Spleen

The spleen is the largest secondary lymphoid organ found on the left side of the abdomen inferior to the diaphragm, in the peritoneal cavity.

The spleen is a soft, highly vascular, lymphatic organ with significant hematologic and immunologic functions in domestic animals.: immune surveillance, proliferation and maturation of lymphocytes and degradation of senescent and damaged erythrocytes

While the basic structure and function are conserved among mammals, there are species-specific variations in morphology, position, and vascular pattern.

1.General Structure and Function

The spleen is enclosed in a fibrous capsule and composed of two main tissue types:

• White pulp: Lymphatic tissue organized around central arterioles. It plays a role in immune surveillance and production of lymphocytes.

• Red pulp: Composed of splenic cords and venous sinuses; responsible for filtering blood, removing senescent erythrocytes, and storing blood.

The capsule contains smooth muscle fibers, enabling contraction and expulsion of stored blood, particularly prominent in certain species like the horse.

2.Surfaces and borders of the spleen

The spleen has two surfaces

The spleen is underneath the left part of the diaphragm, and has a smooth, convex surface that faces the diaphragm, the parietal surface [diaphragmatic]. It is underneath the ninth, tenth, and eleventh ribs.

The other side, the visceral surface of the spleen, is divided into three regions:

• The gastric surface is the cranial part of the visceral surface, in contact with the dorsal part of the stomach.

• The intestinal surface is the caudal part of the visceral surface, in contact with parts of the jejunum and colon. It is absent in ruminants.

• The renal surface is the dorsal part of the visceral surface and is connected to the upper part of the left kidney. It is absent in ruminants.

The spleen has two borders :

• Cranial border: convex and smooth, close proximity to the diaphragm and stomach, particularly the greater curvature of the stomach.

• Caudal border: straighter or slightly concave, lies adjacent to loops of small intestine, the greater omentum, and the left abdominal wall.

The spleen has two extremities

• Dorsal extremity (cranial pole or head):

  • Situated near the fundus of the stomach.

  • More fixed in position by ligamentous attachments (primarily the gastrosplenic ligament).

• Ventral extremity (caudal pole or tail):

  • More mobile and can extend variably depending on splenic size and physiological state.

  • Can sometimes reach as far as the ventral midline or cross to the right abdomen in cases of splenomegaly.

3. Ligamentous Attachments

• Gastrosplenic ligament: Connects the spleen to the greater curvature of the stomach; contains short gastric and splenic vessels.

• Phrenicosplenic ligament: Less prominent in dogs; connects the spleen to the diaphragm near the left crus.

• Splenorenal (lienorenal) ligament: Sometimes described in relation to the left kidney; more developed in other species.

4. Vascularization

The spleen receives arterial blood from the splenic artery, a branch of the celiac artery. Venous drainage is via the splenic vein, which joins the portal vein. The splenic circulation is adapted for slow blood flow, facilitating filtration and immunologic functions.

5.Innervation

It is innervated by sympathetic fibers from the celiac plexus, which regulate the contraction of the splenic capsule and modulate immune responses.

6.Lymphatic Drainage

Lymph drains into the splenic and pancreaticolienal lymph nodes, and then into larger lymphatic vessels directed toward the cisterna chyli.