Changes

The spleen is a major lymphoid and blood filtration organ and is located in the left cranial abdomen. It is responsible for storing and removing [[Erythrocytes - WikiBlood|erythrocytes]] from the blood as well as antigen surveillance of the blood and antibody production.

The spleen is a major lymphoid and blood filtration organ and is located in the left cranial abdomen. It is responsible for storing and removing [[Erythrocytes - WikiBlood|erythrocytes]] from the blood as well as antigen surveillance of the blood and antibody production.

==Development==

==Development==

<p>The spleen develops in association with the digestive system in the dorsal mesogastrium, and as the [[Forestomach - Anatomy & Physiology|stomach]] rotates during development the spleen comes to occupy the left cranial abdomen. Haematopoietic cells in the spleen are derived from the AGM (aorta-gonad-mesonephros) and yolk sac and as the [[Primary Lymphoid Tissue - Anatomy & Physiology|primary lymphoid organs]] become established it becomes populated with [[Lymphocytes - WikiBlood#Types|T and B lymphocytes]].</p>

<p>The spleen develops in association with the digestive system in the dorsal mesogastrium, and as the [[Forestomach - Anatomy & Physiology|stomach]] rotates during development the spleen comes to occupy the left cranial abdomen. Haematopoietic cells in the spleen are derived from the AGM (aorta-gonad-mesonephros) and yolk sac and as the [[Primary Lymphoid Tissue - Anatomy & Physiology|primary lymphoid organs]] become established it becomes populated with [[Lymphocytes#Types|T and B lymphocytes]].</p>

==Structure==

==Structure==

{|align="right"

{|align="right"

<p> The parenchyma is supported by a fine mesh of reticular fibres and is divided into two types of tissue, the red and the white pulp, which are separated by the marginal sinus.</p>

<p> The parenchyma is supported by a fine mesh of reticular fibres and is divided into two types of tissue, the red and the white pulp, which are separated by the marginal sinus.</p>

===Red Pulp===

===Red Pulp===

<p>The red pulp makes up the majority of the spleen and is composed of a network of cell cords in series with vascular sinuses. The splenic cords contain [[Macrophages - WikiBlood|macrophages]], [[B cell differentiation - WikiBlood#Plasma cells|plasma cells]], [[Lymphocytes - WikiBlood|lymphocytes]] and other mature blood cells e.g. [[Granulocyte - WikiBlood|granulocytes]] and [[Erythrocytes - WikiBlood|erythrocytes]].  While the vascular sinuses are wide vascular channels lined with endothelial cells. Blood cells and fluid can pass into the splenic cords through fenestrations in the sinus walls. </p>

<p>The red pulp makes up the majority of the spleen and is composed of a network of cell cords in series with vascular sinuses. The splenic cords contain [[Macrophages - WikiBlood|macrophages]], [[B cell differentiation - WikiBlood#Plasma cells|plasma cells]], [[Lymphocytes|lymphocytes]] and other mature blood cells e.g. [[Granulocyte - WikiBlood|granulocytes]] and [[Erythrocytes - WikiBlood|erythrocytes]].  While the vascular sinuses are wide vascular channels lined with endothelial cells. Blood cells and fluid can pass into the splenic cords through fenestrations in the sinus walls. </p>

===White Pulp===

===White Pulp===

<p>White pulp is organised in relation to the splenic arterioles and consists of discrete lymphoid tissue surrounding a central arteriole. There is a sheath of [[Lymphocytes - WikiBlood#T cells|T cells]] directly around the arteriole, the periarteriolar lymphoid sheath (PALS), which is surrounded by a marginal sinus, and then a zone of [[Lymphocytes - WikiBlood#B cells|B cells]] and [[Macrophages - WikiBlood|macrophages]] (the marginal zone). B cell follicles are associated with the marginal zone and expand and develop germinal centres after antigen activation. The marginal sinuses are linked to the red pulp sinuses. </p><p>White pulp stains basophilic in a H&E stain</p>

<p>White pulp is organised in relation to the splenic arterioles and consists of discrete lymphoid tissue surrounding a central arteriole. There is a sheath of [[Lymphocytes#T cells|T cells]] directly around the arteriole, the periarteriolar lymphoid sheath (PALS), which is surrounded by a marginal sinus, and then a zone of [[Lymphocytes#B cells|B cells]] and [[Macrophages - WikiBlood|macrophages]] (the marginal zone). B cell follicles are associated with the marginal zone and expand and develop germinal centres after antigen activation. The marginal sinuses are linked to the red pulp sinuses. </p><p>White pulp stains basophilic in a H&E stain</p>

====Species Differences====

====Species Differences====

{|align="right"

{|align="right"

*Terminate with open ends in the splenic cords

*Terminate with open ends in the splenic cords

Blood released into the splenic cords, either from the sinuses or capillaries, eventually filters back into the sinus network. The sinuses converge and empty into trabecular veins, which then merge into a single splenic vein which then empties into the portal vein.</p>

Blood released into the splenic cords, either from the sinuses or capillaries, eventually filters back into the sinus network. The sinuses converge and empty into trabecular veins, which then merge into a single splenic vein which then empties into the portal vein.</p>

<p>[[Lymphocytes - WikiBlood|Lymphocytes]] in the arterial blood migrate from the red pulp sinuses, through the splenic cords and through the white pulp. [[Lymphocytes - WikiBlood#T cells|T cells]] specifically migrate through the PALS and [[Lymphocytes - WikiBlood#B cells|B cells]] specifically migrate through the follicles. Antigen in the blood is filtered by the large numbers of [[Macrophages - WikiBlood|macrophages]] in the splenic cords and white pulp.</p>

<p>[[Lymphocytes|Lymphocytes]] in the arterial blood migrate from the red pulp sinuses, through the splenic cords and through the white pulp. [[Lymphocytes#T cells|T cells]] specifically migrate through the PALS and [[Lymphocytes#B cells|B cells]] specifically migrate through the follicles. Antigen in the blood is filtered by the large numbers of [[Macrophages - WikiBlood|macrophages]] in the splenic cords and white pulp.</p>

====Species Differences====

====Species Differences====

<p>The splenic artery:

<p>The splenic artery:

===Lymphoid===

===Lymphoid===

<p>Blood flows through the marginal sinus. This means that most antigens present in the blood come into contact with the [[Lymphocytes - WikiBlood#B cells|B lymphocytes]] and dendritic cells in the spleen. Dendritic cells in the marginal sinus and red pulp take up antigens from the blood and transport them to the primary follicles in the white pulp. If the antigen activates the [[Lymphocytes - WikiBlood#B cells|B lymphocytes]] then a germinal centre will form in the primary follicle and this is called a splenic nodule. Antibody producing cells then migrate to the red pulp and marginal zone.</p>

<p>Blood flows through the marginal sinus. This means that most antigens present in the blood come into contact with the [[Lymphocytes#B cells|B lymphocytes]] and dendritic cells in the spleen. Dendritic cells in the marginal sinus and red pulp take up antigens from the blood and transport them to the primary follicles in the white pulp. If the antigen activates the [[Lymphocytes#B cells|B lymphocytes]] then a germinal centre will form in the primary follicle and this is called a splenic nodule. Antibody producing cells then migrate to the red pulp and marginal zone.</p>

<p>Following splenectomy this doesn’t occur and animals are predisposed to septicaemia and infection with blood [[Protozoa|protozoa]].</p>

<p>Following splenectomy this doesn’t occur and animals are predisposed to septicaemia and infection with blood [[Protozoa|protozoa]].</p>