Variation of Hair Follicle Counts among Different Scalp Areas: A Quantitative Histopathological Study

Abstract

Introduction

Scalp biopsy is a standard method for the definitive diagnosis of alopecia. The hair count parameters of each scalp area remain unclear. This study aimed to determine hair count values at different scalp locations from histopathology and to establish reference values for each part of the scalp.

Methods

We obtained biopsy specimens from the frontal, vertex, temporoparietal, and occipital areas of the scalps of normal deceased subjects. All specimens were evaluated for the number of follicular units, hair counts, hair types, and stages of the hair cycle.

Results

In total, 240 specimens were collected from 60 cadavers. Across all scalp sites, the temporoparietal area showed the lowest mean hair count, number of follicular units, terminal and vellus hairs, and terminal-to-vellus hair ratio. The average anagen-to-telogen hair ratio was comparable across all scalp sites. This study did not observe a significant association of hair parameters with gender differences or increasing age in all scalp areas.

Conclusions

The present study revealed the diversity of the hair index among different scalp areas and suggested that normal hair count values should be separately standardized on each scalp region. Our findings may provide useful reference values for the histopathological evaluation of hair disorders in Asians.

Introduction

Alopecia is a common and distressing clinical problem with many heterogeneous etiologies [1, 2, 3]. It may affect patients' quality of life both emotionally and psychologically, as hair generally influences cosmetic appearance, and treatments may not be satisfactory for some patients [4, 5, 6]. The diagnosis of alopecia not only incorporates history taking and physical examination but also often requires additional investigations. Multiple methods have been introduced for assessing alopecia, such as through a phototrichogram, trichoscopy, or videodermoscopy [7, 8]. However, scalp biopsy remains a standard procedure to assess various hair parameters as it maximizes the diagnostic yield; the horizontal and vertical sections of a scalp biopsy specimen allow for the visualization of hair follicle morphology at different anatomical levels. A scalp biopsy also provides information on the stages of the hair cycle and the pathological features of the surrounding structures. Qualitative and quantitative data obtained from a scalp biopsy can help dermatologists to accurately diagnose hair disorders, particularly nonscarring alopecia [9, 10, 11, 12].

Comparison of hair count values between patients with alopecia and normal controls is necessary to assess histopathological sections of the scalp. Previous studies of scalp biopsy in the general population have provided different reference values for each ethnicity, implicating ethnicity as a factor that influences the variation seen in different hair parameters. Previous studies have reported that Asians have a lower total hair count than Caucasians and Hispanics, but are comparatively similar to Africans [10, 13, 14, 15, 16, 17, 18, 19, 20]. Interestingly, the area of scalp examined may be another factor affecting hair parameter variation, since earlier studies reported unequal hair count values among different scalp locations [15, 16]. Variations in hair density and diameter among different ethnicities and across different scalp areas have also been supported by previous research using quantitative trichoscopic analysis [21, 22, 23, 24, 25, 26].

Currently, variation in hair counts among different scalp locations has not been well investigated, and reference values from each scalp site remain undefined. Therefore, this study aimed to evaluate hair count parameters of different scalp areas through histopathological investigation in the general Thai population and to establish reference values for each part of the scalp to aid a more precise diagnosis of hair disorders.

Materials and Methods

This study was approved by the Mahidol University Institutional Review Board for Ethics in Human Research (ID 07-60-16). Subjects were deceased individuals whose cadavers required medicolegal autopsies. A signed consent form was obtained from the subjects' relatives before their enrollment. Inclusion criteria were deceased subjects aged 18 years or older with clinically normal hair and scalp presentation. Hair-related histories were additionally collected from relatives of the deceased. Exclusion criteria included unavailable demographic data, incomplete medical records, unknown time of death, history of hair and/or scalp disorders, history of systemic diseases or medications affecting the hair growth cycle within the past 6 months, and history of laser or light-based therapy within the past 3 months. We also excluded subjects with a positive hair pulling test at the time of biopsy.

After recording demographic data and medical history, we obtained 4 scalp biopsy specimens from each subject. Four-millimeter punch biopsies were performed on 4 sites of the scalp, including the frontal, vertex, temporoparietal, and occipital areas. Biopsy was performed at 2 cm above the middle of the hairline in the frontal area, at 4 cm behind the middle of an imaginary line crossing between the left and right external ear canals in the vertex area; at 7 cm over the external auditory ear canals in the temporoparietal area; and at 1 cm below the greater occipital protuberance in the occipital area. The duration from the time of death to the time of specimen collection was <8 h to preserve the biopsy specimens, as recommended by a previous study [17]. All biopsy specimens were processed according to Whiting's protocol [10]. Moreover, the histopathological examination was performed by a blinded dermatopathologist. The hair parameters recorded included the number of follicular units and hair follicles, hair type (terminal/indeterminate or vellus), and stage of the hair cycle (anagen or catagen/telogen). We did not differentiate between follicles in the catagen and telogen stages.

All data were statistically analyzed using SPSS, version 18.0 (SPSS Inc., Chicago, IL, USA). The sample size was calculated based on hair counts from a previous study of scalp biopsy [14]. To achieve a power of 80% and a confidence level of 95%, the estimated minimal number of subjects was 20. Linear mixed models were used to compare the hair parameters of different scalp areas. When the overall comparison showed p value <0.05, pairwise comparisons of subgroups were performed. Differences in hair parameters between genders were determined using the χ² test, t test, or Wilcoxon rank-sum test depending on the data characteristics. Correlation coefficients were used to determine the effect of age on hair count values. Statistical significance was determined as a p value <0.05.

Results

A total of 240 scalp biopsy specimens were obtained from 60 normal cadavers, comprising 36 males and 24 females. The mean age was 39.1 ± 11.9 years, ranging from 21 to 82 years. The average number of follicular units in the frontal, vertex, temporoparietal, and occipital areas was 9.2 ± 1.7, 9.3 ± 1.9, 8.7 ± 1.2, and 9.1 ± 1.8, respectively. Regarding total hair counts, the vertex had the highest values of 23.7 ± 5.3, whereas the frontal and occipital areas had relatively comparable values of 22.8 ± 5.1 and 21.4 ± 5.6, respectively. The temporoparietal area showed the lowest total hair counts among all the scalp sites of 18.4 ± 4.1. The temporoparietal area also demonstrated the lowest mean numbers of terminal hairs, vellus hairs, and terminal-to-vellus hair (T:V) ratio compared with the other scalp sites. The average anagen-to-telogen hair (A:T) ratio in each scalp area was comparable. In the linear mixed models, pairwise comparisons showed statistically significant differences between total and terminal hairs of the temporoparietal scalp and those of the other scalp areas. All hair count parameters are summarized in Table 1.

Table 1.

Comparisons of hair count parameters among different scalp areas in the normal Thai population

	Frontal	Vertex	Temporoparietal	Occipital	p value
Follicular units	9.2±1.7	9.3±1.9	8.7±1.2	9.1±1.8	0.21
Total hairs	22.8±5.1	23.7±5.3	18.4±4.1	21.4±5.6	0.01*
Terminal hairs	20.4±4.2	21.2±4.8	16.5±3.6	18.9±4.9	0.01*
Vellus hairs	3 (0–7)	3 (0–6)	2 (0–4)	2 (0–7)	0.82
Terminal/vellus ratio	9.3:1	8.9:1	8.3:1	8.8:1	0.41
Anagen/telogen ratio	91.9:8.2	92.4:7.9	91.1:8.9	92.7:8.1	0.65

^*Pairwise comparisons reveal statistically significant differences between temporoparietal scalp and each of the other scalp areas.

The comparison of hair counts between male and female subjects, including the average number of total hairs, terminal hairs, vellus hairs, and follicular units, as well as the average T:V and A:T ratios, revealed no statistically significant differences between scalp sites (all p > 0.05) (Table 2). No statistically significant correlation was found between hair count values and increasing age for all scalp locations (all r < 0.1) (Fig. 1).

Table 2.

Comparisons of hair count parameters among different scalp areas between males and females

	Frontal	Vertex	Temporoparietal	Occipital
	male	female	male	female	male	female	male	female
Follicular units	9.3±1.8	8.9±1.7	9.4±2.1	9.1±1.8	8.8±1.2	8.3±1.4	9.2±1.6	8.8±1.8
Total hairs	23.2±5.6	21.5±4.8	23.9±5.9	22.1±4.9	18.9±4.2	17.1±4.1	22.2±6.4	20.8±4.2
Terminal hairs	20.8±4.8	18.9±5.1	21.5±5.2	19.8±4.5	16.9±3.9	16.1±4.1	19.1±4.8	17.8±5.1
Vellus hairs	3 (0–7)	3 (0–5)	3 (0–6)	2 (0–6)	2 (0–4)	2 (0–3)	2 (0–7)	2 (0–5)
Terminal/vellus ratio	9.4:1	8.9:1	9.2:1	8.9:1	8.6:1	8.2:1	9.1:1	8.8:1
Anagen/telogen ratio	92.4:8.1	90.9:8.8	92.8:7.8	91.6:8.6	92.1:8.4	91.4:8.3	92.9:7.9	91.4:8.2

No statistically significant differences between genders in any of the parameters.

Fig. 1.

Relationship among the scalp areas of subjects for hair count parameters; the scatterplots show no correlation between hair count values (i.e., follicular unit, total hair, terminal hair, vellus hair, anagen hair, and telogen hair) and increasing age for all scalp locations (i.e., frontal area, vertex area, temporoparietal area, and occipital area).

Discussion

The quantitative and qualitative information obtained from scalp biopsy is important for ensuring the accurate diagnosis of various hair disorders. Previous studies have suggested that hair count values may vary among different scalp areas; our study verified these findings by reporting differences in pathological hair count parameters among different scalp sites in the general Thai population. We observed that the average numbers of follicular units and total hair counts were highest in the vertex area and lowest in the temporoparietal area. We also demonstrated that A:T and T:V ratios were comparable between all scalp areas. There were no differences between genders and no correlations with increasing age for any of the parameters.

The number of total hairs in a normal scalp correlates with the number of follicular units as human hair normally emerges from the scalp in groupings [27]. The present study showed that the number of follicular units and total hair counts differs at different sites of the scalp. Although the average numbers of both indices were highest in the vertex area, these values were comparable to those from the frontal and occipital areas. In contrast, the temporoparietal area presented the lowest number of follicular units and total hair count. This pattern of variation was consistent with the results from a study of scalp biopsies by Mulinari-Brenner et al. [11] as well as previous studies that utilized quantitative trichoscopic analysis [11, 21, 22, 23, 24, 25, 26]. Another study of scalp biopsies from Taiwanese individuals also acknowledged the differences in parameters across different scalp sites, but the number of follicular units and total hairs was the highest in the frontal area, whereas the lowest values were observed in the temporal area [16]. This discrepancy of the results may be due to the different methodology and the small number of biopsy specimens collected from each scalp site in the previous study [16].

The differences in the hair parameters of each scalp area may be explained by their embryological derivation. Human hair follicle formation largely takes place during the embryogenic period from the primitive epidermis via numerous signals arising in both the epithelium and its underlying mesoderm [28]. The anterior scalp consisting of the frontal and temporoparietal areas originates from the neural crest, while the occipital area has a mesodermal origin and creates the posterior scalp [29]. We speculate that scalp areas with different origins may have different patterns of gene expression and signal interaction during the embryogenic stage. There are multiple signaling pathways associated with hair follicle development. The Hox genes, especially Hoxc13, function in processes of hair follicular patterning, hair follicle differentiation, and hair cycle control [30]. One main mechanism of the primary hair follicle patterning process relies on ectodysplasin anhidrotic receptor (EDAR)-bone morphogenetic protein activation-inhibition interactions [31]. The EDAR signaling pathway appears to be among the key regulators of follicle pattern formation, as it participates in the follicular fate decision and stabilizes the Wnt/β-catenin active foci, which is an important event for determining definitive follicle locations [31, 32, 33]. As EDAR and Wnt/β-catenin are restricted to the cell in which they are produced, we hypothesize that the distinguishing number of follicular units and total hair counts in the temporoparietal area may be associated with a differential range of activating molecule production in this particular site of embryogenic skin [31, 34, 35].

Determining the type of hair (terminal, indeterminate, or vellus hair) is beneficial for the diagnosis of nonscarring alopecia, especially androgenetic alopecia [36, 37]. Changes in the T:V ratio can result from the alteration of follicular physiology such as miniaturization. The T:V ratio lower than 4:1 is indicative for androgenetic alopecia [38, 39]. An experimental study suggested that Wnt10b/dickkopf-1 regulated hair follicle size, hair bulb width, and hair shaft thickness during the anagen phase by modulating hair matrix, dermal papillae, and stem cell behaviors [40]. Our study showed a slightly lower T:V ratio at the temporoparietal area compared with the other 3 scalp sites. We hypothesize that the temporoparietal area may have a different expression of the aforementioned signal.

Hair follicles undergo continuous cyclical phases of anagen, catagen, and telogen. The transition between each growth phase is regulated by combinations of epithelial-mesenchymal interactions in the hair follicles, multiple microenvironmental signals, and immune cell regulation [41, 42, 43, 44]. Wnt signaling is one of the fundamental pathways for regulating the hair follicle cycle. Changes in Wnt expression have recently been identified as the key inducer of the anagen-to-telogen transformation [45]. Histopathological evaluation of the amount of anagen hairs, telogen hairs, and their ratio (A:T) is essential for understanding the normal hair growth cycle in each scalp area. Previous studies did not reveal the differences in A:T ratios among different ethnicities [10, 13, 14, 15, 16, 18, 20, 46]. Consistent with previous findings, our study indicated comparable A:T ratios across different scalp sites. We speculate that the proportion of active and resting hair follicles is universally consistent.

As the present study was conducted at a single referral center, our results may not be representative of the general population. Therefore, a further nationwide multicenter study with stratification of a large sample size is encouraged to eliminate this limitation.

Conclusion

The present study used histopathology to investigate various hair count parameters from each scalp area in the normal Thai population. This study demonstrated the diversity of the hair index among different scalp areas and suggested that normal hair count values should be separately standardized on each scalp region. Moreover, the information gained from this study can be applied as reference data for the histopathological assessment of hair disorders in Asian individuals. Therefore, the area of scalp biopsy should be utilized in the standard of histopathological evaluation of transverse sections for a more precise diagnosis.

Statement of Ethics

This study was conducted in accordance with the principles of the Declaration of Helsinki and in compliance with the International Conference on Harmonization-Good Clinical Practice and local regulatory requirements. The study was reviewed and approved by the Mahidol University Institutional Review Board for Ethics in Human Research (ID 07-60-16). A signed consent form was obtained from the subjects' relatives before enrollment.

Conflict of Interest Statement

The authors declare that they have no conflicts of interest, relevant financial activities, or relationships, which could be perceived to have influenced or that gives the appearance of potentially influencing what is written in the submitted work, to disclose.

Funding Sources

Funding was provided by the Division of Dermatology, Department of Medicine, Faculty of Medicine, Ramathibodi Hospital, Mahidol University, Thailand.

Author Contributions

P.S. and K.C. contributed to conceptualization and funding acquisition; P.S., Y.V., and S.S. contributed to methodology; P.S. and S.R. contributed to validation; P.S., K.C., K.P., and K.L. contributed to formal analysis; K.P., K.C., and S.R. contributed to investigation and data curation; S.R., K.C., K.P., and K.L. contributed to writing − original draft preparation; P.S. and K.C. contributed to writing − review and editing. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.