How fingerprints get their one-of-a-kind swirls

The whorls, arches and loops that make fingerprints unique are produced during fetal development by waves of tiny ridges that form on the fingertip, spread and then collide with each other — similar to the process that gives a zebra its stripes, or a cheetah its spots.

In a study¹ published on 9 February in Cell, researchers found that the interplay between two proteins — one that stimulates ridge formation, and another that inhibits it — produces periodic waves of ridges that emerge from three distinct regions on the fingertip.

The precise locations of these regions and the collisions between the waves yields the unique pattern of a fingerprint. “To come up with these different patterns of arches, loops, and whorls, the key isn’t just the molecular ingredients,” says study co-author Denis Headon, a developmental biologist at the University of Edinburgh, UK. “It’s how they are deployed on the anatomy of the hand.”

Identifying marks

Fingerprints are thought to provide added grip and sensitivity to fingertips, and their patterns have long been used to identify individuals and diagnose some developmental conditions. Last year, Headon and his colleagues published work² describing the genes that influence fingerprint patterns, many of which are involved in limb development. These genes seemed to lay the groundwork for fingerprint formation, but many of them were inactive during the process, suggesting that they were not directly involved in forming ridges.

To learn more about fingerprint patterning, Headon and his colleagues tracked how fingerprints emerge over the course of fetal development. Anatomical studies and analyses of gene activity showed that the cells that form fingerprint ridges followed a developmental path that initially mimicked that of a hair follicle. But, unlike a follicle’s gene-activity pattern, the ridge cells failed to incorporate cells from deeper beneath the skin’s surface.

The analyses supported the presence of a ‘Turing reaction–diffusion system’, which can be created when a molecule that activates a developmental process stimulates both itself and an inhibitory molecule. The result is a self-organizing system that creates periodic patterns, says Marian Ros, a developmental biologist at the Institute of Biomedicine and Biotechnology of Cantabria in Santander, Spain.

The maths of patterns

Such systems were proposed³ by mathematician Alan Turing in 1952 as a chemical explanation for developmental processes such as the arrangement of leaves on a plant or tentacles on the small aquatic organisms called hydras. Since then, Turing reaction–diffusion mechanisms have been described as instrumental in establishing a wide variety of familiar biological sights, including the brightly coloured scales of some tropical fish, and feather patterns in birds.

To find the molecules that direct fingertip patterning, Headon and his collaborators studied the ridges on mouse toes, and human cells grown in 3D cultures. They found that a protein called WNT, which is important in hair-follicle development, stimulates ridge formation. Another molecule, called BMP, inhibits them, forming the Turing reaction–diffusion system.

The ridges emanate from three regions: the tip of the finger; the centre of the fingertip; and the crease at the base of the fingertip, where the finger bends (see ‘How prints are patterned’). In simulations, Headon and his team altered the timing, angle and precise location of the waves’ origins in these three sites, and created arches, loops and whorls. “These waves will collide,” says Cheng-Ming Chuong, a developmental biologist at the University of Southern California in Los Angeles. “And when they collide, they provide a turbulence that helps to create the diversity of fingerprint patterns.”

The findings are a significant advance in our understanding of fingerprint patterning, says Ros. Chuong notes that past studies of skin ridges, such as fingerprints, have tended to focus more on theoretical and modelling approaches than on experimental data. But the latest study harnesses advances in cell-culturing techniques and other methods to push the field: “Their work opens this area up,” says Chuong. “Now people might look more at these hidden patterns in our skin.”