Bone Marrow

Original Editor - Lucinda hampton

Top Contributors - Lucinda hampton, Uchechukwu Chukwuemeka and Kim Jackson  

Introduction

The bone framework which stabilizes the human body and allows for mobility has its unique microarchitecture. The cavities derived by the trabecular pattern of the bone's core get filled by a combination of blood cells (of various developmental stages) and adipocytes. This tissue, known as bone marrow, is important for the generation of blood cells (hematopoiesis).

Bone marrow is:

• A component of the lymphatic system.
• the soft and malleable tissue in the bone cavities.
• made up of about 4% of an adult human's weight (about 2.6 kilograms).
• Richly innervated and highly vascularized tissue.
• majorly involved in hematopoiesis; to produce blood cells for oxygen transport, defence against foreign invasion, and hemostasis.
• responsible for the removal of old cells from the circulation.
• made up of a vascular component and a non-vascular component.
• of two major tissue types; red marrow and yellow marrow.

The disease can impact the body's bone marrow.

• Low blood cell production is often a result of damage or disease. To correct this, a bone marrow transplant may be performed so that the body can produce enough healthy blood cells.

Structure and Gross Anatomy

The vascular section and non-vascular sections make up the bone marrow.

1. The vascular part features blood vessels that provide nutrients to the bone and transport blood stem cells and fully formed blood cells from the bone into circulation.
2. The non-vascular part is where haematopoiesis (blood cell creation) occurs. This area comprises immature blood cells, fat cells, white blood cells (macrophages and plasma cells), and thin, spreading fibres of reticular connective tissue. Certain white blood cells grow to maturity in the spleen, lymph nodes, and thymus gland.

• An uncommitted pluripotent stem cell proliferates and differentiates in an orderly manner, creating immature committed progenitors. The progenitors eventually grow into committed cells, that are discharged into the bloodstream.
• Several systems of regulation, including growth factors, influence cell production.
  The availability of recombinant growth factors has prompted clinical trials of these agents in a wide range of haematologic diseases.

Normal bone marrow is classified as red or yellow based on its fat content. Image at R: A piece of femur. The white ring is bone. The inner yellow circle is yellow marrow and the red bone marrow is the red spongy part.

1. Red marrow is composed of

• Haematopoietic stem cells produce two other types of stem cells: myeloid stem cells and lymphoid stem cells. These cells differentiate into red blood cells, white blood cells, or platelets.
• Supporting stroma
• Reticulum (phagocytes and undifferentiated progenitor cells)
• Scattered fat cells
• Rich vascular supply
• Mostly found in the bones of the skull, ribs, spine, pelvis, sternum, shoulder blades, and near the attachment points of the long bones of the arms and legs.
• Red marrow not only produces blood cells, but it also helps to eliminate old cells from circulation. Also, the spleen and liver, filter older and damaged blood cells from the blood.

2. Yellow marrow:

• It has the same constituents as red, with fat cells making up the vast majority of its content (80%, versus 40% in red marrow)
• Poor vascular supply
• Composed of hematopoietic tissue that has become inactive.
• Found in spongy bones and the shaft of long bones.
• Once the circulation of blood is severely depleted, yellow marrow can be transformed into red marrow to create additional blood cells.

Conversion of Bone Marrow

In infancy, red marrow covers the whole ossified skeleton excluding the epiphyses and apophyses.

Red marrow gradually retreats centrally and is almost entirely restricted to the axial skeleton (pelvis, spine, shoulder girdle, skull) by 25 years of age.

• The process of change to yellow from the red marrow starts in the phalanges and progresses to the femora/humeri.
• Within the long bones, the epiphysis is the first to undergo conversion followed by the diaphysis before extending to the metadiaphysis.

Bone Marrow Disease

Damaged or diseased bone marrow produces fewer blood cells. These disorders weaken the body's immunity and deprive organs and tissues of vital oxygen and nutrients.

Bone marrow disease occurs when the body's bone marrow fails to produce sufficient quantities of viable blood cells.

Marrow and blood malignancies, such as leukaemia, have the potential to cause it.

Risk factors include radiation exposure, certain types of infections, and disorders such as aplastic anaemia and myelofibrosis.

A bone marrow transplant may be used to treat blood and marrow disorders.

Bone marrow produces hematopoietic stem cells. These stem cells typically differentiate into healthy red blood cells, white blood cells, and platelets.

• A bone marrow or stem cell transplant is used to replace diseased, defective, or damaged stem cells caused by chemotherapy or radiation.
• Healthy stem cells can be harvested from a donor's blood or bone marrow.
• Bone marrow is taken from bones, such as the hip or sternum.

• Stem cells may also be obtained from umbilical cord blood to be used for transplantation.