When we think of bone, we generally think of a skeleton, hard dead tissue. However, bone is a living, growing tissue made mostly of collagen. The human skeleton is the internal framework of the body.
Collagen is a protein that provides a soft framework, and calcium phosphate is a mineral that adds strength and hardens the framework. This combination of collagen and calcium makes bone strong and flexible enough to withstand stress. The bone mass in the skeleton reaches maximum density around age 21.

Bone marrow is the tissue comprising the center of large bones. It is the place where new blood cells are produced. Bone marrow contains two types of stem cells: hemopoietic (which can produce blood cells) and stromal (which can produce fat, cartilage and bone).