Lymphatic System Development

Lymphatic System Development

• The lymphatic system begins developing at the end of the sixth week of gestation.
• Precursor lymphatic endothelial cells originate from the cardinal veins.
• Lymphatic vessels form similarly to blood vessels, connecting to the venous system.

Primary Lymph Sacs

• Six primary lymph sacs exist at the end of the embryonic period.
• Two jugular lymph sacs are found near the subclavian veins and anterior cardinal veins (internal jugular veins).
• Two iliac lymph sacs are located near the iliac veins and posterior cardinal veins.
• One retroperitoneal lymph sac is situated in the root of the mesentery, on the posterior abdominal wall.
• One cisterna chyli (chyle cistern) is positioned dorsally to the retroperitoneal lymph sac.

Lymphatic Vessel Connections

• Lymphatic vessels connect to lymph sacs, following major veins.
• Vessels from the head, neck, and upper limbs connect to the jugular lymph sacs.
• Vessels from the lower trunk and limbs connect to the iliac lymph sacs.
• Vessels from the gut connect to the retroperitoneal lymph sac, then the cisterna chyli.
• Two large channels (thoracic ducts) connect the jugular lymph sacs with the cisterna chyli.
• An anastomosis (connection) forms between the thoracic ducts.

Thoracic Duct Development

• The thoracic duct arises from the caudal part of the right thoracic duct.
• There is an anastomosis between the thoracic ducts and the cranial part of the left thoracic duct.
• The right lymphatic duct originates from the cranial part of the right thoracic duct.
• Both the thoracic and right lymphatic ducts connect to the venous system via the venous angle (internal jugular and subclavian veins).

Lymph Node Development

• Lymph sacs, except the superior part of the cisterna chyli, transform into lymph nodes in the early fetal period.
• Mesenchymal cells invade each lymph sac, creating lymphatic channels and sinuses.
• Other mesenchymal cells form the lymph nodes' capsules and connective tissue framework.
• Lymphocytes arise from primordial stem cells in the umbilical vesicle mesenchyme, and later, from the liver and spleen.
• Early lymphocytes migrate to the bone marrow, dividing into lymphoblasts.
• Lymph nodes' lymphocytes, before birth, stem from the thymus (a derivative of the third pair of pharyngeal pouches).
• Small lymphocytes leave the thymus, circulating to other lymphoid organs.
• Some mesenchymal cells in lymph nodes also differentiate into lymphocytes.

Spleen and Tonsil Development

• The spleen forms from an aggregation of mesenchymal cells in the dorsal mesogastrium.
• Palatine tonsils develop from the second pharyngeal pouch's endoderm and surrounding mesenchyme.
• Tubal tonsils emerge from lymph nodules next to the pharyngotympanic tubes.
• Pharyngeal tonsils (adenoids) arise from lymph nodules in the nasopharynx.
• Lingual tonsils stem from lymph nodules in the tongue's root.
• Lymph nodules form in respiratory and digestive tracts' mucosa.