Bone marrow is the soft and sponge-like tissue found inside the body's larger bones that produces blood cells. Bone marrow aspiration and biopsy are procedures used to collect and evaluate bone marrow cells and structure.

Bone marrow has a honeycomb or sponge-like structure, consisting of a fibrous network that is filled with liquid. The liquid portion contains blood (hematopoetic), blood cells in various stages of maturation, and "raw materials" such as iron, vitamin B12, and folate that are required for cell production.

The primary function of the bone marrow is to produce red blood cells (RBCs), platelets, and white blood cells (WBCs). The number and type of cell being produced at any one time is based on the cell function, blood loss, and a normal, continual replacement of old cells.

A bone marrow aspiration collects a sample of the fluid that contains cells so that they can be examined under a microscope and/or evaluated with other tests. A biopsy collects a cylindrical core sample that preserves the marrow's structure. The biopsy sample is evaluated to determine the relationships of bone marrow cells to one another and the overall cellularity – the relative ratio of marrow cells to fat and other constituents present in the sample.

Red Blood Cells (RBCs)
Red blood cells, also called erythrocytes, transport oxygen throughout the body. RBCs typically make up about 40-45% of the blood volume and usually survive in circulation for about 120 days. The marrow produces RBCs at a rate that replaces old RBCs that age and degrade or are lost through bleeding, striving to maintain a relatively constant number of RBCs in the blood.

White Blood Cells (WBCs)
There are five different types of white blood cells: lymphocytes, neutrophils, eosinophils, basophils, and monocytes. Each plays a different role in protecting the body from infection.

Platelets
Platelets, also called thrombocytes, are cytoplasmic fragments of very big cells seen in bone marrow called megakaryocytes and are essential for normal blood clotting.