1. Spiny Mice Have Weak Skin but Regrow It After Tearing
The skin of the African spiny mouse (Acomys) is much weaker than that of the common house mouse (Mus). It has been reported that spiny mouse skin tears with about 1/20th the force required to tear house mouse skin. Measurements show that house mouse skin withstands about 2.3 MPa, while spiny mouse skin withstands only about 0.11 MPa [5].
What is surprising is what happens after the skin tears. In experiments, new normal hair grew in wounds on spiny mice and large spine-like hair regrow along with hair follicles between day 21 and day 28. In house mice, the healed area becomes a scar with dense, parallel collagen fibers. In spiny mice, the collagen forms a loose, mesh-like arrangement [5].
It is important to note that the observation of losing about 60% of back skin refers to specific cases and is not the same as the experimental wounds described above. It represents a maximum value seen in some individuals, not a constant occurrence. This leads to the next question: how do human wounds close, and what cells are involved?
2. Fibroblasts Move Into the Blood Clot in a Wound
When an injury occurs, bleeding stops first, and a blood clot (coagulum) forms in the wound. This clot acts as a temporary scaffold to cover the wound, though it is not very strong. Fibroblasts from the surrounding area then multiply and move into the clot. They replace the temporary scaffold, which is rich in fibrin, with stronger granulation tissue. Fibroblasts are considered the main cells responsible for this replacement [1][2].
This granulation tissue has a contracting force. The next section examines how this force pulls the wound edges together.
3. Myofibroblasts Pull the Edges of a Wound Together
Some fibroblasts change into myofibroblasts. These cells are rich in a protein called alpha-smooth muscle actin and have a strong contracting power. Using the actin framework inside the cell, they pull surrounding fibers perpendicular to the wound edge, shrinking the wound opening (wound contraction) [1][2]. This is similar to tightening the drawstring of a pouch, but unlike a pouch, the entire tissue is pulled inward to make the wound smaller.
What triggers fibroblasts to become myofibroblasts? It has been reported that an increase in a substance called TGF-beta and the tension (force) applied to the tissue promote this change, both in the body and in cultures [2]. However, the detailed mechanisms are not fully understood, and the process cannot be explained simply. What happens to these pulling cells once their job is done?
4. Does Lower Tension Help Myofibroblasts Disappear?
Research in Japan is also focusing on this point. A study led by Yamagata University (KAKENHI, 2021–2025) notes that myofibroblasts are abundant during wound contraction. It is thought that excessive tension on the wound affects myofibroblasts and leads to abnormal scarring [3].
In this study, a covering material was used on mouse wounds to reduce tension. This led to the proper regression (disappearance) of myofibroblasts and suggested a potential way to prevent abnormal scars [3]. However, this is a stage where "possibility" has been shown in mouse experiments. It is not yet an established prevention method for humans. The interesting aspect is the emerging view that the reduction of these pulling cells after their work is finished may relate to the quality of the scar.
5. Why Scars Never Return to the Original Skin Structure
The final stage of wound healing is called remodeling. During this process, collagen fibers are reorganized, and tissue strength returns. However, the structure of scar tissue does not completely return to that of uninjured skin. In many cases, barrier function and tensile strength close to normal are recovered [2]. But strength does not always return completely. Materials indicate that the strength of scar tissue is up to about 80% of normal skin [4]. "About 80%" is an upper limit estimate, not a guaranteed number, and it may not be a value measured in mice.
Remembering the spiny mouse, the same wound can lead to a path where hair follicles regrow. In humans, deep scars often end with significant strength recovery but without returning to the original structure. Why is there a difference? The next section looks at an example of healing without scars.
6. Can Fetal Skin Heal Without Leaving Scars?
The skin of human and animal fetuses is said to heal with regeneration-like results, leaving no scars, if injured early in pregnancy. In sheep fetuses, wounds before day 120 of pregnancy healed without scars [6]. The mechanism is not fully understood, and this cannot be directly applied to human gestational weeks.
In mice, research shows there is a lineage of fibroblasts that creates scars. This lineage accounts for about 1% of skin fibroblasts in the early embryo, but exceeds 75% after birth [6]. It is suggested that changes in the composition of fibroblasts as the organism grows are a factor in whether scars remain. Examples of regeneration in spiny mice and scar-free healing in fetuses are clues for ongoing research to find paths that avoid scarring.
7. How to Observe a Scar and Learn More About Skin Healing
If you feel comfortable, you can gently observe an old scar. Is the color, texture, or hair growth different from the surrounding skin? You can look or touch lightly, but do not scratch or pull the scar. If you do not have scars, you do not need to do this.
If you wish to read the materials, the results report of the Yamagata University study (KAKENHI 21K10599) is available in Japanese on the National Institute of Informatics KAKEN website [3]. It is also recommended to draw a cross-section of skin on paper. Adding details like hair follicles on the surface and cells with contracting force under the wound can help you think about what is missing in a scar.
Human scars are not a sign that healing has failed but rather repairs that prioritize strength, while spiny mice take a different path. Where the paths diverge is still a subject of research.