17.2 Lymphatic tissue

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:

  1. T lymphocytes that are responsible for the cellular immune response and mature in the thymus
  2. B lymphocytes that are responsible for the humoral immune reaction and mature in the liver and in the bone marrow.

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.
Fig. 7 - Normal blood-forming bone marrow  Legend

Fig. 7
Among the blood forming cells are preliminary stages of granulopoiesis and erythropoiesis.

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