17.2 Lymphatic tissue

• Introduction
  
• Primary lymphatic organs
  • Bone marrow
  • Thymus
• Secondary lymphatic organs
  • Lymph follicles of the mucous membranes
  • Lymph nodes
  • White pulp of the spleen

Thymus

T cells mature out of stem cells produced in the bone marrow and transported into the thymus. They emigrate out of the thymus, colonizing the secondary lymph organs, and are active there as immunocompetent cells for the defense of the body against infections. The thymus derives from the foregut out of the 3rd and 4th pharyngeal pouches. Its stroma arises out of epithelial cells of ectodermal and also endodermal origins.

Fig. 8
In order to better show the relationships in the foregut, a cross-
section at the level of the pharyngeal arches has been made. One looks at the ventral part of the mouth/throat cavity. The externally visible pharyngeal folds correspond internally to the pharyngeal pouches.

Fig.9
View of the cross-
section of Fig. 8

Up to the sixth week, the thymus is purely epithelial. In the next weeks it becomes dented by an in-growing mesenchymal septal. In this way arise pseudo-lobules that communicate with each other in the center. The mesenchymal anlage material further forms the numerous, partially widened, perivascular spaces that are important for the cell transport into and out of the thymus. The fully developed organ consists of the thymus-epithelial tissue, responsible for the maturation of the T lymphocytes, and the perivascular spaces.
Despite being tightly woven together they are separated from one another by a layer of flat epithelial cells with a basal lamina, the thymus-blood barrier. The anlage material for the non-epithelial stromal cells (precursors of the macrophages) and the free cells of the thymus (prethymic stem cells of the T lymphocytes) derive from embryonic blood formation centers (liver, spleen, bone marrow). They appear in the thymus after the 9th week and initially colonize it relatively uniformly. The thymus (histological picture) acquires its functioning ability only when the cortex and medulla have differentiated. The first indications of this differentiation in a fetus occur after ca. 12 weeks and it is completed at ca. 4 months. With this differentiation the young thymocytes settle into the cortex region.

Fig. 10
In the cortex, the lymphocyte precursors are thickly packed together and are closely associated with the "thymic nurse cells". Their differentiation takes place through migration into the medullar region, where the cells are more loosely arranged. Moreover, the thymus consists of cortical and medullar epithelial cells. A series of smaller epithelial cells that form the blood-thymus barrier surround the individual lobes and the capillaries (not shown). A large number of macrophages also settle in the thymus that, like the thymocytes, derive from liver and bone marrow hematopoietic stem cells.