Perpetually young tissues where cells retain the ability to divide are called stem cell niches.

Here's why:

* Stem cells are undifferentiated cells that have the potential to develop into various specialized cell types. They are the key to the continual renewal of tissues.

* Niches are specific microenvironments within a tissue where stem cells reside. These niches provide the right signals and support for stem cells to maintain their undifferentiated state and to divide when needed.

Examples of stem cell niches in the body include:

* Bone marrow: Produces blood cells.

* Skin: Responsible for skin regeneration.

* Intestinal crypts: Maintain the lining of the intestines.

Let me know if you'd like more details on stem cell niches or their role in tissue regeneration!