Bone Marrow
Introduction
Bone marrow is referred to as red or yellow. Red bone marrow is involved with haemostasis while yellow bone marrow is adipose tissue. Bone marrow occupies the cavities in long bones and spaces in spongy bones.
Development
Pluripotential haematopoietic stem cells (PPSC) migrate into bones from the embryonic yolk sac and the foetal liver during development, a process called homing. The cells then associate closely with the connective tissues within the bone. The PPSCs continue to constantly divide in the bone marrow with one daughter cell remaining a pluripotential stem cell and the other daughter cells developing into multipotential haematopoietic stem cells. The multipotential stem cells also constantly divide with some daughter cells remaining stem cells and the other daughter cells developing into blood cells (haematopoiesis).
Red marrow
Red bone marrow consists of blood vessels, sinusoids and a network of haematopoietic cells. Sinusoids are vascular components with an endothelial layer, basal lamina and an outer adventitial cell layer. The adventitia cells are also called reticular cells and these extend into the haematopoietic cells in sheets to provide structural support. They also produce reticular fibres and cytokines to help stimulate blood cell production. Histological sections show that the haematopoietic cells lie in cords. The cells in these cords form many different blood cell types but cells producing one cell type tend to be located in groups along the cords.
Yellow marrow
In young animals the majority of marrow is red. However as the animal matures into an adult significant portions of the haematopoietic tissues is replaced by adipose tissue. In adults all of the marrow in the long bones is adipose tissue and significant portions of marrow in haematopoietically active bones is adipose tissue as well.
Functions
Functions refer to red marrow
Haematopoietic
The haematopoietic cells produce the vast majority of blood cells in the body (haematopoiesis). In young animals this occurs in most bones in the body but in mature adults this is limited to membranous bones in the body.
To enter circulation newly formed cells press against the sinusoid wall, temporally fusing to it and creating an opening. The cell then passes directly into the circulation and the membrane repairs itself. Mature erythrocytes immediately enter circulation, however the marrow stores leukocytes and consequently contains around ten times more leukocytes than found in circulation
Megakaryocytes reside alongside the sinusoid membrane but do not leave the tissue, rather they release their platelets and then withdraw from the membrane.
Lymphoid tissue
In some primates bone marrow acts as a primary lymphoid organ. Bone marrow is also a secondary lympoid tissue in other species.
It has little involvement in the primary immune response, but the migration of memory cells into the marrow from the spleen and lymph nodes means that during a subsequent exposure to an antigen it produces significant amounts of antibodies.
Others
Bone marrow is a significant source of antibodies as a large population of antibody-producing cells (plasma cells) reside there. Macrophages and dendritic cells in the marrow remove foreign substance from the blood, a process which also occurs in the lymph nodes, spleen and liver.
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