Why are tattoos permanent.

Tattoo Ink Placement

The tattooing process causes damage to the epidermis, epidermal-dermal junction, and the papillary layer (topmost layer) of the dermis. These layers appear homogenized (or in other words, like mush) right after the tattooing process. The ink itself is initially dispersed as fine granules in the upper dermis, but aggregate into more concentrated areas at 7-13 days.

Like any injury, the initial response is to stop bleeding, followed by tissue swelling, and the migration of non-resident immune cells into the area. The "automatic response" immune cells are mostly neutrophils, and macrophages later on. They are phagocytic cells that "swallow" debris to clean up the area and then leave via the lymphatics. This is the extent of an immune response unless an allergic reaction occurs or an infection sets in. The tissue is then repaired and/or regenerated by fibroblasts. Initially the tissue formed is known as granulation tissue (think fresh scar, pinkish and soft), which later matures into fibrous tissue (think old scar).

Stages of Ink Dispersal

Initially ink is taken up by keratinocytes, and phagocytic cells (including fibroblasts, macrophages and mast cells).

At one month the basement membrane of the epidermis (epidermal-dermal junction) is reforming and the basal cells contain ink. In the dermis, ink containing phagocytic cells are concentrated along the epidermal-dermal junction below a layer of granulation tissue that is surrounded by collagen. Ink is still being eliminated through the epidermis with ink present in keratinocytes, macrophages and fibroblasts.

At two to three months the basement membrane of the epidermis is fully reformed, preventing any further loss of ink through the epidermis. Ink is now present in dermal fibroblasts. Most of these ink containing fibroblasts are located beneath a layer of fibrous tissue which has replaced the granulation tissue. A network of connective tissue surrounds and effectively traps these fibroblasts. It is assumed that these fibroblasts are the cells that give tattoos their lifespan.

Then why does the tattoo fade over time?

It is debated whether all the ink particles are in fibroblasts, or if some remain as extracellular aggregations of ink. Also, the lifespan of the ink containing fibroblasts is not known. Presumably, ink particles are moved into the deeper dermis over time due to the action of mobile phagocytic cells (think immune cells), causing the tattoo to look bluish, faded and blurry. Examination of older tattoos (e.g. 40 years) show that the ink is in the deep dermis, and also found in local lymph nodes. Since some types of phagocytic immune cells migrate to lymph nodes to "present their goods", the discovery of ink in lymph nodes is consistent with the theory of phagocytic cells being the cause of ink movement.

Source: http://archives.evergreen.edu/webpages/curricular/1999-2000/humanbio/TattooInk.htm