Hair serves as a protective element, but its internal structure is not uniform across the body. The hair shaft, the portion visible above the skin, is a biological fiber whose composition changes based on its location and diameter. Understanding these variations is key to identifying the specific types of hair that lack a central core, known as the medulla.
Anatomy of Hair: Defining the Medulla
The hair shaft is composed of three distinct concentric layers. The outermost layer is the cuticle, a transparent, protective shield made of overlapping, scale-like cells. Beneath this lies the cortex, which forms the main bulk of the hair shaft and accounts for most of its weight.
The cortex is packed with fibrous keratin protein and contains the melanin pigments that determine hair color. The innermost core is the medulla, a central column of cells that, when present, runs along the length of the hair strand. This layer is composed of loosely packed, rounded cells with air spaces, giving it a soft, spongy structure.
The presence and structure of the medulla are highly variable in human hair, and its exact function remains somewhat unclear. Researchers hypothesize that it may contribute to the hair’s thermal properties or structural integrity, but it is not a universally present feature. Its appearance under a microscope resembles a central canal or a series of air-filled spaces.
Hair Types That Lack a Medulla
The types of human hair that consistently lack a medulla are those with a very fine diameter. The primary category of non-medullated hair is vellus hair, often described as “peach fuzz.” Vellus hair is short, fine, and usually unpigmented, covering most of the human body surface.
Vellus hair universally lacks a medulla because the hair shaft is simply too narrow to accommodate the central core layer. In contrast, terminal hair—the thicker, longer, and usually pigmented hair found on the scalp, eyebrows, and pubic region—often contains a medulla. However, fine terminal hair, such as naturally blonde or very thin scalp hair, may also lack the medullary layer, possessing only a cuticle and a cortex.
The absence of a medulla is directly linked to the thickness of the hair strand. This structural difference explains why fine hair textures can be more fragile compared to coarse hair, which almost always has a prominent medulla. The hair produced during the initial growth phase, known as lanugo hair in infants, is also characteristically non-medullated.
Medullation Patterns and Identification
The presence, absence, and specific configuration of the medulla are significant characteristics used in the forensic analysis of hair samples. When a medulla is present in human hair, it rarely forms a solid, unbroken column. Instead, it displays one of several distinct patterns observed under microscopic examination.
The most common patterns are continuous (unbroken column), interrupted or discontinuous (broken at regular intervals), and fragmented or trace (unevenly or sporadically spaced). Human head hairs often exhibit a fragmented pattern or are entirely absent of a medulla. The state of having no separate pigmentation in the center is considered a distinct pattern category called “none” or absent.
The ratio of the medulla’s diameter to the total hair shaft diameter is calculated as the medullary index, a useful metric in identification. For human hair, this index is generally one-third or less, whereas in animal hair, it is typically one-half or greater. This measurement, along with the medullation pattern, helps forensic scientists distinguish between human and animal hair.