Corns typically are due to mechanical trauma to the epidermal layer of the skin. This is typically due to friction and pressure in between the foot and the foot wear (typically tight/ill fitting). When there is excess friction and pressure applied between the foot and footwear it will cause a physiological response of the in excess growth in the stratus corneum.
Calluses are formed in the same manner as corns such as repeated pressure or friction due to footwear and bones of the foot. This is due to the skin trying to protect itself during the healing process. Calluses will show a markedly dense orthokerototic stratum underlain by hypergranulosis, mild acathosis, and increased collagenization of the dermis with possible mucin deposition in the dermis.
Nanette B Silverberg, M. (2019, November 08). Corns (Clavus). Retrieved October 31, 2020, from https://emedicine.medscape.com/article/1089807-overview