Chapter 128. Development of The Lymphatic System - Review of Medical Embryology Book

The lymphatic system begins to develop at the end of week 5, approximately 2 weeks later than the cardiovascular system. One view states that the lymphatics develop as diverticulae of the endothelium of veins; whereas another states that like other blood vessels they develop from clefts in the mesenchyme that connect with the venous system secondarily. Thus, the cells lining the mesenchymal clefts assume an endothelial shape, and subsequent sprouting of these cells causes the clefts to fuse and form the lymphatic channels

IN WEEKS 6-9, LOCAL DILATATIONS of the lymphatic channels form 6 primary lymph sacs

Two jugular lymph sacs near the junction of the subclavian veins with the anterior cardinals (future internal jugular vein)
Two iliac lymph sacs near the junction of the iliac veins with the posterior cardinal veins
One retroperitoneal lymph sac in the root of the mesentery on the posterior abdominal wall
One so-called cisterna chyli dorsal to the retroperitoneal lymph sac, at the level of the adrenal glands

LYMPH VESSELS GROW OUT from the lymph sacs, along the major veins, to the head, neck, and arms from the jugular sacs; to the lower trunk and legs from the iliac sacs; and to the gut from the retroperitoneal and cisternal sacs

The cisterna chyli is connected to the jugular lymph sacs by 2 large channels, the right and left thoracic ducts. An anastomosis forms between the 2 ducts, thus, the definitive thoracic duct is formed by the caudal portion of the right thoracic duct, the anastomosis, and the cranial portion of the left thoracic duct
The right lymphatic duct is derived from the cranial part of the right thoracic duct
Both the right and left thoracic ducts join the venous system at the angle of the subclavian and internal jugular veins at the base of the neck

EXCEPT FOR THE UPPER PORTION OF THE CISTERNA CHYLI, which persists, the lymph sacs are transformed into groups of lymph nodes during early fetal life, at about month 3.

Surrounding mesenchymal cells invade each sac and break it up into lymphatic channels or sinuses. The mesenchymal cells give rise to the lymph node capsule and the connective tissue framework of the node
The lymphocytes seen in the node before birth come from the thymus gland
The lymph nodule and germinal centers of lymphocyte production do not appear in the nodes until just before or after birth
Lymph nodes also develop along the course of other lymph vessels

THE SPLEEN develops from an aggregation of mesenchymal cells in the dorsal mesentery of the stomach
THE PALATINE TONSILS form from the second pair of pharyngeal pouches
THE TUBAL (PHARYNGOTYMPANIC) TONSILS develop from aggregations of lymph nodules around the openings of the auditory tubes
THE PHARYNGEAL TONSILS (adenoids) develop from an aggregation of lymph nodules in the nasopharyngeal wall
THE LINGUAL TONSILS develop from aggregations of lymph nodules in the root of the tongue
LYMPH NODULES also are seen in the mucosa of the digestive tract and respiratory tract