The functioning and interrelationships of blood capillaries and lymphatics

Aqueous outflow channels and its lymphatic association: A review

The human eye has a unique immune architecture and behavior. While the conjunctiva is known to have a well-defined lymphatic drainage system, the cornea, sclera, and uveal tissues were historically considered "alymphatic" and thought to be immune privileged. The very fact that the aqueous outflow channels carry a clear fluid (aqueous humor) along the outflow pathway makes it hard to ignore its lymphatic-like characteristics. The development of novel lymphatic lineage markers and expression of these markers in aqueous outflow channels and improved imaging capabilities has sparked a renewed interest in the study of ocular lymphatics. Ophthalmic lymphatic research has had a directional shift over the last decade, offering an exciting new physiological platform that needs further in-depth understanding. The evidence of a presence of distinct lymphatic channels in the human ciliary body is gaining significant traction. The uveolymphatic pathway is an alternative new route for aqueous outflow and adds a new dimension to pathophysiology and management of glaucoma. Developing novel animal models, markers, and non-invasive imaging tools to delineate the core anatomical structure and physiological functions may help pave some crucial pathways to understand disease pathophysiology and help develop novel targeted therapeutic approaches for glaucoma.

Neglected interstitial space in malaria recurrence and treatment

The interstitial space, a widespread fluid-filled compartment throughout the body, is related to many pathophysiological alterations and diseases, attracting increasing attention. The vital role of interstitial space in malaria infection and treatment has been neglected current research efforts. We confirmed the reinfection capacity of parasites sequestrated in interstitial space, which replenish the mechanism of recurrence. Malaria parasite-infected mice were treated with artemisinin-loaded liposomes through the interstitial space and exhibited a better therapeutic response. Notably, compared with oral administration, interstitial administration showed an unexpectedly high activation and recruitment of immune cells, and resulted in better clearance of sequestered parasites from organs, and enhanced pathological recovery. The interstitial route of administration prolongs the blood circulation time of artemisinin and increases its plasma concentration, and may compensate for the inefficiency of oral administration and the nanotoxicity of intravenous administration, providing a potential strategy for infectious disease therapy.

Lymphatic Vessel Network Structure and Physiology

The lymphatic system is comprised of a network of vessels interrelated with lymphoid tissue, which has the holistic function to maintain the local physiologic environment for every cell in all tissues of the body. The lymphatic system maintains extracellular fluid homeostasis favorable for optimal tissue function, removing substances that arise due to metabolism or cell death, and optimizing immunity against bacteria, viruses, parasites, and other antigens. This article provides a comprehensive review of important findings over the past century along with recent advances in the understanding of the anatomy and physiology of lymphatic vessels, including tissue/organ specificity, development, mechanisms of lymph formation and transport, lymphangiogenesis, and the roles of lymphatics in disease. © 2019 American Physiological Society. Compr Physiol 9:207-299, 2019.

Lymphatic vessels of the eye - old questions - new insights

Due to its accessible position and tissue heterogeneity, the eye is ideally suited for studying the lymphatic system. As early as the 19th century, questions about the origin and function of this system were discussed. For example, whether Schlemm's canal, which is of particular importance in the pathogenesis of glaucoma, is a lymphatic vessel, or does this vascular system begin with finger-shaped protuberances? Despite the discovery of lymphatic endothelial molecules and the use of molecular imaging technologies, these questions are still discussed controversially today. Leber demonstrated in 1873 with a solution consisting of two dyes of different particle size that only the smaller particles from the anterior chamber of the eye filled the episcleral and conjunctival veins around the corneal margin. He believed to have proven - to be read in the historical review of our article - that the Canalis Schlemmii in humans is a venous circular vessel and not a lymphatic vessel. In our own investigations, we reduced the rather contradictory and complex question of whether there are lymphatic vessels in the eye to the question of whether there are drainage connections between the different sections of the eye and the lymphatic system or not. With different radioactive tracers and combined with unilateral ligation of cervical lymph vessels, we observed outflow from the subconjunctival and retrobulbar space, from the anterior chamber and the vitreous body. The rate of discharge of the radioactive tracer was determined by the radiopharmaceutical and injection site. In analogy to the lymphatic drainage of the head we found a segmental drainage of lymphatic substances on the eye. Vitreous humour and retrobulbar space were drained by lymphatic vessels, predominantly to the deep cervical lymph nodes, while anterior chamber and subconjunctival space drains predominated over the superficial cervical lymph nodes. Eyeball tattoos - as loved by some fan communities - should therefore cause a coloured staining of the superficial cervical lymph nodes. The boundary of the drained segments would be in the area of the eyeball's equator. According to the textbooks, the lymph is actively removed from finger-shaped initial segments via pre-collectors and collectors with properly functioning intraluminal valves and smooth muscle cells in the vessels' media. In patients with spontaneous conjunctival bleeding, however, we observed phenomena in the conjunctival lymph vessels, which ca not be explained with old familiar ideas. At nozzle-shaped vessel constrictions separation of blood components occurred. The erythrocytes formed partially a so-called fluidic "resting bulk layer". Parallel vessel parts caused a retrograde filling of already emptied segments. These observations led our experimental investigations. In the literature, there are different scanning electron microscopy (SEM) images of lymphatic endothelial surfaces; nevertheless they are unassigned to a particular vessel segment. In the conjunctiva, we studied the question whether there is a dependence between vessel diameter and the surface characteristics of endothelial cells (after unfolding by lymphography). A constantly applied photo-mathematical procedure for all specimens allowed determining the size of the cross sections. The specimens were randomized into seven groups with diameters of 0.1-1.0mm and above and examined by SEM. In the smallest vessels (diameter=0.11mm), the impressions of the occasionally occurring nuclei in the lumen were clearly impressive. With increasing diameter, these impressions were lost and the individual endothelial nuclei could no longer be identified. Rather, one recognized only wall-like structures. In vessels of intermediate diameter (0.3-0.4mm), structures could be seen on the surface similar to reticular fibres. With increasing diameters, their prominent character weakened. In the group with diameters above 0.5mm, wavy surface structures were shown. Finally, in vessels of diameters over 1.0mm, a uniform, flat surface was observed. Regardless of the collection site of the specimens, we found certain surface characteristics related to the vessels' calibre. In further investigations by means of interstitial dye lymphography, we were able to demonstrate in the conjunctiva that under increasing injection pressure, additional vessels stained from finger-shaped processes. At least in the conjunctiva, the existence of so-called "blind-ending initial segments" seems doubtful (despite the fact that initial segments or "initial lymphatics" would begin in periphery, not end). Rather, these are likely to be temporary filling states. SEM investigations were carried out on the internal structure of these dome-shaped vessel parts by means of a specially developed preparation technique. Despite numerous variants in the lymphographic design of the blind bags - in the form of finger, balloon, dome, piston, pyramidal, double-humped and spearhead-like endings - slot-shaped, lip-shaped and saw blade-like structures were repeatedly found, similar to a zipper. These findings suggest preformed connections to the next segment and may control lymphatic flow. To clarify the retrograde fluid movements, we examined the lymph vessels' valves or those structures that were previously interpreted as valves. The different structures found could be subdivided into three groups. The lack of common bicuspid structures provides an explanation for retrograde fluid movement. That nevertheless a directional flow is possible, is explained by the flow model developed by Gerhart Liebau. Conjunctival lymphatics show intraluminal structures by double contrast injection, which we divided into four groups due to anatomical differences: An accurate statement about the occurrence of certain intraluminal vascular structures in certain vascular calibres was possible only conditionally. However, complex and extended structures (group d) were found almost exclusively in larger vessel calibres (diameter>0.9mm). The structures are reminiscent of published findings in the "collector channel orifices of Schlemm's canal". They should play an important role in the regulation of the intraocular pressure, or the balance between production and outflow of the aqueous humour. The influence of such structures on the function of the lymphatic vessels is not yet known. As an approach models could be used, which for instance are applied in the water industry for the drainage, the degradation of introduced substances, or the detention pond. The latter serves for the retention and purification of drainage water (storage, treatment and reuse of drainage water). Dead zones, barriers, short-circuit currents and swirling are further hydraulic terms. Can intraluminal vascular structures, for example, affect the lymphatic flow and thus the mechano-sensitivity of lymphatic endothelial cells? Whatever interpretation model we use, the warning of the Swiss anatomist His from 1862 is still true today that all theories about the formation and movement of lymph should be based on precise anatomical basics. This review article therefore tries to make a contribution therefore. Despite knowing of lymphatic endothelial molecules, despite the discovery of the role of lymphangiogenic growth factors in diseases and the use of molecular imaging technologies, we still know too little about the anatomy and function of the lymphatic system.

A Model for Interstitial Drainage Through a Sliding Lymphatic Valve

This study investigates fluid flow and elastic deformation in tissues that are drained by the primary lymphatic system. A model is formulated based on the Rossi hypothesis that states that the primary lymphatic valves, which are formed by overlapping endothelial cells around the circumferential lining of lymphatic capillaries, open in response to swelling of the surrounding tissue. Tissue deformation and interstitial fluid flow through the tissue are treated using the Biot equations of poroelasticity and, the fluid flux (into the interstitium) across the walls of the blood capillaries, is assumed to be linearly related to the pressure difference across the walls via a constant of proportionality (the vascular permeability). The resulting model is solved in a periodic domain containing one blood capillary and one lymphatic capillary starting from a configuration in which the tissue is undeformed. On imposition of a constant pressure difference between blood and lymphatic capillaries, the solutions are found to settle to a steady state. Given that the magnitude of pressure fluctuations in the lymphatic system is much smaller than this pressure difference between blood and lymph, it is postulated that the resulting steady-state solution gives a good representation of the state of the tissue under physiological conditions. The effects of changes to the Young's modulus of the tissue, the blood-lymphatic pressure difference, vascular permeability and valve dimensions on the steady state are investigated and discussed in terms of their effects on oedema in the context of age- and pregnancy-related changes to the body.

Nanotheranostics of circulating tumor cells, infections and other pathological features in vivo

Many life-threatening diseases are disseminated through biological fluids, such as blood, lymph, and cerebrospinal fluid. The migration of tumor cells through the vascular circulation is a mandatory step in metastasis, which is responsible for ∼90% of cancer-associated mortality. Circulating pathogenic bacteria, viruses, or blood clots lead to other serious conditions including bacteremia, sepsis, viremia, infarction, and stroke. Therefore, technologies capable of detecting circulating tumor cells (CTCs), circulating bacterial cells (CBCs), circulating endothelial cells (CECs), circulating blood clots, cancer biomarkers such as microparticles and exosomes, which contain important microRNA signatures, and other abnormal features such as malaria parasites in biological fluids may facilitate early diagnosis and treatment of metastatic cancers, infections, and adverse cardiovascular events. Unfortunately, even in a disease setting, circulating abnormal cells are rare events that are easily obscured by the overwhelming background material in whole blood. Existing detection methods mostly rely on ex vivo analyses of limited volumes (a few milliliters) of blood samples. These small volumes limit the probability of detecting CTCs, CECs, CBCs and other rare phenomena. In vivo detection platforms capable of continuously monitoring the entire blood volume may substantially increase the probability of detecting circulating abnormal cells and, in particular, increase the opportunity to identify exceedingly rare and potentially dangerous subsets of these cells, such as circulating cancer stem cells (CCSCs). In addition, in vivo detection technologies capable of destroying and/or capturing circulating abnormal cells may inhibit disease progression. This review focuses on novel therapeutic and diagnostic (theranostic) platforms integrating in vivo real-time early diagnosis and nano-bubble based targeted therapy of CTCs, CECs, CBCs and other abnormal objects in circulation. This critical review particularly focuses on nanotechnology-based theranostic (nanotheranostic) approaches, especially in vivo photoacoustic (PA) and photothermal (PT) nanotheranostic platforms. We emphasize an urgent need for in vivo platforms composed of multifunctional contrast nanoagents, which utilize diverse modalities to realize a breakthrough for early detection and treatment of harmful diseases disseminated through the circulation.

Intralobular pulmonary lymphatic distribution in normal human lung using D2-40 antipodoplanin immunostaining

It has been assumed for a long time that except for limited areas close to respiratory bronchioles or their satellite arteries, there is no evidence of lymphatic vessels deep in the pulmonary lobule. An immunohistochemical study using the D2-40 monoclonal antibody was performed on normal pulmonary samples obtained from surgical specimens, with particular attention to the intralobular distribution of lymphatic vessels. This study demonstrated the presence of lymphatics not only in the connective tissue surrounding the respiratory bronchioles but also associated with intralobular arterioles and/or small veins even less than 50 mum in diameter. A few interlobular lymphatic vessels with a diameter ranging from 10 mum to 20 mum were also observed further away, in interalveolar walls. In conclusion, this study, using the D2-40 monoclonal antibody, demonstrated the presence of small lymphatic channels within the normal human pulmonary lobules, emerging from interalveolar interstitium, and around small blood vessels constituting the paraalveolar lymphatics. This thin intralobular lymphatic network may play a key pathophysiological role in a wide variety of alveolar and interstitial lung diseases and requires further investigation.

A model for mechanics of primary lymphatic valves

Recent experimental evidence indicates that lymphatics have two valve systems, a set of primary valves in the wall of the endothelial cells of initial lymphatics and a secondary valve system in the lumen of the lymphatics. While the intralymphatic secondary valves are well described, no analysis of the primary valves is available. We propose a model for primary lymphatics valves at the junctions between lymphatic endothelial cells. The model consists of two overlapping endothelial extensions at a cell junction in the initial lymphatics. One cell extension is firmly attached to the adjacent connective tissue while the other cell extension is not attached to the interstitial collagen. It is free to bend into the lumen of the lymphatic when the lymphatic pressure falls below the adjacent interstitial fluid pressure. Thereby the cell junction opens a gap permitting entry of interstitial fluid into the lymphatic lumen. When the lymphatic fluid pressure rises above the adjacent interstitial fluid pressure, the endothelial extensions contact each other and the junction is closed preventing fluid reflow into the interstitial space. The model illustrates the mechanics of valve action and provides the first time a rational analysis of the mechanisms underlying fluid collection in the initial lymphatics and lymph transport in the microcirculation.

A mathematical model of flow through the terminal lymphatics

Proper understanding of the mechanisms of fluid absorption and flow through the terminal lymphatics is essential for the control of several pathological conditions such as edema, bedsores and cancer. A mathematical model of the terminal lymphatics was developed using the principles of mechanics. Computer simulation results substantiate the hypothesis that fluid absorption and flow through the terminal lymphatics occur due to suction mechanisms of the adjacent contractile lymphatic segments and due to periodic fluctuations in the interstitial fluid pressure. In addition, the results suggested that increasing the length of a terminal lymphatic vessel beyond a certain limit does not cause further increase in fluid flow into the terminal lymphatic.

Developmental morphology of vascular and lymphatic capillaries in the working myocardium and Purkinje bundle of the sheep septomarginal band

The normal development of vascular and lymphatic capillaries in the right ventricular septomarginal band of the sheep heart was studied in 9 fetuses aged 60-143 days (term = 147 days), 14 lambs aged 1 day to 16 weeks, and 3 adults. Tissue was fixed by perfusion and examined with light and transmission electron microscopy. The septomarginal band is composed of working myocardium and a well-defined peripheral bundle of Purkinje cells. Vascular capillaries of the working myocardium were closely apposed to myocardial cells. By contrast, vascular capillaries of the Purkinje bundle were situated within the connective tissue sheath and septa, at variable distances from the Purkinje cells. After birth, the capillaries of the Purkinje bundle were also found in grooves and tunnels within the Purkinje strands. The ultrastructure of fetal vascular capillaries associated with myocardial and Purkinje cells was initially similar, and characterized by an abundance of synthetic organelles in endothelial cells and pericytes. However, after 115 days in utero, capillary endothelium with diaphragmed fenestrae, 40-60 nm in width, were observed within the Purkinje bundle. The fenestrae attained an average frequency of 1 per 11 capillary cross sections just before term, and this was maintained in lambs and adults. The ultrastructure of lymphatic capillaries, which were not observed in the septomarginal band until just before term, changed little during development.

Structure and distribution of lymphatic capillaries and fenestrated blood capillaries in the conduction system of the rabbit heart

The distribution and structure of lymphatic and blood capillaries in the rabbit heart conduction system were investigated by transmission electron microscopy. The sinuatrial node, atrioventricular node, and atrioventricular bundle possessed a rich network of lymphatic capillaries, which were situated not only at the periphery but also in the interior of the conduction system. The fine structure of these lymphatic capillaries was essentially similar to those within the atria and the ventricles. Although blood capillaries within working myocardium were nonfenestrated, the heart conduction system was often supplied by fenestrated blood capillaries. In the atrioventricular node and bundle especially, fenestrated blood capillaries and lymphatic capillaries were topographically associated, forming an extensive microcirculatory system. The presence of fenestrated capillaries suggests that a fast transcapillary passage of metabolites occurs in these regions, while the lymphatic capillaries may play an important role in the removal of macromolecules and excess intercellular fluid.

Interstitial fluid flow in cortical bone

After injection of ferritin into the tibial nutrient artery of adult dogs, ferritin was seen in the interstitial fluid compartment of the tibial cortex immediately and up to 25 min after injection. The pattern of movement was consistent with bulk interstitial fluid flow influenced by hydrostatic pressure. In osteons sectioned transversely, evidence was seen of centrifugal movement of ferritin in a halo pattern around the central capillary. Haloes in the superficial part of the cortex coalesced to form a ferritin front which moved toward the periosteal surface. Movement of ferritin was delayed by appositional bone of the periosteal surface. Ferritin was seen in channels in the matrix and perivascular spaces termed "matrix prelymphatics" and "perivascular prelymphatics," respectively, because they lacked an endothelial lining.

The radial protein concentration profile in the interstitial space of the rat ileal mesentery

The radial distribution of the protein concentration in the interstitium between arteriolar and venular vessels of the ileal mesentery of the rat was examined. Protein mass was determined by means of uv ultramicrospectrophotometry (UMS) and the relative volume distribution by means of fluorescence microscopy (FM) using the Na fluorescein and FITC-dextran (10,000 mol wt). UMS revealed gradients for protein mass from the vessels out into the interstitial space. FM showed a uniform distribution of fluorescence in the interstitium between the vessels. A gradient for protein mass without a gradient for volume distribution signifies the presence of a concentration gradient for protein in the interstitial space. The protein concentration across the arteriolar wall drops from 5.4 +/- 0.24 (SD) to 2.6 +/- 0.65% and across the venular wall from 5.4 +/- 0.24 to 3.3 +/- 0.43%. From the perivascular site the protein concentration declines exponentially reaching a minimum average interstitial concentration of 1.6 +/- 0.56%. Minimal protein concentration occurred at a point 37 +/- 6.4% of the 295 +/- 37 micron distance from the arteriolar to the venular vessels. In view of this distribution, it is unlikely that lymph or direct samples of interstitial fluid are representative of the perivascular protein concentration.

Tissue fluid pressure, lymph pressure, and fluid transport in rat intestinal villi

Tissue fluid pressure (Pt) and lymph pressure (Pl) as well as fluid transport between initial lymphatics and tissue matrix were determined in the villi of the jejunum in vivo and in vitro. When the intestine was absorbing fluid, free tissue fluid as a micro pool appeared in the villi. Pt and Pl were 2.4 +/- 0.7 and 2.9 +/- 0.8 mm Hg (mean +/- SE), respectively, in the villi in vivo (26 villi of 5 rats) or 0.8 +/- 0.4 and 1.2 +/- 0.5 mm Hg, respectively, in the villi in vitro (30 villi of 5 rats). Pl was significantly (P less than 0.01) higher than Pt. When Evans blue in saline was continuously infused into an initial lymphatic, dye rapidly leaked out into the tissue matrix, but when it was infused into the free tissue fluid, dye did not enter the lymphatics but leaked out of the epithelial layer, apparently due to low hydraulic conductance of the tissue matrix to fluid transport in this direction. Furthermore, in the villi with fat absorption, a retrograde flow of chylous lymph out of the villous tips always occurred, indicating that there are large pores at the villous tips to allow free passage of fluid and chylomicrons. From these findings and other evidences, it is inferred that during fluid absorption a fraction of the lymph may be formed by the transport of the luminal fluid directly into the terminal end of the initial lymphatics via large pores at the villous tips presumably by inhibition of the tissue matrix or by lymphatic suction or both.

Transient ultrastructural variations of pulmonary lymphatic capillaries during the respiratory cycle of the rat lung

The effect of respiratory movements on the endothelial wall of lung lymphatic capillaries was investigated by a morphometric study of their ultrastructure using a rodent model. The juxta-alveolar lymphatic capillaries from lungs were fixed in the inflated phase by vascular perfusion with aldehydes and compared to those from lungs fixed in a deflated phase. Important quantitative differences in lymphatic endothelial wall architecture were observed between the two states. The most salient finding was a marked reduction of the mean endothelium thickness from 320 +/- 55 nm in deflated lungs to 210 +/- 36 nm (P less than .01) in inflated lungs. The endothelial attenuation observed with inflated lungs was accompanied by a reduced frequency of interdigitation, the intricate pattern of intercellular contact (P less than .02), and a decreased number of lumenal and ablumenal endothelial projections (P less than .05). Despite these important variations in the dilated lymphatic capillaries, no typical open junctions (greater than 30 nm) were found among 105 intercellular contacts. The vesicular system varied slightly after lung inflation. Dilated capillaries showed a significant reduction in cytoplasmic vesicles during inspiration (P less than .01); however, the observed increases in lumenal and ablumenal vesicles were not found to be significant. As for numerical density and vesicular diameter, the fraction of cytoplasm occupied by the vesicles remains unchanged. Contrary to previous assumptions, these results suggest that intercellular clefts are not widened to the extent of open junctions (greater than 30 nm) by the mechanical movement of inspiration in normal lungs. Moreover, transient changes to the endothelial architecture seem to preclude any excessive widening of the intercellular cleft.

Mechanisms of tumor invasion: evidence from in vivo observations

The major mechanisms of tumor invasion in vivo are discussed in the present review. A special emphasis is placed on tumor dedifferentiation which has proved to be of paramount importance for the invasion process. Based on in vivo observations obtained from various human and animal tumors a concept for the mechanism of tumor invasion is proposed which mainly comprises the following basic events: the first and essential step in tumor invasion is the tumor dedifferentiation and dissociation at the invasion front. This apparently temporary and reversible process mobilizes the tumor cells out of the main tumor bulk and enables them to invade the host tissue by active locomotion. This mechanism is essentially supported by an interstitial edema in the host tissue adjacent to the tumor periphery, which causes an 'opening and widening' of the host intercellular spaces. Enzymatic changes in the micromilieu of the extracellular matrix may contribute to this process. The tumor cell proliferation completes the invasion process in so far, as the invading tumor cells are still able to proliferate, leading this way to expanding tumor cell nests in the host tissue which have the potency to redifferentiate. The expansive growth of these tumor cell nests results in a progressive atrophy of the host tissue, mainly caused by an increasing compression and a competitive withdrawal of oxygen and other nutrients by the tumor cells. The overall picture of tumor invasion can therefore be considered as a repetitive cycle of active tumor cell locomotion followed by focal tumor cell proliferation in the host tissue.

Topography and ultrastructure of the uterine lymphatics in the rat

An integrated light and electron microscopic study of the rat uterine lymphatic microcirculation reveals that the functional layer of the endometrium lacks an intrinsic lymphatic system. Only some scanty lymphatic capillaries may be observed in the basal endometrium near the endo-myometrial junction; these are in fact initial lymphatic capillaries belonging to the well-developed plexus of myometrial lymphatic capillaries which rarely permeate the entire myometrial internal muscle layer, ending blindly in the basal endometrium. The ultrastructure of these initial 'endometrial' lymphatic capillaries is detailed. The myometrium, in contrast, has a rich supply of lymphatic capillaries as well as lymphatic vessels, the latter provided with many valves. Both the topography and the fine structure of myometrial lymphatic capillaries and lymph collecting vessels-including their valves-are described and discussed.

In situ observations of spontaneous contractions of the peripheral lymphatic vessels in the rat mesentery: effects of temperature

By means of an intracellular glass microelectrode, action potential changes were successfully recorded in situ from the endothelial cells of rat mesenteric lymphatics over the temperature range of 27-40 degrees C. The frequency of action potential and the lymphatic contraction rate correlated well with temperature.

Lymphatic metastasis

Lymphatic metastasis is an important mechanism in the spread of human cancer. During its course, tumor cells first penetrate the basement of membrane of the epithelium, in which they arise, and then the underlying connective tissue, carried partly by hydrostatic pressure. They enter the lymphatic partly by active movement, pass up the lymphatic trunk; they then settle and proliferate in the subcapsular sinus, penetrate its endothelium and proliferate and destroy the node. There are varied forms of immune response in the node and in human nodes often a complex fibrous and vascular response. The degree of lymphocytic response may be important for prognosis. The nodal reaction may be stimulated by release of antigens from the tumor. One of the most studied animal models of lymphatic metastasis is that which occurs in the politeal node after injection of tumor into the footpad. This model has been used to show that tumor cells enter lymphatics through gaps in endothelium, probably between endothelial cells, and that lymph nodes can destroy small numbers of tumor cells. Local immunotherapy and chemotherapy can sterilize a lymph node of tumor cells; the modes of treatment used have included intralymphatic injection and encapsulation of chemotherapeutic agents in liposomes. Prior radiotherapy may accelerate metastasis possibly by making tumor cells shed into lymphatic vessels. Lymph nodes are rather poor barriers to tumor cells. The prognostic significance of lymph node metastasis varies within tumor type; if hematogenous metastasis is early, then the presence of lymph node metastasis is of lesser prognostic significance. Lymph nodes can probably destroy only small numbers of tumor cells. Tumor cell heterogeneity is of importance in many aspects of metastasis; while clonal variation may be of importance in determining lymph node metastasis, it is not yet clear how important this is, nor whether specific clones metastasize specifically to lymph nodes. Lymphography is well established in diagnosis of lymphatic metastasis. A recent interesting development has been to inject antibodies labeled with a radioactive label, and image the label in lymph nodes with a gamma-camera. If anti-tumor antibodies are used in this way it may be possible to detect lymph node metastasis. Within the expanding field of tumor metastasis, lymphatic metastasis needs much more attention, particularly in relation to the diagnosis and treatment of the lymphatic spread of human cancer.

Fluid spaces in canine bone and marrow

In order to provide a firm basis for the analysis of experiments on the kinetics of ion and solute exchange in bone, the total water space, extracellular space, and vascular (plasma plus erythrocyte) space in the tibial diaphysis in adult dogs were estimated. Tracers were injected intravenously into 40 dogs after ligation of the renal pedicles. Estimates were made of the volumes of distribution of radioactive tracers of varying characteristics (molecular weight, solubility, and charge), by taking samples of blood, plasma, and bone 3 hr after injection. The volumes of distribution (milliliters per milliliter of cortical bone) were: for tritiated water (a total water marker) 0.245 ± 0.003 (mean ± SD); for [¹⁴C]sucrose (an extracellular marker) 0.043 ± 0.001; for ¹¹¹In-labeled transferrin (a plasma marker) 0.008 ± 0.003; and for red blood cells 0.005 ± 0.002. Interstitial fluid space was estimated by subtracting the estimated plasma space from the estimated extracellular space and it was 0.034 ml/ml of bone. Estimates of haversian canal volumes, obtained morphometrically, were 0.015 ml/ml bone and were compatible with the estimates of the vascular space.

Lymphatic metastasis: invasion of lymphatic vessels and efflux of tumour cells in the afferent popliteal lymph as seen in the Walker rat carcinoma

When twenty million Walker rat carcinoma cells are injected into the footpad of albino outbred rats, there is progressive metastasis to the draining popliteal and thence para-aortic lymph nodes. The lymphatic duct efferent from the footpad and afferent to the popliteal node has been cannulated; it has been shown that there is a continuous and progressively increasing output of tumour cells, small and large lymphocytes, macrophages and polymorphs from the footpad. About 20 per cent of the cells are tumour cells. The number of tumour cells in the popliteal and para-aortic nodes has been counted using a Coulter counter and subsequent differential counting of stained smears; the nodes contain a progressively increasing number of both tumour cells and lymphoreticular cells. The early accumulation of tumour cells in the para-aortic nodes makes it evident that tumour cells pass rapidly through the primary node. Examination of the simulated primary tumour by transmission electron microscopy suggests that tumour cells move actively toward lymphatics and protrude cytoplasmic processes through gaps in the endothelium. The endothelial cell then degenerates in close proximity to tumour cell processes. This leaves gaps through which tumour cells may pass and ultimately results in lymphatics with large defects in their walls.

Simultaneous measurement of pressure in the interstitium and the terminal lymphatics of the cat mesentery

1. Simultaneous measurements of the pressure in terminal lymphatics and interstitial tissue have been made in the exteriorized cat mesentery superfused with either physiological salt solution (Krebs solution) or a water-immiscible fluorocarbon, FC-80. 2. The pressures within individual terminal lymphatics were measured using glass micropipettes attached to a servo pressure-measuring system. Tissue pressures were recorded using saline-filled cotton-wool wicks. 3. Mean pressure recorded in the terminal lymphatics of the Krebs-superfused mesentery were slightly above atmospheric (+0.2 mmHg, n = 45), while those recorded in the FC-80-superfused mesentery were slightly below atmospheric (-0.2 mmHg, n = 46). 4. Tissue pressures were also slightly subatmospheric in the in situ mesentery, and the recently exposed tissue. Continuous superfusion with Krebs solution caused the tissue pressure to rise to atmospheric pressure or above; with FC-80-superfusion the tissue pressure also rose, but never to above atmospheric pressure. 5. Isolated strips of mesentery immersed in Krebs solutions of different concentrations gained weight, but when immersed in FC-80 no change in weight was detected. 6. It was concluded that the interstitial gel of the mesentery is normally unsaturated and that superfusion with Krebs solution leads to tissue oedema. This tendency is less marked in FC-80-superfused preparations. Possible mechanisms for lymph formation and propulsion are discussed.

Absence of restricted diffusion in cutaneous capillaries

Capillary permeability in cutaneous tissue for 57Co-cyanocobalamin (57 Co-B12) was determined by the single injection, external registration method. The capillary diffusion capacity, CDC (the permeability--surface area product, PS) was 2.3 ml/100 g-min. Capillary extraction was 0.48 at a plasma flow of 4.1 ml/100 g-min. Results were compared to 51Cr-EDTA data from a previous study with identical method and preparation. As CDC(51Cr-EDTA)/CDC(57Co-B12) was 1.61 and as D(51Cr-EDTA)/D(57Co-B12), the ratio between the free diffusion coefficients in water at 37 degrees C, was 1.79 it is concluded that restricted diffusion does not occur in cutaneous tissue for 57Co-B12 as compared to 51Cr-EDTA, i.e. 51Cr-EDTA and 57Co-B12 diffuse across the capillary membrane of cutaneous tissues at rates proportional to their respective free diffusion coefficients in water. The Pappenheimer equivalent pore radius estimate of 30 A and the Karnovsky interendothelial 40 A slit width are both defective in explaining the experimental data. The transendothelial patent channel system of fused vesicles (Simionescu, Simionescu and Palade 1975) is a possible structural equivalent for the present findings. The results support the hypothesis that capillaries of continuous type exhibit similar permeation characteristics regardless of the tissue in which they are located.

Protein concentration in interstitial and lymphatic fluids from the subcutaneous tissue

The protein content of interstitial fluid and lymph from the same tissue was determined fluid samples of nanolitre volumes being taken from the subcutaneous tissue of rabbits by micropuncture. In the same area lymph was collected from lymphatic vessels of 30-100 mum. An electrophoretic technique in polyacrylamide gels in glass capillaries was used for the analysis of proteins. The gels were stained and then scanned on a microscope equipped with a moving table and photomultiplier. The area under each protein band was calculated from the recorded densitograms. The method required only a few nanolitres of fluid for analysis. Using solutions of known protein concentrations the relationship between the amount of protein and the area under the densitogram band was investigated. This relationship was found to be linear, making it possible to quantify the protein content of both interstitial fluid and lymph samples. The interstitial fluid/plasma concentration ratios for albumin, transferrin, globulins and total proteins were 0.42, 0.42, 0.32 and 0.37. The albumin/globulin ratio in interstitial fluid was found to be 1.60 of the plasma value although the values obtained for lymph were similar to those found for interstitial fluid. The similarity between the protein concentration of interstitial fluid and lymph indicated that the endothelial lining of the terminal lymphatics did not restrict the passage of macromolecules into the lymphatics.