Science Focus (issue 25)

Introduction Before we delve into the complexities of blood tests and DNA analyses to identify who we are, society has already made a simple yet straightforward method that we always see in movies and when we are crossing the border: our fingerprints which are also known as “friction ridge skin”. Just like our faces, fingerprints are key to our individuality and identity. But have you ever wondered why each of us has unique fingerprints? The answer lies deep in the interaction between our genes, especially those that control limb development, and the environment. This results in the formation of unique dermatoglyphic patterns which can be classified into three categories: arch, loop, and whorl (Figure 1). Fingerprint Formation There are multiple theories supporting fingerprint development but dermatologists believe the folding hypothesis is the most promising one [1]. Skin tissue consists of three tightly connected vertical layers: epidermis, basal layer and dermis. The different rates of cellular growth in the top epidermis and the bottom dermis create a tension across the fast-growing basal layer, resulting in the folding of the basal layer at individual sites to relieve the stress (Figure 2) [2, 3]. Cell proliferation continues at those sites while the folds combine and merge into clusters to form linear ridges in a rather random fashion, creating the unique pattern of wrinkles in our fingerprints (Figure 3) [2]. Volar pads are well known to play a role in determining fingerprint pattern. They are transient tissue swellings present on certain areas of our palms, including each fingertip, during embryonic development (embryogenesis) (Figure 4). Coinciding with the process of ridge formation, these structures start shrinking from the 10th week [1]. The shrinkage introduces extra mechanical stresses across the skin, affecting the directions of ridge formation [1]. Scientists generally agreed that the height and size of volar pads can influence the pattern of fingerprints [2, 4]. Whorl-type patterns are Figure 2 Schematic diagram of the folding hypothesis [3]. The three vertical layers are epidermis (top), basal layer (middle) and dermis (bottom), respectively. Figure 3 Ridge formation through the combination of the centers of cell proliferation [2]. By Charlton Sullivan 蘇柏安 Figure 1 The three categories of fingerprint: arch, loop and whorl.