1. ^*Department of Dermatology, University Hospital Hamburg-Eppendorf, University of Hamburg, Hamburg, Germany
2. ^†Department of Physiology, University of Debrecen, Debrecen, Hungary

Correspondence: Ralf Paus, Department of Dermatology, University Hospital Hamburg-Eppendorf, University of Hamburg, D-20246 Hamburg, Germany. Email: paus@uke.uni-hamburg.de

Received 2 October 2003; Revised 22 October 2003; Accepted 22 October 2003; Published online 23 March 2004.

To the Editor:

While it is undisputed that estrogens (17--estradiol, E₂) can profoundly modulate hair growth in practically all mammalian species investigated, usually exhibiting hair growth inhibitory properties (^{Emmens, 1942;Williams et al, 1946;Stumpf et al, 1974}; Ebling et al, 1991; Smart et al, 1999; Chanda et al, 2000), it is still rather unclear what exactly E₂ administration does to human scalp hair growth.

Given the profound clinical relevance of this question and the frequent use of E₂-containing topical preparations in trichological practise (^{Sinclair, 1999;Sterry and Paus, 2000}), this question deserves a more careful and more systematic dissection than it has experienced in the past. This letter calls attention to a few salient points that should be taken into account in this respect.

For human scalp hair, topical E₂ has long been used in the management of telogen effluvium and androgenetic alopecia, especially in women (^{Wüstner and Orfanos, 1974;Schuhmacher-Stock, 1981;Sterry and Paus, 2000}). Even though this remains to be unequivocally proven in vivo, it is felt that E₂ inhibits hair shaft formation, thus lowering the rate of hair growth, i.e., how much new hair shaft is generated by the anagen hair bulb per time unit, yet prolongs anagen duration, thus decreasing the telogen rate (^{Schuhmacher-Stock, 1981;Sinclair et al, 1999}). This would help to explain the clinical observation that topical E₂ or high systemic E₂ levels during estrogen-based contraception and during pregnancy increase the telogen/anagen ratio, thus notably improving a pre-existing telogen effluvium, while causing a telogen effluvium postpartum supposedly due to E₂ withdrawal (^{Lynfield, 1960;Barman et al, 1969;Ebling et al, 1991}). In view of the very complex, multiple concomitant endocrine changes during and after pregnancy and lactation (including, e.g., dramatic fluctuations in gestagen and prolactin levels (^{Braunstein, 2003})), it is however exceedingly difficult to dissociate strictly E₂-based hair growth effects from those that other hormones might exert on the human scalp hair follicle in vivo during this time.

Therefore, it is reasonable to explore the isolated effects of E₂ on human hair shaft elongation, anagen duration, and hair follicle keratinocyte proliferation/apoptosis in microdissected human anagen VI scalp hair follicles that are organ-cultured according to the method pioneered by^{Philpott et al (1990)}—in the absence of the sebaceous gland, a key compartment for steroid hormone synthesis and metabolism (^{Zouboulis, 2000}). We found only two corresponding reports in the published literature, one using human anagen hair follicles from an unspecified scalp skin location of what appears to be two male individuals aged 17 and 35 y (^{Kondo et al, 1990}), and one recent meeting abstract based on the use of female occipital scalp skin follicles (^{Nelson et al, 2003}). Both studies report that E₂ (Kondo et al: 18 nM; Nelson et al: 10 nM), significantly inhibits human scalp hair shaft elongation in vitro. In addition, Kondo et al (1990) report that E₂ does not influence the "decay rate" of organ-cultured human anagen hair follicles (as measured by morphology and autoradiographic 3H-thymidine incorporation).

Recently, we have also studied the effects of E₂ (1 nM–1 M, Sigma St. Louis, MO) on female occipital scalp hair follicles, and have essentially confirmed hair shaft elongation-inhibitory properties of E₂, which were maximal at 1 M (^{Ohnemus et al, 2003}). In view of the extreme dependence of androgen effects on the exact integumental location of human hair follicles however (^{Ebling, 1991;Jahoda and Reynolds, 1996}), we were curious to learn whether E₂ effects on human scalp hair follicles are location- and/or sex-dependent. This has already been demonstrated for the E₂ response of pelage hair follicles from mice and rats, which is profoundly influenced by sex and body site (^{Emmens, 1942;Mohn, 1958}).

Therefore, we have investigated in a single, large, frontotemporal scalp skin sample (healthy male individual, no medications, 46 y; obtained with informed consent during routine facelift plastic surgery; all experiments were performed in order to the Declaration of Helsinki Principles) how E₂ addition to the medium (1–100 nM, Sigma, diluted in serum-free William's E medium, supplemented with L-glutamine, penicillin, streptomycin, insulin, and hydrocortisone) affected hair shaft elongation, anagen duration, hair follicle pigmentation and hair matrix keratinocyte proliferation in microdissected, organ-cultured male anagen VI hair follicles from the frontotemporal scalp skin region.

Surprisingly, compared to the vehicle control, the hair shaft elongation of male frontotemporal scalp hair follicles was significantly stimulated by 1–100 nM E₂ already as early as 1 d after the start of organ culture, and this stimulation became even more pronounced at the end of organ culture (days 7 and 9) Figure 1. This stimulation of hair shaft formation (which is the result of stringently coordinated proliferation and differentiation of hair matrix keratinocytes (^{Stenn and Paus, 2001}) corresponded to a significant stimulation of hair matrix keratinocyte proliferation by 10 nM E₂ at day 9 (average number of Ki-positive-cells: in the control group 14 cells (SEM 3.21) and 26 cells in the E₂-treated (10 nM) group (SEM 4.38); level of significance: p0.05, Mann–Whitney test). While no evident differences were noted by H&E or Fontana–Masson histochemistry between E₂- and vehicle-treated hair follicles in the hair follicle pigmentary unit or in the degree of hair follicle degeneration during organ culture (data not shown), a slight, though not statistically significant, anagen-prolonging effect of E₂ was seen in E₂-treated test hair follicles as compared to vehicle controls (data not shown).

Figure 1.

Hair shaft elongation of organ-cultured human scalp hair follicles (%, male, frontotemporal), measurements of the increase of the length of the follicles on days 0, 1, 3, 5, 7, and 9 (compared to day 0), using an inverted microscope fitted with an eyepiece measuring graticule SEM, Level of significance: ^*p0.05; ^**p0.01; ^***p0.001. Mann–Whitney test, test groups are compared to the vehicle control. n=36 hair follicles per test group, n=18 hair follicles in the control group.

Full figure and legend (20K)

Therefore, organ-cultured male frontotemporal scalp hair follicles in vitro respond to E₂ treatment (by a mode of E₂ administration that mimics systemic drug application) with a stimulation of both hair shaft generation and of hair matrix keratinocyte proliferation, and a tendency towards anagen prolongation, while no hair pigmentation effects are seen. This is well in line with the ill-documented, but widely shared clinical experience of topically applied E₂ on the male scalp in vivo (i.e., hair growth stimulation; Schumacher-Stock, 1981) and supports the anagen-prolonging effect of E₂.

Our observation in a single, yet carefully analyzed male patient^* raises five basic questions that must be addressed much more systematically by subsequent work on the effects of E₂ on human hair growth in order to better explain the seemingly contradictory results obtained with occipital (^{Kondo et al, 1990;Nelson et al, 2003}) versus frontotemporal scalp hair follicles Figure 1:

1. What are the differences between male and female hair follicles with respect to estrogen receptor (ER-, ER-) expression (^{Thornton, 2003}) and aromatase activity (^{Sawaya and Price, 1997;Hoffmann, 2001,2002})?

2. How do occipital and frontotemporal hair follicles, as well as various other integumental sites, differ from each other in this respect?

3. Is there any indication that E₂ exerts similarly "paradoxical, site-dependent" effects on human hair growth as androgens (^{Jahoda and Reynolds, 1996})? Does the signalling and gene expression response of a defined human hair follicle population to E₂ stimulation differ in a stringently location-dependent manner, as has been postulated, e.g., for the response of beard hair versus scalp follicles to androgen stimulation with respect to TGF1 expression in the dermal papilla (^{Inui et al, 2002})?

4. Which important regional differences in the extrafollicular estrogen metabolism of defined integumental sites must be taken into account when estrogens are administered topically (e.g., with respect to epidermal, sebaceous, and dermal activities of key enzymes like aromatase, 17-hydroxysteroid dehydrogenase, or steroid sulfatase)?

5. How is ER expression and/or the metabolism of topically applied estrogens in loco influenced by the choice of vehicle?

Only when these basic questions are finally addressed systematically can we expect to solve the ancient enigma of what estrogens really do to human hair growth in a defined integumental site in vivo, on the scalp and elsewhere, and can rationally select estrogen receptor ligands for therapeutically desired hair growth modulation.

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References

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