**9. Hair regrowth treatment**

Up to the present, although many treatments have been tested, hair loss continues to be a frequent dermatological condition [46].

Two FDA-approved hair loss treatment drugs: Finasteride (acting on the hormonal cause of alopecia—the excess of androgens) and Minoxidil (acting on the physical cause—the hypoxia due to vasoconstriction), are commonly used in clinical practice in order to treat androgenetic alopecia, which represents 95% of all hair loss causes [38, 47–49]. Minoxidil (1 mg per day) is a topical formulation available in 2 and 5% concentration. It stops hair loss and promotes hair growth as it is a vasodilator and potassium channel opener, allowing more oxygen, blood, and nutrients to reach the follicle [50–53]. It has no therapeutic action on the hormonal and genetic causes of hair loss; therefore, it must be used as a continuous support for the hair follicles, otherwise the hair regrowth will cease and hair loss will begin again in 1–2 months [54–57]. Finasteride is a dihydrotestosterone-suppressing 5-alpha-reductase inhibitor, recommended for male use only, decreasing the serum levels of dihydrotestosterone, stopping hair fall (in 48% of the cases), and stimulating hair regrowth (in 51% of the cases). Studies have shown that 1 mg of finasteride oral treatment has an efficacy similar to daily topical application of minoxidil [58–60]. Given the temporary efficacy of finasteride and minoxidil and the limited number of treatments available in alopecia, new therapies are needed to prevent hair loss and enhance hair regrowth [61, 62].

Pharmaceutical hair loss management also includes different substances (arginine, aminexil, caffeine, and taurine), different peptides, B spectrum vitamins, zinc, or different procedures (application of stem cells or plasma-rich platelets and low-level laser therapy), even if clinical studies in this respect are lacking. A large variety of over-the-counter products claim to treat hair loss pathology: hair tonics, hair balms, hair masks, shampoos, leave in conditioners, topical solutions, or foams function as potential anti-hair loss agents [43, 44, 63–68].

repressed enhanced interferon-γ mRNA expression in alopecic skin. Also, restoration of hair shaft elongation occurred due to Th1 transcription factor T-box 21 small interfering RNAs conjugated to cationized gelatin [71]. Another recent study showed that the sonic hedgehog (*shh*) gene stimulated the hair shaft production and anagen phase in C57BL/6 mice, after being

Evaluation of Animal Models Suitable for Hair Research and Regeneration

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Gene therapy is currently available only in research settings. It represents a promising therapeutic option for several diseases (especially those with no cure for the moment), but this procedure needs more research and improvement that need to be considered safe and to prove its effectiveness. So far, scientists have encountered difficulties in finding proper delivery pathways of the genes to the body, targeting them to particular cells, controlling the new

Human hair follicles as research material for hair loss and regeneration involve ethical problems, an invasive collection method and a limited quantity of follicles available for extraction

The first methods of isolation and maintenance of hair follicles in cell cultures go back to 1990, when several researchers used this method in order to study the biology of the hair cycle [74, 75]. Follicles were usually taken during face lifting surgery, but only a third were suitable for the isolation phase of the hair transplant, due to improper collection procedures. The follicles needed to be isolated from human scalp in a few hours, maintained at 2–6°C, in an Earl medium, combined with phosphate buffered saline solution, with calcium and magnesium

*In vitro* hair research was supported by the identification of growth factor function in the process of hair regrowth and differentiation [76–79]. Philpott et al. have reported that in the absence of insulin, follicles prematurely enter the catagen stage [80]. Subsequent *in vitro* and *in vivo* studies, in murine and human models of hair follicles, have demonstrated that IGF-1 level is a regulation factor of hair growth and together with IGF-1 receptor influence hair growth cycle. Other studies performed in 1990 have shown that transforming growth factor beta 2 (TGF-ß2) promotes anagen to catagen transition. Several inhibitors of hair follicle growth *in vitro* have been identified such as interleukins (IL-1 alpha and beta) and tumor necrosis factor (TNF-alpha). Researchers concluded that these cytokines play a significant part in the pathophysiology of hair inflammatory diseases. Although the factors that perform the transition *in vitro* from anagen to catagen have been discovered, inducing a full hair growth cycle has not been made possible yet. Murine models of hair follicles, isolated at different growth stages *in vitro* seem to maintain their

On the other hand, healthy human dermal papilla cells, isolated from hair follicle, lose the ability to produce hair growth when being outside the body. Also, cycling hair follicles cannot

gene(s) and their effect after they have been inserted into the body [73].

delivered with an adenovirus vector [72].

**11. Hair regrowth studies** *in vitro*

added. Only the follicles that seemed intact were used.

cyclic activity and to illustrate their status *in vivo* [81].

be maintained in culture for any length of time [82].

and testing [60, 61].

Alternatives to traditional treatment are laser (low-level laser therapy) and platelet-rich plasma (PRP) injections [47, 69].
