Why Use Horizontal and Vertical LPP Biopsy Sections?

Lichen planopilaris (LPP) is an inflammatory condition that can permanently destroy hair follicles, replacing them with scar tissue. If your doctor recommends a scalp biopsy to confirm LPP, they may examine tissue using two different microscopic views: horizontal (transverse) sections and vertical (longitudinal) sections. Sometimes, a doctor will take two separate small skin samples (punch biopsies) to allow for both views, or the laboratory might divide a single sample ^[1][2].

Using both methods can provide a more complete picture. Horizontal sections give a “bird’s-eye view” to estimate the density and distribution of hair follicles across the sampled tissue, while vertical sections provide a “side view” to evaluate a lengthwise portion of the follicles and surrounding skin ^[3][4].

What Horizontal Sections Show

Horizontal sections involve cutting the biopsy sample crosswise, like slicing a carrot into rounds. This allows the dermatopathologist (the doctor examining the tissue) to look at multiple hair follicles at the same time ^[3]. This “cross-section” view is especially helpful for evaluating:

• Follicle Density: An estimate of the number of hair follicles in the sampled area and whether there is an overall loss of hairs ^[5].
• Hair Cycle Stage: The proportion of actively growing hairs versus resting hairs, though this depends heavily on the quality and levels of the tissue examined ^[3].
• Miniaturization: Whether some hair follicles are shrinking over time. This finding is common if another condition, such as pattern hair loss, is occurring alongside LPP ^[1][5].
• Inflammation Pattern: How inflammatory cells are distributed around the middle of the follicles (the isthmus) and the oil glands (sebaceous glands) ^[3].

What Vertical Sections Show

Vertical sections involve slicing the tissue straight down from the skin surface into the deeper fat layer, like cutting a slice of cake. This preserves a top-to-bottom view of the structures ^[3]. Vertical sections are better for seeing:

• Surface Skin Changes: The relationship between the top layer of the skin (epidermis) and the hair follicles ^[3].
• Location of Inflammation: Vertical views help localize where inflammation is concentrated around the follicle. In LPP, inflammation typically targets the upper to middle portion of the hair follicle (the bulge region). This is where hair follicle stem cells reside, and their destruction is what makes the hair loss permanent ^[4].
• Scarring (Fibrosis): The extent of prior structural damage and scar tissue tracking along the follicle ^[4]. Keep in mind that finding scar tissue indicates past damage, while active inflammatory cells suggest ongoing disease activity. However, the absence of inflammation in a small biopsy does not necessarily prove the disease is entirely inactive ^[2].

Why the Combination Matters

The two cutting techniques answer different but complementary questions. While horizontal sections are often better for estimating follicle numbers and overall architecture, vertical sections are better for following the length of a follicle downward to evaluate targeted inflammation ^[3][4]. Neither orientation is universally sufficient by itself, and using both is particularly helpful when LPP might coexist with other conditions, or when clinical signs are difficult to classify ^[1][6].

It is important to note that clinical practice varies. Some laboratories or clinicians may select only one approach based on the specific clinical question or the size of the specimen ^[1]. If your report only mentions one orientation, it does not mean the biopsy was done incorrectly.

The usefulness of a biopsy also depends on where it was taken. Dermatologists typically try to sample a clinically active edge of hair loss to find ongoing inflammation, but they may sample a completely scarred area if they are evaluating late-stage disease ^[2][7]. Ultimately, because biopsy features can overlap with other types of scarring hair loss, you should always ask your dermatologist how your pathology results fit with their actual examination of your scalp rather than interpreting the biopsy report on its own ^[8][9].