Equine Spleen - Horse Anatomy

Introduction

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 from the blood as well as antigen surveillance of the blood and antibody production.

Structure

Normal Ultrasound (Equine)	Histological section

^{©Nottingham Uni 2008}

The spleen lies vertically on the left side of the cranial abdomen, under the last three 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 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.

Red Pulp

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, plasma cells, lymphocytes and other mature blood cells e.g. granulocytes and 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.

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 T cells directly around the arteriole, the periarteriolar lymphoid sheath (PALS), which is surrounded by a marginal sinus, and then a zone of B cells and 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 in the arterial blood migrate from the red pulp sinuses, through the splenic cords and through the white pulp. T cells specifically migrate through the PALS and B cells specifically migrate through the follicles. Antigen in the blood is filtered by the large numbers of macrophages in the splenic cords and white pulp.

Innervation

Innervation is purely sympathetic and nerve fibres travel with the artery into the spleen.

Histology

Functions

The spleen has a number of functions:

It filters the blood removing ageing erythrocytes and antigens
It stores erythrocytes and platelets
Secondary lymphoid organ

Erythrocytes & Platelets

In the foetus the spleen also has a role in haematopoiesis when it becomes the main 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 (along with the liver and bone marrow). As 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. If a splenectomy is performed the number of aged erythrocytes in circulation increases.

The red pulp also acts as a storage site for 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.

Lymphoid

Blood flows through the marginal sinus. This means that most antigens present in the blood come into contact with the 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 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.