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

Secondary lymphatic organs

The secondary lymphatic organs represent the location where the defense battles take place. In them the T- and B-lymphocytes that have matured in the thymus and in the bone marrow develop further when they come into contact with antigens, leading to a clonal proliferation. Specific proliferation zones form for the two lymphocyte groups. Thereby effector and regulator cells arise (see: achieving immunocompetence). The anlage material for the secondary lymphatic organs has a mesenchymal origin. It forms in connection with the differentiation of the lymph- and vascular systems.

Lymph follicles of the mucous membranes

The lymph follicles of the mucous membranes have a mesenchymal origin, but also have a close relationship to the endoderm. Their development is thus connected with that of the endoderm derivative in a cranio-caudal gradient. A reciprocal developmental influence results from this. The beginnings are always marked by an accumulation of round cells and a rich vascularization in this region. At first the tonsil anlagen appear with the formation of the pharyngeal pouches. They are located in the throat region as the palatine, lingual and the unpaired pharyngeal tonsils. The palatine tonsils arise on both sides in the tonsillar sinus, a relict of the second pharyngeal pouch.

Fig. 11
The pharyngeal pouches differentiate to become various derivatives of the endocrine and lymphatic systems.

The epithelium congeals and forms a protuberance. Below it congealing of elements of the mesenchyma can be recognized and smaller blood and lymph vessels converge there. From the endodermal epithelium smaller and larger epithelial cords soon grow into the mesenchyma and form the later tonsillar crypts. With the beginning of the fetal period T- and B-lymphocytes also settle into T- and B-specific regions of the tonsils.

To the B-specific regions number the primary follicles; between them the T-lymphocytes settle and form the parafollicular regions. As in the thymus, the stroma cells in the B- and T-cell-specific regions are also here an important precondition for the maturation of the various subpopulations of immunocompetent lymphocytes.

The lingual tonsil on the tongue floor and the pharyngeal tonsil on the roof of the posterior wall of the nasopharynx also arise in a similar way in that endodermal epithelial cords grow into the depths of the congealed mesenchyma that lies below it.

The lymphatic tissue of the intestines, Peyer's plaque, and the vermiform appendix appear clearly later than the tonsillar anlage, namely only in the second half of the pregnancy. Initially, accumulations of lymphocytes gather, mainly in the ileum, where they accumulate in the submucosa at various locations and thus mark the beginning of the formation of Peyer's plaque. The first primary follicles appear in the 7th to 8th months.

In the cecum region - between the 4th and 5th month - the first accumulations of lymphatic cells have also been shown to be present immediately below the intestinal epithelium. Here, the close relationship to the intestinal epithelium that lies just above it is especially pronounced.

In the lung anlage, which also has an endodermal origin, the lymphatic tissue develops still later. The first lymph nodes form only after the 7th month. One can recognize primary follicles only in the 8th month.