Twins and Fingerprints

http://www.nytimes.com/2009/10/06/science/06qna.html?_r=2

Twins and Fingerprints-Q. Why do identical twins have different fingerprints? Why do we have fingerprints to begin with?

A. The probable answers to both questions are related to the minute differences in the mechanical forces each developing fetus experiences in the uterus as its cells proliferate.

Researchers have found that identical twins have a very high correlation of loops, whorls and ridges, but a review study last year in Circulation Research examining how complex structures like the circulatory system develop says that “the detailed ‘minutiae’ — where skin ridges meet, end or bifurcate — are different even between identical twins.” Even twins that develop from one zygote occupy different positions in the womb, and the variations are enough to make a difference.

At the crucial stage of development, the study explains, ridges are thought to form as compressive stresses develop in the dermal cell layer of the skin, sandwiched between the epidermis and the subcutaneous tissue. “Like the buckling of land masses under compression,” the study continues, regular ridges form to relieve the stress.

Where the skin is flat, the ridges are parallel; but primates have raised pads on their fingertips at this stage, so the ridges form along lines of equal stress. “Surrounding the highest point of the raised pad,” the study says, “ridges form in concentric circles.” Meanwhile, the pads are regressing, but where they remain high longer, ridges form whorls. Pads that have largely regressed give rise to a simple arch pattern. Where the processes of ridge formation and pad regression overlap, an intermediate loop pattern results. C. CLAIBORNE RAY