Bone marrow is soft, gelatinous tissue found in the centres of some bone also known as medullary cavities.

The two types of bone marrow are red bone marrow, known as myeloid tissue, and yellow bone marrow, or fatty tissue. Both are contained with capillaries and blood vessels. Bone marrow undergoing haematopoiesis is coloured red due to the presence of red blood cells, and are considered as the blood forming stem cells.

Bone marrow that is not undergoing haematopoiesis is yellow in color, and is known to produce fat, cartilage, and bone. The red marrow consists of long trabeculae (beam-like structures) within a sponge-like reticular framework. Spaces around this framework are filled with fat cells, stromal fibroblasts and blood cell precursors. Yellow marrow tends to be located in the central cavities of long bones, and is generally surrounded by a layer of red marrow with long trabeculae (beam-like structures) within a sponge-like reticular framework.

The blood-forming stem cells in red bone marrow can multiply and mature into three significant types of blood cells, each with their own job:

  1. Red blood cells (erythrocytes) transport oxygen around the body
  2. White blood cells (leukocytes) help fight infection and disease. White blood cells include lymphocytes – the cornerstone of the immune system – and myeloid cells which include granulocytes: neutrophils, monocytes, eosinophils, and basophils
  3. Platelets (thrombocytes) help with clotting after injury. Platelets are fragments of the cytoplasm of megakaryocytes, another bone marrow cell.
  4. After maturation, these blood cells move from the bone marrow into the bloodstream, where they perform their functions that are required to keep the body alive and healthy

Stem cells are the immature cells that have the ability to differentiate into a number of different types of cells. Stem cells are constantly dividing and producing new cells. Some new cells remain as stem cells and others go through a series of maturing and differentiation stages, as precursor or blast cells, before becoming mature blood cells.

Bone marrow is where the circulating blood cells are produced by a process called as haematopoiesis. It produces more than 200 billion new blood cells every day.  In the early stages of the human’s life, haematopoiesis takes place in many bones, but during developmental stages of the human life haematopoiesis increasingly centres on flat bones so that by puberty, blood production takes place predominantly in the sternum, vertebrae, iliac bones and ribs.

HSCs (haematopoeitic stem cells) are self-renewing stem cells that have the ability to differentiate into any blood cell type.

MPPs (multipotent progenitors) also have the potential differentiate into any cell type, but cannot divide continuously so must be renewed by the differentiation of HSCs.

CLPs (common lymphoid progenitors) differentiate into lymphoid cell types.

CMPs (common myeloid progenitors) differentiate into myeloid cells types via the GMP (granulocyte-macrophage progenitor) or MKEP (megakaryocyte-erythrocyte progenitor).

Mesenchymal stem cells (MSC) are found in the bone marrow cavity and have the ability to differentiate into a number of stromal lineages such as chondrocytes (cartilage generation), osteoblasts (bone formation), adipocytes (adipose), myocytes (muscle), endothelial cells and fibroblasts.

Mesenchymal stem cells are found in the bone marrow cavity. They differentiate into a number of stromal lineages, such as:

  1. chondrocytes (cartilage generation)
  2. osteoblasts (bone formation)
  3. osteoclasts
  4. adipocytes (adipose tissue)
  5. myocytes (muscle)
  6. macrophages
  7. endothelial cells
  8. fibroblasts

Functions

The life span of White blood cells is from a few hours to a few days, platelets about 10 days, and red blood cells about 120 days. These cells, therefore, must constantly be replaced by the bone marrow, as each blood cell has a set life expectancy.

The bone marrow produces many types of white blood cells. The main types of white blood cell, or leukocyte, are:

1) Lymphocytes

Lymphocytes are produced in bone marrow. They make natural antibodies to fight infection caused by viruses that enter the body through the nose, mouth or other mucous membrane, or through cuts and grazes. Specific cells recognize the presence of foreign invaders (antigens) that enter the body and send a signal to other cells to attack the antigens. There are two major types of lymphocyte: B- and T-lymphocytes.

2) Monocytes

Monocytes are produced in the bone marrow. Mature monocytes have a life expectancy in the blood of only 3 to 8 hours, but when they move into the tissues, they mature into larger cells called macrophages. Macrophages can survive in the tissues for long periods of time where they engulf and destroy bacteria, some fungi, dead cells, and other material foreign to the body.

3) Granulocytes

Granulocyte is the family or collective name given to three types of white blood cells: neutrophils, eosinophils and basophils.

Bone marrow produces platelets in a process known as thrombopoiesis. Platelets are needed for blood to coagulate and for clots to form, to stop bleeding.

The lymphatic system consists of lymphatic organs such as bone marrow, the tonsils, the thymus, the spleen and lymph nodes.

All lymphocytes develop in the bone marrow from immature cells called stem cells. Lymphocytes that mature in the thymus gland (behind the breastbone) are called T-cells. Those that mature in the bone marrow or lymphatic organs are called B-cells.

Stem cells are primarily located in four places:

  1. an embryo
  2. bone marrow
  3. peripheral blood, found in blood vessels throughout the body
  4. cord blood, found in the umbilical cord and collected after birth