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Equine Spleen - Horse Anatomy (view source)
Revision as of 17:08, 28 November 2012
, 17:08, 28 November 2012→Structure
|colspan="2"|<center><sup>©Nottingham Uni 2008 </sup></center>
|colspan="2"|<center><sup>©Nottingham Uni 2008 </sup></center>
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The spleen lies vertically on the left side of the cranial abdomen. It is attached to the greater curvature of the [[Equine Alimentary System - Horse Anatomy#Stomach|stomach]] by the '''gastrosplenic ligament'''. The spleen is enclosed in a capsule of fibrous and elastic tissue that extends into the parenchyma as trabeculae.
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|<center>[[Image:LH_Spleen_Equine_photo.jpg|110px]]</center>
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The spleen lies vertically on the left side of the cranial abdomen, under the last three [[Ribs and Sternum - Anatomy & Physiology|ribs]]. Dorsally it is broad but narrows as it extends cranially and ventrally. On rectal palpation it is located against the body wall and feels smooth with a sharp border. It is attached to the greater curvature of the [[Equine Alimentary System - Horse Anatomy#Stomach|stomach]] by the '''gastrosplenic ligament'''. The spleen is enclosed in a capsule of fibrous and elastic tissue that extends into the parenchyma as trabeculae.
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.
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.
===White Pulp===
===White Pulp===
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. White pulp stains basophilic in a H&E stain.
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. White pulp stains basophilic in a H&E stain.
===Vasculature===
The '''splenic artery''', a branch of the '''celiac artery''', supplies the spleen. The artery branches into arterioles and capillaries regularly as it passes through the spleen, which may either:
*Connect with the venous sinuses, or
*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'''.
[[Lymphocytes - Introduction|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.
===Innervation===
Innervation is purely sympathetic<ref>{{citation|initiallast = Nance|initialfirst = D.M|finallast = Sanders|finalfirst = V.M|year = 2007|jtitle = Autonomic innervation and regulation of the immune system (1987-2007)|jor = Brain, Behavior, and Immunity|vol = 21(6)|range = 736-745}}</ref> and nerve fibres travel with the artery into the spleen.
===Histology===
<center><gallery>
Image:LH_Spleen_Rat_Histology.jpg|<p>'''Gross view (rat)'''</p><sup>©RVC 2008</sup>
Image:LH_Spleen_Rat_Higher_Histology.jpg|<p>'''Red & white Pulp (rat) '''</p><sup>©RVC 2008</sup>
Image:LH_Spleen_Rat_Higher_2_Histology.jpg|<p>'''Central artery & PALS (rat) '''</p><sup>©RVC 2008</sup>
Image:LH_Spleen_Rat_Higher_3_Histology.jpg|<p>'''Trabecula and capsule (rat) '''</p><sup>©RVC 2008</sup>
Image:LH_Spleen_Rat_Higher_4_Histology.jpg|<p>'''Trabeculae & erythrocytes (rat)'''</p><sup>©RVC 2008</sup>
Image:LH_Spleen_Mouse_Higher_Histology.jpg|<p>'''Megakaryocyte & macrophages (mouse)'''</p><sup>©RVC 2008</sup></gallery></center>
==Functions==
The spleen has a number of functions:
*It filters the blood removing ageing [[Erythrocytes|erythrocytes]] and antigens
*It stores [[Erythrocytes|erythrocytes]] and [[Thrombocytes|platelets]]
*[[:Category:Secondary Lymphoid Tissue|Secondary lymphoid organ]]
===Erythrocytes & Platelets===
In the foetus the spleen also has a role in [[Haematopoiesis - Overview|haematopoiesis]] when it becomes the main [[Erythrocytes|erythrocyte]] producing organ during the haematopoietic transitional phase. In the developed animal the red pulp is involved in the removal of aged, damaged or abnormal [[Erythrocytes|erythrocytes]] (along with the [[Equine Alimentary System - Horse Anatomy#Liver|liver]] and [[Bone Marrow - Anatomy & Physiology|bone marrow]]). As [[Erythrocytes|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|erythrocytes]] in circulation increases.
The red pulp also acts as a storage site for [[Erythrocytes|erythrocytes]]. The degree of storage is particularly notable in horses which, during exercise under sympathetic activity, can contract their spleen to increase the concentration of circulating [[Erythrocytes|erythrocytes]].
===Lymphoid===
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. Following splenectomy this doesn’t occur and animals are predisposed to septicaemia and infection with blood [[Protozoa|protozoa]].