BASIC BIOLOGY AND CHEMISTRY OF HAIRS

• Jurisdiction: Australia

The hair follicle .............................................................................................. [88B.100]

Hair growth dynamics ................................................................................... [88B.120]

Hair growth forms.......................................................................................... [88B.140]

Hair chemistry ............................................................................................... [88B.160]

[88B.100] The hair follicle

Hairs grow from the skin or epidermis and originate from hair follicles. The follicle is an invagination or down growth of the epidermis into the dermis. Much of what is known about the hair follicle comes from the field of developmental biology where molecular biologists study the hair follicle as a model for cell differentiation, tissue development, gene expression and control. This is because the hair follicle has very high rates of cell division. Research into the development of sheep animal wool fibres has also contributed greatly to our knowledge of the hair follicle. The initial development of the follicle is a continuous process but it is often described as a series of stages. The important point from a forensic perspective is that it is an ongoing and a dynamic process, with the hair follicle capable of producing many hairs during its lifetime and hairs of different types. Figure 1 shows a diagram of a mature hair follicle and its component "zones" and structures, while Figure 2 shows different stages in the growth cycle.

Figure 1. Mature Hair Follicle

(Schematic diagram of the pilosebaceous unit showing various main features and the regions where the main events of cell proliferation and keratinisation take place. The cells in the bulb region move into different steams that give rise to the cortex, the cuticle, the medulla and the inner root sheath. Modified from "The role of Keratin proteins and their genes in the growth, structure and properties of hair" by B.C. Powell and G.E. Rogers, in Formation and structure of Human Hair, edited by PJolles, H.Zahn and H.Hocker, © 1998 Birkhauser Verlag Basel/Switzerland)

Figure 2. Diagrammatic representation of hair follicles at the different stages of the growth cycle.

(Anagen is the active growth phase during which follicle development takes place and the hair fibre is produced. Catagen is the regression phase in which tissue changes occur as the follicle approaches telogen, the resting phase. The next anagen is thought to be initiated by an interaction between dermal papilla cells and pluripotent stem cells located in the bulge region. B, bulge; APM, arrector pili muscle; SG, sebaceous gland; C, hair cortex and cuticle; M, medulla, DP, dermal papilla, ORS, outer root sheath, IRS, inner root sheath. Modified from Cotsarelis et al. (1990), copyright Cell press.)

From Figures 1 and 2, it can be seen that cell division and proliferation takes place in the germinative cells in the basal area of the follicle. During development two swellings appear on the slanting (angled) side of the follicle. The upper swelling develops into the sebaceous gland. The lower swelling is glycogen-rich and is the point at which the arrector pili muscle becomes attached. This area in the hair follicle is called the bulge and marks a significant point in the follicle. Above the bulge...