Growth and Structure of the Lymphatic Tree

Work on the lymphatic system began in the 17th century, and by the beginning of the 19th century, the anatomy of most of the lymphatic system had been described. Much of the work carried out late in the 19th century and in the first half of the last century was aimed at determining the embryonic origin of lymphatic endothelium. Two theories were proposed. The first suggested that lymphatic endothelium derives by sprouting from blood vascular (venous) endo-thelium, the so-called centrifugal theory. The second, the so-called centripetal theory, suggested that lymphatic endothelium differentiates in situ from primitive mes-enchyme, and secondarily acquires connections with the blood vascular system at a limited number of sites. More recent studies have provided support for both hypotheses. Thus, the homeobox transcription factor Prox-1, which is a master switch for inducing the lymphatic phenotype, is expressed at sites of budding of new lymphatics from veins during development. There is also evidence for the existence of lymphangioblasts that give rise to the de novo differentiation of lymphatic endothelial cells in tissue interstitium during development.

In postnatal life, new lymphatic capillaries grow by lymphangiogenesis during acute and chronic inflammation. This occurs by sprouting from preexisting lymphatic capillaries in much in the same way as new blood capillaries arise by sprouting from preexisting blood capillaries or postcapillary venules during angiogenesis. The appearance of new lymphatic capillaries is secondary to that of blood capillaries, although linear growth occurs at a comparable speed. Lymphatic capillaries are less labile than blood capillaries: They send out fewer sprouts, anastomose less frequently, and show much less tendency to retract or undergo changes in size or form. Although temporary lymphedema occurs following lymphatic disruption, this usually resolves due to spontaneous regeneration or reconnection of lymphatics. In resolving inflammation, newly formed lymphatic capillaries tend to involute more rapidly than do blood capillaries.

The lymphatic vascular system originates in the interstitial space of virtually all tissues of the body as a series of blind-ending thin-walled capillaries (Figure 1). Lymphatic capillaries lack a continuous basement membrane and peri-cytes, but contain anchoring filaments that aid in maintaining luminal patency in the face of increased interstitial pressure (Table 1). Tissues that are purported to be devoid of lymphatic vessels include, among others, the central nervous system, placenta, bone, and islets of Langerhans. Careful immunohistochemical studies with the newly described lymphatic markers (see later discussion) will be required to carefully map the distribution of lymphatics throughout the body. Lymphatic capillaries drain into larger collecting lymphatics that contain valves. Collecting lymphatics also contain smooth muscle. Lymph nodes are interposed along the

Lymphatic Collecting capillaries lymphatic Lymph nodes

Lymphatic Collecting capillaries lymphatic Lymph nodes

Figure 1 Schematic representation of the lymphatic vascular tree.

path of collecting lymphatics. Segments of collecting lymphatics between valves are called lymphangions. Collecting lymphatics progressively converge and ultimately end up as two large trunks, the thoracic duct and the right lymphatic duct, which anastomose with the large veins at the base of the neck (Figure 1). Lymphatic ducts are reinforced by smooth muscle cells, and the adventitia of large lymphatic ducts contains vasa vasorum and a rich neural network. Unlike the blood vascular system, in which flow is ensured by the rhythmic contractile activity of the heart, flow of lymph within the lymphatic tree is aided initially by external mechanical forces (contraction of surrounding skeletal muscle) and later by the contraction of smooth muscle cells in the walls of larger lymphatics. Unidirectional flow of lymph and cells is ensured by the presence of valves. The blood and lymphatic vascular systems very rarely anastomose.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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