The early development of the lymphatic system in mouse embryos

The early development of the lymphatic system was studied in embryos of an inbred strain of the laboratory mouse. During the first stage of its development the system is represented by a more or less regular series of small and blind-ending outgrowths of the major embryonic veins which develop in a cranio-caudalward direction from the jugular to the pelvic region. As a result of differences in growth rates of adjacent anatomical structures this series of early lymphatic primordia becomes subdivided into 4 singular primordia and 12 groups of primordia. After the constituents of each group of early primordia have fused, 16 isolated lymphatic plexuses (sacs) are formed of which 14 are in bilaterally symmetric and 2 are in a median line position: i.e. bilaterally: (1) the jugulo-axillary lymph sac situated lateral to the anterior cardinal vein and dorsal to the primitive ulnar vein and its major branch, the external mammary vein, (2) the paratracheal lymph plexus situated medial to the anterior cardinal vein, (3) the internal thoracic lymph plexus situated lateral to the thoracic part of the posterior cardinal vein, (4) the thoracic ducts situated medial to the thoracic part of the posterior cardinal vein, (5) the lumbar lymph plexus situated dorso-lateral to the abdominal part of the posterior cardinal vein, (6) the subcardinal lymph plexus and (7) the iliac lymph plexus situated ventro-lateral to the abdominal part of the posterior cardinal vein; and in the median line: (8) the subtracheal lymph plexus situated at the confluence of the pulmonary veins and (9) the mesenteric lymph plexus situated near the confluence of the splenic and the superior mesenteric veins. Except for some openings at the jugulo-subclavian confluence all connections with the veins disappear. From the primordia extensions grow out centrifugally. They invade the surrounding tissues and, in part, fuse with similar sprouts of adjacent primordia. In this way a continuous system of lymph truncs is formed that opens into the venous system at the jugulo-subclavian confluence.