It is pretty crowded in the bone marrow. Many types of stem and progenitor cell, including progenitors of immune cells, coexist side-by-side1,2 and are supported by nearby cells that generate specialized protective environments for the stem cells, called niches.
https://www.nature.com/articles/d41586-021-00419-y
https://www.nature.com/articles/d41586-021-00419-y