Which type of bone marrow is fatty tissue found in the medullary cavity of most adult long bones?

• A. Fatty tissue found in the medullary cavity of most adult long bones
• B. Red bone marrow
• C. Articular cartilage
• D. Endosteum

Answer: (A)

In adults, the medullary cavity of long bones is filled with yellow bone marrow, which is composed mainly of adipose (fat) tissue. As people age, red marrow (the hematopoietic tissue that makes blood cells) is gradually replaced by fat in these cavities, so the fatty tissue becomes the predominant content in the medullary cavity of most long bones. Yellow marrow stores energy in the form of lipids and has little active role in blood cell production unless needed.

Red bone marrow, in contrast, is the site of hematopoiesis and is found in specific bones or regions, not primarily in the medullary cavity of most adult long bones. Articular cartilage lines joint surfaces and is avascular, not fatty marrow. The endosteum is the thin membrane lining the inner bone surfaces, including the medullary cavity, and contains cells important for remodeling, not fat storage. So the fatty tissue description specifically identifies yellow bone marrow.