The entire surface of the skin is supplied with hairs, but on the head the hairs are longer and more pigmented. The structure of both head and body hair is practically identical yet they differ in their physiological responses, especially to hormones. Only two areas of the body, the palm of the hand and the sole of the foot, are devoid of hairs. Men and women have the same number of hair follicles, the organs that produce the hairs. In fact, the number of hair follicles is all but equal in all individuals, even the bald. In these subjects hairs are not completely absent; it is only that they have become so small as to be invisible. There are three types of hair:
1. Vellus hair
2. Terminal hair
3. Lanugo hair
The lanugo is the thin and unpigmented hair present in the foetus before and just after birth, which is subsequently transformed into terminal hair (like that of an adult). Vellus hair is that present in women, adolescents, and also on the scalp of bald individuals. All three hair types have the same structure and even the same histological profile.
Hairs develop from the hair follicles, which are true organs formed by the invagination of the epidermis during the foetal development. These organs are divided into three main parts:
1. The infundibulum is the portion from the opening on the skin surface to the mouth of the sebaceous gland duct.
2. The isthmus is the section between the sebaceous gland duct and the point of intersection with the hair erector muscle.
3. The inferior portion extends from the erector muscle to the bulb of the hair follicle.
The hair root is a generic term that includes all of the inferior portion and the isthmus. The bulb is the enlarged terminal part of the hair follicle in the root. The root produces the hair shaft, which is defined as the hair structure visible above the skin surface. Below the bulb the dermis re-enters the hair follicle, producing the dermal papillae, as the hair follicle is formed by introtlexions of the epidermis; this re-establishes the junction with the dermis. In this way the hair follicle is surrounded by the basal membrane. In this region it is particularly thick and assumes the name vitreous membrane, because together with the basal membrane it forms a kind of connective sheath that surrounds the whole follicle.
The part on the hair follicle between the bulb and the isthmus is known as the keratogenous zone. The maturation of the cells that produce the hair shaft occurs in this area. The most important part of the hair follicle is the bulb. In the basal layer, immediately above the papillae, cells continuously divide and begin their ascent. Slowly as this process proceeds the cells subdivide in the layers that make up the follicle. In fact, if a hair is cut perpendicularly to the follicle one can easily see how, independently of the zones and structure described earlier, each of these is formed by the union of layers and from inwards towards the outside of the follicle we can recognize the following layers: the medulla, cortex, and cuticle. These three layers form the hair shaft.
The cuticle is followed by the cuticle of the internal epithelial sheath, Huxley's layer, and Henle's layer. Beyond these layers the external epithelial sheath begins and this, too, is composed of cell layers becoming increasingly thinner from the epidermal surface to the bulb. Inthe germinative layer, or hair matrix, the keratinocytes are in contact with melanocytes, which are present only in this region. Here the melanocytes produce the pigment that will give rise to our hair colouring. The connective sheath also envelops the sebaceous gland and the hair erector muscle. The latter structure, important in furry animals in controlling the angle of the hair and so regulating heat exchange, is all but useless in man-nevertheless, this thin muscle is able to raise the hair. The follicles are set obliquely to the skin's surface and the erector muscle, by contracting and nipping both the follicle and the epidermis, is able to raise the hair. This is the phenomenon of horripilation, commonly called goose flesh, which occurs during shivering due to cold or fear.
Hairs live according to particular cycles. They do not all grow together and consequently their life and loss are staggered. In furry animals hair growth and loss are synchronized, allowing these animals to change their fur entirely with the seasons. Inman, on the other hand, hair lives in a way that is defined mosaic. There are approximately 180,000 hairs on the scalp. The cycle of each hair is staggered with respect to the others and thus each is at a different phase of growth. Three distinct phases can be identified in the hair life cycle:
1. Anagen or hair growing phase
2. Catagen or quiescent phase
3. Telogen or hair loss phase
The growing phase (anagen) lasts about three years and involves 90-95% of the hair at anyone time. During this period the cells of the matrix divide continuously and the hair shaft grows in length. After this phase the hair follicle enters a resting state that will normally last about three months. During this period, which will daily involve approximately 13% of the hair, the follicle undergoes profound changes and the bulb becomes thinner and loses its connections with the papilla. Later in catagen the follicle can regain the growing phase or enter the final stage (telogen). If the latter occurs a new hair follicle will develop at the bottom of the old one after approximately three weeks. This new growth pushes the old hair out of the follicle, which is subsequently lost. One percent of the hairs in the telogen phase are lost daily and therefore, even in healthy subjects, 100-180 hairs can be lost in a day. The important thing is that these hairs get replaced by new ones.
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