The spleen is a mesodermal derivative but its development is discussed here because it develops in the dorsal mesentery of the foregut.
As described earlier (under the topic of development of stomach), the spleen appears during the 5th week of development as a small swelling produced by the proliferation .of mesen-chymal cells between the two layers of the dorsal mesogastrium. This swelling enlarges and projects from the left surface of the dorsal mesogastrium.
In the early stages of its development the spleen consists of a number of mesenchymal masses which later fuse. Hence, the fetal spleen is lobulated, but the lobulation normally
disappears before birth. The notches seen in the superior border of the adult spleen are remnants of the grooves that separated the lobules of the developing spleen.
As a result of rotation of the stomach, a portion of the dorsal mesogastrium fuses with the posterior abdominal wall. The spleen divides the free (ie, unfused) dorsal mesogastrium into two parts: (1) gastrolienal ligament extending from the greater curvature of stomach to the spleen, and (2) lie-norenal ligament extending from the spleen to the posterior abdominal wall in the region of the upper pole of the left kidney.
The mesenchymal cells in the developing spleen give rise to the capsule, the trabeculae and reticular framework. The developing spleen is invaded by the T-lymphocytes and B-lymphocytes during the 5th and 6th months of development. The spleen begins to function as a hematopoietic organ in the 5th month of development. However, the hematopoietic activity ceases by the 8th month.