Very fine hair is the type most likely to lack a medulla entirely. The medulla, the spongy innermost channel that runs through some hair shafts, appears reliably in thick terminal hairs but shrinks or disappears altogether in thin ones. Vellus hair, the wispy, nearly invisible fuzz that covers most of your body, is the classic example of hair without a medulla. But the story is more interesting than a simple thick-versus-thin divide, because factors like where hair grows on your body, what stage of its growth cycle it is in, and even your genetic background all influence whether that central core shows up.
Why Hair Diameter Matters More Than Anything Else
The single biggest predictor of whether a hair contains a medulla is how wide the shaft is. Research on human scalp hair has shown that the maximum size of the medulla is strongly associated with the overall cross-sectional area of the hair. In terminal hairs, the thick, pigmented strands that make up the hair on your head and eyebrows, the medulla tends to be large and clearly visible. In very small hairs, it is small or absent altogether.1PubMed. The size and form of the medulla of human scalp hair is regulated by the hair cycle and cross-sectional size of the hair shaft This relationship holds both when comparing different hairs on the same person’s head and when comparing terminal hairs to finer ones across the body.
Think of the hair shaft as having three concentric layers. The outermost is the cuticle, a shingle-like protective covering. Beneath that sits the cortex, which provides the hair’s structural strength and carries most of its pigment. The medulla, when present, occupies the center. In a thick terminal hair, there is enough room in the shaft for all three layers to develop fully. In a very fine hair, the shaft may be so narrow that the medulla never forms at all, or forms only as a faint, fragmented trace.
This is why vellus hair, the soft peach-fuzz type, almost never has a medulla. Vellus hairs are typically less than about 30 micrometers across, too thin to accommodate a well-developed central channel. By contrast, coarse beard hairs or thick scalp hairs can exceed 100 micrometers in diameter, and these almost always contain one. The relationship is not perfectly linear, but it is consistent enough that forensic scientists use it as a rough rule for identifying hair types.
How the Growth Cycle Strips the Medulla Away
Even within a single strand of hair, the medulla is not uniform from root to tip. Human hair grows in a cycle: an active growth phase called anagen, a brief transitional phase, and a resting phase before the hair sheds. The medulla tracks this cycle closely. During the first half of anagen, when the hair follicle is producing cells at its fastest rate, the medulla tends to be continuous, running as an unbroken column down the center of the shaft. During the next quarter or so of anagen, it becomes discontinuous, showing up as scattered islands or fragments. By the final quarter of growth, the medulla is virtually absent or gone entirely.2British Journal of Dermatology. The size and form of the medulla of human scalp hair is regulated by the hair cycle and cross‐sectional size of the hair shaft
What this means in practical terms is that the tip end of an older hair, the part produced months or even years ago when the follicle was winding down its previous growth cycle, may be medulla-free even if the root end, produced during vigorous early growth, has a robust one. The tip of the hair shaft itself also tapers and often lacks a medulla regardless of hair type, simply because the strand narrows.3Research & Reviews: Journal of Veterinary Sciences. Hair Shaft Anatomy of Mammals: A Comprehensive Overview So if you plucked a single thick scalp hair and examined it under a microscope from root to tip, you might see a continuous medulla near the base, a fragmented medulla in the middle, and no medulla at all near the end.
Body Location and Hair Type
Not all the hair on your body is the same, and the presence or absence of a medulla varies accordingly. Humans grow three general categories of hair at different life stages. Lanugo is the ultra-fine hair that covers a fetus in the womb; it is mostly shed before birth and has no medulla. Vellus hair replaces it and persists into adulthood as the barely visible fuzz on your forearms, cheeks, and forehead. Vellus hairs are thin enough that they almost universally lack a medulla. Terminal hair is the thick, pigmented type found on the scalp, eyebrows, eyelashes, and, after puberty, in the armpits, groin, and on the face and chest.
Within terminal hair, there is still plenty of variation. Scalp hairs in many individuals are on the borderline: some have a continuous medulla, some a fragmented one, and many have none. One study of young adults found that absent medulla was the single most common category, appearing in roughly 62% of hair samples examined, while a continuous medulla showed up in only about 1% of samples.4Journal of Forensic Sciences & Criminal Investigation. A Study on The Presence of Medulla Types of Hair Among the Young Jaat Residents of Western Uttar Pradesh That finding surprised even the researchers. For many people, the majority of their scalp hairs have no medulla or only a fragmented one, despite being full-fledged terminal strands.
Coarser body hairs tend to have a medulla more often than fine scalp hairs. Beard hairs, in particular, are among the thickest hairs the human body produces and usually contain a well-developed medulla. Eyebrow and eyelash hairs are short and relatively fine, so they may or may not have one.
The Forensic Angle and the Medullary Index
One of the reasons anyone outside a biology classroom cares about the medulla is forensic science. When a hair is recovered from a crime scene, one of the first things an examiner does is measure the medullary index: the ratio of the medulla’s width to the total width of the hair shaft. In human hair, that ratio is consistently less than one-third.5PubMed Central. Analysis of hair samples using microscopical and molecular techniques to ascertain claims of rare animal species In most animal hair, the medulla is proportionally much wider, often filling half or more of the shaft.
This distinction is one of the quickest ways to determine whether a hair came from a person or an animal. Human scalp hair, with its frequently absent or narrow medulla, looks strikingly different under a microscope from, say, deer hair, which has a thick ladder-like medulla that nearly fills the entire shaft. The medullary index is not a DNA test, but it gives investigators a fast preliminary answer and has been used in forensic casework for well over a century.
The fact that many human hairs lack a medulla entirely actually reinforces its usefulness as a species marker. If an examiner finds a hair with a continuous, wide medulla occupying more than a third of the shaft, that is strong evidence it came from an animal rather than a person.
Population and Individual Variation
How common medullation is varies across human populations. A study examining hair from twelve different ethnic groups and geographic regions found that both the incidence of medullation and hair diameter varied enough to differentiate populations statistically.6PubMed. Variation in hair histological variables: medulla and diameter Populations with generally finer hair tend to show lower rates of medullation, while those with coarser hair show higher rates, which aligns with the diameter relationship described earlier.
There is also significant variation between individuals within any population. One person’s scalp may produce mostly medullated hair, while their neighbor of the same background has almost none. Even on the same head, some strands will have a medulla and others will not. This kind of individual variability is part of why forensic hair comparison has moved toward DNA analysis as the definitive tool. Microscopic features like medullation can narrow possibilities, but they are rarely unique enough to identify a specific person.
Age and sex, interestingly, do not seem to have a large effect on the medullary index itself. Research on Thai scalp hair from cadavers ranging from young adults to people over 80 found that the medullary index did not significantly differ between males and females, or across age groups.7PubMed Central. Relative studies between hair index, hair area, and medullary index with age and sex in Thai scalp hair The overall size of the hair shaft did change with age, peaking in the early twenties and gradually decreasing thereafter, but the medullary index stayed relatively stable. So while your hair may get finer as you age, the proportion of the shaft occupied by the medulla does not shift dramatically.
What Controls Whether a Medulla Forms
At the molecular level, the medulla is not just leftover space in the center of the hair. It is an actively constructed tissue regulated by specific signaling pathways. Research in mice has identified that fibroblast growth factor (FGF) signals, working through a protein called IGF-binding protein 5, are key regulators of medulla formation.8Development. FGF signals specifically regulate the structure of hair shaft medulla via IGF-binding protein 5 When this signaling pathway is disrupted, the medulla structure changes dramatically. These signals essentially tell the cells at the center of the developing hair shaft to undergo a specific program of growth, differentiation, and eventually cell death that creates the characteristic air-filled or protein-filled spaces of the medulla.
In very fine hairs, the follicle is small enough that this signaling program may never kick in fully. The cells in the center of the shaft simply differentiate into cortex instead, producing a solid strand with no central channel. In thicker hairs, the follicle is large enough that the medullary program activates and a distinct core of cells is set aside for medulla formation. This is why the diameter-medulla connection is so consistent: it reflects a genuine developmental threshold. Below a certain follicle size, the signaling environment does not support medulla development.
Thin Versus Thick Medullas
When a medulla is present, it is not always the same structure. Electron microscope work has revealed at least two distinct types of medulla in human hair: a thin medulla and a thick one, which differ in their internal architecture at the cellular level.9PubMed. Electron microscopic observations of human hair medulla The thin medulla appears as a narrow streak, sometimes only a single cell wide, and is common in finer scalp hairs that are just above the threshold for medulla formation. The thick medulla, by contrast, occupies a broader portion of the shaft and is packed with larger, more clearly defined air spaces. It is the thick type that gives coarse hairs their slightly different feel and optical properties, since those air pockets scatter light differently than solid cortex does.
This distinction matters for cosmetic and textile purposes as well. Hair with a prominent medulla refracts light differently, which can subtly affect color appearance and shine. In the fiber and textile industry, animal hairs with wide medullas, like cashmere or mohair, have distinct thermal and luster properties partly because of the air spaces in their cores. Human hair dye and conditioning treatments penetrate differently depending on whether a medulla is present, since the central core creates channels that products may or may not reach.
Animal Hair Without a Medulla
Although this question is usually asked about human hair, it is worth noting that even in the animal kingdom, not all hair contains a medulla. A study constructing identification keys for Korean mammals found that the hair of bats (order Chiroptera) did not show any medulla structures at all.10PubMed Central. Species identification key of Korean mammal hair Bat fur is extremely fine, consistent with the general rule that very thin shafts lack the structural space for a medulla to develop. Most other mammals have well-developed medullas in at least some of their hair types, and many wild mammal hairs have medulla patterns so distinctive that wildlife biologists can identify species from a single strand.
Domesticated sheep bred for ultrafine wool also produce fibers with absent or minimal medullas. This is actually a desirable trait in the wool industry, because medulla-free fibers are smoother, softer, and take dye more evenly. Breeding programs for fine-wool sheep have, whether intentionally or not, selected against heavy medullation.
How Scientists See the Invisible
Determining whether a medulla is present in a given hair used to require slicing it into thin cross-sections and examining them under a standard light microscope. The air-filled spaces of the medulla appear as a dark or translucent streak running through the center of the hair when viewed this way. But thin or fragmented medullas can be hard to detect, especially in fine hair where the core may appear as little more than a faint shadow.
Newer imaging technology has improved the picture considerably. Micro-computed tomography, or micro-CT, can now produce three-dimensional reconstructions of the entire hair shaft without cutting or chemically treating it. One recent study demonstrated that micro-CT could reveal the presence of a medulla in tightly curled hair fibers, which are difficult to section cleanly for traditional microscopy.11PubMed Central. Quantifying whole human hair scalp fibres of varying curl: A micro-computed tomographic study Synchrotron radiation micro-CT, an even higher-resolution version using powerful X-ray beams, has allowed researchers to visualize the internal and external structures of hair in three dimensions without any sample preparation at all.12New Physics: Sae Mulli. Comparison of the 3D Structures of Hairs from Humans and from Animals by Using Synchrotron Radiation Micro-CT
These advances are mostly relevant to researchers and forensic laboratories, but they have started to change our understanding of how common medullation actually is. Older studies that relied on basic light microscopy may have undercounted fragmented or very thin medullas in fine hair, simply because the resolution was not good enough to see them. As imaging gets better, the line between “absent” and “extremely thin” becomes harder to draw. It is possible that some hair classified as medulla-free in earlier surveys actually contains a wispy remnant of one, too small to see with the tools available at the time.
When Hair Disorders Alter the Medulla
Certain hair shaft disorders can change the internal structure of the hair, including the medulla. In conditions that cause the hair shaft to become irregularly shaped, with twists, nodes, or fracture points along its length, the medulla may be disrupted, fragmented, or absent in sections where it would normally be present. These are relatively rare conditions, and diagnosing them requires careful microscopic examination of the shaft itself rather than just looking at overall hair thickness or density.
Miniaturization, the process by which terminal hairs gradually shrink back toward vellus-like dimensions, is the hallmark of androgenetic hair loss (common male- and female-pattern baldness). As a terminal hair follicle miniaturizes over successive growth cycles, the hairs it produces become thinner, shorter, and lighter in color. As they shrink past the diameter threshold for medulla development, they lose the medulla as well. So if you pulled a miniaturizing hair from the front of a receding hairline and compared it to a thick hair from the back of the same scalp, the thinning hair might have no medulla at all while the thick one had a well-developed continuous core. The loss of the medulla in this context is not the disease itself; it is a downstream consequence of follicle shrinkage driven by hormonal sensitivity.