Lymphatic organs

The thymus develops from the 3rd pharyngeal pouch. Its epithelium comes from the endoderm of this pouch and probably also from the ectoderm of the 3rd pharyngeal cleft. Its connective tissue develops from the mesenchyme, which is thought to originate from the neural crest. The paired primordia of the thymus are shifted more caudally (descensus) and fuse together in the middle. During month 3, cells in the thymus differentiate to form a cortex and a medulla, and the thymus begins production of T lymphocytes. It increases continuously in size, reaching its greatest volume at puberty. Thereafter it regresses. Most of its substance is then replaced by fat and connective tissue.
Lymph nodes occur in the early fetal period, firstly in the axillary and inguinal regions. They start off as aggregations of lymphoblasts. Elements of connective tissue are added later.
At S13, the spleen can first be identified as a local mesenchymal condensation in the dorsal mesogastrium. It is located intraperitoneally. As its thickness increases, it causes the left leaf of the mesogastrium to bulge out towards the peritoneal cavity. The spleen divides the mesogastrium into two parts: the part between the dorsal abdominal wall and the spleen (splenorenal ligament), and the part between the stomach and the spleen (gastrosplenic ligament). The spleen is moved into the upper left part of the abdomen due to rotation of the stomach. By week 17, lymphocytes settle close to the small arteries. Follicles develop by week 24. T and B lymphocytes can already be observed around this period.
The palatine tonsil arises from the growing endoderm and the adjacent mesenchyme of the 2nd pharyngeal pouch. Lymphoblasts soon invade the mesenchyme and form follicular aggregations of lymphocytes. By week 16, areas with T or B lymphocytes can already be identified. Similar aggregations of lymphatic tissue in conjunction with the mucosa can be observed elsewhere: tubal tonsil, pharyngeal tonsil, and lingual tonsil. Together with the palatine tonsil, they form the lymphatic pharyngeal ring.