Earliest lymphoid colonization of neonatal rat lymph nodes: an antigen-specific process?

The present work studied the little known process of lymphoid cell colonization of neonatal lymph nodes, while considering the nodal site of entry of circulating lymphoid cells and the either random or antigen-specific character of the process. Tissue sections of a mesenteric, cervical and popliteal node from each of 57 rats, aged 4 hours to 3 weeks, were analysed. Observations bear on the relative importance of the implication of the subcapsular sinus versus venules of nodes, and the composition of their emerging lymphoid cell population by determining the proportion of lymphocytes and blast-related cells. At 16-20 hours after birth, cell counts yielded a mean proportion of 84% for blast-related cells which decreased to 18% at 3 weeks. These percentages are compatible with values expected for a selective antigen-specific entry of lymphoid cells in nodes, not with values that would result from a random entry of lymphocytes. Moreover, observations revealed that by far most colonizing cells initially enter nodes carried by the afferent lymph, little via their venules.

[Emigration of Quebec physicians: motivation for departure and return]. / L'émigration des médecins québécois: motifs de départ et de retour.

Since the 80's, outmigration of physicians from Quebec is steadily increasing. About 46 percent of outmigrating doctors explain their move by factors related to their occupational life (higher income, greater opportunity in the academic career, larger amount of resources devoted to the health care system). Nearly 40 percent relate their decision to personal factors (greater job opportunity for their wife/husband, quality of family life...). The factors linked to the context of the receiving place (political climate, linguistic regulations, income tax level...) play a minor role on the migration decision. As concerns the returning physicians, 80 percent explain their decision by personal factors. The factors linked to the occupational life have a lower role. It appears therefore that doctor outmigration from Quebec is not directly determined by manpower policies adopted by the Province during the last two decades, except the policies directly linked to the income level of professionals.

[Beyond the statistics: why are Quebec physicians emigrating?]. / Au-delà des statistiques: qu'en est-il de l'émigration des médecins québécois?

During the period 1986-1999, about 16% of the annual average number of active physicians in Quebec have left for the US or an other Canadian province. The absolute number was 2367. During the same period, 661 would have returned to Quebec. The majority of the outmigrating doctors were graduated from the anglophone university of the Province (although 44 percent are francophone) whereas the majority of the returning doctors were graduated from the 3 francophone universities of the Province. As a matter of fact, 78 percent of the returning doctors are francophone. The migration decision is equally linked to post-graduate training and occupational opportunities and job satisfaction. This feature should be related to an other: those who have returned to Quebec have had a geographic and occupational mobility level far lower than those who have not returned. Finally, it is noteworthy that there are significant differences between the doctors emigrating to the US and their colleagues going to an other Canadian province.

Ectasias of the subcapsular sinus in lymph nodes of athymic and euthymic rats: a relation to immunodeficiency.

This paper describes a morphologically unusual feature occurring in lymph nodes of some aged euthymic animals but mostly athymic animals. It initially consists of small alveole-like excrescences of the cortical wall of the subcapsular sinus. With dilatation, an excrescence becomes an ectasia which expands into the cortex. Observations suggest that ectasias enlarge under the influence of an increased pressure of the afferent lymph of a node. Such condition conceivably results from a greater lymph formation due to inflammation of the drained tissue site, combined with an impairment to the flow of lymph from the subcapsular sinus into medullary sinuses. A probable relation of ectasia formation to immunodeficiency is discussed. This formation results in the atrophy of the affected lymphoid cell populations of a node which likely contributes to aggravate the deficiency of the immune system.

High endothelial venules of the rat lymph node. A review and a question: is their activity antigen specific?

BACKGROUND: The high endothelial venules (HEVs) of the lymph nodes are sites for transvascular lymphocyte traffic. Due mostly to the wide scale of variations manifested by the HEVs and to frequently restricted conditions of observation, reports often differed on their morphological or functional features, which has led to opposing views on aspects of the functioning of HEVs. METHODS: In the present review, we analyze previous reports and attempt to derive comprehensive proposals to reconcile variations in actual observations under diverse conditions. RESULTS: This analysis shows that the features typical of the HEV endothelial cells (HEV cells) are stimulated to emerge by antigens and the proper lymphocytes and mediators. The stimulation would implicate drained lymphocytes migrating in the perivascular channel, immediately cuffing an HEV's endothelium. A marked pleomorphism of HEV cells betrays the fact that they undergo individual stimulation and a somewhat heterogeneous activity. Other facts indicate that the subendothelial spaces of HEV cells are sites of interactions between drained lymphocytes, HEV cells, and recruited blood lymphocytes. Facts also reveal time- and site-related variations in the intensity of recruitment of blood lymphocytes by HEV cells and topographically related variations in the nature of the recruited cells. CONCLUSIONS: Analysis of some other observations, often ignored, lead to the conclusion that recruitment of lymphocytes by HEV cells for the sake of participating in local specific immune activities is antigen specific, despite the implication of homing receptors of lymphocytes.

A study on binding of suspended nodal lymphocytes to high endothelial venules in sections of frozen rat lymph nodes.

Live lymphocytes were previously shown to bind selectively to endothelial cells (HEV cells) of the high endothelial venules (HEVs) in sections of frozen lymph nodes. This study examines aspects of the assay that have so far not been considered or which have been incompletely addressed. It was found that bound lymphocytes form a cytoplasmic veil, and that about half of them form small groups in which they are linked together by cytoplasmic bridges. It was also found that at least 83% of the lymphocytes bind to HEV walls, but very unevenly, and that 5% also bind to medullary venules. In addition, 31% of the lymphocytes were estimated to bind to the abluminal face of HEV cells and probably to tissue lymphocytes present in subendothelial spaces or in perivascular channels (spaces). This would reflect cell interactions occurring, in vivo, between HEV cells and/or subendothelial lymphocytes. It is suggested that antigen specificity of lymphocyte retention by HEV cells accounts for the uneven binding to HEVs.

Association of neutrophils with high endothelial venules in the neonatal rat lymph nodes: a probable relation to immunoincompetence.

The endothelium of the high endothelial venules (HEVs) of lymph nodes is normally considered to inhibit an association with neutrophils. The present paper shows that for a few weeks after birth, however, neutrophils are commonly associated with the walls of HEVs, the extent depending on the site of the lymph node. Overall, neutrophils increase in numbers in rat nodes from birth until about day 11, and vanish progressively thereafter. Moreover, neutrophils are more abundant in the nodes of standard neonates than in the nodes of pathogen-free neonates raised in an aseptic milieu. It is concluded that the postnatal recruitment of neutrophils by nodal HEVs relates to the then prevailing state of immaturity of the immune system. An explanation is proposed as to why neonatal HEVs of nodes recruit neutrophils and not only lymphocytes, as is the case later in ontogeny.

Dilatation of high endothelial venules in compartments of rat lymph nodes with abundant cortical mast cells.

Previous work has shown that the method of killing animals and of fixation of lymph nodes determines the appearance of high endothelial venules. Routine procedures produce a narrowing of the high endothelial venules whilst in situ fixation of lymph nodes results in their dilatation. More recently, routine procedures have demonstrated that lymph nodes from athymic and some euthymic ageing rats have occasional compartments in which the high endothelial venules are dilated in association with an increased population of cortical mast cells. These observations suggest that mast cell products may influence the physiological state of the high endothelial venules and affect the recruitment of blood lymphocytes.

A modified silver impregnation technique for the observation of thick sections of lymph nodes perfused with colloidal carbon.

For many years, a variant of the silver impregnation technique of Bielchowsky has been used to study the lymph node because it clearly outlines the various structures which are usually hard to contrast with standard staining methods. Like other variants of silver impregnation, this method blackens the cell nuclei as well as the reticular fibers; however, it inhibits the impregnation of the nuclear chromatin immediately adjacent to fibers. Hence, this variant selectively darkens the lymphoid cell populations of the nodal structures which contain a loose fiber network. To study the blood vascular network of the lymph node based on perfusion of colloidal carbon, a staining procedure was needed which would contrast nodal structures on thick sections, while allowing the carbon-filled small blood vessels to be distinguished from the impregnated coarse reticular fibers. In an attempt to adapt this variant of Bielchowsky's technique, 10, 20, 40 and 60 microns thick sections from rat nodes, fixed in a solution of Bouin-Hollande for 72 hr, were silver impregnated with serial dilutions (1:2 to 1:128) of the ammoniacal silver solution. Forty-micrometer thick sections impregnated with a 1:16 dilution of the original silver solution at 37 C and for 30 min provided the best results for the conditions.

Retention of lymphocytes in the subcapsular sinus of lymph nodes: a physiological event?

This study examined the lymphocyte content of the subcapsular sinus of lymph nodes of diverse anatomical sites, from euthymic and athymic animals of various ages. One unusual feature which prevailed in young euthymic animals consisted of the accumulation of lymphocytes on the outer wall of the subcapsular sinus, following differential patterns with respect to diverse domains or areas of the subcapsular sinus of a node compartment. It is concluded that such an accumulation is due to the retention of lymphocytes on the sinus outer wall. Whether the retention reflects a step in unspecific defence mechanisms, in an immunological reaction pathway, or a transient state of the misfunctioning of lymph-carried cells, is considered. Some findings favor the latter possibility. In this case, retention would be due to a mild form of lymphocyte alteration caused by the emergence of an abnormal milieu in a drained tissue, and conceivably involving mast cell products. Whatever the case, the retention on the outer wall of the subcapsular sinus, instead of on its inner wall, would prevent any hindering of the usual activity of the latter wall.

Atrophy of lymph node compartments lacking lymph-carried lymphocytes in athymic rats.

This paper reports on a form of atrophy of lymph node compartments occurring in mesenteric nodes of athymic nude rats whose perinodal tissue had become unusually rich in mast cells and fibrotic. The subcapsular sinus of an affected compartment was depleted of lymphocytes, while the medullary sinuses were loaded with macrophages. A progressive expansion of medullary sinuses into the medullary cords, the extrafollicular zone, the deep cortex unit and, finally, the folliculo-nodules of the compartment was accompanied by a gradual atrophy of the usual lymphoid cell populations of these nodal components. The occurrence of such a mode of atrophy of a compartment, associated with a lack of lymphocytes in the afferent lymph, agrees with our previous proposal that lymphocytes of the afferent lymph provide the stimuli for the development and maintenance of the structures of the node. The involvement of mast cells and fibrosis in the emergence of the present form of nodal atrophy, which further weakens the immune system of an immunodeficient animal, is considered.

Atrophy of compartments of rat lymph nodes related to an entry of lethally altered lymphocytes.

This paper reports the occurrence of an accumulation of lethally altered lymphocytes in the subcapsular sinus of a compartment or compartments of some lymph nodes, an unusual feature best developed in nodes of the mesenteric site in aging athymic animals. Many of these cells are rod-like. In other compartments, similar lymphocytes occurred at various depths in the nodal parenchyma. This was accompanied by the disappearance of a compartment's populations of normal lymphoid cells. The observations reveal that lymphocytes, altered in a tissue, may reach the subcapsular sinus of the draining node compartment and migrate into its parenchyma which then undergoes atrophy. The likely involvement of mast cells is discussed.

Blood vascular network of the rat lymph node: tridimensional studies by light and scanning electron microscopy.

Many aspects of the blood vascular network of the lymph node are unknown, and others need confirmation. We have studied the blood vasculature of rat peripheral lymph nodes by means of carbon perfusion and vascular cast corrosion techniques. At the hilus of the node, an artery gives off arterioles running in medullary cords towards the cortex. Some reach the peripheral cortex directly, branching there into slender cortical vessels. Other arterioles enter the periphery of the deep cortex units, and then head towards the peripheral cortex. Upon reaching it, they curve part way above the center of the deep cortex units and provide slender branches to the overlying peripheral cortex. Dense plexuses of capillaries arise from arterioles in the medullary cords, in the periphery of the deep cortex units, and in the outermost stratum of the extrafollicular zone of the peripheral cortex. In the cortex, the draining high endothelial venules are restricted to the extrafollicular zone and to the periphery of the deep cortex units. At the cortico-medullary junction, these peculiar venules transform into regular medullary venules which form the hilar veins. In contrast, the folliculo-nodules and center of the deep cortex units are little vascularized by a loose capillary network, while no vessels occur in the subsinus layer. These features of the node vascular network are of interest in relation to the node architecture.

Mast cells and fibrosis in compartments of lymph nodes of normal, gnotobiotic, and athymic rats.

Reports vary on the amount and distribution of mast cells in lymph nodes. We analysed the mast-cell population in compartments of nodes of diverse sites, from euthymic and athymic animals of various ages. Nodal mast cells were few in young animals, occurring mostly in medullary sinuses. Aging is often accompanied by a moderate increase of nodal mast cells. In compartments of a few nodes of some aged athymic and euthymic animals, the mast cells were greatly increased in the extrafollicular zone overlying medulla directly. In certain cases, this great increase was accompanied by pronounced mast-cell degranulation and by fibrosis in the mast cell-rich extrafollicular zone. It is suggested that the mast cells of medullary sinuses relate to non-immunological events, while those of the lymphoid parenchyma relate to elements that can induce humoral immune responses or are somehow involved in nodal processes of such responses. It is further suggested that an occasional emergence, with aging, of a deficiency of particular humoral immune responses may induce an excessive increase of cortical mast cells, and that activities of the resulting dense mast-cell population contribute to the onset of fibrosis.

Formation of morphologically unusual features, associated with immunodeficiencies, in lymph nodes of gnotobiotic rats exposed to a conventional milieu.

Athymic animals are characterized by unusual features in lymph nodes, which are indicative of immunodeficiencies. These features include hypertrophy of follicles, atrophy of the peripheral cortex above the center of deep cortex units, and the formation of lymphocyte clusters at the periphery of these units as well as of compartment replicas in the capsules. Such features were recently observed in some nodes of a minority of aged euthymic animals and we concluded that they probably also reflected immunodeficiencies, since immunodeficiencies may emerge in aging euthymic animals. In an attempt to validate this conclusion, we exposed one-year-old gnotobiotic animals to a conventional milieu, thereby presumably rendering these euthymic animals somewhat immunodeficient, and checked their nodes for unusual features. Nodal unusual features, similar to those encountered in nodes of athymic and of some aged euthymic animals, arose rapidly and the ex-gnotobiotic animals either manifested signs of infections or died. These findings support our previous conclusion that the arising of such features reflects a progressive emergence of a certain state of immunodeficiency with aging.

Morphological anomalies associated with immunodeficiencies in the lymph nodes of aging mice.

Immunodeficiencies can emerge in aging normal animals. We wondered whether the morphological taking place in nodes of normal but aging animals comprise anomalies associated with immunodeficiencies. We analyzed the nodes of various anatomical sites in 20 normal mice, aged 12 or 24 months. The same morphological anomalies described recently in nodes of young athymic animals and reflecting immunodeficiencies were found to develop in present mice, though varying in extent with age, individuals, anatomical site of a node, and compartments of a given node. Some anomalies reflect deficiencies of cellular immune responses; others reflect deficiencies of humoral immune responses. Practicality restricts the variety of antigens which can be tested to recognize the possible emergence of immunodeficiencies in an aging animal. A morphological analysis of its nodes can thus indicate if immunodeficiencies occur with respect to a fraction of the challenging antigens.

The formation of "compartment replicas" in the lymph nodes of athymic animals.

This communication describes a new anomaly that can affect the capsule of lymph nodes of athymic animals. Lymphocytes infiltrate a segment of the capsule above the variably atrophied peripheral cortex overlying the center of the deep-cortex unit of a node compartment. Lymphocytes thereafter form a capsular mass. The developing mass of lymphocytes is invaded by outgrowths of the node's subcapsular sinus while it fuses with the parenchyma of the related node compartment. Eventually, this new nodal element acquires structures resembling those of nodes and becomes a more or less exact replica of the original node compartment. Replicas stem from node compartments that are overchallenged by uncontrolled antigens. Aspects of the formation of replicas are explained by recent findings on events occurring in nodes of athymic animals and on the pattern of antigen distribution in the subcapsular sinus of a node. It is concluded that the formation of a compartment replica constitutes a mechanism allowing the organism to compensate somewhat for the partial atrophy or deficiency of a node compartment.

The stroma of the thymus of the rat: morphology and antigen diffusion, a reconsideration.

This work reconsiders aspects of the morphology of the capsule, of the blood vasculature, of the distribution of reticular fibers, and of the diffusion of intramediastinally injected antigens in the stroma of the thymus of the rat. This was done by an analysis of standard sections of normal thymuses, of sections of thymuses perfused with colloidal carbon, of silver-impregnated sections, and of sections of thymuses of rats injected intramediastinally with a fluorescent antigen or intravenously with Trypan blue, and by electron microscopy of the thymic capsule. The capsule consisted of two layers: an outer layer covering the entire periphery of a thymic lobe, and an inner layer which outlined the entire convoluted peripheral cortex of a lobe. Cortical vessels entered the capsule and septa in which they formed a capillary network. These capsular capillaries were fenestrated and leukocytes were often present near them. Adipocytes were also seen near these vessels in some areas of the capsule, and often at the bases of septa and trabeculae. Furthermore, much of the medulla had a dense network of coarse reticular fibers, whereas the remainder of the medulla and the cortex contained a loose network of fine fibers stretching out from the capsule, septa, and trabeculae. Intramediastinally injected fluorescent antigens were observed to spread in the capsule and septa and to diffuse in the fiber networks stretched across the cortex and the medulla. Fluorescence also highlighted cortical reticular cells but not the thymocytes. Intravenously injected Trypan blue stained the capsule, the septa, the cortical reticular cells, and the autofluorescent cells outlining the corticomedullary junction of each lobule. The unusual penetration of capillaries from the thymic parenchyma into the thymic capsule suggested that the capsular capillaries participate in peculiar thymic events, such as the recruitment of blood stem-cells. It is concluded that small amounts of blood antigens normally exude from capsular capillaries and diffuse into the fibers extending from the capsule across the cortex. The phenomenon would be increased under conditions causing thymic involution. An explanation is proposed to account for the development of involution which involves the exudation of antigens from the capsular capillaries. A comparable mechanism could also account for the development of a particular experimental immune tolerance.