Page 329 - Veterinary Immunology, 10th Edition

VetBooks.ir Sources of Lymphocytes

Lymphoid stem cells are first found in the fetal omentum, liver, and
yolk sac. In older fetuses and in adults, these stem cells are mainly

found in the bone marrow. The bone marrow has multiple
functions in adult mammals. It is a hematopoietic organ containing
the precursors of all blood cells, including lymphocytes. In some
mammals, such as primates, it is also a primary lymphoid organ (a
site where newly produced lymphocytes can mature). Like the

spleen, liver, and lymph nodes, the bone marrow is also a
secondary lymphoid organ. It contains many dendritic cells and
macrophages and thus removes foreign material from the blood. It

contains large numbers of antibody-producing cells and is therefore
a major source of antibodies. Because of these multiple functions,
the bone marrow is divided into a hematopoietic compartment and
a vascular compartment. These compartments alternate, like slices
of cake, in wedge-shaped areas within long bones. The

hematopoietic compartment contains stem cells for all the blood
cells as well as macrophages, dendritic cells, and lymphocytes, and
is enclosed by a layer of adventitial cells. In older animals, these

adventitial cells may become so loaded with fat that the marrow
may have a fatty yellow appearance. The vascular compartment,
where antigens are mainly trapped, consists of blood sinuses lined
by endothelial cells and crossed by a network of reticular cells and
macrophages.

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