• Alex Savin
• March 14, 2022
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Human Hair Growth in Vitro

Hair has always had a special meaning for the human race if we put aside its biological purpose of protecting the body and helping keep the temperature its importance lies in its decorative purpose. Hair can tell stories of people’s positions in society, their title, and nowadays their opinions and attitudes. That is why the growth of hair is very important.

What is Human Hair

Human hair grows everywhere on the body except for the soles of the feet, the lips, the palms of the hands, and the eyelids, apart from eyelashes. Like skin, hair is a stratified squamous, keratinized epithelium made of multi-layered, flat cells with overlying keratin (a protein), whose rope-like filaments provide structure and strength to the hair shaft.

wig hair follows a specific growth cycle with three distinct and concurrent phases: anagen, catagen, and telogen phases. Each phase has specific characteristics that determine the length of the hair. All three phases occur simultaneously; one strand of hair may be in the anagen phase, while another is in the telogen phase.

We report for the first time the successful maintenance and growth of human hair follicles in vitro. Human anagen hair follicles were isolated by microdissection from human scalp skin. Isolation of the hair follicles was achieved by cutting the follicle at the dermo-subcutaneous fat interface using a scalpel blade. Intact hair follicles were then removed from the fat using watchmakers’ forceps. Isolated hair follicles maintained free-floating in supplemented Williams E medium in individual wells of 24-well multiwell plates showed a significant increase in length over 4 days. The increase in length was seen to be attributed to the production of a keratinized hair shaft and was not associated with the loss of hair follicle morphology. [methyl-3H]thymidine autoradiography confirmed that in vitro the in vivo pattern of DNA synthesis was maintained; furthermore, [35S]methionine labeling of keratins showed that their patterns of synthesis did not change with maintenance. The importance of this model to hair follicle biology is further demonstrated by the observations that TGF-beta 1 has a negative growth-regulatory effect on hair follicles in vitro and that EGF mimics the in vivo depilatory effects that have been reported in sheep and mice.

The factors that regulate hair follicle growth are still poorly understood. In vitro models may be useful in elucidating some aspects of hair follicle biology. We have developed an in vitro human hair growth model that enables us to maintain isolated human hair follicles for up to 10 days, during which time they continue to grow at an in vivo rate producing a keratinized hair fiber. We have shown that the epidermal growth factor (EGF) in our system mimics the in vivo depilatory action of EGF in sheep, and suggest that this occurs as a result of EGF stimulating outer root sheath (ORS) cell proliferation which results in the disruption of normal mechanisms of cell-cell interaction in the hair follicle. We identify transforming growth factor-β (TGF-β) as a possible negative regulator of hair follicle growth and show that physiological levels of insulin-like growth factor-I (IGF-I) can support the same rates of hair follicle growth as supraphysiological levels of insulin. Furthermore, in the absence of insulin hair follicles show premature entry into a catagen-like state. This is prevented by physiological levels of IGF-I. Finally, we demonstrate that the hair follicle is an aerobic glycolytic, glutaminolysis tissue and discuss the possible implications of this metabolism.

Whole human scalp hair follicles were cultured. The follicles were dissected from skin pieces of the normal scalp and put into 1.5 ml of incubation medium in a closed 5 ml glass tube under an atmosphere of 95% O2 and 5% CO2. The tube was rolled at 15 rpm at 36C. Remarkable hair growth was noticed for 7 to 8 days. Hair root sheaths also grew with the hair shafts. The structure of the hair bulbs was well maintained for at least 6 days, and then the hair matrix cells started to degenerate. Fetal calf serum was not essential for hair growth in vitro but increased the growth rate slowly. Testosterone and estrogen inhibited hair growth in vitro to a similar extent. The minimum effective doses of both hormones to suppress hair growth were around 5 ng/ml, which corresponds well to the normal plasma level of testosterone in adult males in vivo, suggesting that scalp hair growth may be critically controlled by testosterone in adult males.

We studied the effect of cyclosporin A on human hair growth using a recently described model in which isolated hair follicles are grown in vitro. Cyclosporin A had no effect on the rate of hair growth, but at 10-7 M, a dose within the therapeutic blood range, it maintained hair growth for longer than control to give a 42% greater mean follicle elongation after 15 d (p < 0.05). Eighteen of 42 cyclosporin-treated follicles (43%) were still growing after 15 d compared with one of 42 control follicles (2%). These results suggest that the hypertrichosis action of cyclosporin A may be due to the prolongation of the anagen phase of the hair-growth cycle.

We have examined the growth capacity of keratinocytes isolated from human scalp hair follicles. Like the keratinocytes of the glabrous epidermis, most of the colony-forming cells are classified as holoclones or meroclones when analyzed in a clonal assay. Some of them have extensive growth potential, as they are able to undergo at least 130 doublings. Therefore, the hair follicle, like the epidermis, contains keratinocytes with the expected property of stem cells: an extensive proliferative capacity permitting the generation of a large amount of epithelium. We have also examined the distribution of clonogenic keratinocytes within the hair follicle. Several hundred colony-forming cells are concentrated at a region below the midpoint of the follicle and outside the hair bulb. This region lies deeper than the site of insertion of the arrector pili muscle, which corresponds with the position of the bulge when the latter can be identified. In contrast, few colony-forming cells are present in the hair bulb, where most of the mitotic activity is observed during the active growth phase of the follicle. Paraclones, which are present both in the midregion and in the bulb of hair follicles, are unlikely to be the transient amplifying cells expected from kinetic studies.

The in vitro properties of cells cultured from the dermal papilla of human hair follicles were studied and compared with those of lines of dermal fibroblasts derived from the same material. In serial subcultures, the dermal papilla cells displayed a spread-out, polygonal cellular morphology at stationary growth phases and a tendency to form multi-layered aggregates before reaching confluence. Aggregation was particularly marked when papilla cells were grown on collagen gels. In contrast, dermal fibroblasts grew as branching, parallel arrays of spindle-shaped cells which remained as monolayers until confluence. Compared with dermal fibroblasts, papilla cells also exhibited a shorter in vitro survival time. The properties of cultured human papilla cells are similar to those of rat vibrissa papilla cells.

Conclusion

Hair growth will always be researched with the aim to improve it and prevent its decrease and even stagnation. With science advancing at such a rate and all new products and techniques being created, hair fall and reduced hair growth will soon become the thing of the past.