Primary lymphatic organs
In humans the largest part of the lymphocyte development ccurs in specialized tissue of the primary lymphatic organs: bone marrow (liver in the fetal period) and thymus. There a large number of immunocompetent lymphocytes are produced that colonize the secondary lymphatic tissue.
One distinguishes two types of immunocompetent cells:
- T lymphocytes that are responsible for the cellular immune response and mature in the thymus
- B lymphocytes that are responsible for the humoral immune reaction and mature in the liver and in the bone marrow
The production of blood cells in bone marrow begins roughly 4-5 months after conception. Stem cells immigrate from the liver into the bone marrow, where the «microenvironment» is decisive for the development of stem cells.
This stroma consists of endothelial cells, fat cells, osteoblasts and fibrocytes. Here, among others, mature the B lymphocytes.
Macrophages also colonize the stroma, but they stem from hematopoietic stem cells. This creates an environment that, according to need, stimulates the proliferation and differentiation of the precursor cells. As soon as these cells are mature they proceed through the openings in the sinusoids from the bone marrow into the blood stream.
T cells mature out of stem cells produced in the bone marrow and transported into the. 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.
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 () 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.