The average person is covered by 16 to 21 square feet of skin. It’s the largest organ in (on?) the human body. A key role of skin (aside from simply helping keep us all together) is to serve as a physical barrier to our surrounding environment and its assorted, myriad pathogens.
Indeed, our skin (or more particularly, its outer layer called the epidermis) is home to its own ecosystem of microorganisms, including yeasts and bacteria that cannot be removed by any amount of cleaning. On average, it’s estimated that roughly 50 million microbes inhabit each square inch of epidermis, though that number varies greatly by location. Oily or moist skin, such as under the arm or between the toes, harbors higher numbers than a similarly sized patch of forearm.
The scanning electron micrograph above, created by Thomas Deerinck at UC San Diego’s National Center for Microscopy and Imaging Research, depicts a patch of skin. The epidermis contains no blood vessels, receiving all of its nutrients via diffusion from capillaries in the underlying dermis layer.
The bottom layer of the epidermis contains proliferating cells, which divide to form keratinocytes. These daughter cells migrate up through the epidermis and eventually die as their nutrient supply dwindles. The keratinocytes lose their cytoplasm, which is replaced with keratin, a structural protein that forms tough, insoluble filaments similar to those found in your hair and nails.
After 27 days or so, the dead keratinocytes reach the surface of the skin and are sloughed off. It’s estimated humans shed about 600,000 particles of skin per hour, about 1.5 pounds a year or 105 pounds of skin by the time they are 70 years old. This translates to an entirely new outer layer of skin cells every 27 days, almost 1,000 new skins in an average lifetime.