Final answer:
The third major site to study bone marrow from in the adult is the thymus gland.
Step-by-step explanation:
The third major site to study bone marrow from in the adult is the thymus gland.
In the adult, hematopoiesis mostly occurs in the red bone marrow, which is a loose collection of cells where blood cell production takes place. However, the thymus gland plays a key role in the development and maturation of T cells, a type of white blood cell involved in the immune response.
While the primary site for bone marrow study is the red bone marrow, the thymus gland is an important site to understand the differentiation and maturation process of certain cells.