Changes

<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|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|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#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>

<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|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|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>

<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|macrophages]] in the splenic cords and white pulp.</p>

====Species Differences====

====Species Differences====

<p>The splenic artery:

<p>The splenic artery:

===Erythrocytes & Platelets===

===Erythrocytes & Platelets===

<p>In the foetus the spleen also has a role in [[Overview of Haematopoiesis - WikiBlood|haematopoiesis]] when it becomes the main [[Erythrocytes - WikiBlood|erythrocyte]] producing organ during the haematopoietic transitional phase.</p>

<p>In the foetus the spleen also has a role in [[Overview of Haematopoiesis - WikiBlood|haematopoiesis]] when it becomes the main [[Erythrocytes - WikiBlood|erythrocyte]] producing organ during the haematopoietic transitional phase.</p>

<p>In the developed animal the red pulp is involved in the removal of aged, damaged or abnormal [[Erythrocytes - WikiBlood|erythrocytes]] (along with the [[Liver - Anatomy & Physiology|liver]] and [[Bone Marrow - Anatomy & Physiology|bone marrow]]). As [[Erythrocytes - WikiBlood|erythrocytes]] age they become less supple and this causes them to become damaged when they pass through the very narrow capillaries of the spleen, after which they are phagocytised by splenic [[Macrophages - WikiBlood|macrophages]]. If a splenectomy is performed the number of aged [[Erythrocytes - WikiBlood|erythrocytes]] in circulation increases.</p>

<p>In the developed animal the red pulp is involved in the removal of aged, damaged or abnormal [[Erythrocytes - WikiBlood|erythrocytes]] (along with the [[Liver - Anatomy & Physiology|liver]] and [[Bone Marrow - Anatomy & Physiology|bone marrow]]). As [[Erythrocytes - WikiBlood|erythrocytes]] age they become less supple and this causes them to become damaged when they pass through the very narrow capillaries of the spleen, after which they are phagocytised by splenic [[Macrophages|macrophages]]. If a splenectomy is performed the number of aged [[Erythrocytes - WikiBlood|erythrocytes]] in circulation increases.</p>

<p> The red pulp also acts as a storage site for [[Erythrocytes - WikiBlood|erythrocytes]]. The degree of storage is variable between species but is particularly notable in horses which, during exercise under sympathetic activity, can contract their spleen to increase the concentration of circulating [[Erythrocytes - WikiBlood|erythrocytes]]. In some species such as cats and rodents the red pulp acts as a storage site for [[Thrombocytes|platelets]] and contains [[Thrombopoiesis - WikiBlood#Megakaryocyte|megakaryocytes]].</p>

<p> The red pulp also acts as a storage site for [[Erythrocytes - WikiBlood|erythrocytes]]. The degree of storage is variable between species but is particularly notable in horses which, during exercise under sympathetic activity, can contract their spleen to increase the concentration of circulating [[Erythrocytes - WikiBlood|erythrocytes]]. In some species such as cats and rodents the red pulp acts as a storage site for [[Thrombocytes|platelets]] and contains [[Thrombopoiesis - WikiBlood#Megakaryocyte|megakaryocytes]].</p>