Lymphatic Pathophysiology

Macroscopic Anatomy

In general, lymph from the lower part of the torso and viscera enters the bloodstream via the thoracic duct at the left subclavian–jugular venous junction (see Figs. 10.1 and 10.2 ). Lymphatics from the head and neck and from the upper extremities enter the central veins either independently or by a common supraclavicular cistern. Numerous interconnections exist within this rich vascular network, and subvariant anatomic pathways are plentiful. For example, the bulk of cardiac and pulmonary lymph empties into the great veins in the right side of the neck. In contrast, intestinal lymph, which transports cholesterol, fat-soluble vitamins, and long-chain triglycerides as chylomicra, courses retroperitoneally to the aortic hiatus and with other visceral and retroperitoneal lymphatics forms the multichannel cisterna chyli and thoracic duct. The bulk of lymph formed in the liver flows retrograde or countercurrent to portal blood and joins intestinal lymph collectors just before the origin of the thoracic duct. Although these topographic variants influence development and progression of peripheral (lymph) edema only indirectly, they are nonetheless essential for a broad understanding of edema syndromes, including those accompanied by abnormal visceral lymphatics, celomic effusions, and chylous reflux.

Although the retina and brain do not technically have lymphatic apparatuses, they possess analogous circulations, such as the aqueous humor canal of Schlemm (the anterior chamber of the eye) or the cerebrospinal fluid/subarachnoid villus (pacchionian bodies) connections (the brain). The initial path postulates that the natural vasomotion of cerebral intraparenchymal arteries/arteriole generates a force that forces fluid from the perivascular space into the brain parenchyma crossing the glial perivascular sheet via water pores present in the membrane of glial cytoplasmic projections. Once in the parenchyma, the interstitial fluid travels following either a convective or diffusive trajectory ^, towards adjacent perivascular spaces (the intraparenchymal peri-arterial drainage pathway towards adjacent paravascular spaces (the intraparenchymal peri-arterial drainage pathway, ^, where it is transported to the meningeal space, thus entering the second pathway. In the meningeal space, interstitial fluid is collected into initial lymphatic capillaries present within the dura mater, which drains directly into afferent lymphatic vessels located in the cervical region, in particular into deep cervical lymphatic vessels. ^, In addition, other pathways exist within the central nervous system and eyes that drain interstitial fluid and cerebrospinal fluid including the aqueous humor. Glial elements and non-endothelial-lined intracerebral perivascular (Virchow–Robin) spaces form the glymphatic system drains the CSF, perivascular spaces and interstitial fluid of the brain swept up by astrocytic foot processes and empties into the endothelial-lined meningeal lymphatic capillaries and thence to the contractile collecting cervical lymphatics and lymph nodes.

Extensive interruption of the cervical lymphatic trunks (e.g., after bilateral radical neck dissection) therefore causes prominent facial suffusion and a transient neurologic syndrome resembling pseudotumor cerebri, whereas an infusion of crystalloid solution directly into the canine cisterna magna causes an elevation in intracranial pressure and increases lymph flow from draining neck lymphatics. ^, Although abundant lymphatic pathways thus exist for surplus tissue fluid to return to the bloodstream, homeostasis still depends on an unimpeded, intact, interstitial–lymph fluid circulatory system (see Figs. 10.1 and 10.2 ).

Embryonic Development

Controversy has persisted since the early 1900s about the embryologic origin of lymphatics, that is, lymphovasculogenesis (endothelial precursors or stem cells, such as lymphangioblasts, differentiate and proliferate into a primitive tubular network) and subsequent lymphangiogenesis (sprouting from existing vessels; see Box 10.1 ). According to Sabin, the primary lymphatic plexuses derive from central veins and their growth progresses centrifugally by sprouting toward the periphery and ultimately forming the superficial lymphatic system ( Fig. 10.3 ). In contrast, Kampmeier, after a review of serial tissue sections, including Sabin’s original human embryo preparations, and phylogenetic considerations, proposed the centripetal theory that the lymphatic system arises independently from tissue mesenchyme in peripheral tissues and the surrounding primary lymph sacs and only later joins the central venous system.

Recent studies of the growth regulatory gene PROX1 and the receptor for lymphatic vascular endothelial growth factor (VEGFR-3) supported the centrifugal theory (origin from venous spouting). Yet, other elegant work has demonstrated a substantial centripetal contribution from mesenchymal lymphangioblasts in the engrafted wing lymphatic system of chimeric quail chick embryos and early avian embryos through PROX1 staining. Both processes may contribute in various degrees to the ultimate link between the lymph and blood vasculature (e.g., the cervical thoracic duct/venous junction in humans).

Light Microscopic Anatomy

As an efferent vascular system, the lymphatics originate within the interstitium as specialized capillaries, although in certain organs, such as the liver, they seem to emanate from nonendothelialized precapillary channels (e.g., the spaces of Disse). Lymphatic capillaries are remarkably porous and readily permit the entry of even large macromolecules (molecular weight >1000 kD). In this respect, they resemble the uniquely “leaky” fenestrated sinusoidal blood capillaries of the liver but are in distinct contrast to most other blood capillaries, which are relatively impervious to macromolecules even the size of albumin (molecular weight, 69 kD).

Under light microscopy without treatment, it is difficult to distinguish between blood and lymph vessels, although the latter are usually thin walled and tortuous, have a wider, more irregular lumen, and are largely devoid of red blood cells. Many staining features have been advocated to differentiate between blood and lymph microvasculature, such as the endothelial marker factor VIII-related antigen: von Willebrand factor (vWF). Although staining characteristics vary in both normal and pathologic states and at different sites (perhaps related to endothelial cell de-differentiation), in general, lymphatic staining resembles but is less intense than its blood vessel counterpart. In other words, the staining differences have been more quantitative than qualitative.

Lymphatic-Specific Markers

Lymphatic vessels can increasingly be distinguished from blood vessels in tissue sections by whole-mount staining with specific markers. Only 20 years ago, little histochemical specificity existed to distinguish lymphatic vessels from blood capillaries and veins, and identification was based primarily on morphology, distinctive ultrastructure, or both. One of the most commonly used and most specific markers in use today is LYVE-1 ^, with D-240 widely used to stain clinical specimens. LYVE-1 has been applied to tissues ranging from early mouse embryo to adult human and highlights initial collecting vessels and lymphatic capillaries (but not larger-caliber lymphatics). The LYVE-1 receptor appears to be a key entry point for dendritic cells to selectively access the lymphatic system. Another strong marker localizing to the nucleus of lymphatic endothelial cells and adaptable to multiple tissues is the transcription factor PROX1. Podoplanin recognizes a transmembrane glycoprotein in lymphatics but not blood vessel endothelial cells in the mouse, whereas its human analogue, D2-40, ³⁵ also sharply distinguishes lymphatic from blood vessel endothelium and larger collecting lymphatics which may not stain with LYVE-1 but stains other distinguishable cells. This feature has been useful in identifying preexisting and new lymphatics in tumor specimens and in generating quantitative differentials from blood vessels and indices of lymphatic invasiveness and tumor dissemination. 5-Nucleotidase staining is used by research laboratories for its lymphatic specificity and other common markers showing some cross-reactivity to veins include VEGFR-3 and neuropilin-2 (reviewed elsewhere). ^, Thus an array of lymphatic markers are now available to distinguish lymphatics from blood vessels, although there is overlap of cell types in normal conditions and even more so in pathologic states.

Ultrastructure

Ultrastructurally, lymph capillaries display both “open” and “closed (tight)” endothelial junctions, often with prominent convolutions and these capillaries can dramatically adjust their shape and lumen size. Unlike blood capillaries, a basal lamina (basement membrane) is tenuous or lacking altogether in lymph capillaries. ^, Moreover, complex elastic fibrils, termed anchoring filaments, tether the outer portions of the endothelium to a fibrous gel matrix in the interstitium. These filaments allow the lymph microvessels to open wide, which causes a sudden increase in tissue fluid load and pressure, in contrast to the simultaneous collapse of adjacent blood capillaries ( Fig. 10.4 ). Just beyond the lymph capillaries, which have a “button” structure, are the terminal lymphatics, which have a “zipper” structure. In contrast to more proximal and larger lymph collectors and trunks, the terminal lymphatics are devoid of smooth muscle, although the endothelial lining is rich in the contractile protein actin. Intraluminal bicuspid valves are also prominent features and serve to partition the lymphatic vessels into discrete contractile segments termed lymphangions . These specialized microscopic features support the delicate lymphatic apparatus’s functions of absorbing and transporting elements and the large protein moieties, cells, and foreign agents of the bloodstream (e.g., viruses, bacteria) that gain access to the interstitial space ( Figs. 10.5 and 10.6 ).

Structural–Functional Imaging

Early physicians were able to visualize the lymphatic system only by observing chyle-filled mesenteric lymphatics. Asellius’s initial publication included what has been reported as the first color anatomic plate in history. This was followed by intralymphatic injection of mercury into cadavers by Nuck in 1692, which depicted fine channels, then the detailed and elegant work of Mascagni in 1787, and subsequently, the classic images of both subcutaneous and deep vessels by Sappey in 1874. Von Recklinghausen used silver nitrate, which allowed imaging to take place without removal of surrounding tissue and facilitated visualization of lymphatic vessels as distinct from blood capillaries. Gerota developed a technique in 1896 of injecting a mixture of Prussian blue and turpentine to highlight the vessels, and this was followed in 1933 by the intracutaneous injection of vital dyes that bind to tissue proteins by Hudack and McMaster, which is a technique still used today for investigation and in the clinic ( Fig. 10.7 ).

Modern imaging techniques also include direct (intralymphatic) injection of oily contrast agents, termed lymphangiography, first described by Kinmonth in 1954, and whole-body lymphangioscintigraphy after subcutaneous or intradermal injection of protein-bound radiotracers ^, (see Fig. 10.7 ), which can provide a dynamic whole body image of the lymphatic system – peripheral and central – but with limited resolution spatially, and is useful for screening and monitoring the course of disease and effects of intervention noninvasively. SPECT-CT can greatly improve the sensitivity and localization with 3D higher resolution LAS images. Other agents used for indirect lymphography include various fluorescent or magnetic particles, ^, infrared particles and dyes, immunoglobulin conjugates, and microbubbles for detection with fluorescent microscopes, optical imaging systems, computed tomography (CT), magnetic resonance imaging (MRI; with and without contrast) and ultrasound, including in combination with light as photoacoustics, with an expanding potential for highly specific molecular imaging. New contrast agents and techniques continue to be developed. Most important has been the refinement of percutaneous MR imaging with direct puncture of central lymphatics and also inguinal node injection of contrast to open up the central lymphatic system to clear view and, with it, the opportunity to intervene therapeutically through gluing leaks or performing endovascular stent graft shunts, with other image-guided assistance or direct surgical lymphatic venous shunts for thoracic duct decompression. Photoacoustic imaging for resolution and depth improvement shows promise. ICG lymphography has been used effectively for staging of severity and operability in preparation for supermicrovascular lymphatic–venous shunt procedures to treat limb lymphedema.

General Principles

As a fine adjuster of the tissue microenvironment, the lymphatic system is often neglected in most treatises on vascular diseases. Yet this delicate system, so inconspicuous during life and collapsed after death, helps to maintain the liquid, protein, and osmotic equilibrium around cells and aids in absorption and distribution of nutrients, disposal of wastes, and exchange of oxygen and carbon dioxide in the local milieu intérieur.