The Oldie: Profitable Wonders – December 2010
Skin Deep
We are, for the most part, so comfortable in our skins as to rarely reflect how this seamless stocking, flexing around the contours of the body, should combine so many variations on a single theme:delicate and callused, radiant and wrinkled, smooth and hairy, lax over the joints yet tight over the shins. Nor indeed how it reconciles in a single structure the two contradictory properties of resilience and sensitivity—both a robust barrier against the physical assault of heat, cold and pollutants of all kind, yet also exquisitely sensitive to the subtlest of changes in the external world.
Charles Darwin in a famous passage from The Voyage of the Beagle captures this paradox in a memorable image, describing how when anchored off Tierra del Fuego, “A canoe with six quite naked Fuegians came alongside and remained all day out of curiosity". He was particularly struck by seeing a woman “suckling a recently born child whilst sleet fell and melted on her naked bosom, and on the skin of her naked baby".
The same year, far away in the Czech (at the time) city of Breslau, the recently appointed Professor of physiology, translator of Shakespeare and general polymath Jan Purkinje was refining the techniques for visualising down the microscope the then hidden microstructure of the body’s tissues. “Every day brings new discoveries!” he wrote excitedly in his diary—including the profundities of that seamless stocking.
First, its resilience poses the obvious question how the fragile delicate cells from which it is fashioned can nonetheless form that inviolable protective barrier. Here Professor Purkinje noted how the layer of living cells just beneath the surface of the skin grew outwards to form a single flattened layer of dead cells—the stratum corneum. This is then discarded at the rate of hundreds of millions of cells a day to be replaced by those moving up behind—just like successive waves of infantrymen marching into battle.
So, the whole of life, our existence and that of all living creatures, is utterly dependent on this layer of dead cells, no thicker than a sheet of tissue paper—that has the further vital property of being doubly waterproof, both preventing water getting in (or otherwise we will become water logged every time we took a bath), while ensuring the sustaining internal fluids do not leak outwards.
And as for the skin’s antithetical virtue of sensitivity, it can be difficult to grasp its staggering acuity in being able to discriminate between an ethereal puff of wind and a single molecule of water on the skin. Here professor Purkinje and his colleagues scrutinising their slides down the microscope discovered an abundance of different shaped nerve endings, round, thin, oval and many others that mediate those diverse sensations of touch, pain, pressure, heat and cold.
And if that were not enough the microscope also revealed another wonder of a different kind of direct relevance to those naked Fuegians—how it is that though the temperature that day must have been hovering around freezing, their body temperature would remain constant at 37 degrees centigrade—thus melting, as Darwin noted, those flakes of sleet on that naked bosom.
The explanation lies in the network of capillaries just below the surface of the skin that is far too extensive than would be required were it just supplying oxygen and nutrients to the surrounding tissues. Rather the network’s purpose is to ensure the constancy of that core temperature –dilating on a hot summer’s day to dissipate the body’s heat outwards and, on that bitterly cold day in Tierra del Fuego, constricting to conserve it. The most optimistic of bioengineers could never aspire to create a covering with such extraordinary specifications—and, pace Charles Darwin, one might profitably wonder why.