Learning aims

At the end of this module you should be able to:

• know the development from stem cells to differentiated blood cells
• know the location where erythropoiesis occurs
• have a concept of the functions of the various blood cells both before and after birth.
• know the organs of the lymphatic system
• know how they arise
• know the difference between cell-derived and humoral immunity
• have a concept of how immunological competence arises

What you should already know

• Embryonic disk

Introduction

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.
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.

Delving deeper

• Which factors determine what a pluripotent stem cell should become?
• Why do erythrocytes formed during the extraembryonic phase contain nuclei while those of the intraembryonic phase no longer have them?
• Why do various hemoglobins form during the embryo-fetal development?
• How can so many various and different antibodies form?
• What influences cause the thymus to migrate from the neck region into the anterior mediastinal region?