A cut generally needs to reach the dermis, the second layer of skin, before it will leave a permanent scar. The epidermis, your outermost skin layer, is only about as thick as a few sheets of paper in most places, and wounds confined to it heal by regenerating identical cells without lasting marks. Once a wound breaches the basement membrane and damages the dermis, the body shifts from regeneration to repair, filling the gap with collagen that never quite matches the original tissue. But depth alone does not tell the full story, because two wounds of the same depth can produce very different scars depending on where on the body they occur, how they are cared for, and who you are genetically.
Why the Dermis Is the Dividing Line
Your skin has two main layers. The epidermis is the outermost barrier, composed of tightly stacked cells that constantly shed and replace themselves. Beneath it lies the dermis, a thicker layer packed with collagen fibers, blood vessels, nerve endings, hair follicles, and sweat glands. The boundary between them, called the basement membrane, is where the scarring threshold sits.
When a cut damages only the epidermis, the surrounding skin cells migrate across the wound bed and close the gap. Because the underlying scaffold of the dermis is still intact, the new epidermis forms on top of it in the same arrangement as before. There is no need for a structural patch, and no scar results. A paper cut is a good everyday example: it stings sharply because it clips nerve endings near the surface, but it heals cleanly within days.
Once a wound extends into the dermis, the body can no longer simply slide new skin cells across a stable foundation. The damaged collagen matrix must be rebuilt, and the replacement tissue that fills the wound is structurally different from what was there before. Adult mammalian wound healing results in scar formation without regeneration of skin appendages like hair follicles or sweat glands.1PubMed Central. Wound healing and skin regeneration That rebuilt patch is scar tissue.
What Makes Scar Tissue Different
Scar tissue is not just “healed skin.” Its internal structure is fundamentally different. In normal, uninjured skin, collagen fibers are arranged in a loose, basket-weave pattern that gives the skin its flexibility and strength in multiple directions. In scar tissue, those fibers line up in a more parallel arrangement, like threads running in roughly the same direction. Research comparing the two has found that normal skin has a much lower collagen orientation index than scar tissue, meaning the fibers are more randomly organized in healthy skin and more aligned in scars.2PubMed. Collagen morphology in human skin and scar tissue: no adaptations in response to mechanical loading at joints This parallel alignment holds true across different scar types, from flat normotrophic scars to raised keloids.3PubMed. Differences in collagen architecture between keloid, hypertrophic scar, normotrophic scar, and normal skin: An objective histopathological analysis
That parallel collagen is why scars feel stiffer and look different. They lack the pliability of normal skin, they do not tan the same way, and they cannot grow hair. Over time, the body continues to remodel scar collagen, which is why many scars soften and flatten over months to years. But the tissue never fully returns to its original basket-weave architecture.
How the Body Builds a Scar
The process begins almost immediately after a dermal wound. Blood clots form a temporary plug, and inflammatory cells rush in to clean up debris and fight potential infection. Within days, specialized cells called fibroblasts begin depositing new collagen to bridge the gap. Some of these fibroblasts transform into myofibroblasts, contractile cells that pull the wound edges together and speed closure. Myofibroblasts play a key role in this process: they promote wound closure and deposit new matrix material, then normally die off through programmed cell death once the wound is sealed.4PubMed. Mechanisms of pathological scarring: role of myofibroblasts and current developments
The early collagen laid down during repair is mostly type III collagen, which is thinner and weaker than the type I collagen that dominates mature skin. Over weeks and months, the body gradually replaces type III with type I, and the scar gains strength. Even so, a mature scar typically reaches only about 70 to 80 percent of the tensile strength of uninjured skin. The ratio of collagen types matters for how the scar turns out. Animal studies have shown that when type III collagen is deficient, wounds develop more myofibroblasts and produce more scar tissue than normal.5PubMed Central. Diminished type III collagen promotes myofibroblast differentiation and increases scar deposition in cutaneous wound healing Fetal wounds, which heal with minimal scarring, are naturally rich in type III collagen and hyaluronan, along with a much milder inflammatory response.6PubMed Central. Fetal wound healing: implications for minimal scar formation
Depth Is Not the Only Factor
A wound that barely grazes the dermis and a wound that slices deep into the fat layer both produce scars, but those scars can look wildly different. Several factors beyond raw depth determine whether you end up with a thin, barely visible line or a raised, discolored mark.
Mechanical Tension
How much the surrounding skin pulls on a healing wound is one of the strongest predictors of scar quality. Wounds on joints, the chest, and the shoulders sit in areas of high tension, and they tend to produce wider or more raised scars than wounds in low-tension areas like the eyelids or inner wrist. Increased mechanical stress in the wound environment drives scar tissue formation through signaling pathways that push cells toward proliferation and fibrosis.7PubMed Central. Mechanical Forces in Cutaneous Wound Healing: Emerging Therapies to Minimize Scar Formation A randomized trial found that paper tape applied to surgical incisions to offload tension helped prevent hypertrophic scarring, and that scars became hypertrophic or stretched only after the tape was removed, suggesting that tension is the trigger rather than just a contributing factor.8Plastic & Reconstructive Surgery. A Randomized, Controlled Trial to Determine the Efficacy of Paper Tape in Preventing Hypertrophic Scar Formation in Surgical Incisions that Traverse Langer’s Skin Tension Lines
Infection and Prolonged Inflammation
The inflammatory phase of wound healing is necessary, but when it drags on too long or becomes excessive, scarring worsens. A study of thermal burns found that deep scars were present in 93 percent of wounds that developed persistent infection, compared with only 29 percent of non-infected wounds.9PubMed. Persistent wound infection delays epidermal maturation and increases scarring in thermal burns Experimental work has shown that artificially prolonging inflammation in wounds leads to significantly elevated scar tissue compared with normally healing controls.10PubMed. Exacerbated and prolonged inflammation impairs wound healing and increases scarring Keeping a wound clean is one of the most practical things you can do to minimize scarring, not because cleanliness directly changes collagen deposition, but because it prevents the drawn-out inflammatory cycle that amplifies it.
Body Location and Dermal Thickness
The dermis is not the same thickness everywhere. It is thinnest on the eyelids and inner arms, and thickest on the upper back and scalp. Research in women with darker skin types found that areas with thinner dermis produced narrower scars than areas with thicker dermis.11PubMed. Effect of Dermal Thickness on Scars in Women with Type III-IV Fitzpatrick Skin This means the same depth of cut can produce different scars depending on where on the body it occurs. A cut on your forearm that just barely enters the dermis may heal with a faint line, while a cut of the same absolute depth on your upper back, where the dermis is much thicker, might still be relatively superficial in context.
When Scars Grow Beyond Normal
Most dermal wounds produce a flat or slightly raised scar that fades over time. But some people develop abnormal scars that are raised, itchy, painful, or continue to grow. The two main types are hypertrophic scars and keloids, and telling them apart matters because they behave differently and respond to different treatments.
Hypertrophic scars stay within the boundaries of the original wound. They are raised and often red or pink, and they tend to improve on their own over months to years. Keloids, by contrast, grow beyond the original wound edges, sometimes extending well past the area that was injured. This outward growth is the defining feature that separates them. While both conditions involve excessive scar tissue, the differences between them are mostly quantitative rather than qualitative, with many of the same cellular abnormalities present in both.12PubMed Central. Hypertrophic scars and keloids: Overview of the evidence and practical guide for differentiating between these abnormal scars
Keloid formation has a strong genetic component. Certain families and ethnic groups have a much higher prevalence, and identical twins tend to develop them in parallel, reinforcing the idea that genes play a major role.13PubMed Central. Keloid scarring: understanding the genetic basis, advances, and prospects People of African, Asian, and Hispanic descent are at higher risk, while those of Northern European ancestry develop keloids less frequently. If you have a family history of keloids, even a shallow dermal wound can trigger one, which makes the “how deep” question somewhat personal. For keloid-prone individuals, even ear piercings and minor nicks can produce large, raised scars.
What You Can Do During Healing to Reduce Scarring
Since scarring is driven by dermal damage, inflammation, infection, and mechanical tension, the most effective interventions target those factors while the wound is still healing. You cannot prevent scarring entirely once the dermis is involved, but you can influence how the scar turns out.
Keeping the wound moist and covered is the single most impactful step. A dry wound forms a thick scab that forces new tissue to grow underneath it, slowing healing and increasing scar formation. A moist wound environment, maintained with a simple occlusive bandage or petroleum-based ointment, lets new skin cells migrate across the wound bed faster and with less inflammation.
Silicone-based products, either sheets or gels, are one of the most studied scar-prevention tools. They work primarily by hydrating the skin over the wound site, which suppresses the overactivity of scar-producing cells.14PubMed Central. The Use of Silicone Adhesives for Scar Reduction In animal studies, early silicone gel application reduced scar thickness substantially, with total occlusion cutting scar elevation by roughly 80 percent compared with partial coverage. The effect appeared to come from hydrating the outer skin cells, which in turn calmed the collagen-producing cells beneath them.15PubMed. The role of the epidermis in the control of scarring: evidence for mechanism of action for silicone gel
Wound closure technique also plays a role, though perhaps less than you might expect. A trial comparing wound tape to sutures for traumatic wounds found no significant overall difference in scar width at two months. For shorter wounds under 20 mm, however, tape produced significantly narrower scars than sutures.16PubMed Central. Comparison of Wound Tape and Suture Wounds on Traumatic Wounds’ Scar The takeaway for most people is that getting a clean wound closed promptly matters more than the specific method used.
Age and Scarring
Children and younger adults tend to form more noticeable scars than older adults, which is the opposite of what most people assume. The reason is that younger skin has a more robust inflammatory response and lays down collagen more aggressively, both of which contribute to thicker scars. Hypertrophic scars and keloids are more common in people between puberty and their thirties.
As skin ages, the dermis thins and its collagen becomes more cross-linked and fragmented.17PubMed Central. Skin Structure-Function Relationships and the Wound Healing Response to Intrinsic Aging Older skin heals more slowly, which means wounds in elderly people are more vulnerable to complications like infection or breakdown. But the scars that do form in older skin tend to be flatter and less conspicuous than those in younger people, because the diminished collagen production that slows healing also produces less exuberant scar tissue. The trade-off is real: older skin scars less dramatically but heals less reliably.
Painful Scars and Nerve Involvement
Scarring is not always just cosmetic. When a wound damages nerve fibers in the dermis, the scar that forms can trap or compress regenerating nerve endings, leading to chronic pain, heightened sensitivity, or persistent itching. Painful scars can range from mild itchiness to severe, disabling pain that qualifies as a neuropathic condition.18PubMed Central. Diagnosis, Treatment, and Management of Painful Scar: A Narrative Review Deeper cuts, burns, and surgical incisions are more likely to injure nerve structures and leave behind this kind of problematic scar.
If a scar is painful to touch, hypersensitive to temperature changes, or causes a pins-and-needles sensation, that usually indicates nerve involvement rather than a purely structural problem. Treatment approaches differ from those used for cosmetic scar improvement, often involving desensitization therapy, injections, or in some cases surgical revision to free trapped nerves.
Treating Scars After They Have Formed
Even after a scar has matured, the collagen within it can still be remodeled. The body continues to break down and rebuild collagen in scar tissue for years after the initial injury, though the process slows with time. Several interventions take advantage of this ongoing remodeling.
Fractional laser therapy and microneedling both work by creating tiny, controlled injuries in the scar tissue that restart the wound-healing process on a micro scale, prompting the body to lay down new collagen in a more organized pattern. A meta-analysis comparing the two found that fractional laser therapy had a statistically significant advantage over microneedling in improving scar appearance and promoting collagen fiber regeneration.19PubMed Central. Fractional laser therapy versus microneedling for non-acne scars and scar-like dermal fibrotic lesions Both approaches typically require multiple sessions, and neither erases a scar completely, but they can meaningfully improve texture, color, and pliability.
For raised scars like keloids and hypertrophic scars, steroid injections remain a common first-line treatment. Corticosteroids reduce collagen synthesis and can flatten a raised scar over a series of sessions. Pressure garments, cryotherapy, and surgical excision followed by adjunct treatment are also used, particularly for keloids that do not respond to injections alone. Keloids have a high recurrence rate after excision, which is why surgery is almost always combined with another therapy to reduce the chance of the scar growing back.
Silicone sheets and gels, described earlier as a prevention tool, also have a role in treating existing scars. Consistent daily use over several months can soften and flatten raised scars, and they are often the first recommendation for anyone dealing with a scar they want to improve without more invasive procedures.
Why Fetuses Heal Without Scars
One of the more striking findings in wound-healing research is that early-gestational fetuses can heal skin wounds with essentially no scarring. The wound closes, and the tissue that grows back is indistinguishable from the surrounding skin, complete with normal collagen architecture and even hair follicles. This ability is lost as the fetus matures, and by the third trimester, fetal wounds begin to scar much like postnatal ones.
Fetal scarless healing involves several factors working together: a distinct profile of growth factors, a much milder inflammatory response dominated by anti-inflammatory signals, an extracellular matrix rich in type III collagen and hyaluronan, and lower mechanical stress on the wound.6PubMed Central. Fetal wound healing: implications for minimal scar formation Researchers have been studying these differences for decades, hoping to find ways to recapitulate fetal-like healing in adults. The challenge is that adult immune systems and wound environments are fundamentally different, and the conditions that allow scarless healing in utero are difficult to reproduce in a body that has to contend with bacteria, mechanical strain, and a fully mature inflammatory system. Progress has been slow, but understanding the fetal model continues to guide experimental therapies aimed at reducing scar formation in adults.