The appearance of a noticeable gap or area of sparser growth in the center of the mustache is a common observation for many people growing facial hair. This central area is known anatomically as the philtrum, the distinct vertical groove located directly beneath the nose and above the upper lip. This difference in hair coverage is a normal biological pattern, resulting from specific anatomical structures and the localized way hair follicles respond to hormonal signals.
The Unique Anatomy of the Philtrum
The philtrum is a remnant of facial development during the embryonic stage, forming a distinct area with unique tissue characteristics compared to the surrounding skin. This central depression is outlined by two vertical ridges, which are formed in part by the underlying orbicularis oris muscle fibers. These muscle fibers decussate, or cross over, at the philtrum’s ridges, which contributes to the formation of the distinct central structure.
The hair follicles in the philtrum area are often different in concentration and type from those on the lateral parts of the upper lip that form the bulk of the mustache. All facial skin is covered in hair, but it exists in two forms: fine, light vellus hair, and thick, pigmented terminal hair. The philtrum naturally tends to have fewer follicles capable of producing thick terminal hair, or those follicles are primarily programmed to produce the finer vellus hair.
This difference in follicle population and density is a key factor in the visible gap. While the sides of the mustache are densely packed with follicles that mature into coarse terminal hairs, the philtrum often retains a higher proportion of the nearly invisible vellus hairs. The result is a lighter, thinner appearance in the center, which gives the impression of a split or a lack of growth.
Androgen Sensitivity and Follicle Distribution
The primary physiological reason for the mustache pattern lies in the differential sensitivity of hair follicles to androgens, the male sex hormones. Facial hair growth is stimulated by androgens, specifically testosterone and its more potent derivative, dihydrotestosterone (DHT). These hormones trigger the transformation of vellus hair follicles into terminal hair follicles during and after puberty.
Hair follicles across the face are not uniformly responsive to these hormonal signals. The follicles on the chin, jawline, and the lateral sides of the upper lip possess a higher density of androgen receptors or are more sensitive to lower concentrations of androgens. This increased sensitivity allows them to activate and begin producing thick, dark terminal hair relatively easily.
The follicles located in the philtrum area, however, often have a lower sensitivity threshold to androgens. They require a higher concentration of circulating hormones or a more prolonged period of stimulation to fully convert from vellus to terminal hair production. This localized, reduced sensitivity means that while the sides of the mustache are robustly growing, the central philtrum area lags behind or fails to produce the same quality of terminal hair, resulting in the characteristic gap.
Individual Genetic Variation in Facial Hair Patterns
The specific pattern of facial hair an individual develops, including the size and prominence of the philtrum gap, is significantly influenced by genetics. Genetic makeup dictates both the overall level of circulating androgens and, more importantly, the hair follicle’s sensitivity to these hormones. It is the inherited differences in the number and distribution of androgen receptors on the facial skin that determine the final growth map.
Variations in genes, such as LNX1 or EDAR, have been linked to differences in hair follicle density and architecture, which directly impacts how thickly or sparsely facial hair grows. If one inherits genes that program the philtrum follicles to have a particularly low androgen sensitivity, the central gap will be more pronounced. This explains why some people have a seamless, full mustache, while others have a distinct separation.
These inherited traits also account for familial or ethnic tendencies toward specific facial hair patterns, from full coverage to more patchy or separated growth. The degree to which the philtrum area fills in is essentially pre-programmed by one’s DNA.