Histology Fact Sheet: Lymphatic System

The spleen, the thymus, lymph nodes and bone marrow are all part of the lymphatic system.

Spleen

The spleen is involved in destruction of old or damaged red blood cells. Storage of blood occurs in the spleen. In the fetus, the spleen is involved in blood cell formation. Lymphocyte and antibody production occurs in the spleen.

The spleen has a connective tissue capsule. The invaginations of the capsule into the splenic parenchyma are trabeculae.

PALS stands for periarterial lymphatic sheath. Periarterial lymphatic sheaths are the lymphocytes which surround the central artery in the spleen.

The parenchyma of the spleen can be divided into the white pulp and the red pulp. The white pulp of the spleen is the lymphatic portion of the spleen. Within the white pulp, splenic nodules are found. Splenic nodules are also called Malpighian corpuscles.

The red pulp is made up of the splenic sinuses and splenic cords. The splenic cords are also called the cords of Billroth.

Thymus

Most lymphatic organs are made of a framework of reticular fibers and reticular cells. However, the thymus is made of epithelioreticular cells instead.

Hassall's corpuscles are the ring like structures found in the thymus. Hassall's corpuscles are visible on a histology slide.

T cells gain their immunocompetence in the thymus.

Histology slide of the thymus.
Histology slide courtesy of Histology-World.

Lymph Node

A lymph node is an encapsulated lymphatic organ. (Read the Lymph Node Fact Sheet here.)

Diffuse Lymphatic Tissue

The random distribution of lymphocytes seen in the lamina propria of the respiratory tract, genitourinary tract, and gastrointestinal tract is called diffuse lymphatic tissue. Diffuse lymphatic tissue is non-encapsulated lymphatic tissue. In the gastrointestinal tract it is referred to as GALT (gut associated lymphatic tissue). In the respiratory tract it is referred to as BALT (bronchi associated lymphatic tissue). Diffuse lymphatic tissue is not found in the central nervous system.

Peyer's Patches

The large aggregates of lymphatic tissue in the ileum are called Peyer's patches.  Peyer's patches are visible on a histology slide.