Blood cells are made in the bone marrow. During a bone marrow punction a big, large needle is inserted in the bone and a part of the bone marrow is aspirated. The bone marrow sample is then applied on a glass slide and examined under the microscope. A bone marrow punction is done when anomalies are found in the bloodwork (such as a low or very high level of red blood cells, blood platelets or white blood cells, or abnormal cells).