|
Blood can be viewed most generally as a highly specialized connective tissue in that the intercellular substance is a fluid. The first signs of the formations of blood and vessels are seen already quite early at 17 days in stage 6,  6 .
The extraembryonic mesenchyma forms blood vessels in the chorionic villi. On the umbilical vesicle so-called blood islands also arise that form the first precursor cells (stem cells), giving rise, as well, to the vessels and all of the blood cells. Some of these predecessor cells remain lifelong pluripotent and can also still be found in adult bone marrow. They are responsible for the lifelong renewal of all blood cells.
In the lymphatic system, a specific and a non specific system can be differentiated. The specific immunity is the domain of bone marrow and thymus, forming the primary lymphatic organs as well as the white pulp of the spleen, lymph nodes and the lymph follicles of the mucous membranes, known as the secondary lymphatic organs. They perform their immunological functions only in cooperation with specialized blood cells, the phagocytes and lymphocytes, which belong to the non-specific system. This non-specific immunity develops partially already before birth despite the absence of external stimulating antigens. Thus at the time of birth an immunity, based on the activity of macrophages, is already relatively mature, while the specific immunity is only weakly developed.
|
|
|
