Scalpel removes a portion of the capsule to see deeper tissues.
Spleen Anatomy & Histology
Gross Anatomy
- The splenic artery and vein enter at the hilum.
- Blood vessels travel within the lieno-renal ligament (aka, splenorenal ligament), which connects the intraperitoneal spleen to the posterior abdominal wall.
Histology
Capsule
- Invaginates into the parenchyma as trabeculae, which divide the tissues into lobules; although omitted here for simplicity, blood vessels pass through the trabeculae.
- Comprises collagen, elastic, and smooth muscle fibers.
- The mesothelium, which is the outermost layer, forms the trabeculae.
Red pulp
- Is highly vascularized; this tissue is responsible for filtering of damaged red blood cells and other particles.
- Stroma comprises reticular cells and fibers, plasma cells, and macrophages.
- These cells and fibers constitute the splenic cords (of Billroth), which are interspersed between the sinusoids.
- Sinusoids comprise elongated endothelial cells on a discontinuous basement membrane; though not visible here, reticular fibers also encircle the sinusoids.
- Blood flow through the spleen is complicated, and authors disagree on whether circulation is open and/or closed. In closed circulation, the blood flows from arterial vessels directly to the sinusoids; in open circulation, arterial vessels open into the red pulp, through which the blood percolates towards the sinusoids.
White pulp, which comprises nodules of lymphoid aggregations; this is the immune component of the spleen.
- A nodule comprises lymphocytes, primarily B cells, and antigen-presenting cells; recall that B cells participate in the immune response against blood-borne antigens.
- Perilymphoid red pulp surrounds the nodule.
- Germinal center is at center of the nodule; its size diminishes with age, and that inter-individual variation exists.
- Mantle and marginal zones form rings of darker-staining areas.
- A central artery passes through each nodule; it is a branch of the trabecular artery, and, in humans, is located peripherally in the nodule.
- As it passes through the white pulp, T-cells surround the vessel, forming the peri-arteriolar lymphoid sheath (PALS).
- Though not shown here, the central artery exits the white pulp and terminates in macrophage-sheathed capillaries, of the red pulp. These capillaries may either drain directly into the splenic sinusoids or, instead, into the spaces of the red pulp.
Notice that the spleen does NOT have a cortex and medulla.
Be aware that we've simplified some aspects of splenic histology; there is a great deal of intertextual variation regarding the details of splenic tissue structure and function.
Furthermore, although rodent models are often applied to studies of human anatomy and physiology, they are not viable in the case of the spleen because there are significant differences in the rodent and human spleens.
Images:
Spleen (Mark Braun, MD http://medsci.indiana.edu/c602web/602/c602web/virtual_nrml/nrml_lst_pad.htm)