Hello! Welcome to our next lesson on the pathophysiology of PCOS.
In our last session, we uncovered a crucial part of the PCOS puzzle: how low levels of Sex Hormone-Binding Globulin (SHBG) dramatically increase the amount of biologically active androgens circulating in the body. We now understand that hyperandrogenism in PCOS is a two-part problem: the ovaries and adrenals produce too much androgen, and the liver produces too little SHBG to bind it, creating a vicious cycle.
Today, we will connect this systemic hormonal imbalance to the clinical signs that are often the most distressing for patients. Our learning outcome is to describe how androgens affect target tissues like the hair follicle and sebaceous gland to cause clinical signs. We will explore the specific cellular mechanisms that lead to hirsutism (excess hair growth), androgenic alopecia (hair loss), and acne.
The General Mechanism: How Androgens "Talk" to Skin Cells
Before we look at specific tissues, let's review the general pathway of androgen action, which you'll recognize from our foundational lessons. The effects of androgens on the skin are primarily driven by free testosterone and its more potent derivative, dihydrotestosterone (DHT).
The process unfolds as follows:
- Entry: Free, unbound testosterone from the blood enters a target cell in the skin, such as a cell in a hair follicle or sebaceous gland.
- Conversion: Inside the cell, an enzyme called 5-alpha reductase converts testosterone into dihydrotestosterone (DHT). DHT is significantly more potent than testosterone and is the primary driver of androgenic effects in the skin.
- Binding: DHT binds to an Androgen Receptor (AR) located in the cell's cytoplasm or nucleus.
- Gene Regulation: The DHT-AR complex moves into the nucleus and binds to specific DNA sequences. This action changes the expression of target genes, either increasing or decreasing the production of specific proteins.
- Physiological Effect: These changes in protein levels alter the cell's function, leading to the visible clinical signs.
Now, let's explore how this single mechanism produces such different effects in various skin structures.
Hirsutism: The Transformation of Vellus to Terminal Hair
One of the most common signs of hyperandrogenism is hirsutism. It's important to be precise with our definitions.
Topic 44: Hirsutism and Virilization
First, let's watch a short clip from the Association of Professors of Gynecology and Obstetrics (APGO) that clearly defines hirsutism and distinguishes the types of hair involved.
Please watch from 00:32 to 01:11. Pay attention to the distinction between vellus hair and terminal hair, and the typical male-pattern distribution of hirsutism.
As the video explained, hirsutism is the growth of coarse, pigmented terminal hair in androgen-sensitive areas like the chin, upper lip, chest, and lower abdomen. This is distinct from the fine, soft, unpigmented vellus hair (often called "peach fuzz") that covers most of the body.
In women with PCOS, high levels of androgens—specifically DHT—act on the vellus hair follicles in these sensitive areas, causing them to transform into terminal hair follicles.
This transformation involves several changes at the cellular level:
- Increased Follicle Size: Androgens signal the follicle to grow larger and more robust.
- Prolonged Anagen Phase: The anagen, or growth phase, of the hair cycle is extended.
- Increased Hair Diameter: The larger follicle produces a thicker, coarser hair shaft.
The following reading provides an excellent, detailed explanation of this process.
Deciphering the Role of Androgen in the Dermatologic ...
The article 'Deciphering the Role of Androgen...' explains the physiological and pathological effects of androgens on the skin. We will focus on the section describing how androgen excess leads to hirsutism.
Please read the subsection titled '6.1.1. Androgen Excess'. Focus on how androgens increase hair follicle size, hair diameter, and the duration of the anagen stage, leading to the vellus-to-terminal hair transformation.
An important concept highlighted in the reading is that the severity of hirsutism doesn't always correlate perfectly with blood androgen levels. This points to the importance of local tissue sensitivity. Factors within the hair follicle itself, such as the activity of 5-alpha reductase and the number of androgen receptors, play a huge role. This explains why some women may have significant hirsutism with only mildly elevated androgens, a condition sometimes referred to as "idiopathic hirsutism."
Androgenic Alopecia: The Scalp Hair Paradox
While androgens promote hair growth on the body, they have a paradoxical and opposite effect on the scalp in genetically predisposed individuals, leading to androgenic alopecia, or female pattern hair loss (FPHL).
To understand this, we first need to recall the hair growth cycle.
Hair Loss: Androgenic Alopecia
This short animated video provides a clear overview of the three phases of the hair growth cycle.
Please watch from the beginning to 01:42. First, focus on the description of the anagen (growth), catagen (transition), and telogen (resting) phases. Then, pay close attention to how androgens alter this cycle to cause hair thinning.
As the video explained, DHT acts on scalp hair follicles to:
- Shorten the anagen (growth) phase.
- Lengthen the telogen (resting) phase.
This leads to a process called follicular miniaturization. With each successive hair cycle, the follicle shrinks and moves closer to the skin surface. It produces a hair that is shorter, finer, and less pigmented. Over time, these terminal hairs are replaced by vellus-like hairs, causing a visible reduction in hair density, typically over the central part of the scalp.
The following reading delves into the mechanisms behind this paradoxical effect.
Deciphering the Role of Androgen in the Dermatologic ...
Let's return to the 'Deciphering the Role of Androgen...' article to explore this paradoxical effect of DHT on the scalp.
Please read the subsection titled '6.2.1. Role of Androgens in Female Pattern Hair Loss'. Note how DHT does the opposite of what it does in body hair follicles: it shortens the anagen phase and causes miniaturization, transforming terminal hairs into vellus hairs.
This section highlights that FPHL is a result of an interplay between androgen excess and a genetic predisposition of the scalp follicles to be sensitive to DHT.
Acne Vulgaris: Overstimulation of the Sebaceous Gland
The third major clinical sign of hyperandrogenism is acne. Acne develops in the pilosebaceous unit, which consists of the hair follicle and its attached sebaceous gland. Androgens are the primary driver of the first and most crucial step in acne formation.
The pathophysiology of acne involves four key processes:
- Hyperseborrhea (Excess Sebum Production): This is the androgen-dependent step. Androgens, particularly DHT, stimulate the proliferation of sebocytes (the cells of the sebaceous gland) and drastically increase the production of sebum, an oily substance that normally lubricates the skin.
- Abnormal Follicular Keratinization: The cells lining the hair follicle shed abnormally and clump together, forming a plug (a microcomedone) that blocks the pore.
- Bacterial Colonization: The excess sebum creates an anaerobic, lipid-rich environment perfect for the proliferation of the bacterium Cutibacterium acnes (formerly Propionibacterium acnes).
- Inflammation and Immune Response: C. acnes and the breakdown products of sebum trigger an inflammatory response, leading to the red, swollen lesions (papules, pustules, nodules) characteristic of inflammatory acne.
Let's read a final section that connects these steps directly to androgen excess in PCOS.
Deciphering the Role of Androgen in the Dermatologic ...
The 'Deciphering the Role of Androgen...' article also provides an excellent summary of how androgen excess drives the development of acne.
Please read the subsection '6.3.1. Androgen Excess'. Focus on how androgens promote sebum production and how this creates the perfect environment for the subsequent steps of acne pathogenesis.
Test your understanding!
A class of drugs called 5-alpha reductase inhibitors (e.g., finasteride) works by blocking the conversion of testosterone to the more potent DHT. Based on what you've learned, explain how this single mechanism could theoretically be used to treat both hirsutism and androgenic alopecia in a woman with PCOS.
Show answer
By inhibiting 5-alpha reductase, the drug reduces the amount of DHT available to bind to androgen receptors in target tissues.
- For Hirsutism: In androgen-sensitive body hair follicles (like on the chin or chest), less DHT means less stimulation for vellus hairs to transform into terminal hairs. It would reduce the growth signal, leading to finer, slower-growing hair over time.
- For Androgenic Alopecia: In genetically susceptible scalp follicles, less DHT means a reduction in the signal that causes miniaturization. This would help to lengthen the anagen (growth) phase and shorten the telogen (resting) phase, allowing follicles to produce thicker, stronger hairs and slowing the progression of hair loss.
This demonstrates how understanding the specific role of DHT in different tissues is key to understanding both the problem and the potential treatments.
Conclusion
Today we have bridged the gap between the hormonal chaos of PCOS and its visible manifestations. We've seen how a single class of hormones—androgens—can produce such varied and even paradoxical effects depending on the target tissue and its local environment.
Key Takeaways:
- The clinical signs of hyperandrogenism are caused by the action of androgens, primarily dihydrotestosterone (DHT), on target cells in the skin.
- Hirsutism is the result of DHT stimulating vellus hair follicles on the face and body, causing them to transform into large, terminal hair follicles by prolonging the growth phase.
- Androgenic alopecia is a paradoxical effect where DHT on the scalp causes follicular miniaturization by shortening the growth phase, leading to thinner, finer hairs.
- Acne is initiated by DHT-driven hyperseborrhea (excess sebum production), which creates the ideal environment for follicular plugging, bacterial growth, and inflammation.
- The clinical severity of these signs depends not only on systemic androgen levels but also on local factors like 5-alpha reductase activity and androgen receptor sensitivity in the skin.
Preview of the Next Lesson:
We have now thoroughly examined the causes and consequences of hyperandrogenism. In the next module, we will shift our focus back to the core reproductive dysfunction in PCOS. Our first lesson will be: "Describe the abnormal gonadotropin secretion pattern in PCOS, including the elevated LH:FSH ratio." We will explore how the disruption of the HPO axis leads to arrested follicular development and anovulation, the other key components of the syndrome.