This chapter integrates references that address small facets of the underlying biology of friction ridge skin, so that latent fingerprint examiners can use this chapter as a single reference for understanding why friction ridge skin is unique and persistent for each individual.
The basis for persistence in friction ridge skin is found in morphology and physiology, which have determined that the epidermis faithfully reproduces the three-dimensional ridges due to physical attachments and constant regulation of cell proliferation and differentiation. On the other hand, embryology explains why friction ridge skin is unique for each individual. This is the focus of the chapter. The unique features of the skin are established between approximately 10 1/2 and 16 weeks estimated gestational age (EGA) due to developmental patterns. During growth, signal substances are partitioned into different daughter cells, endowing them with distinct developmental instruction. The embryo is pre-patterned to continue developing with unique cell orientation. The chapter explains late embryological development (3-8 weeks EGA); fetal growth (9-12 weeks EGA); second trimester; and third trimester. This is followed by a section on limb development, with attention to the hands and friction ridge development. The differentiation of the friction ridge skin is explained in detail, including primary ridge formation and the propagation of secondary ridge formation. This is followed by an explanation of pattern formation. A section on genetics addresses genetic diversity and friction ridge skin, with attention to relevant familial studies. Causes of the uniqueness of friction ridge skin for each individual are summarized under the concept of "developmental noise," which pertains to the myriad of random forces that are themselves influenced by a seemingly infinite number of factors. Extensive figures and approximately 80 references