Corpus: Bone marrow

The term bone marrow refers to the specialised connective and stem cell tissue located in the centre of the large bones, which serves, among other things, to form blood cells.

The bone marrow is located in the medullary cavities of most bones, especially the long tubular bones and the flat bones of the skullcap, ribs or sternum, which are interspersed with cancellous bone. It makes up approx. 5% of the body weight.

From a macroscopic point of view, a distinction is made between two different forms of bone marrow:

• Red bone marrow (medulla ossium rubra)
• Yellow bone marrow (medulla ossium flava)

A pathological variant of the bone marrow is the greyish-glassy gelatinous marrow that occurs in cachexia, among other things.

Red bone marrow[Bearbeiten]

The red bone marrow contains stem cells and precursor cells of monocytes, erythrocytes, granulocytes and thrombocytes (cells of the "myeloid series") of different maturation stages, which are embedded in reticular connective tissue. The fibroblastic reticulum cells produce reticular fibres and growth factors that influence blood cell formation. They can store fat and then resemble the adipocytes of the connective tissue. The bone marrow also contains mesenchymal stem cells that can differentiate into connective tissue and muscle cells.

The basic tissue of the bone marrow is traversed by numerous thin-walled blood vessels, which are known as bone marrow sinuses. These are blood capillaries with a very wide lumen, which can have a diameter of 30-70 µm or more. The wall of the sinusoids is formed by a delicate, irregularly perforated endothelium that has no continuous basal lamina. It is part of the bone marrow-blood barrier, which prevents haematopoietic stem cells from entering the blood vessel system. When the mature blood cells migrate into the lumen of the sinusoids, the endothelium forms passages called migration pores.

In adults, the red bone marrow is mainly found in the flat and short bones (e.g. sternum, ilium, ribs, vertebral bodies, proximal femur). In infants, however, the red bone marrow is found in all bones. In the diaphysis of the long bones, the red bone marrow is replaced by fat-rich yellow bone marrow with increasing age.

Yellow bone marrow[Bearbeiten]

The yellow bone marrow is also called fatty marrow. It is mainly found in the diaphyses of the long bones. The colour is caused by reticulum cells in which large amounts of fat are stored. Yellow bone marrow is no longer involved in haematopoiesis as it no longer contains pluripotent stem cells.

Under physiological conditions, the red bone marrow is the exclusive site for the formation of most blood cells. This process is called haematopoiesis. It is further subdivided into:

• Erythropoiesis
• Leukopoiesis
  • granulopoiesis
  • Monocytopoiesis
  • Lymphopoiesis
• Thrombocytopoiesis

The precursor cells of the blood cells are embedded in a network of connective tissue in the bone marrow and divide continuously and increasingly in response to hormonal stimuli. The connective tissue provides the environment that the precursor cells need to differentiate. The mature cells are then released into the vessels running through the marrow and thus reach the peripheral blood.

The differentiation of T cells from their precursor cells in the context of lymphopoiesis takes place outside the bone marrow (mainly in the thymus).

Defects in the bone marrow usually result in disorders of the haematocrit composition. The causes of such disorders are manifold and range from drug-induced damage to the marrow to tumours (myelomas).

Diseases[Bearbeiten]

Important diseases of the bone marrow are

• Primary myelofibrosis (osteomyelofibrosis)
• Myeloproliferative diseases
• Aplastic anaemia

Diagnostics[Bearbeiten]

In many blood disorders, involvement of the haematopoietic system must be investigated. The method of choice for this is a bone marrow puncture to analyse the structure and cell composition. Today, this puncture is usually performed in the area of the iliac crest. Sampling from the sternum is now obsolete due to the risk of pericardial or cardiac puncture.

In addition to the exact cell composition, the total number of haematopoietic stem cells in the bone marrow is also relevant in comparison to normal findings. A reduced number of cells is referred to as hypocellularity. It occurs, for example, in aplastic anaemia. If the cell count is increased, this is characterised as hypercellularity. One possible cause is polycythaemia vera.

Therapeutic significance[Bearbeiten]

If necessary, donor marrow can be transplanted for bone marrow diseases (see bone marrow transplantation). In addition, hormones (e.g. erythropoietin) can be used to stimulate the formation of new blood cells. In preclinical emergency medicine, medication can be administered into the medullary cavity using an intraosseous access.

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