Protocolos de seguridad en redes WLAN

Hair is a filamentous, keratinised structure forming part of the Pilosebaceous Unit, which in addition, consists o f the follicle, the arrector pilli muscle, sebaceous gland and depending on the site, an apocrine gland (see Figure 1.1). Keratins are a group of insoluble proteins o f which there are two types, hard and soft. Hair contains hard keratin whilst desquamating tissues such as skin contain soft keratin. Keratins form long fibres which are bound tightly via di-sulphide bonds and cross-linking with other proteins. Consequently, they show great resistance to changes in pH, temperature and enzymatic digestion (Rushmer, et al., 1966). Hair is essentially dead except at its base where the growing region, or hair bulb, is situated. Hair tracts have been mapped in man showing that a general “flow” o f hair exists particularly noticeable on the forearm and fingers in the ulnar direction. There are parts o f the body where hair is absent. These are the palms of the hands, soles o f the feet, the umbilicus, nipples, glans penis, clitoris and labia minora where it is probable that their presence would be a hindrance to function.

Hair is contained within a follicle whose depth can vary from between 1 and 3 mm below the dermis so the lower end protrudes into subcutaneous fat. The hair emerges from the skin surface at an oblique angle which can be changed by action of the arrector pilli muscle. This muscle originates from the papillary dermis and attaches to the follicle at approximately l/3rd the way dovm the hair shaft. Action of the muscle, which is influenced by cold and the sympathetic nervous system, makes the angle between skin and hair more acute and so the hair “stands on end”. The diameter of hair can vary between 15 to 120 pm and is dependent on the type and the site the hair originates from (see Figure 1.2).

Microscopically, the mature hair shaft has been described as usually having 3 concentric zones differing in the type of keratin present. From inside out the zones are described as the medulla, the cortex and the cuticle. The medulla may be absent in thinner hairs. It is formed of disintegrating cells producing columns which contain air cavities above the level of the epidermis. The cortex is the largest component of the

Chapter 1 - General Introduction

Figure 1.2: Modified SACPIC stain of a transverse section taken from human hair- bearing skin showing the different keratinised regions of a hair shaft. The viable hair kératinocytes are stained green, whilst those in the keratogenous zone are stained red. The fully keratinised, mature cells of the hair shaft are stained yellow. HB is the hair bulb (xlOO).

hair-shafl and is composed of many tightly packed keratinised cells set in a dense matrix. The cells contain melanosomes and nuclear remnants. The cuticle, which forms the surface o f the hair, consists of several layers of keratinised squamous epithelial cells directed in an upward and slightly outward direction similar to the tiles on a roof. They are fused with the cells of the inner root sheath at the level of the isthmus (just below the sebaceous gland) so helping to fix the hair-shaft in its containing piliary canal. The outer surface o f the cuticle cells is rich in high sulphur protein which increases their environmental resistance. Nevertheless, as the hair shaft emerges from the skin so wear and tear causes it to become jagged as the cells are removed until ultimately it gains a brush-like appearance.

The inner root sheath is a layer which lies adjacent to the hair shaft almost up to the level o f the duct of the sebaceous gland (isthmus) but then degenerates just before reaching it. Its role has been described as that of a temporary filling material for the growing hair shaft. It consists of 3 layers; an inner cuticle, a middle Huxley’s layer and an outer Rente’s layer. The cuticle layer undergoes kératinisation as the cells ascend the canal. Huxley’s layer keratinises after those o f the cuticle at the level o f the middle o f the follicle and Rente’s at the level o f the upper bulb. All layers then degenerate at the level of the isthmus.

The outer root sheath extends from the level o f the upper bulb as a single or double layer o f undifferentiated cells becoming multilayered higher up the follicle and then continuous with the superficial epithelium of the skin. Melanocytes and Langerhans cells are contained within it. At the level o f the sebaceous duct the outer root sheath forms the wall of the piliary canal after the breakdown o f the inner root sheath. Two types of cells containing different keratin and antigen expressions during differentiation have been described recently within the outer root sheath (Ito, et al., 1986). Further to this, a bulge region situated at and below the insertion of the arrector pilli muscle has been described. The cells within this region have features consistent with those of stem cells (Cotsarelis, et al., 1990), (Rochat, et al., 1994) and could play a fundamental part in hair cycling to be discussed later (see physiology section). Melanocytes have been described in the bulge region interspersed with the

Chapter 1 - General Introduction

kératinocytes which have occasionally been seen to contain melanin under electron microscopy (Narisawa, et a l, 1995). Narisawa looked at scalp and eyebrow hair with reference to the bulge and found that melanisation was independent o f hair cycling (Narisawa, et al., 1997). The bulge of eyebrow hair follicles always contained melanin in varying amounts whilst the bulge of scalp hair was free o f melanin, it only being seen in the bulb and during specific growth phases. Thus the site and distribution of melanocytes and the melanin they produce would appear to vary throughout the body, particularly in the two regions most associated with hair growth, which could have an important influence on ruby laser-assisted dépilation described later. Surrounding the outer root sheath is the glassy membrane which is a non-cellular layer equivalent to the basal layer of the skin. The whole follicle is invested in a connective tissue sheath called the outer dermal sheath into which is inserted the arrector pilli muscle.

The base of the pilosebaceous unit is called the hair bulb being the lowermost part of the follicular epithelium. It encloses the dermal papilla, which during hair growth consists predominantly o f highly cellular connective tissue continuous with the outer dermal sheath, but during development induces formation of the hair germ. The hair bulb generates the hair and its inner root sheath and can be divided into two sections. The lower section, called the germinal matrix, consists of close-packed, mitotically active pluripotential kératinocytes interspersed with melanocytes and Langerhans cells. The upper section, or the upper bulb, consists o f cells derived from the lower germinal matrix which move apically, differentiating along several lines. Those at the centre form the hair medulla whilst successive, radial cells give rise to the cortex and cuticle o f the hair and finally the layers of the inner root sheath respectively.

Hair pigmentation derives from melanocytes present in the bulb adjacent to the apex of the dermal papilla. Melanocytes are also present in the outer root sheath and other parts o f the follicle as described earlier. Within the bulb the melanocytes donate melanosomes to the medulla and cortex of the hair mainly and are active during hair growth alone producing both eumelanin (dark brown) and pheomelanin (red/yellow) in varying quantities depending on genetic predisposition. Hair that contains less

melanin (eu or phaeo) becomes lighter in colour. The melanin granules present in the hair cortex are at a stronger concentration at the periphery.

Hair colouration appears to have no protective role against environmental irradiation. The colour may change, usually during adolescence, as a result o f the change in the dominant melanin. This is primarily under genetic control via the probable production o f melanocyte stimulating hormone. However, other factors can affect hair colour such as drugs (Chloroquine), nutritional deficiencies and metabolic disorders (Phenylketonuria). Hair greying results from a reduction in the number and activity of melanocytes. Albinism occurs when the melanocytes are present at the normal quantity but are inactive. Both circumstances appear to be genetically determined.

The blood supply o f the hair follicle is via collateral vessels from the reticular arteriolar plexus to the dermal papilla and from ascending branches to networks around the bulb and the inferior segment of the follicle. These latter vessels travel with the sensory nerve endings within the outer dermal sheath.

Embryologically, the hair follicle is composed of epidermal and dermal tissue. The latter, namely the dermal papilla and dermal sheath, are derived from mesenchymal tissue. This is situated just below the epidermis and instructs the epidermis to form a downward projection which progresses and envelopes the mesenchymal grouping so becoming the dermal papilla and sheath. This is accomplished by a series of messages sent between the two embryological cell lines during the 2nd and 5th month of fetal life. Approximately equal numbers of such groupings occur in all regions o f the body initially taking the form o f focal crowding of basal cell nuclei in the epidermis of the eyebrow, upper-lip and chin (Pinkus, 1958). These are the primary germs and subsequently more primary, and then secondary germs form between existing follicles so forming groups of three. Production o f hair follicles is complete by 6 months gestation and is equal in amount in both males and females (Szabo, 1958). The total number has been estimated at 5 million. After birth, new follicles are not thought to appear (Billingham, 1958) except in certain tumours, namely trichofblliculomas, but

Chapter 1 - General Introduction

uneven regional growth o f the skin causes the differing densities noted over the body. After its development, a certain follicle may produce several different types of hair.

Three types of hair are recognised on the human; lanugo (lana = wool), vellus (Fleece) and terminal (Rook, 1965). Lanugo covers the fetus by the fifth or sixth month in- utero and is shed before birth except in the region of the eyelids, eyebrows and scalp where it becomes initially stronger hut then replaced several months post-partum. New hair growth then occurs over the body giving a downy appearance. These are the vellus hairs which persist until puberty. They are soft, rarely exceed a length o f 2cm, are occasionally pigmented and do not have a medulla. At puberty, in certain regions o f the body and under hormonal control, coarse hairs develop. This occurs most notably in the axilla and groin in both sexes and the back and chest to a greater or lesser extent in the male. They are termed, along with the hairs of the scalp and eyebrows, the terminal hairs. These hairs are longer and coarser than vellus hairs and are often medullated and pigmented, although there are gradations between the two. The process from vellus to terminal may reverse with ageing.

O f the estimated 5 million hair follicles on the body, approximately 1 million are present on the scalp. 100 000 of these have been stated to be terminal but this number appears to vary according to the hairs pigmentation; light hairs 140 000, dark 102 000 and red 88 000 (Friedenthal, 1908). Ageing seems to produce a reduction in follicular count on the scalp firom 615 per cm^ between 20 and 30 years to 435 per cm^ between 80-90 years and baldness reduces this figure still further (305 per cm^ on average between ages 45 and 85) (Giacommetti, 1965). Distribution o f hair follicles in other bodily regions has been classified by Szabo and tabulated below.

Area Number of Specimens Investigated

Number of Hair Follicles +/- s.e. (mean per cm^) Cheek 11 880 +/- 60 Forehead 4 770 +/- 60 Forearm 5 100 +/- 50 Thigh 21 55 +/- 5 Leg 5 50 +/- 20 Upper Arm 10 40 +/- 10 Abdomen 3 40 +/- 30

In men, 90% o f the hairs on the chest, trunk, shoulders, arms and legs are stated to be terminal. In women, this figure has been estimated to be approximately 35% (Danforth, 1925).