This study describes the conditions for the long-term culture of the renal hematopoietic tissue of the rainbow trout, Oncorhynchus mykiss, the characterization of the stromal cells, and their relationships with myelopoietic cells. The... more
This study describes the conditions for the long-term culture of the renal hematopoietic tissue of the rainbow trout, Oncorhynchus mykiss, the characterization of the stromal cells, and their relationships with myelopoietic cells. The long-term cultures consisted of stromal cells, which supported active myelopoiesis. Stromal cells were analyzed by enzyme-cytochemical techniques and electron microscopy. Major stromal cell types in long-term cultures consisted of fibroblastic reticular cells and epithelioid cells. Myelopoietic cells differentiated in close association with the fibroblastic reticular cells, and mature granulocytes were released into the culture medium.
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The aroA gene of Aeromonas hydrophila SO2/2, encoding 5-enolpyruvylshikimate 3-phosphate synthase, was cloned by complementation of the aroA mutation in Escherichia coli K-12 strain AB2829, and the nucleotide sequence was determined. The... more
The aroA gene of Aeromonas hydrophila SO2/2, encoding 5-enolpyruvylshikimate 3-phosphate synthase, was cloned by complementation of the aroA mutation in Escherichia coli K-12 strain AB2829, and the nucleotide sequence was determined. The nucleotide sequence of the A. hydrophila aroA gene encoded a protein of 440 amino acids which showed a high degree of homology to other bacterial AroA proteins. To obtain an effective attenuated live vaccine against A. hydrophila infections in fish, the aroA gene was inactivated by the insertion of a DNA fragment containing a kanamycin resistance determinant and reintroduced by allelic exchange into the chromosome of A. hydrophila AG2 by means of the suicide vector pSUP202. The A. hydrophila mutant AG2 aroA::Ka(r) was highly attenuated when inoculated intraperitoneally into a rainbow trout, with a 50% lethal dose of >2 x 10(8) CFU. The mutants were not recoverable from the internal organs after 48 h postinoculation. Immunohistochemical studies de...
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Aeromonas hydrophila is a Gram-negative rod, motile, non-sporing and facultative anaerobic bacterium widely distributed in nature. It is an opportunistic pathogen of poikilothermic and homeothermic animals, including humans. As with other... more
Aeromonas hydrophila is a Gram-negative rod, motile, non-sporing and facultative anaerobic bacterium widely distributed in nature. It is an opportunistic pathogen of poikilothermic and homeothermic animals, including humans. As with other motile aeromonads, A. hydrophila may adhere to solid surfaces and form biofilms in aquatic environments. Typically, a biofilm is composed of microbial cells, extracellular polymeric substances (EPS) secreted by the cells, metabolic products, and a variety of colloidal and dissolved substances. Bacteria growing in biofilms show specific structural and physiological properties and, remarkably, increased resistance to antimicrobial drugs. All these features have been related to a higher persistence and severity of several bacterial diseases affecting human and animal health, including that of aquacultured fish. The particular features of the biofilms formed by virulent bacterial strains may facilitate a higher waterborne dispersal capacity, a feature ...
Owing to the lack of data about thymic non-lymphoid cells in fish we decided to perform a histochemical characterization of these cells in order to ascertain their relationships to other thymic components. In the present study we analyze... more
Owing to the lack of data about thymic non-lymphoid cells in fish we decided to perform a histochemical characterization of these cells in order to ascertain their relationships to other thymic components. In the present study we analyze the enzyme-histochemical patterns for acid phosphatase, alkaline phosphatase, non-specific sigma-naphthyl acetate esterase and 5' nucleotidase activities, as well as the presence of keratin demonstrated by immunoperoxidase staining, in the non-lymphoid cell populations of the thymus of the rainbow trout, Salmo gairdneri. According to their location in the organ, morphology and histochemical reactivities, we were able to define seven different subpopulations of keratin-positive epithelial cells: 1) Epithelial cells limiting with the capsular and septal connective tissues; 2) Subcapsular epithelial cells; 3) Stellate epithelial cells of the inner thymic zone; 4) Large, ovoid epithelial cells of the inner thymic zone; 5) Acidophilic epithelial cell...
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This study reports the ultrastructure of subpopulations of epithelial cells of the thymic parenchyma during the post-hatching development of the rainbow trout, Salmo gairdner, kept at 14 degrees C. At hatching, the thymus contained a... more
This study reports the ultrastructure of subpopulations of epithelial cells of the thymic parenchyma during the post-hatching development of the rainbow trout, Salmo gairdner, kept at 14 degrees C. At hatching, the thymus contained a small number of medium and large thymocytes interspersed among three different types of epithelial cells: (1) epithelial cells adjacent to the connective tissue capsule; (2) ramified dark epithelial cells with electron-dense cytoplasm; and (3) pale electron-lucent epithelial cells displaying secretory-like features. All these cells types were anchored to one another by desmosomes and had apparently differentiated from the pharyngeal epithelium. At 4 days after hatching, the thymus enlarged, and numerous gaps occurred between the cell processes of contiguous epithelial cells adjacent to the capsular connective tissue. In 21-day-old trout, thymic trabeculae developed carrying blood vessels, and a subcapsular zone became evident containing lymphoblasts and large subcapsular epithelial cells. In 30-day-old trout, an outer thymic zone developed consisting of spindle-shaped epithelial cells which formed a dense network. At this stage, scattered cystic cells, which apparently differentiated from the pale epithelial cells, were present.
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Viral hemorrhagic septicaemia virus (VHSV) infected the hematopoietic stromal cells (7,8) derived from pronephritic tissue of the rainbow trout, Oncorhynchuss mykiss, W., at their ninth passage in vitro. Viral infection resulted in the... more
Viral hemorrhagic septicaemia virus (VHSV) infected the hematopoietic stromal cells (7,8) derived from pronephritic tissue of the rainbow trout, Oncorhynchuss mykiss, W., at their ninth passage in vitro. Viral infection resulted in the development of lytic cytopathic effects on confluent in vitro tridimensional network stromal cell cultures. Replication of VHSV in the stromal cell cultures was demonstrated by the increase in infectivity by epithelioma papulosum cyprini (EPC) cell culture assays and by the increase of the nucleoprotein antigen of VHSV by ELISA. By using anti-VHSV monoclonal antibodies (MAbs), flow cytometry studies demonstrated that only the infected stromal cells contained cytoplasmic viral antigens. The lytic infection of trout hematopoietic stromal cells in vitro could be relevant to the hemorrhagic pathology seen in the kidney of fish infected with VHSV.
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Aeromonas hydrophila is a pathogen that causes disease in a wide range of homeothermic and poikilothermic hosts due to its multifactorial virulence. We have previously described the characterisation and use of an auxotrophic aroA mutant... more
Aeromonas hydrophila is a pathogen that causes disease in a wide range of homeothermic and poikilothermic hosts due to its multifactorial virulence. We have previously described the characterisation and use of an auxotrophic aroA mutant of the A. hydrophila AG2 strain as a live attenuated vaccine against A. hydrophila infections in rainbow trout (Oncorhynchus mykiss). In this study we report the expression of extracellular proteolytic activities and of quorum-sensing molecules by this mutant grown under different culture conditions, and in vaccine inocula. The aroA strain expresses extracellular proteases efficiently during in vitro growth and this ability is retained in vaccine inocula that were prepared by washing the bacterial cultures and resuspending the cells in phosphate-buffered saline. Since proteases are considered to be major bacterial antigens, the expression of these enzymes in the live attenuated vaccine may contribute to the superior protection afforded by these kind of vaccines. On the other hand, the production of serine- and metalloprotease activities in A. hydrophila has been described as controlled in a cell density-dependent fashion, through a mechanism known as quorum sensing. A microtiter method was developed that allowed correlation of the production of quorum-sensing molecules and of proteases produced by the aroA strain during in vitro growth and in the vaccine inocula. The production of both products was related to the type of culture medium and conditions used to grow the aroA mutant, whereas there was no correlation between the concentration of acyl homoserine lactones and protease production.
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A morphometrical evaluation was made of the seasonal changes affecting the numbers of lymphocytes in the thymus, spleen and pronephros of wild brown trout, Salmo trutta, while the size of the thymus and the three thymic zones were also... more
A morphometrical evaluation was made of the seasonal changes affecting the numbers of lymphocytes in the thymus, spleen and pronephros of wild brown trout, Salmo trutta, while the size of the thymus and the three thymic zones were also determined. Results reveal statistically significant changes throughout the year in the number of lymphocytes in the lymphoid organs studied. The spleen and pronephros have similar annual patterns of lymphocyte distribution with high numbers in two seasons, spring and autumn, and two periods of lymphoid involution in winter and summer. The highest numbers of thymocytes occur in trout caught in May and August, and the lowest in winter. In addition to normal lymphocytes, degenerated lymphoid cells that show pale cytoplasm devoid of cell organelles, also occurred in all the lymphoid organs. A negative correlation exists between the numbers of normal lymphocytes and that of degenerated lymphoid cells. The thymic size, as well as that of the subcapsular, inner and outer thymic zones, undergo very significant changes over the year. We discuss the relevance of cell proliferation, cell migration and in situ cell death for the circannual variations observed in the cell content of trout lymphoid organs, together with the possible causes.
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ABSTRACT A comparative analysis was made of the thymic cytoarchitecture of healthy and Saprolegnia-infected wild brown trout. In Saprolegnia-infected fish, even when the thymus was not invaded by fungal hyphae, the thymic architecture was... more
ABSTRACT A comparative analysis was made of the thymic cytoarchitecture of healthy and Saprolegnia-infected wild brown trout. In Saprolegnia-infected fish, even when the thymus was not invaded by fungal hyphae, the thymic architecture was lost without signs of regionalization. Intercellular oedema, some thymocytes, hypertrophic and degenerated epithelial cells and increased phagocytic activity were also observed. In thymi invaded by the fungus, hyphae occurred isolated or in close apposition to epithelial cells, and the thymic disorganization was more noticeable. Most thymocytes were pyknotic and both epithelial cells and macrophages contained engulfed dead cells. However, no inflammatory response to the fungal invasion was observed. These results confirm the immunodepressed condition of Saprolegnia-infected wild brown trout previously observed in peripheral lymphoid organs. We discuss the secretion of fungal products and/or the high levels of corticosteroids observed in these fish as possible origins of the condition.
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The number of circulating leukocytes and the structure of splenic and renal lymphoid tissue were comparatively analysed in healthy and Saprolegnia-infected wild brown trout, Salmo trutta fario. Sick trout showed lymphopenia, mainly due to... more
The number of circulating leukocytes and the structure of splenic and renal lymphoid tissue were comparatively analysed in healthy and Saprolegnia-infected wild brown trout, Salmo trutta fario. Sick trout showed lymphopenia, mainly due to decreased numbers of circulating small lymphocytes, and heterophilia. The splenic and renal lymphoid tissue of infected trout exhibited similar changes, consisting of cellular depletion, lymphoid cell degeneration, and vascular alterations with blood vessel enlargement and hypertrophy of sinusoidal endothelial cells. Furthermore, the endothelial cells in the spleen and kidney of the infected trout contained cytoplasmic vesicles filled with material of possible fungal origin. The absence of a reticular sheath was also evident in the splenic ellipsoids. These results suggest some immunodepression in Saprolegnia-infected trout which might favour the course of the disease.
Research Interests: Zoology, Fish Diseases, Spleen, Brown trout, Kidney, and 8 moreFemale, Animals, Male, Oomycetes, Salmo trutta, Trout, Veterinary Sciences, and Lymphoid Tissue
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Research Interests: Physiology, Zoology, Morphology, Animals, Intestines, and 4 moreEsophagus, Urodela, Stomach, and Lymphoid Tissue
ABSTRACT The ultrastructure of hemopoietic bone marrow of the Spanish lizard, Lacerta hispanica, has been studied for the first time. The organ consists of a stroma formed by venous sinuses and reticular cells. Erythropoiesis takes place... more
ABSTRACT The ultrastructure of hemopoietic bone marrow of the Spanish lizard, Lacerta hispanica, has been studied for the first time. The organ consists of a stroma formed by venous sinuses and reticular cells. Erythropoiesis takes place in the lumen of blood vessels, while granulopoiesis is extravascular. Pluripotent stem cells are structurally differentiated into erythrocytes and granulocytes. Two types of granulocytes, heterophils and acidophils, have been found, and a third granular cell type is tentatively identified as granular leukocyte. Remarkably, plasmacytopoiesis occurs in the bone marrow of Lacerta hispanica. The possible functional significance of these results is discussed with emphasis on their importance for the reptilian immune system.
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Triton X-100, sodium dodecylsulphate, sodium cholate, and beta-octylglucoside increase the size of sonicated, but not of unsonicated, phospholipid vesicles above the Tc gel to liquid-crystalline transition temperature.... more
Triton X-100, sodium dodecylsulphate, sodium cholate, and beta-octylglucoside increase the size of sonicated, but not of unsonicated, phospholipid vesicles above the Tc gel to liquid-crystalline transition temperature. Lysophosphatidylcholine or glyceryl monooleate do not increase liposome size under these circumstances. The observed phenomenon of vesicle growth is virtually unaffected by phospholipid composition, surface potential, calcium ions, EDTA, or albumin. The presence of cholesterol makes the vesicles more susceptible to detergent-induced growth. The presence of detergents, even at concentrations much lower than required to observe any increase in vesicle size, favors the liberation of vesicle contents. These results may have implications concerning membrane fusion as well as the reconstitution of membrane proteins in the presence of detergents.
Research Interests: Microbiology, Electron Microscopy, Kinetics, Medical Microbiology, Membrane Biology, and 13 moreCalcium, Cell fusion, Cholesterol, Ultrasonics, Membrane Fusion, Liposomes, Surface Active Agents, Molecular Conformation, Potential Energy Surface, Edetic Acid, Membrane Fluidity, Biochemistry and cell biology, and Liquid Crystalline
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ABSTRACT The role of rainbow trout (Oncorhynchus mykiss, Walbaum) gill cells in uptake of the salmonid pathogen Renibacterium salmoninarum was examined using in vitro, whole animal and isolated perfused head models. Uptake of the pathogen... more
ABSTRACT The role of rainbow trout (Oncorhynchus mykiss, Walbaum) gill cells in uptake of the salmonid pathogen Renibacterium salmoninarum was examined using in vitro, whole animal and isolated perfused head models. Uptake of the pathogen was observed only when dissected gill tissue was exposed to the live pathogen. In contrast, live cells of Yersinia ruckeri interacted with the gill epithelia in the isolated perfused head model, confirming the findings of previous studies with whole animal and in vitro systems. These results are discussed in relation to the role of gill tissue in bacterial kidney disease (BKD) pathogenesis and in antigen trapping.
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Gill explants were maintained for one week in culture, and infected withR. salmoninarum. The explants were collected at two weeks post-infection and processed for ultrastructural examination. A marked tissue reaction occurred in the... more
Gill explants were maintained for one week in culture, and infected withR. salmoninarum. The explants were collected at two weeks post-infection and processed for ultrastructural examination. A marked tissue reaction occurred in the infected explants in contrast with the control (non-infected) cultures, partially resembling an inflammatory response, and involving a marked increase in the number of eosinophilic granular cells. Several
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The Aeromonas hydrophila aroA is an attenuated strain that has been assessed as a live vaccine in rainbow trout, Oncorhynchus mykiss. In this study the effects of different culture media used to grow the strain on its survival after in... more
The Aeromonas hydrophila aroA is an attenuated strain that has been assessed as a live vaccine in rainbow trout, Oncorhynchus mykiss. In this study the effects of different culture media used to grow the strain on its survival after in vitro exposure to rainbow trout serum, and on its immunogenicity in rainbow trout were compared. Four culture media were tested: Luria broth (LB), Luria broth with 0.25% glucose, trypticase soy broth (TSB), and brain-heart infusion broth (BHIB). Bacteria grown in culture media with glucose (TSB, BHIB and LB with 0.25% glucose) showed reduced complement consumption and a lower serum susceptibility. O. mykiss vaccinated with inocula prepared with BHIB- and LB-grown aroA cells resuspended in phosphate-buffered saline (PBS) showed higher and longer-lasting serum agglutinating antibody titres than those vaccinated with TSB-grown bacteria. Thus, a direct relationship between serum resistance and immunogenicity could not be established, but BHIB and LB culture media were the most effective in increasing the immunogenicity of the A. hydrophila aroA vaccine.
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This study describes the culture conditions and the phenotypic features of different types of splenic cultures established from explants. Using the same culture technique it was possible to grow splenic explants from which monolayers of... more
This study describes the culture conditions and the phenotypic features of different types of splenic cultures established from explants. Using the same culture technique it was possible to grow splenic explants from which monolayers of reticular origin, long-term haematopoietic cultures, and subcultures were obtained. The cultures were characterised by light and electron microscopy, cytochemical and immuno-cytochemical analyses, phagocytic activity and
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An auxotrophic aroA mutant of the Aeromonas hydrophila AG2 strain is a live attenuated vaccine against A. hydrophila infection in rainbow trout (Oncorhynchus mykiss). The protection conferred by the live attenuated vaccine against A.... more
An auxotrophic aroA mutant of the Aeromonas hydrophila AG2 strain is a live attenuated vaccine against A. hydrophila infection in rainbow trout (Oncorhynchus mykiss). The protection conferred by the live attenuated vaccine against A. salmonicida strains is reported here, and several parameters of the specific and non-specific immune response in vaccinated trout were characterised. Vaccination with a dose of 10(7)cells/fish of the aroA mutant elicited significant protection against the Hooke and DK30 strains of A. salmonicida (relative percent survival RPS >60%). This cross-protection correlated moderately with the activation of the humoral and cellular specific immune responses, which show cross-reactivity against antigens shared by the two bacterial species, and a moderate increase in the lysozyme and antiprotease activities in the serum of vaccinated trout.
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This study describes angiogenic processes taking place in the in vitro micro-environment of a trout pronephric stroma cell line (TPS) under specific culture conditions, in which fetal calf serum, horse serum and... more
This study describes angiogenic processes taking place in the in vitro micro-environment of a trout pronephric stroma cell line (TPS) under specific culture conditions, in which fetal calf serum, horse serum and hydrocortisone-sodium-21-hemisuccinate were used as supplements to the culture medium. When TPS cultures were kept in the same flask, i.e. without passages, for longer than 7 months, epithelioid cells differentiated into endothelial cells. Early stages of such differentiation were characterised by the presence of intracellular tubular vacuoles in clusters of neighbouring epithelioid cells. Subsequently, the endothelial cells reorganised and gave rise to microvascular structures, which branched over and into the TPS multilayers. The lining cells of the microvasculature showed typical characteristics of endothelial cells, such as ovoid or cubical shape, bundles of microfilaments and microtubules, and particularly numerous small vesicles at the apical pole, some of them fused to the plasma membrane. Similar angiogenic processes were also observed in long-term haemopoietic co-cultures formed by the TPS cell line and trout pronephric cell suspensions. Developing haemopoietic cells were observed at the basal pole of the vessels, and in the vascular lumen, where some immature cells appeared in close contact with the endothelium. These results indicate that the TPS cell line contains endothelial cell precursors, which are able to differentiate under certain culture conditions.
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A new stromal adherent cell line, called trout pronephric stroma (TPS), was initiated from a long-term pronephric culture of an adult rainbow trout, and subcultured 104 times over a period of 4 years. This study describes the culture... more
A new stromal adherent cell line, called trout pronephric stroma (TPS), was initiated from a long-term pronephric culture of an adult rainbow trout, and subcultured 104 times over a period of 4 years. This study describes the culture conditions and characterization by enzyme-cytochemistry, electron microscopy, isoenzyme profile, cytogenetic techniques, and viral susceptibility. The cell types in TPS cultures consisted of
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To rapidly and efficiently establish haemopoietic cultures from rainbow trout,O. mykiss, a method to elicit haemopoiesis on a trout pronephric stromal cell line (TPS) was developed. The system used cocultures of pronephric cell... more
To rapidly and efficiently establish haemopoietic cultures from rainbow trout,O. mykiss, a method to elicit haemopoiesis on a trout pronephric stromal cell line (TPS) was developed. The system used cocultures of pronephric cell suspensions with subconfluent cultures of TPS cells. The initiation of haemopoiesis in these cocultures occurred rapidly, and the duration of sustained haemopoiesis, after a single inoculum of
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The permeability of the pharyngeal epithelium was analysed, using ruthenium red and the entry of water-borne ferritin, into the thymus and gills of developing rainbow trout. In post-hatching trout, staining with ruthenium red did not... more
The permeability of the pharyngeal epithelium was analysed, using ruthenium red and the entry of water-borne ferritin, into the thymus and gills of developing rainbow trout. In post-hatching trout, staining with ruthenium red did not reveal any gaps between the cells of the pharyngeal epithelium covering the thymus, the intercellular spaces being sealed by tight junctions. Developing trout (from hatching
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Research Interests: Zoology, Macrophages, Innate immunity, Toll Like Receptors, Cell line, and 12 moreReactive Oxygen Species, Nitric oxide, Animals, Fisheries Sciences, Lipopolysaccharides, Phagocytosis, Veterinary Sciences, Aeromonas Hydrophila, Oncorhynchus Mykiss, Levamisole, Gene Expression Regulation, and Gene expression profiling
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... Histopathological changes in several tissues of fish naturally and experimentally infected withReni-bacterium salmoninarum have been described (Wood & Yasutake 1956, Young & Chapman 1978, Bruno 1986, Sami et al. 1992, Evenden... more
... Histopathological changes in several tissues of fish naturally and experimentally infected withReni-bacterium salmoninarum have been described (Wood & Yasutake 1956, Young & Chapman 1978, Bruno 1986, Sami et al. 1992, Evenden et al. 1993). Few de-...
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In a previous report we described the existence of two structural and antigenic variants of light chains in serum trout Igs, defined by mAbs 2H9 and 2A1. In this report, we analyse the cell distribution, the "in... more
In a previous report we described the existence of two structural and antigenic variants of light chains in serum trout Igs, defined by mAbs 2H9 and 2A1. In this report, we analyse the cell distribution, the "in vitro" kinetics of production, and the ontogenic appearance of these two variants. Both variants were already present at day 8 before hatching and their kinetics of appearance correlated with that of total Ig, showing a peak around hatching. These mAbs stained two distinct lymphoid cell populations in the pronephros, mesonephros, spleen, and peripheral blood. Both mAbs reacted with approximately 1% of thymocytes. The ratio between these two cell populations (2A1+/2H9+) was about 2/1 in the lymphoid organs analysed. No differences were found between the profiles of the "in vitro" production of these variants in pronephric cell cultures stimulated with LPS. In these cultures, the 2A1 and 2H9 Igs together accounted for 10-70% of the total trout Ig produced, suggesting the existence of at least one additional L chain variant.
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Thymus glands of two salmonid species, Salmo trutta and Oncorhynchus mykiss, caught monthly throughout the year, were examined by light and transmission electron microscopy. Erythropoietic foci, consisting of both developing and mature... more
Thymus glands of two salmonid species, Salmo trutta and Oncorhynchus mykiss, caught monthly throughout the year, were examined by light and transmission electron microscopy. Erythropoietic foci, consisting of both developing and mature erythroid cells, occurred in the subcapsular, inner, and outer thymic zones from April to November. We discuss the possible physiological significance of this seasonal erythropoietic activity, together with the role played by the thymic cell microenvironments and endocrine factors.
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ABSTRACT We have analyzed the ultrastructure of normal and immunized jugular bodies in adult frogs to point out new information on these primitive lymphoid organs. They consist of cell cords arranged in a sinusoidal network. The cell... more
ABSTRACT We have analyzed the ultrastructure of normal and immunized jugular bodies in adult frogs to point out new information on these primitive lymphoid organs. They consist of cell cords arranged in a sinusoidal network. The cell cords are formed by a stroma of reticular cells with intercellular spaces filled by free cells. The free cells consist of small and medium lymphocytes, lymphoblasts and developing and mature plasma cells, and in lesser amounts of free macrophages and granulocytes. After immunizing with SRBC there is an increase in plasma cells at 7 days correlating with the increase in PFC response and in serum antibody titers. However, at 14 days we have observed a diminution in PFC response in the jugular body. At this time, there occurs an increase in the number of eosinophils accompanied by degranulation. The results are discussed from a functional and phylogenetic view.
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The present study analyzes ultrastructurally the non-lymphoid elements occurring in the diffuse deep cortex and medulla of the postnatal developing lymph nodes of the rat. The supporting meshwork of the organ consist of connective... more
The present study analyzes ultrastructurally the non-lymphoid elements occurring in the diffuse deep cortex and medulla of the postnatal developing lymph nodes of the rat. The supporting meshwork of the organ consist of connective reticulum cells joined together themselves by intercellular junctions. Two morphologically different macrophage cell types can be described in the developing lymph nodes. The first one consists of free-migrating macrophages homing the cortical parenchyma, apparently related with phagocytosis of necrotic cells. In contrast, macrophages located in the lumen of the medullary lymphatic sinuses are anchored to connective reticulum cell processes. They form lymphocyte-macrophage clusters and are involved also in erythrophagocytosis phenomena. Postcapillary venules lined by a low cuboidal endothelium, which allow lymphocyte diapedesis, appear in the diffuse deep cortex of the neonatal lymph nodes. The possible significance of all these cellular elements is discussed with respect to their role as constituents of the local lymph node microenvironments.