The science of your skin

Lift­ing the lid on the body’s largest and most sen­si­tive or­gan

How It Works - - SCIENCE -

Weigh­ing in at 2.7 kilo­grams, your skin is by far the largest or­gan in, or rather on, your body. Wrapped around you from head to toe, it pro­vides a wa­ter­proof bar­rier that sep­a­rates your tis­sues from the out­side world. Skin keeps mois­ture in, blocks out the light, stores fat, senses touch, reg­u­lates tem­per­a­ture and shields you against in­fec­tion. To do all this it has three sep­a­rate lay­ers, each packed with a dif­fer­ent set of spe­cial­ist cells.

The out­er­most layer of the skin is the epi­der­mis. It con­tains four or five lay­ers of skin cells, which come from cube-shaped stem cells deep un­der the sur­face. These stem cells make enough new skin cells to com­pletely re­place your skin ev­ery four weeks. The skin cells them­selves are called ker­atinocytes, be­cause they make the pro­tein ker­atin. This is the same tough fi­bre that makes hair and nails. As new ker­atinocytes ap­pear, they push the old ones up­wards and, as the cells get closer to the sur­face, they be­come flat­ter and tougher. The cells die as they reach the very outer layer, form­ing a hard and wa­ter-re­sis­tant bar­rier.

Col­la­gen fi­bres con­nect the epi­der­mis to the next layer of skin via a series of fin­ger-shaped folds. This layer, called the der­mis, con­tains blood and lym­phatic ves­sels, nerves, hair fol­li­cles and sweat glands. These struc­tures all sit in a layer of flex­i­ble fi­bres, which are made by spe­cialised cells called fi­brob­lasts. The fi­bres – elastin and col­la­gen – give skin its strength and abil­ity to stretch.

The very bot­tom layer of skin is the hy­po­der­mis, and it links the skin to the in­side of the body, con­nect­ing it up with mus­cle, bone and tis­sue. Here, cells called adipocytes store ex­cess en­ergy as fat, pro­vid­ing a layer of in­su­la­tion and cush­ion­ing against im­pacts.

Cells called fi­brob­lasts make the elas­tic tis­sue that sup­ports our skin

Lev­els of col­la­gen and elastin in the der­mis drop as we age

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