Chapter 23: The Lymphatic System
Chapter 23: Lymphatic System; Embryological Developmentby John F. Neas
The lymphatic vessels, lymph nodes, and spleen develop from lateral plate mesoderm. Myeloid tissue, the tissue in bone marrow for production of various classes of blood, including lymph cells, also develops from lateral plate mesoderm. The epithelial parts of tonsils develop from foregut endoderm (pouches I and II and other foregut endoderm), as does the thymus (pouch III and occasionally pouch IV). The lymphatic system begins to develop by the end of the fifth week.
The lymphatic vessels may develop as an outgrowth of the venous system or more probably, like blood vessels, by union of small mesenchymal clefts or spaces into vessels and development of an endothelial lining.
Lymph sacs develop from fusion and dilation of mesenchymal spaces. Adjacent spaces fuse into a network of dilated lymphatic capillaries that establish a primitive lymphatic system by the end of the second month. Lymphatic vessels drain excess tissue fluid (lymph) from the interstitial spaces to the blood vascular system. As such, the lymphatic system closely follows the course of and establishes secondary connection with the venous system. Lymphatic vessels acquire valves that direct the flow of lymph toward the larger sacs.
The fist lymph sacs to develop are the paired jugular lymph sacs at the junction of the internal jugular and subclavian veins, lateral to the precardinal (internal jugular) veins. Capillary plexuses extend from the jugular lymph sacs to the thorax, upper extremities, head, and neck. Some of the plexuses enlarge and produce lymphatics in their respective areas.
The jugular lymph sacs communicate inferiorly with the single retroperitoneal (mesenteric) lymph sac at the root of the mesentery of the intestine and with the cisterna chyli, a lymph sac below the developing diaphragm on the posterior abdominal wall. The retroperitoneal lymph sac develops from mesonephric (primitive kidney) veins and the primitive vena cava. Capillary plexuses and lymphatics expand from the retroperitoneal lymph sac to the abdominal viscera and diaphragm. Channels that join the jugular lymph sacs to the cisterna chyli become the unpaired thoracic duct (left lymphatic duct) and the right lymphatic duct that drains the right cephalic region. The cisterna chyli drains lymphatic plexuses within the gut and communicates with paired iliac lymph sacs. The retroperitoneal sac establishes connections with the cisterna chyli but loses its connections with nearby veins. The cisterna chyli loses its connections with surrounding veins but produces the inferior portion of the thoracic duct. The primitive lymph sacs become united by longitudinal connections, and each sac associates with lymphatic vessels that drain specific areas.
The paired posterior lymph sacs develop from the iliac veins at their junction with the posterior cardinal veins and produce capillary plexuses and lymphatics of the abdominal wall, pelvic region, and lower extremity. The posterior lymph sacs join the cisterna chyli and lose connections with adjacent veins.
New vessels sprout from the primitive lymphatic system and extend to most tissues of the body, mostly along the course of the main veins. Lymphatics never occur in the central nervous system, meninges, eyeball and cornea, internal ear, cartilage, epidermis, and spleen. Eventually, lymphatic vessels replace the lymph sacs.
Lymph Nodes (Glands)
Except for the anterior part of the sac that produces the cisterna chyli, all lymphatic capillary plexuses become invaded by mesenchymal cells that proliferate and aggregate to form groups of lymph nodes. The mesenchymal cells form connective tissue cells that produce the cellular components of a lymph node, including a fibrous capsule, trabeculae, and a reticular net. Afferent and efferent lymphatic capillaries conduct lymph to and away from the lymph node.
Lymph nodes function in hematopoiesis during the medullolymphatic phase but erythropoiesis later shifts to red bone marrow. Lymph nodes primarily produce lymphocytes.
The spleen develops as a circumscribed condensation of mesenchymal cells between the layers of the dorsal mesentery. Mesenchymal cells in the developing spleen differentiate to form the capsule, trabeculae, and reticular framework. Development involves establishment of mesenchymal trabeculae within a blood vascular network consisting of a large number of endothelial sinuses. The splenic artery supplies the sinuses. The spleen initially consists of several mesenchymal masses that later incompletely fuse.
The spleen is the largest organ of the reticuloendothelial (lymphatic) system. It serves as an hemopoietic (blood-forming) organ during the hepatosplenothymic phase (beginning in the second month). The spleen mainly produces cells of the erythroid lineage in the fetus and lymphocytes in the newborn. Thymic and B-lymphocytes migrate to the spleen and populate the white pulp associated with the trabeculae. (The sinuses contain erythroblastic tissue, or red pulp.) The spleen contains phagocytes that remove "exhausted" erythrocytes and foreign bodies from the blood and it is a major reservoir for blood.
The thymus arises as endodermal diverticula of the ventral part of the third pharyngeal pouches that later fuse. The two thymic diverticula grow inferiorly in the neck to reach the superior mediastinum and fuse into a two-lobed organ. The thymus achieves maximum size at puberty and gradually regresses thereafter, being replaced by fatty tissue.
Endodermal cells proliferate to form solid clusters called Hassalls corpuscles. Surrounding mesenchymal cells invade the thymus and differentiate into a reticular framework, capsule, and trabeculae. The thymus becomes subdivided into lobules.
The thymus is the first lymphatic organ to receive lymphocytes, first from the wall of the yolk sac and later from bone marrow. Lymphocytes enter the reticulum where they differentiate into thymic lymphocytes (T-lymphocytes). Descendants of thymic lymphocytes populate other peripheral lymphoid organs.
Tonsils consist of aggregates of incompletely encapsulated lymphoid tissue beneath and in contact with digestive tract epithelium.
The surface epithelium and the lining of the tonsillar crypts of the palatine tonsil develop from endoderm of the second pharyngeal pouch. The mesenchymal cells that surround the crypts form a reticular framework. Lymphocytes appear in the reticulum by the third month and become arranged as lymphatic nodules. The capsule of the tonsil develops from a condensation of mesenchyme.
Similar aggregations of incompletely encapsulated lymphoid tissue create the tubal tonsil of the first pouch, the lingual tonsil on the dorsum of the tongue, and the adenoids (nasopharyngeal tonsil) in the roof and dorsal wall of the nasopharynx. Lymphocytes accumulate in the connective tissue of the mucous membrane, mesenchymal cells lay down a reticular framework, and lymphocytes organize into nodules.
The dark-staining cortex consists of densely packed free adult lymphocytes. Lymphocytes (L1) of superficial cortex (Cx) are larger than those of the deeper cortex. (L2). Lymphocytes are fewer in the lighter-staining medulla (M). In the center of the medulla are eosinophilic lamellated structures called Hassalls corpuscles (H) that represent degenerate epithelial cells. The thymus is one of the blood-forming organs in the fetus. Hematopoiesis is most prominent in the connective tissue septa between lobules where it is possible to see developing blood cells (BC) of the erythrocytic and granulocytic series. C = capsule; T = trabecula.
The spleen is covered by a capsule (C) of connective tissue that in turn is invested by mesothelium of the peritoneum. From its inner surface branching and anastomosing connective tissue trabeculae (T) project into the organ to form its skeleton. Most of the spleen is red pulp (RP) that consists of reticulum cells with their fibers supporting free cells of the blood and forming the splenic cords (B). Between these cords, there are sinusoids lined by special endothelial cells. Scattered through the red pulp is the white pulp (WP), typical lymphoid tissue packed with lymphocytes in a reticulum network. This white pulp is distributed as sheaths around small branching arterial vessels.
Lymph Nodes (Glands)