Development of the lymphatic organs

• Primary lymph sacs


  • Two jugular lymph sacs near the junction of the subclavian veins with the anterior cardinal veins (the future internal jugular veins)
  • 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 abdominal wall
  • One cisterna chyli located dorsal to the retroperitoneal lymph sac
• There are six primary lymph sacs
• The lymphatic system begins to develop at the end of the 5th week
• Lymphatic vessels develop in a manner similar to that blood vessels and make connections with the venous system
• The lymph sacs are transformed  from groups of lymph nodes during the early fetal period.
• Mesenchymal cells invade each lymph sac and break up its cavity into a network of lymphatic channels-the primordia of the lymph sinuses.
• Other mesenchymal cells give rise to the capsule and connective tissue framework of the lymph nodes.
• The lymphocytes are derived originally from stem cells in the umbilical vesicle (yolk sac) mesenchyme and later  on from the liver and spleen.
• Early lymphocytes eventually enter the bone marrow, from where they divide to form lymphoblasts.
• The lymphocytes that appear in the lymph nodes before birth are derived from the thymus
• Small lymphocytes leave the thymus to other lymphoid organs through circulatation.
• Later, some mesenchymal cells in the lymph nodes also differentiate into lymphocytes.
• he lymph nodules do not appear in the lymph nodes until just before and/or just after birth, as there is no exposure to foreign antigens.
• There may be diffuse swelling of a part of the body-congenital lymphedema. This condition may result from dilation of primordial lymphatic channels or from congenital hypoplasia of lymphatic vessels
• • More rarely, In cystic hygroma,large swellings usually appear in the inferolateral part of the neck, and consist of large, single or multilocular, fluid-filled cavities.
• • PHARYNGEAL APPARATUS
  • The key to understanding craniofacial development are the Pharyngeal Apparatus (PA)
  • Also known as Branchial Apparatus
• • Pharyngeal Apparatus include:
  • Pharyngeal arches (mesoderm)

  Pharyngeal clefts (ectoderm)

  Pharyngeal pouches      (endoderm)

  • Pharyngeal membranes
• Development of lymphatic system

  1. Lymphatic vessels develop in a manner similar to blood vessels

  2. Lymph sacs later become interconnected by lymph nodes
• Primary lymph sacs


  • Two jugular lymph sacs
  • Two iliac lymph sacs
  • One retroperitoneal lymph sac
  • One cisterna chyli
• Development of lymph nodes

  1. Mesenchymal cells invade each lymph sac and break up its cavity into a network of lymphatic channels

  2. Other mesenchymal cells give rise to the capsule and connective tissue framework of the lymph nodes
• Lymphocytes


  Derived originally from stem cells in the umbilical vesicle (yolk sac) mesenchyme, later from the liver and spleen
• Development of lymph nodes

  1. Early lymphocytes enter the bone marrow and divide to form lymphoblasts

  2. Lymphocytes that appear in the lymph nodes before birth are derived from the thymus

  3. Later, some mesenchymal cells in the lymph nodes also differentiate into lymphocytes
• Lymph nodules do not appear in the lymph nodes until just before and/or just after birth, as there is no exposure to foreign antigens
• Congenital anomalies of the lymphatic system

  • Congenital lymphedema
  • Cystic hygroma
• The 4th week is important for the development of the pharyngeal apparatus
• Development of the thymus

  1. Epithelial tubes form solid cords that proliferate side branches

  2. Each side branch becomes the core of a lobule of the thymus

  3. Some cells of the epithelial cords form Hassal's corpuscles

  4. The mesenchyme between the epithelial cords forms thin incomplete partitions (septa)
• Growth and development of the thymus are not completed at birth
• DiGeorge syndrome


  Infants are born without a thymus and parathyroid glands
• Accessory thymic tissue is often found in abnormal sites due to extensive migrations during early embryogenesis
• Development of the palatine tonsils

  1. The central part of the endoderm buds breaks down to form crypts

  2. The pouch endoderm surface epithelium lines the crypts

  3. At about 20th week, the mesenchyme surrounding the crypts differentiates into lymphoid tissue and lymphatic nodules
• Other tonsils


  • Tubal tonsils
  • Pharyngeal tonsils
  • Lingual tonsils
• Development of the spleen

  1. Mesenchymal cells located between the layers of the dorsal mesogastrium condense to form small mesenchymal masses (lobules of splenic tissue/spleniculi)

  2. The lobules later fuse to form a single mesenchymal mass (splenic mass)

  3. The mesenchymal cells form the capsule, septa, and connective tissue network including reticular fibers

  4. Some of the free cells form lymphoblasts while the others differentiate into hemopoietic cells
• The spleen functions as a hematopoietic center until late fetal life, but it retains its potentiality for blood cell formation in adult life
• Accessory spleens (polysplenia) may exist near the hilum of the spleen or the tail of the pancreas
• The notches on the superior (anterior) border of the adult spleen are a reflection of its lobular origin