Development of the lymphoid organs.

Development of the thymus.

In man the thymus arises as a paired ventral outgrowth from the third branchial pouch. Each outgrowth has a narrow lumen at first, but this quickly is obliterated by proliferation of the lining epithelial cells. The epithelial cells differentiate, and some transform into epithelial reticular cells at about the end of the second month of pregnancy. Thymocytes (or lymphocytes) appear at this time also. It is thought that they arise from mesenchymal cells which invade the developing thymus. The lymphocytes proliferate rapidly and the epithelium is converted into a reticular cell mass. Lobules form at this time, and connective tissue invades the lobes to form septa and trabeculae. Hassall’s corpuscles first appear during fetal life and continue to form until involution is initiated. They are thought to arise from hypertrophied and degenerating epithelial cells.

Development of the spleen.

The primordium of the spleen in human embryos appears as a thickening of the mesenchyme in the dorsal mesentery of the stomach during the fifth week of embryonic development. At this time it consists of a mass of mesenchymal cells that later divide actively. The mass of the primordium is added to by apposition of cells from the covering mesothelium of the body cavity. The mesenchymal cells differentiate into cells of the reticulum and into primitive free cells resembling lymphocytes. Later the tissue becomes myeloid in type, containing all stages in the development of megakaryocytes, granulocytes, and erythrocytes. The development of lymphocytes and monocytes continues throughout life, but myeloid elements dissappear shortly after birth.

In the early stages of development, the spleen is supplied by a rich capillary plexus, but as a characteristic distribution of vessels is established, lymphocytes become compactly arranged around the arteries to form the white pulp. Definite nodules do not appear until later in fetal life, and germinal centres are not present until after birth. The venous sinuses develop as irregular spaces that later become connected with the established blood vessels.

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