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.
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.
The formation of the lymph nodes (see development lymphatic tissue).) follows that of the lymph vessels. The primary lymph nodes develop in regions that are occupied by lymphatic sacs. Between the branched lymphatic sacs mesenchymal solidifications form that join up incompletely. This leads to the formation of a plexus of lymph vessels (
As a first step - at the end of the embryonic period - the deep cervical lymph nodes form out of the jugular lymphatic sacs and soon afterwards the axillar, parasternal, mediastinal lymph nodes also form in the region of the shoulder girdle and the lymphatic nodes under the trapezius muscle.
The cysterna chyli, the retroperitoneal, lumbar and inguinal lymph nodes as well as the lymph nodes of the small pelvis develop out of the extended lumbar lymphatic sac.
Soon, reticulum cells and macrophages also grow into these fetal lymph nodes and, somewhat later, the first lymphocytes appear as well as cells of the granulopoietic series. These lymph nodes also exhibit the first postcapillary venules.
White pulp of the spleen
The spleen, a derivative of the mesoderm, is visible for the first time in stage 13 (ca. 32 days). It arises in direct contact to the stomach in the dorsal mesogastrium, which at this point is still very knobby. As the first differentiation arises a thickening of the visceral mesothelium and within it an accumulation of mesenchymal cells. Through the leftward shifting of the stomach the spleen is also moved to the left. The dorsal mesogastrium gets placed at the posterior abdominal wall and adheres in regions where the pancreas has grown in. Only the lieno-renal ligament remains, which connects the spleen with the posterior abdominal wall and contains the spleen vessels. The part of the original dorsal mesogastrium, which forms the connection between the spleen and the stomach, remains as the gastro-lienal ligament. Thus the spleen stays an intraperitoneal organ.
During the first trimenon additional types of cells migrate into the spleen, e.g., macrophages and precursor cells of the erythropoiesis. The mesenchymal cells begin to form a network. At this time hematopoiesis can also appear in the spleen. One also calls this developmental stage of the spleen a primary vessel reticulum. In the following weeks the spleen is modified and lobulated structures arise via the in-growth of trabeculae. After the 15th week the white spleen pulp with an accumulation of lymphocytes can be distinguished from the red spleen pulp that consists of the meshwork with sinusoids, described above, and lies in the periphery of the lobules.
The colonization of the spleen with lymphocytes occurs at the beginning of the second half of the pregnancy. First T-lymphocytes surround the central arteries that diverge from the trabecular arteries. The further branchings of the blood vessels, the follicle arteries, lead to accumulations of B-lymphocytes. Additional, more in the periphery lying branchings, the penicillar arterioles and the sheathed capillaries bring blood via an open or closed circulation system into the peripheral sinusoids.
In the open circulation system, the capillaries discharge openly into the meshwork of a reticular fibrous framework (open circulation system). The blood must pass this meshwork before reaching the sinusoids. Thereby it is freed from old, no longer flexible erythrocytes.
In a second circulation system the capillariess empty directly into the sinusoids. This represents the closed circulation system. The purified blood gets into the large circulation system again via the efferent veins.