Scalp Apocrine Glands: The Neglected Component of the Hair Follicle Complex

Abstract

Background

In humans, apocrine glands are generally associated with chemical signaling and body odor. Their presence in the scalp is poorly documented, commonly being associated with benign and malignant tumors. If apocrine glands are consistently present in the normal scalp or become apparent only under pathological circumstances remains unclear. Moreover, their potential physiological role in relation to the hair follicle and microbiome modulation has yet to be fully elucidated.

Summary

The role of scalp apocrine glands is often considered rudimentary and may be underestimated. However, the higher excretions of urea and potassium compared to eccrine glands suggest a potential role in the removal of metabolic waste products and electrolyte balance. Additionally, the openings of apocrine ducts drain into the hair follicle, mixing with follicular contents and sebum before reaching the skin’s surface. This highlights a possible close interaction between the apocrine glands and microbiome. Furthermore, given the anatomical and functional association between follicular units and apocrine glands, it is likely that they participate in the pathogenesis of follicular occlusion diseases.

Key Messages

This article aimed to review existing research on scalp apocrine glands, clarifying this underexplored topic and highlighting them as a potentially relevant component of scalp physiology and pathology.

Introduction

In primates, scalp apocrine glands play a role in thermoregulation, social and sexual signaling. In humans, their presence in the scalp is poorly documented in the medical literature, commonly receiving attention only in the context of pathological conditions, such as benign or malign scalp tumors or other dermatoses [1, 2]. Whether apocrine glands are consistently present in the normal scalp or exhibit latent activity that becomes apparent only under pathological circumstances remains unclear. Furthermore, their potential physiological role in relation to the hair follicle and microbiome modulation has yet to be fully elucidated. This article aimed to review the existing research on the role of apocrine glands in the scalp and discuss how those structures might play a role in the tetrad of follicular occlusion.

The authors performed a literature search in the scientific database PubMed and selected articles published from 2010 until June 2025. We also conducted a review of the main textbooks of dermatology. The search terms were “apocrine glands,” “trichology,” “scalp histology,” “scalp histopathology,” “apocrine scalp tumors,” and “scalp”. We focused on articles published in English and excluded duplicate titles, prioritizing reviews and the most relevant studies that specifically addressed apocrine gland’s function and development, as well as their clinical settings on the scalp. After screening and eligibility assessment, 11 articles and 5 book chapters were selected due to their relevance, technical depth, and frequent citation in the indexed literature. Notably, the inclusion of these chapters was crucial, adding information not fully covered by the articles.

Discussion

Hair follicle distribution and phenotype are already genetically determined in intrauterine life [3]. Throughout embryologic development, hair follicle formation versus sweat glands is determined by a fine-tuned balance of mesenchymal-epithelial signaling. Both structures share early morphogenetic pathways, but their fate diverges due to antagonistic signals. Wnt/β-catenin and Bone Morphogenetic Protein are the major pathways that promote either hair follicle or gland formation, in an exclusive way [4]. Sweat glands usually surround hair follicles. They are classified into two main types: eccrine and apocrine. Each type possesses distinct anatomical, physiological, and developmental characteristics that determine their specific roles in health and disease [5]. Both eccrine and apocrine glands originate from the embryonic ectoderm [6]. However, their developmental paths diverge, as follows: eccrine glands develop as epithelial cell buds grow into the underlying mesenchyme, forming coiled tubular glands with ducts that open onto the skin surface. They appear early in gestation (12th week) and are partially functional at birth [7]. Eccrine glands are primarily responsible for thermoregulation though the production of watery sweat. Eccrine glands and hair follicles are classically considered independent structures, but it has been demonstrated that they might function in association, as a hair cluster [8].

On the other hand, apocrine glands originate from hair follicle differentiation (around the 20th week) and their duct opens into the follicular infundibulum [5]. It seems reasonable that apocrine glands are related to hair follicles in some way, anatomically and physiologically. In the embryo, apocrine glands are initially distributed across the entire skin surface, but most regress overtime, resulting in the characteristic adult distribution in the axillae, perianal region, areolas of the breasts and external ear channel [8, 9]. They are also found in the scalp, as consistently evidenced by clinical conditions such as apocrine scalp chromhidrosis and primary cutaneous tumors, such as apocrine carcinoma of the scalp [10, 11]. While the function of these glands is yet to be fully understood, they are generally associated with chemical signaling and body odor [10].

On the scalp, the role of apocrine glands is often considered rudimentary and may be underestimated [10]. Although present at birth, they become functionally active only after puberty, triggered by the influence of sex hormones. The composition of apocrine secretion is described as viscous, lipid-rich, and contains proteins, sugars, and ammonia, in addition to possibly including electrolytes, steroids, and vitamins (shown in Fig. 1) [10, 12]. Apocrine glands showed significantly higher excretions of urea and potassium compared to eccrine glands, especially during dynamic exercise [12]. It suggests a potential role for apocrine glands in the removal of metabolic waste products and electrolyte balance, particularly under conditions of elevated metabolic rate. This idea expands the understanding of their physiological contribution to homeostasis beyond their thermoregulatory or odor-producing functions [10]. Furthermore, the innervation of apocrine glands remains poorly understood, limiting our ability to fully explain how emotional or physical stimuli specifically influence apocrine secretion, particularly in the scalp.

Fig. 1.

Schematic representation of the pilosebaceous unit and associated adnexal structures, highlighting the anatomical relationship with the apocrine gland and its main physiological functions.

The openings of the apocrine gland ducts drain into the hair follicle infundibulum, mixing with other follicular contents and sebum before reaching the skin’s surface [13]. This interaction alters the final composition of the secretion and influences its relationship with the scalp’s microbial environment, which is very complex, thus affecting both odor formation and the skin surface composition. In this scenario, the apocrine function will also depend on how their secretions will be processed. Consequently, changes in the scalp microbiome can significantly impact apocrine production properties, even if glandular secretion remains unchanged. This hypothesis highlights the microbiome as a possible key modulator of apocrine function [13].

Apocrine glands have been associated with both benign and malignant tumoral conditions. Malignant lesions such as primary cutaneous apocrine carcinoma may arise on the scalp, often exhibiting aggressive behavior and requiring wide surgical excision followed by close surveillance [9]. Moreover, aggressive adnexal tumors with apocrine differentiation, such as apocrine hidradenocarcinoma, belong to a broader spectrum of malignancies that may involve both eccrine and apocrine components [14]. Benign cystic tumors, such as apocrine hidrocystoma, are also observed and are typically characterized by positive transillumination and distinctive histopathological features [15]. Classical sebaceous nevi of Jadassohn, an adnexal hamartoma, also demonstrate epithelial, follicular, sebaceous, and apocrine hyperplasia, with the scalp being the most affected site [16].

Furthermore, the pathogenesis of the follicular occlusion tetrad, comprising hidradenitis suppurativa, acne conglobate, dissecting cellulitis, and pilonidal sinus, has traditionally centered on infundibular obstruction of the hair follicle. Given the anatomical and functional association between follicular units and apocrine glands, it is likely that these may play an amplifying role. When occlusion occurs, it impairs the drainage of sebaceous, keratinous material and apocrine products, leading to retention of bioactive secretions, which may act as inflammatory adjuvants. This may contribute to follicular rupture, neutrophilic recruitment, and chronic tissue damage. The predilection of these conditions for apocrine-rich areas further supports this hypothesis. Therefore, we proposed that the tetrad could be more accurately conceptualized as a disorder of the follicular-sebaceous-apocrine unit, in which follicular obstruction initiates the disease while apocrine dysfunction contributes to the sustenance and exacerbation of chronic inflammation. They may be a “missing link” in the pathogenesis of these disorders.

Conclusion

Finally, this article shed light on the underexplored human scalp apocrine glands, which might potentially be an important component of scalp physiology and pathology. It is likely that their functions extend beyond odor production and involvement in tumoral processes. These may include roles in waste excretion, electrolyte balance, and modulation of the local microbiome. However, it is important to better understand their development, innervation, and regulation by hormonal and microbial factors. As previously hypothesized in this article, it would be interesting to explore the role of apocrine glands in the scenario of the follicular occlusion tetrad and to clarify their interaction with the hair follicle. A deeper understanding of these neglected glands may ultimately reveal novel insights into scalp health and disease, pointing for targeted therapeutic approaches.

Conflict of Interest Statement

The authors have no conflicts of interest to declare.

Funding Sources

This study was not supported by any sponsor or funder.

Author Contributions

Fernanda Nogueira Torres conceived the idea, designed the study, wrote the manuscript, reviewed the text, and approved the final version to be published. André Luiz Vairo Donda prepared the draft, wrote the manuscript, and approved the final version to be published. Daniel Fernandes Melo designed the study, wrote the manuscript, reviewed the text, and approved the final version to be published.

Funding Statement

This study was not supported by any sponsor or funder.