Lymphatic System Development

Study Notes

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.

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 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.

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 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.

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.