WHAT ARE FRICTION RIDGES?
Friction ridges appear on the palms, soles, and the ends of the fingers and toes (see Figure 1). These ridges are found on the palms and soles of all primates (humans, apes, monkeys, and prosimians); in primates with prehensile tails (“fingerlike” tails, such as spider monkeys), friction ridges also appear on the volar surface of the tails. All primates have an arboreal evolutionary heritage: Trees have been and continue to be the primary habitat for most apes and monkeys, and humans share this arboreal heritage. Primates’ hands and feet show adaptations for locomotion and maneuvering in the branches of trees. The opposable thumb provides a flexible and sturdy means of grasping branches or the food that hangs from them. Primates, unlike other mammals such as squirrels or cats, have nails instead of claws at the distal end of their phalanges. Claws would get in the way of grasping a branch (imagine making a fist with two-inch nails) and would provide insufficient structure to hold an animal with a high body weight (a one-pound squirrel is highly maneuverable in a tree, but a 150-pound jaguar is not). The fingerpads and friction ridges are part of a complex manipulation and sensory system that is still being explored and researched . Friction ridges begin forming in the ninth or tenth week of fetal development. These primary friction ridges develop deep in the dermal layer of the skin, as shown in Figure 2. At about 14 weeks of gestation, sweat glands and sweat ducts begin to form, proliferating from the primary friction ridges. They infiltrate into the dermis and develop into mature ducts and glands. The primary friction ridges proliferate until about the fifteenth or seventeenth week of gestation; at this point, the primary friction ridges stop proliferating and secondary friction ridges appear. Secondary friction ridges develop from week 17 and mature by week 24. The interface or margin between the epidermis and the dermis provides a template of the configuration of the friction ridges on the surface. Numerous factors may affect the patterning and arrangement of friction ridges, including the fetus’ genetics, environmental factors, drugs, disease, and perhaps even the shape of the volar pad itself.
FIGURE 1 The terminology for the palms is important for the proper identification and description of friction ridges.
FIGURE 2 The primary friction ridges develop deep in the dermal layer of the skin.
Friction ridges develop in utero and remain the same throughout life, bar ring some sort of scarring or trauma to the epidermal–dermal margin of a fric tion ridge area. This interface between the epidermis and the dermis acts as a template for the configuration of the friction ridges seen on the surface of the skin. Although humans grow and increase in size, the friction ridge patterns on their bodies, which became permanent and fixed in their patterns from about 17 weeks of embryonic development, do not change like other parts of their bodies
