How our distinctive swirls are formed

A team of researchers has deciphered how the uniqueness of our fingerprints with their loops, arcs and whorls arises. The patterns are already formed in the womb and remain unchanged throughout life. Specific genes, the individual shape of the developing fingers and the concept of a famous British mathematician are decisive for their formation, reports an international team from a wide range of disciplines in the specialist journal Cell.

The same body size, the same color of eyes, the same genetic make-up: At first glance, identical twins are often difficult to tell apart. But even their fingerprints are different. Their appearance is determined by the so-called papillary ridges - fine lines, which are also found on the fingertips of primates and koala bears and whose purpose is probably to improve grip and sense of touch.

In humans, the individual patterns are formed in the embryo as early as the 13th week. Just last year, a study showed that certain genes influence their expression. But since not even identical twins have the same fingerprint, other factors must also play a role. A multidisciplinary team led by Denis Headon from the University of Edinburgh has now investigated what this could be.

To do this, the scientists sequenced genetic information from the skin cells of the finger cells of human embryos that had been made available for research after abortions. In this way, they tracked down three different molecular signaling pathways and the genes WNT, BMP and EDAR that influence the growth of the papillary ridges.

The research group used mice to examine exactly how this works: the skin on the rodents' paws has a simple groove pattern that resembles human fingerprints. When the scientists artificially suppressed the signaling pathways in the mice, they found that signals from the WNT and BMP genes act in opposite ways: WNT appears to stimulate cell growth, causing bumps to form in the outer layer of the skin, while BMP suppresses cell growth, which leads to the formation of furrows. EDAR signals, on the other hand, would determine the size and spacing of the grooves.

If the WNT signaling pathway was switched off in the mice, no furrows were found on the undersides of their paws, while the furrows became wider when the BMP signaling pathway was switched off. In contrast, dotted instead of linear patterns were observed in animals without EDAR activity. According to the authors, the opposing relationship between WNT and BMP pathways is characteristic of a so-called Turing pattern.

The concept, developed by the British mathematician Alan Turing, describes how different, overlapping chemical activities lead to complex patterns that are widespread in nature and explain, for example, the stripes on zebras or the spots on giraffes. While genes and their signaling pathways now control the formation of the primary papillary ridges, their further development depends on the respective anatomy of the finger and the timing of growth, according to the researchers.

In their study of human embryonic tissue, they found that the primary ridges of the fingerprint form in three zones: the extreme end of the fingertip, the pad at the center of the fingertip, and the crease between the fingertip and the middle knuckle. From these three locations, the ridges spread out in waves across the fingertip, Headon explained in an article on the Cell study published online by the journal Science. "Each bar serves to define the position of the next."

In the next step, the individual finger shape plays a role: Large and symmetrical finger pads with early groove formation tend to form whorls, while long, asymmetrical pads tend to develop loops. As the study authors summarize in a statement from the University of Edinburgh, the formation of fingerprints is influenced by many factors, including the course of growth, finger length and shape of the fingertips and local gene activities: "This results in an infinite variety of fingerprint patterns that are generated can."

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