Microanatomy Of The Lymphatic System

Although there are remarkable similarities in the lymphatic microanatomy within different organs, there are also some essential differences [2]. This review will deal with the common ultrastructural features first and then examine the differences between organs. The lymphatic system (Fig. 1) begins as a network of blind ending capillaries which absorb lymph from the interstitial spaces. These capillaries are called initial lymphatics and are also known in the literature as prelymphatics, lymphatic capillaries, or terminal lymphatics [2,3]. The initial lymphatics then form the lymphatic collecting vessels (also known as the lymphatic collectors, collecting ducts, or conducting lymphatics). These lymphatic collecting vessels in turn feed into afferent lymph trunks. Lymph nodes are lo

Lymphatic System Collecting Vessels

Figure 1 Principal elements of the lymphatic system.

INTERSTITIAL SPACE CAPILLARY VESSEL

Figure 1 Principal elements of the lymphatic system.

cated along the lymphatic system so that one or several afferent lymphatic collecting vessels drain into a node, and the efferent lymphatic collecting vessels leave the nodes and drain toward major (large central) lymphatic trunks which transport the lymphatic fluid into large veins in the neck. The lymphatic collectors drain into local or sometimes remote lymph nodes which are arranged in regional groups. Nodes within a regional group are often interconnected. Lymph from most parts of the body finally drains into the venous circulation via the thoracic duct, at the junction of the left subclavian and jugular veins. In general, lymph passes through a series of lymph nodes before reaching a major collecting duct. The exceptions to this general arrangement are the lymph vessels of the thyroid gland, the esophagus, and the coronary and triangular ligaments of the liver, which drain directly to the thoracic duct without passing through lymph nodes. The flow rate in the large lymphatic conduits, including the thoracic duct, is approximately 1-3 mL/min. Numerous valves are present along the lymphatic collecting vessels and trunks, and they aid in directing lymphatic drainage from the periphery toward the subclavian vein.

The initial lymphatics exist either as blind terminal sacs or as an anastomosing network of lymphatic capillaries 10-50 m in diameter in the interstitial space, in close relationship to the blood capillaries and small venules. These are sometimes connected to irregular-shaped sinusoids which have no basement membrane.

The walls of the initial lymphatics are made up of a single layer of endothe-lial cells with a discontinuous basement membrane, tethered to the surrounding connective tissue by collagen filaments (Fig. 2). Each lymphatic endothelial cell

OVERLAPPING ENDOTHELIAL CELLS FORMING

OVERLAPPING ENDOTHELIAL CELLS FORMING

Initial Lymphatics

FLUID

Figure 2 Initial lymphatics.

FLUID

Figure 2 Initial lymphatics.

is very thin, measuring 0.1 m or less at its edge where it meets the adjacent cell and thickens only in the perinuclear area to measure 2-4 |J,m. It characteristically has sparse rough endoplasmic reticulum scattered throughout the cytoplasm. It also contains fine contractile filaments (40-60 A in diameter) in the cytoplasm, lying parallel to the long axis of the cell.

The interendothelial junctions are approximately 10-25 nm wide, rendering the initial lymphatics highly permeable to plasma proteins and particulate material though occasionally gaps of several micrometers are present between cells. The overlapping junctions run obliquely and therefore function like flap valves, allowing the entry of fluid but closing when the intraluminal pressure increases above interstitial pressure. There is bulk flow of fluid, proteins, macromolecules, and other substances through these interendothelial junctions into the lumen of the initial lymphatics. Plasma membrane invaginations are present on both the luminal and abluminal surfaces of the endothelial cells, suggesting that pino-cytosis may be responsible for vesicular transport of particles larger than 30 nm across the lymphatic endothelium.

The anchoring filaments are microfibrils approximately 8 nm in diameter. They are attached to the walls of the initial lymphatics at one end and to collagen fibers in the interstitium at the other end (Fig. 3). These filaments pull the endothe-lial walls of the initial lymphatics apart as the matrix of the interstitial space swells with increased interstitial fluid volume.

Lymph Anchoring Filaments
Figure 3 Anchoring filaments of initial lymphatics.

Thus, the microanatomy of the initial lymphatics allows the endothelial lining to function as a microvalve system between the interstitium and itself. As the anchoring filaments prevent the collapse of the initial lymphatics, large interendothelial gaps open between the overlapping junctions, allowing the entry of interstitial fluid, cells, bacteria, and solid particles. When the luminal lymphatic pressure is higher than the interstitial pressure, an apposition of the overlapping margins of the endothelial cell results, thereby closing the interendothelial junction and preventing leakage of lymph into the interstitial space. The lymphatic collecting vessels are similar to the initial lymphatics with respect to the structure of their endothelium but, in addition, have bicuspid semilunar valves which promote lymphatic flow toward the central lymphatics and prevent the backflow of lymph. The valves are made up of thin collagen sheets between two endothelial layers. Thus, the valves are funnel shaped and can operate at very low flow rates irrespective of the size or shape of the lumen of the lymphatics. Viscous fluid stresses appear to be the main physical factor influencing the functional status of the valves. The intervalvular distance is 1-3 mm in the small lymphatics and increases to 6-12 mm in the larger lymphatics. There are approximately 30-35 valves in the thoracic duct, where the intervalvular distance is about 12-15 mm.

The walls of the lymphatic collecting vessels consist of an intima lined by endothelial cells and a basement membrane, a media with longitudinal and circular layers of smooth muscle, and an adventitia with connective and elastic fibers, fibroblasts, and nerve endings (Fig. 4). The collecting lymphatics form a series of discrete contractile units, each separated from the next by a valve. Each unit or compartment between two consecutive valves forms a lymphatic ''micro-heart'' called a lymphangion. The lymphangion is the basic functional unit of the lymphatic system, and its length varies between 6 and 20 mm in different organs.

Spontaneous peristalsis of about 10 contractions per minute occurs in lymphangions, producing synchronized opening and closing of the valves which prevent reflux of fluid. The contractility of lymphangions is regulated by their filling pressures, humoral mediators, and neural mechanisms. As the intraluminal pressure increases, the frequency of the peristaltic contractions increases, indicating a myogenic autoregulation. Alpha-adrenergic stimulation due to sympathetic activity increases the rate and force of contractions of lymphangions. Serotonin (5-hydroxytryptamine) is a potent stimulator of lymphatic contractions. Prostaglandins E1, E2, and I2 inhibit lymphatic motility. Anesthetic agents also inhibit the intrinsic contractility of the lymphatic vessels. It has been shown, for example, that halothane decreases the rate of lymph flow by 25-59% [2].

The walls of the major lymphatic trunks essentially have a structure similar to that of lymphatic collectors but with a thicker muscle layer and abundant elastic fibers in the media, and more nerve endings in the adventitia. They contain valves

Anchoring Filaments Lymphatic System
Figure 4 Lymphatic collectors.

which are essential for the forward flow of lymph fluid and for preventing the generation of high hydrostatic forces due to gravity. The intervalvular distance is approximately 6-10 cm. The lymphatic pressures can vary from less than 25 mmHg in the absence of lymphatic obstruction to over 100 mmHg during obstruction.

Large lymphatic conduits from the lower limbs and the abdominal viscera converge, and the lymph (now known as chyle) flows into a large saccular dilatation of the lymphatic trunk called the cisterna chyli. This acts as a temporary reservoir for chyle, which then passes onward in the thoracic duct. The thoracic duct receives about 75% of the body's lymph and drains into the left subclavian vein at its junction with the jugular vein.

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Responses

  • Ennio
    What is the initial lymphatic made up of?
    5 years ago
  • KAIJU KILKKA
    Are collecting ducts bigger than lymphatic vessels?
    5 years ago

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