A callus is a localized area of hyperkeratosis, a medical term for a patch of hardened and thick skin. While most common calluses are the body’s protective response to external friction and pressure, the degree to which an individual forms them can be influenced by inherited factors. Genetic makeup determines overall skin resilience, but in rare cases, a genetic disorder is the sole cause of severe skin thickening. Calluses are overwhelmingly caused by environmental forces, though genetics influence susceptibility.
The Biological Mechanism of Skin Thickening
Callus formation is a specific physiological defense mechanism initiated by the epidermis, the skin’s outermost layer. When the skin experiences repetitive, mild injury from forces like rubbing or shearing, it activates basal layer cells known as keratinocytes. These cells significantly increase their proliferation rate in the stressed area.
Accelerated cell division leads to a rapid increase in keratinocytes moving upward through the epidermal layers. As they migrate, these cells produce larger amounts of keratin, a tough, fibrous structural protein. This process, known as hyperkeratosis, results in a dramatically thickened stratum corneum, the outermost layer of dead skin cells. The dead keratinocytes in a callus create a durable, protective pad.
Environmental Factors and External Triggers
The most frequent cause of callus development is chronic friction, pressure, and shearing forces applied to the skin. The body interprets these mechanical stresses as threats, prompting the defensive hyperkeratotic response. This is why calluses commonly form on the soles of the feet and the palms of the hands.
Ill-fitting footwear is a major contributor, as shoes that are too tight, too loose, or have small toe boxes cause constant rubbing. High heels shift body weight forward, creating excessive pressure on the balls of the feet, which often leads to callus formation. Repetitive occupational and athletic activities also act as external triggers.
Manual laborers, weightlifters, and musicians often develop calluses on their hands from gripping tools or instrument strings. A person’s intrinsic structure also influences where calluses form. Biomechanical issues like an abnormal gait, high-arched feet, or foot deformities such as bunions create localized pressure points that increase the likelihood of callus development. While genetics may predispose individuals to these foot issues, the callus will not form without the environmental trigger of friction or pressure.
Genetic Conditions That Cause Excessive Callusing
While common calluses react to the environment, certain inherited conditions cause severe skin thickening without external friction. These disorders, known as hereditary palmoplantar keratodermas (PPK), are exceptions to environmental causation. PPK is a heterogeneous group characterized by marked hyperkeratosis confined to the palms and soles.
These conditions are caused by specific mutations in genes that code for structural proteins within the skin’s epidermis. For instance, mutations in the KRT9 gene, which codes for keratin expressed in the palmoplantar skin, result in diffuse thickening present from birth or early childhood. Other genes, such as KRT1 and AAGAB, have also been implicated in various forms of PPK. These genetic disorders differ from common calluses because the skin thickening is a primary symptom of the mutation, not a secondary protective response to mechanical stress.