Skin

The skin was the outer covering of animals. According to the Eleventh Doctor, human skin was fragile, like parchment. (TV: Journey to the Centre of the TARDIS)

The Foamasi (TV: The Leisure Hive, PROSE: Placebo Effect) and the Slitheen (TV: Boom Town) used skin suits to disguise themselves as humans.

By the time the Ninth Doctor and Rose Tyler met Lady Cassandra O'Brien.Δ17, she was little more than a piece of skin stretched onto a frame with eyes and a mouth, connected to a brain. After that piece of skin ripped apart, (TV: The End of the World) her brain was attached to stretched out skin taken from the back. (TV: New Earth)

Toby, possessed by the Beast, had markings on his skin. (TV: The Impossible Planet)

When Toby Zed was possessed by the Beast, markings appeared on his skin. (TV: The Impossible Planet / The Satan Pit)

Historians who had been allowed access to the "Monster Vaults" of the databanks in the Doctor's TARDIS identified Abzorbaloffs as being covered in folds of loose, bright green skin. Existing in a permanent state of suction, Abzorbaloffs constantly drew sustenance through their skin and were naturally as naked as possible, assimilating the heads of their victims into their digestive tract. (PROSE: The Monster Vault, TV: Love & Monsters) Victor Kennedy's green skin was saggy and not unlike the Slitheen. (PROSE: Creatures and Demons)

Jenny was created in a progenation machine from a skin sample taken from the Tenth Doctor. (TV: The Doctor's Daughter)

Amy, with marks on her skin to remind herself she had met the Silence. (TV: Day of the Moon)

One way that individuals reminded themselves they had encountered the Silence was to make tally marks on their own skin. (TV: Day of the Moon, The Wedding of River Song)

The Leaf Dragon's mate became stuck on Coach Dawson's skin. When later confronting the Leaf Dragon, Ram Singh suggested that it could use his skin like leather to make a "chair or something". (TV: The Coach with the Dragon Tattoo)

When Mels regenerated into River Song, her skin colour changed. (TV: Let's Kill Hitler) The General's skin colour also changed in at least one regeneration, (TV: Hell Bent) as did Karlax's. (PROSE: Engines of War)

Sleep dust largely consisted of skin cells. (TV: Sleep No More)

Olivia Steele had "flawless" caramel coloured skin. (PROSE: Exodus Code)

When investigating the Boneless, the Twelfth Doctor recognised what he and Clara Oswald had earlier dismissed as a mural was a blown-up human skin cell, flattened into two dimensions. (TV: Flatline)

A tall woman on the 68 to Bolton had pale skin. (PROSE: Have You Seen This Man?)