The fingertips are one of the most relied-on resources of the central nervous system. We use our fingertips to detect hot and cold, feel various textures, and provide us with sensory information for everything from a handshake to a cup of coffee. As much as humans use their fingertips in day-to-day life, there is still a lot of mystery concerning their design and function. Sometimes the flesh in the fingertips can become depressed for a variety of reasons. and there are several possibilities as to why this occurs. Under normal circumstances, pressing a thumbnail into a fingertip can cause a temporary depression as the blood under the skin surface moves out of the way as the pressure from the thumbnail pushes the skin cells inward. The depression then fills again as the blood flows back into the area and pushes the skin cells back out into their normal position. There are several other causes for longer-term flesh depression.
Extreme indentation of the fingertip could be a symptom of lymphedema, which is the accumulation of lymphatic fluid in a particular area of the body. This is most common in the arms or legs but can happen in other body parts as well. This can develop when the lymphatic vessels designed to keep this fluid flowing through your body are damaged and not working properly or if vessels are missing or obstructed. Under these conditions, the amount of fluid in the area exceeds the body's ability to transport it and the fluid seeps into surrounding tissues, resulting in swelling. The fluid is rich in protein and can cause the tissues to harden. More fluid continues to build and the afflicted body part becomes highly susceptible to bacterial infection. If the hand or fingers are affected, an early symptom of stage 1 lymphedema would be if you created a depression in the fingertip (such as using your thumbnail) and the depression did not go away. As the condition turns into stage 2, the swelling would increase to where no indentation would be formed even with the application of pressure as the tissue has a more "spongy" quality.
Water and Wrinkling
Leaving your fingers wet for extended periods of time can cause numerous depressions in the fingertips as the skin temporarily wrinkles. This phenomena is still not completely understood, but it is believed that the epidermis of the fingertip (the outermost layer of flesh) expands as it takes in water. Since it is tightly attached to the skin layers underneath it, it wrinkles to accommodate for its increased surface-area. The epidermis itself has layers and the top portion is made of dead keratin cells, which is most sensitive to water-immersion. The reason the wrinkling-phenomena occurs almost exclusively in hands and feet is because those areas are the ones with the most keratin cells. The other theory on fingertip depression via wrinkling concerns the narrowing of blood vessels called vasoconstriction. When hands are immersed in water, the nerve fibers trigger a shrinking of the blood vessels and glomus bodies in the hands that regulate body temperature start to lose volume, which pulls the skin further down and produces wrinkles and fingertip depression.