**8. Consequences of hair loss**

Current concepts of hair loss pathogenesis include genetic, genomic, hormonal, and immune contributions. Furthermore, the patient's behavior influences the hair density and its strength. In recent years, evidence has suggested that hair loss is a multifactorial disease, and the contributing factors include the resistance to insulin, local pathologies (inflammation, hypoxia, and vascular insufficiency), predisposing physiological factors (menopause and aging), association with other diseases (polycystic ovary syndrome, hirsutism, acne, hormonal imbalances, thyroid pathologies, and other autoimmune diseases). Hair loss remains a consequence of the genotype (hereditary information of the organism)-phenotype (morphology, behavior,

The most common form of hair loss is known as "androgenetic alopecia" (AGA), which represents almost 95%. In this case, hair loss is generated by hair cycle abnormalities, such as the shortening of the anagen, within an abnormal hair cycle, and the anagen-telogen rate shifting from 6:1 to 2:1. Also, hair loss can be due to a small-sized dermal papilla. Both situations lead to shortening of hairs, decreasing hair diameter, shaft loss, and an increased number of hairs in telogen phase. In most situations, the changes of hair diameter (hair thinning) are followed by the loss of pigment: final hair (thick and pigmented) can turn back into vellus (thin and white). Studies point out that another cause of hair loss is the fact that the scalp suffers from

Hair cycle disturbances are mainly caused by an excess of androgens, which alters the production of regulatory factors (soluble paracrine factors and extracellular matrix components) by the dermal papilla cells [13]. Some specific sites of the body (beard, axillary, and pubic hair) react differently than hair from the scalp, as they are androgen-sensitive [4]. Hair miniaturization and thinning, followed by hair fall is most common in the vertex and the crown-

The occipital part of the scalp is an androgen insensitive area that is why in alopecia, hair is still present in this region, and hair follicles are suitable to be used in hair transplants [31, 32]. The androgen effect on hair can be summarized by the metabolization of the testosterone into 5-alpha-dihydrotestosterone by 5-alpha reductase. A good metabolization limits the hair length (in case of the beard, for example) and deficiencies of the 5-alpha reductase generate

Another form of alopecia is Alopecia areata (AA), a cell-mediated disease directed against active growing hair follicles. It is a nonscarring alopecia, with limited alopecic patches on the scalp or the body, sometimes affecting also the nails. The pathogenesis of AA includes an autoimmune etiology, linked to human leukocyte antigen (HLA) class II alleles and to the T

Several studies including recent genome-wide association analyses concluded that a large number of single nucleotide polymorphisms (SNPs) are associated with AGA susceptibility.

enlarged hair diameter (thicker hair in the axillary and pubic area) [19, 33–35].

and development) interaction [25, 26].

238 Experimental Animal Models of Human Diseases - An Effective Therapeutic Strategy

vasoconstriction and hypoxia [27, 28].

frontal area of the scalp [29, 30].

lymphocytic co-stimulatory cascade [30].

**7. Genes associated with hair loss**

Alopecia can be a part of the normal aging process. Still, hair loss represents a great concern for patients. Several studies have shown that it generates anxiety and distress especially in females, affecting couple and social relationships [37–39].

Hair loss is defined as a stressful experience for both sexes, patients being unable to cope with the progression of the disease [40, 41]. Stress functions not only as a cause, a risk factor, but also as a consequence of hair loss. Alopecia determines a poor quality of life by the physical and psychological sequelae: low self-esteem, depression, distorted social perception, and psychosocial functioning [42–45].
