In late anagen, follicle growth stops, apparently on cue from an as-yet unidentified signal. This marks the time when catagen begins and precipitates the involution of the cycling, deep portion of the follicle, a catastrophic-like event in which the follicle collapses inward and rapidly degenerates in function. Nonetheless, catagen is a normally occurring phase and a highly controlled process of regression characterized by eight sub-phases of cell differentiation and apoptosis. 3

Anagen: The anagen phase begins at the moment a new hair starts to grow and can last as long as seven years. As much as 90% of the hair follicles may be in anagen phase at any one time.

4 Chapter 1 The precise molecular mechanisms are beyond the scope of this book, but eventuate the cessation of cell growth and pigmentation, release of the papilla from the bulb, loss of layered differentiation in the lower follicle, substantial extracellular matrix remodeling, and distal vectorial shrinkage of the inferior follicle. Telogen: By the end of catagen, the hair follicle, now called a telogen follicle, is no more than a skeleton of its former self a small finger-like projection of quiescent epithelial cells lying above a tight cluster of papilla fibroblasts in the dermis and extending only as far as its upper layer. Although the cells of the anagen and telogen papilla are seemingly alike, those of the telogen papilla are deficient in extracellular matrix and have little cytoplasm in their fibroblasts. Furthermore, the epithelial cells of the lower telogen follicle show little or no evidence of DNA or RNA synthesis of proteins, although the epithelial sac to which the telogen hair fiber is tethered continues to synthesize keratin 14. Embedded within this epithelial sac is the telogen shaft, sometimes called club hair, which has a brush-like base composed of small, tightly packed cells known as the hair germ. Flattened at the papilla interface, the hair germ lies dormant until the telogen follicle transits to another anagen phase, at which time it swells and grows down to enclose the papilla. Again, no marker initiating this new anagen phase has been identified. Very little is known about telogen, but many researchers believe that there is much more to the placid persona of those quiescent cells, and its descriptor as a resting phase may come to be an oxymoron. Exogen: The consensus among hair biologists is that hair shedding is an innate phenomenon whereby each fiber grows to a certain length over a certain time period before being shed from the scalp. In animals, new hairs regularly grow in before old ones fall out, but this is purely a protective mechanism. Because telogen hairs in humans have also been found to last for more than one follicular cycle, it is believed that fiber release probably involves more than a mechanical stimulus alone; most likely, it utilizes a separate set of controls, and seems to occur indepen- 4

Telogen: By the end of catagen, the hair follicle, now called a telogen follicle, is no more than a skeleton of its former self a small finger-like projection of quiescent epithelial cells lying

5 Hair Chemistry dently of anagen and telogen. For these reasons, hair shedding has been coined the exogen phase, earning its place as a separate cycling entity. 4 Hair Structure All hair, regardless of ethnic origin, shares common characteristics in its chemical makeup, molecular structure, and morphology. The hair shaft, lying in the center and growing up through the follicle, emerges from the scalp as threadlike structures. These hair fibers are each composed of three distinct regions: the cuticle, the outermost area consisting of lamellar layers of structural tissue; the cortex, the inner area comprising the bulk of the fiber; and the medulla, the innermost area lying at the center of the fiber. On cross-section analysis, one can see that the hair fiber consists of a hierarchy of subunits (see Figure 1.1). Figure 1.1. Cross section of hair shaft; image courtesy of JA Swift The cuticles consist of thin scale-like cells and collectively constitute the hair surface visible to the naked eye. While the cuticles contribute to the esthetics of the hair, their primary function is to 5

The hair shaft, lying in the center and growing up through the follicle, emerges from the scalp as threadlike structures.

Notes on Hair Analysis

Notes on Hair Analysis I have found local veterinarians very uncooperative when trying to get samples of dog and cat fur. I have found neighbors, friends and relatives a much better source of fur. There