Hair Structure and Hair Removal A Summary Report of Scientific Literature on “Structure of Hair” and “Chemistry and Kinetics of Depilatories” Procter & Gamble Prepared by https://www.dolcera.com/client/p8g5/wiki/index.php?title=Hair_removal/depilation 1 Hair Structure and Hair Removal Table of Contents [A] Executive Summary 3 [B] Structure of Hair 1. Morphological Structure of Hair 4 [1.1] Introduction 4 [1.2] Hair Follicle 5 [1.3] Hair Fibre (Shaft) 10 2. Structure of Hair across Various Body Sites 19 3. Chemical Composition of Hair 21 [C] Chemistry and Kinetics of Depilatories 1. Reducing Human Hair 25 [1.1] Bonding in Keratin 25 [1.2] Depilation by Reduction of Hair 26 2. Kinetics of Reduction 29 [2.1] Determination of Equilibrium Constant 30 [2.2] Mechanism 30 [2.3] Factors Affecting the Kinetics of Reduction 31 [2.4] Effect of Mercaptan Structure on Reaction Rate 35 [2.5] Kinetics/Reactivity in Microemulsion Media 36 [D] References 38 2 Hair Structure and Hair Removal [A] Executive Summary: The present review report involves the study of scientific literature to elaborate on the “structure of hair” and "the chemistry and kinetics of depilation”. In the first section, the report summarizes the morphological structure of different hair components, the structure of hair across various body sites and the chemical composition of hair. In the second section, the study of bonding in hair keratin protein, reduction of hair by depilatories and kinetics of reduction of hair is incorporated. 3 Hair Structure and Hair Removal [B] Structure of Hair : [1] Morphological Structure of Hair: [1.1] Introduction: Hair is a primary characteristic of mammals, and exerts a wide range of functions including thermoregulation, physical protection, sensory activity, and social interactions. Hair (the stratified epithelium) is an appendage of skin that proliferates from large cavities or sacs called follicles [1]. The length of the hair extends from its root or bulb embedded in follicles through the dermis, epidermis, stratum corneum, skin, then continues into a shaft and terminates at the tip end [1] [Figure: 1]. (Figure 1: Diagrammatic Representation of Structure of Hair) 4 Hair Structure and Hair Removal [1.2] Hair Follicle [2]1: The mature (anagen) hair follicle can be divided into two parts: [1.1.1] a ‘permanent’ upper part, which does not cycle visibly, and [1.1.2] a lower part, which is continuously remodelled in each hair cycle. [1.2.1] Upper Part of Hair Follicle: The upper part of the hair follicle consists of the (a) Infundibulum, and (b) Isthmus. (a) Infundibulum: It is the most proximal part of the hair follicle relative to the epidermis, extending from the sebaceous gland duct to the epidermal surface. It includes the hair canal and the distal Outer root sheath. It is the opening of the hair canal to the skin surface. At the proximal end, the infundibulum joins the isthmus region of the outer root sheath, where the arrector pili muscle is inserted [Figure: 2A]. • Sebaceous Gland: It is Acinar gland composed of lipid‐filled sebocytes, localized close to the insertion of the arrector pili muscle. It secretes sebum to the epidermal surface via a holocrine mechanism. Sebum helps making hair and skin water‐proof. It forms the pilosebaceous unit together with the hair follicle and the arrector pili muscle. • Outer Root Sheath: The outermost layer of the hair follicle which merges proximally with the basal layer of the interfollicular epidermis and distally with the hair bulb. (b) Isthmus: It is middle part of the hair follicle extending from the sebaceous duct to the ‘Bulge region’. The upper isthmus is joined with the infundibulum; while The lower isthmus also harbours epithelial and melanocytic hair follicle stem cells in the ‘Bulge region’, which is the end of upper part of hair follicle. • Bulge region: It is a convex protrusion of the outer root sheath in the most distal permanent portion of the hair follicle. It is located just below the sebaceous gland 1 The information provided in this section is taken from: [2] Marlon R. Schneider, Ruth Schmidt‐Ullrich, and Ralf Paus. Current Biology 19, R132–R142, February 10, 2009. 5 Hair Structure and Hair Removal and at the insertion site of the muscle arrector pili. It contains the hair follicle stem cells. • The Arrector Pili Muscle: The arrector pili muscle is an appendage to the hair that attaches itself to the underside of the hair at the dermal papilla and the hair shaft, midway between the bulb and the sebaceous gland. This tiny muscle fiber is responsible for lifting the hair, thereby trapping a layer of air on the skin’s surface. As a result, the arrector pili is partially responsible for heat regulation. Both fear and cold stimuli cause the arrector pili muscle fiber to contract, lifting the hair straight upward. [1.2.2] Lower Part of Hair Follicle: The lower, cycling part represents the actual hair shaft factory, the ‘Anagen Bulb’ [Figure: 2A] [3]. The anagen bulb contains the matrix keratinocytes and the hair follicle pigmentary unit. Bulge and Anagen Bulb, are separated by a long stretch of suprabulbar hair‐follicle epithelium [Figure: 2A, 2C]. 6 Hair Structure and Hair Removal (Figure 2. Histomorphology of the hair follicle: (A) Sagittal section through a human scalp hair follicle (anagen VI) showing the permanent (infundibulum, isthmus) and anagen associated (suprabulbar and bulbar area) components of the hair follicle. (B) High magnification image of the isthmus. The dashed square indicates the approximate location of the bulge. (C) High magnification image of the bulb. insulation (BM: basal membrane; APM: arrector pili muscle; CTS: connective tissue sheath; DP: dermal papilla; M: matrix; HS: hair shaft, IRS: inner root sheath; ORS: outer root sheath; SG: sebaceous gland)) (Figure 2D: Light micrograph of the different layers of the hair follicle. In the bulb region, a proliferating epithelial matrix surrounds the mesenchymal dermal papilla. The hair shaft of cortex, medulla and cuticle layers enclosed by the inner root sheath move outwards within the outer root sheath which is continuous with the epidermis) • Papilla: At the base of the follicle is a large structure that is called the papilla. The papilla is made up mainly of connective tissue and a capillary loop. Cell division in the papilla is either rare or non‐existent. While infundibulum, isthmus, bulge and hair bulb are all part of the hair follicle epithelium, i.e. of ectodermal origin, the dermal papilla is mesoderm‐derived. The dermal papilla [Figure: 2C, 2E], which consists of a small cluster of densely packed fibroblasts, dictates hair bulb size, hair shaft diameter and length, and anagen duration [4, 5, 6, 7]. 7 Hair Structure and Hair Removal The inner root sheath (IRS) is a layer extending from the base of the bulb to the isthmus, which surrounds and protects the development of the hair fiber. The IRS can be subdivided into several layers. A single cell thick IRS cuticle layer, adjacent to the hair fiber, closely interdigitates with the hair fiber cuticle layer. The next IRS layer is called the Huxley layer that may consist of up to four cell layers. Outside of this there a single cell layer called the IRS Henle layer. The Henle layer runs adjacent to the outer root sheath (ORS) layer. The outer root sheath forms a non‐keratinizing region at the periphery of the follicle and is continuous with the epidermis. It extends all the way to the tip of the bulb. When looking at a cross‐section of the hair follicle, its epithelium forms a cylinder with at least eight different concentric layers, each one expressing a distinct pattern of keratins [3]. Starting from the periphery, these layers include the outer root sheath, the companion layer, the inner root sheath,and finally the hair shaft [Figure: 2E]. In its bulge region, the outer root sheath contains the epithelial hair follicle stem cells [6]. The central part of the (Figure 2E: Schematic drawing illustrating the concentric layers of the outer root sheath (ORS), inner root sheath (IRS) and shaft in the bulb. The inner root sheath is composed of four layers: Companion layer (CL), Henle’s layer, Huxley’s layer, and the inner root sheath cuticle. The companion layer cells are tightly bound to Henle’s layer, but not to the outer root sheath, thus allowing the companion layer to function as a slippage plane between the stationary outer root sheath and the upwards moving inner root sheath. Further inwards, the inner root sheath cuticle is composed of scales that interlock with the scales of the hair shaft cuticle, anchoring the shaft in the follicle and enabling both layers to jointly move during hair follicle growth. (CTS: connective tissue sheath; DP: dermal papilla; IRS: inner root sheath; ORS: outer root sheath)) 8 Hair Structure and Hair Removal hair follicle epithelium holds the hair shaft [Figure: 2E]. The entire hair follicle epithelium is surrounded by a mesoderm‐derived connective tissue sheath [Figure: 2C, 2E], a loose accumulation of collagen and stromal cells resting upon a basement membrane. The hair shaft is enwrapped by the cuticle, and further components are the cortex and the medulla [Figure: 2E]. 9 Hair Structure and Hair Removal [1.3] Hair Fibre (Shaft): Morphologically, three distinct varieties of cells or units are produced within the hair follicle which results in the formation of three basic structural layers of any human hair fibre [1]. The three layers are: (i) (Outer) Cuticle (ii) Cortex (iii) (Central) Medulla, which may be absent or discontinuous along the hair shaft. [1.3.1] Cuticle: The outermost or external layer of the fibre consist of flattened overlapping scales known as cuticle, which is responsible for the much of the chemical resistance and stability of the hair [1][Figure: 3].