16.8 Development of the lymphatic vessels

• Introduction
  
• The lymphatic vessel system

Introduction

The lymphatic vessels form, like the blood vessels, from hemangioblastic stem cells which aggregate to form fine tubular vessels. The first signs of developing lymph nodes are found already in the 5th week 15 (36th day) as so-called lymph sacs near where the inferior cardinal vein and superior cardinal vein flow together and form the common cardinal vein. Bilateral evaginations of the venous system above the later subclavian vein arise as the jugular lymph sac and somewhat later below as the axillar lymph sac. In other regions of the body further such evaginations arise: mesenteric, lumbar, iliac and retroperitoneal sacs.

The lymphatic vessel system

The connections between the lymphatic sacs are formed by a fine network of lymphatic vessels. Later several of these lymphatic plexuses come together to form larger vessels. Like the venous system this is also originally arranged bilaterally and symmetric and experiences an asymmetric development. Ultimately, the thoracic duct also arises through the growing together of several local plexuses.

Fig. 31
The development of the lymphatic systems at the 6th week. Three bilateral lymphatic sacs (jugular, axillary and lumbo-iliac) as well as the incompletely formed lymphatic vessel plexus are shown.

Fig. 32
The most important lymphatic vessels arise bilaterally and form a fine network.
The site where plexuses are located transform later into lymph nodes, the lymph filtering stations.

At the end of the embryonic period 3 bilateral systems have formed:

• jugular-axillar lymphatic sac
• mesenterial lymphatic plexus
• lumbo-inguinal lymphatic plexus

They are connected through a very variable lymphatic vessel system. The thoracic duct drains the lymph from the lower as well as the left upper half of the body into the left venous angle between the jugular vein and the left subclavian vein. The right arm and the right half of the head are drained by the jugular and axillary lymphatic plexus, respectively, which empties into the right venous angle.

The lymph nodes develop in the early fetal period through a septation of the lymph sacs by mesenchymal cells. The spaces thus delimited become the sinus of the adult lymph nodes. Other mesenchymal cells build the connective tissue framework of the lymph node and its capsule.

Fig. 33
As in the venous system the lymphatic vessels also atrophy selectively and unilaterally. At the thoracic and abdominal level only a thoracic duct remains to drain the lymph of the entire lower part of the body and the left head and arm region. This lymph empties into the venous system at the junction of the jugular and left subclavian vein. Dashed lines indicate the atrophied portion of the lymphatic system.