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Bone marrow is referred to as [[#Red marrow|red]] or [[#Yellow marrow|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==
<p>Haematopoietic stem cells migrate into bones during embryo development, a process called homing. The cells then associated closely with the connective tissues within the bone. </p>
==Red marrow==
<p>Red bone marrow consists of blood vessels, sinusoids and a note work of haematopoietic cells. Sinusoids and 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.</p>
==Yellow marrow==
<p>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.</p>
==Functions==
''Functions refer to red marrow''
=== Haematopoietic===
<p>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.</p>
<p>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 - Anatomy & Physiology|leukocytes]] than found in circulation</p>
<p>[[Thrombopoiesis - Anatomy & Physiology#Megakaryocyte|Megakaryocytes]] residue alongside the sinusoid membrane but do not leave the tissue, rather they release their platelets and then withdraw from the membrane.</p>
===Others===
*In some primates it acts as a [[Primary Lymphoid Tissue - Anatomy & Physiology|primary lymphoid organ]]
*It is a significant source of antibodies as a large population of antibody-producing cells ([[B cell differentiation- Anatomy & Physiology#Plasma cells|plasma cells]]) reside in the bone marrow
*[[Macrophages - Anatomy & Physiology|Macrophages]] and dendritic cells in the marrow remove foreign substance from the blood
**A process which also occurs in the [[Lymph nodes - Anatomy & Physiology|lymph nodes]], [[Spleen - Anatomy & Physiology|spleen]] and [[Liver - Anatomy & Physiology#Functions|liver ]]