The entire epidermal layer of your skin turns over every forty-eight days

We do not think of skin as an organ in the conventional sense.  It lacks the solidity and the discrete locus of the more familiar viscera.  That elastic but porous covering stretched over your frame, folds of flesh and imperfections that you know well enough to take for granted, fulfills an essential task.  It is no less important to your continued survival than a heart or a pair of lungs.

Skin is the first line of defense against the microbial hordes massing on its surface; it prevents the excessive evaporation of the body's precious fluids; it harbors an exquisitely sensitive array of detectors that warn us of harm and allow us to respond fast enough to avoid further injury; and it thermoregulates to keep us warm when it is cold or cool us down when it is hot. 

Skin is deeper than you think; in some areas of the body it is up to a fifth of an inch thick.  The stuff at the top is dead, a keratinized layer that serves the purpose of physical protection.  Below that layer is living, moist tissue that needs nutrients and a blood supply and is vulnerable to attack and injury.

A cross section of human skin taken with microscope.

Skin is organized into distinct strata.  The topmost layer, the epidermis, forms the tough barrier with which we feel so familiar.  The cells of the epidermis are densely packed and further subdivided into layers.  The base layer consists of stem cells that boast large purple-staining nuclei.  These cells mature, eventually losing their nuclei and acquiring filaments of keratin, making them more rigid.  As they develop, they ascend through the epidermal layer toward the surface, finishing at the top to form a tough protective layer of dead cells.

That layer tends to reinforce our image of the epidermis as a durable but passive barrier to the outside world.  Yet it is anything but passive.  The layers of epidermal cells, constantly being born and marching forward, are like a never-ending conveyor belt of foot soldiers throwing themselves at the wire.  They mount a spirited defense: They create a dry and acidic environment hostile to bacterial growth; their tentacle-like appendages seek out and destroy foreign bacterial cells, and they secrete enzymes and fatty chemicals to further deter would-be colonists.  The fight at the surface is fierce -- a war against perpetual mechanical, chemical, and biological attack. 

Consequently the rate of attrition among these cells is high.  For a single epidermal cell, that journey -- from birth in the basal layer to combat maturity on the surface of the epidermis-takes something like six weeks.  The rate of replacement must match the rate of loss, and the entire epidermal layer turns over every forty-eight days. 

But the epidermis, the layer that we casually refer to as our skin, represents only what we can see.  Beneath this there is the dermis, which under the microscope looks like a vertical section through a chaotically planted vegetable garden.  There are microscopic structures here that look like the cut surfaces of onions.  That bacon-like connective tissue is found here, dotted with strange-looking whorls, blood vessels, and tubes.  Here the skin becomes more recognizable as an organ, run through with a network of glands and vessels and studded with organelles.  It is from this layer that the skin derives both its elasticity and its supply of blood and nourishment.

Together the epidermis and dermis form a waterproof but breathable layer.  They have pores that are small enough to prevent ingress of water droplets but large enough to let molecules of water vapor out.  But it is the sensory array that is perhaps the skin's most remarkable feature.  Able to resolve point contacts little more than a millimeter apart, it's capable not only of registering heat and cold but also of differentiating between a lover's caress and pain from a needle tip.
Extreme Medicine: How Exploration Transformed Medicine in the Twentieth Century

Author: Kevin Fong M.D.    Publisher: The Penguin Press 

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