Causes
Normally, a callus will form on any part of the skin exposed to friction over a long period of time. For example, people often develop calluses on the middle finger of their dominant hand due to writing with a pen or pencil. Another cause is from playing string instruments like the guitar or the violin; calluses will develop on the four left hand fingers used in holding the strings down to the fingerboard, and sometimes right hand fingers used for pizzicato or strumming. Calluses are also very common on the fingers of bassists who use both the pizzicato and slapping techniques. This also applies to rock climbers on almost all of their fingers. There are many activities that can result in the formation of a callus, and having one is sometimes viewed as a badge of honor. Activities that are notorious for causing calluses include (but are not limited to) construction work, many sports, wood carving, playing musical instruments, rock climbing, hiking, martial arts, weight training, dancing (especially ballet), praying, chopping wood, and wearing high heels. Tenpin bowlers will often develop calluses on their thumbs and occasionally their middle fingers from frequent bowling. Although often found on the foot (where the most pressure and friction are applied), calluses can occur anywhere on the body as a reaction to moderate, constant "grinding" pressure. It is the natural reaction of the palmar or plantar skin. Too much friction occurring too fast for the skin to develop a protective callus will cause a blister or abrasion instead.
Biologically, calluses are formed by the accumulation of terminally differentiated keratinocytes in the outermost layer of skin. Though the cells of calluses are dead, they are quite resistant to mechanical and chemical insults due to extensive networks of cross-linked proteins and hydrophobic keratin intermediate filaments containing many disulfide bonds.
Sometimes a callus occurs where there is no rubbing or pressure. These hyperkeratoses can have a variety of causes. Some toxins, such as arsenic, can cause thick palms and soles. Some diseases, such as syphilis, can cause thickening of the palms and soles as well as pinpoint hyperkeratoses. There is a benign condition called keratosis palmaris et plantaris, which produces corns in the creases of the fingers and non-weight bearing spaces of the feet. Some of this may be caused by actinic keratosis, which occurs due to overexposure to sun or with age and hormonal shifts.
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