Interleukin 1beta and GABA-transaminase activity in rat thymus

The occurrence and distribution of 4-aminobutyrate:2 oxoglutarate transaminase (GABA-t) activity were examined in the rat thymus of normal and immunostimulated rats using biochemical and histoenzymatical methods. Specific GABA-t reactivity was confined primarily to the arteries and, to a lesser extent, to the veins. Only a few activities could be observed in association with the subcapsular and medullar part of the parenchyma and nerve fibers. GABA-t was considered a linking enzyme between the immune and the nervous system and it was studied with the aim of analyzing the relationships between these two systems. Our findings indicate that the GABA-t activity in the thymus is specifically located in the wall of the blood vessels. Moreover, our results demonstrate the presence of a GABA-t activity in the peripheral blood vessels. Treatment with interleukin 1beta induces an increase of protein content of the amounts of GABA-t biochemically assayed and of the levels of histoenzymatically stained GABA-t. Furthermore, staining of the different structures of the thymus in treated or untreated rats shows that the significant modifications concern the parenchyma, the structures resembling nerve fibers and finally, the whole thymus. On the contrary, the highest activity of the GABA-t is located in the walls of arteries, veins and lymphatic vessels.

[Recovery processes in the cerebral cortex, myocardium and thymus of rats with experimental atherosclerosis exposed to low-frequency electromagnetic fields on the head]

Studies of animals with experimental sclerosis has shown that a course of 10 procedures of alternative magnetic field (AMF) (50 Hz, 30 mT, 3 min daily) promotes partial recovery of the lipid spectrum and corrects vasomotor-metabolic disturbances in the cerebral cortex, myocardium and thymus caused by atherosclerosis. Combination of AMF with constant magnetic field in the same regime and location does not produce a hypolipidemic effect in atherosclerotic animals and this, in combination with increased vascular permeability may aggravate the condition. Activated microcirculation, antioxidant and antiproteinase effects in activation of biosynthetic processes in the cerebral cortex reflect inhibition in the CNS in this combined effect and create conditions for a hypotensive effect.

Thymic vasculature: organizer of the medullary epithelial compartment?

The epithelial component of the thymic environment is organized into discrete cortical and medullary compartments that mediate different aspects of thymocyte differentiation. The processes controlling the growth and organization of these epithelial compartments are poorly defined. In this study we have used a novel approach to define the three-dimensional organization of thymic epithelial (TE) compartments to demonstrate that the organization of the medullary TE compartment is very complex. A spatial relationship of medullary thymic epithelium with vascular elements of the thymus was demonstrated by simultaneous immunohistochemical labeling of vascular elements and medullary TE. Medullary TE was often arranged as perivascular cuffs surrounding intermediate-sized vessels, but was not associated with either the capillary network or large centrally located vessels. Similar analyses of RAG-2(-/-) thymi revealed a striking physical association of medullary TE with vascular elements. Ultrastructural analysis of the RAG-2(-/-) thymus indicated a preferential association of focal accumulations of medullary TE with post-capillary venules. These data suggest that discrete segments of the thymic vasculature provide cues that act in concert with thymocyte-derived stimuli to effect normal development of the thymic environment.

Anomalous thymic branch of the right common carotid artery

A previously unreported anomalous thymic artery that branched from the anterior aspect of right common carotid artery approximately 1 cm above bifurcation of the brachiocephalic artery was found during routine dissection. It traveled inferiorly through a plexus of inferior thyroid veins for 6 cm in front of the brachiocephalic artery and crossed the anterior surface of the trachea where it divided into two branches that supplied the right and left lobes of the thymus. The development and blood supply of the thymus and their clinical anatomy are reviewed.

Surgical technique for vascularized thymus transplantation in mice

Traditionally, mouse nonvascularized thymus implants have been used to investigate various aspects of thymus function. However, these grafts are easily damaged by ischemia and fail to reproduce the normal anatomy of the thymus. In addition, the function of these grafts has not been fully examined. We have recently developed a vascularized thymus transplant model in mice. The donor operation consists of isolating the right lobe of the thymus and creating a single vascular pathway. In the recipient surgery, end-to-side anastomoses between donor brachycephalic artery and recipient right common carotid artery, and between donor superior caval vein and recipient right external jugular vein, were performed. We performed 10 consecutive isografts in BALB/c mice with a success rate of 90%. The thymus grafts had a normal histology and function. This study illustrates that it is technically possible to transplant a mouse vascular thymus graft. This model has several advantages that make it a useful tool to study many aspects of thymus function. We plan to use this model further to study the potential for induction of tolerance by thymus grafts.

P2 receptors in the thymus: expression of P2X and P2Y receptors in adult rats, an immunohistochemical and in situ hybridisation study

The expression of the seven P2X receptor subtypes and of two P2Y receptors was examined immunohistochemically and by in situ hybridisation in thymi of adult male rats. P2X4, P2Y2 and 4 receptor mRNA colocalisation studies combining in situ hybridisation and immunohistochemistry were also carried out. P2X and P2Y receptors were found on thymocytes. P2X receptors were also abundant in cells of the thymic microenvironment, involved in control of T-cell maturation in vivo. We are the first to describe the expression of P2X4 receptors on thymocytes and confirm the finding of P2X1 and P2Y2 receptors on subpopulations of lymphocytes. P2X1,2,3,4 and 5 receptors were present in blood vessels of the thymus. P2X1,2 and 4 receptors were detected in vascular smooth muscle, while P2X3 receptors appeared to be associated with endothelial cells; some small arteries were positive for P2X5, possibly labelling vascular smooth muscle or fibroblasts in the adventitia. P2X2,3,6 and 7 receptors were found on thymic epithelial cells. P2X2 and 3 receptors were abundant on medullary epithelial cells, whilst P2X6 receptors were prominent in Hassall's corpuscles. P2X2 receptors were found on subcapsular and perivascular epithelial cells. P2X2,6 and 7 receptors were detected in epithelial cells along the thymic septa. Expression of P2X receptors was also investigated by Western blotting of crude thymic tissue extracts under reducing conditions. All seven P2X receptor subtypes were found to be dimers of approximately 70 kDa and 140 kDa molecular weight. ATP-mediated apoptosis and cell proliferation of thymocytes are discussed.

Acetylcholinesterase activity in rat thymus after immunostimulation with interleukin beta

The occurrence and distribution of Acetylcholinesterase (AChE) activity were examined in the thymus of normal and immuno-stimulated adult and aged rats using biochemical and enzymehistochemical methods. Specific AChE reactivity was found primarily in the arteries and, to a lesser extent, in the veins. Only a small amount of activity could be observed in association with the subcapsular and medullary part of the parenchyma and nerve fibers. Our findings indicate that AChE activity in the rat thymus increases after treatment with interleukin beta. In fact treatment with interleukin beta induces an increase of protein content, of the amounts of AChE biochemically assayed and at the levels of AChE histoenzymatically stained. Furthermore, staining of the different structures of the thymus in treated or untreated rats shows that the significant modifications concern the parenchyma, the structures resembling nerve fibers and the whole thymus, while only small changes are observed in AChE activity located in the walls of arteries, veins and lymphatic vessels.

Thymic transplantation in miniature swine. II. Induction of tolerance by transplantation of composite thymokidneys to thymectomized recipients

Previous studies in our laboratory have demonstrated that the presence of the thymus is essential for rapid and stable tolerance induction in allotransplant models. We now report an attempt to induce tolerance to kidney allografts by transplanting donor thymic grafts simultaneously with the kidney in thymectomized recipients. Recipients were thymectomized 3 wk before receiving an organ and/or tissues from a class I-mismatched donor. Recipients received 1) a kidney allograft alone, 2) a composite allogeneic thymokidney (kidney with vascularized autologous thymic tissue under its capsule), or 3) separate kidney and thymic grafts from the same donor. All recipients received a 12-day course of cyclosporine. Thymectomized animals receiving a kidney allograft alone or receiving separate thymic and kidney grafts had unstable renal function due to severe rejection with the persistence of anti-donor cytotoxic T cell reactivity. In contrast, recipients of composite thymokidney grafts had stable renal function with no evidence of rejection histologically and donor-specific unresponsiveness. By postoperative day 14, the thymic tissue in the thymokidney contained recipient-type dendritic cells. By postoperative day 60, recipient-type class I positive thymocytes appeared in the thymic medulla, indicating thymopoiesis. T cells were both recipient and donor MHC-restricted. These data demonstrate that the presence of vascularized-donor thymic tissue induces rapid and stable tolerance to class I-disparate kidney allografts in thymectomized recipients. To our knowledge, this is the first evidence of functional vascularized thymic grafts permitting transplantation tolerance to be induced in a large animal model.

Expression of CD146 adhesion molecules (MUC18 or MCAM) in the thymic microenvironment

The thymic microenvironment is thought to play a critical role in T-lymphocyte development, providing signals both via cell surface molecules such as adhesion molecules and soluble molecules. The present investigation is focused on immunoelectron microscopical analysis of distribution patterns of CD146 adhesion molecules (MUC18 or MCAM) in the microenvironment of normal human thymus, using the pre-embedding indirect immunoperoxidase technique. The anti-CD146 monoclonal antibody (mAb) revealed strong membraneous labelling of immature thymocytes at both the light and electron microscopical level. Proliferating thymocytes, most of the epithelial cells, macrophages, endothelial cells and smooth musle cells of small vessels and capillaries showed both membraneous and cytoplasmic labelling with anti-CD146 mAb as was demonstrated by electron microscopy. In contrast, these cells displayed a strong cytoplasmic immunoreactivity at the light microscopical level. The extracellular matrix was also stained with the anti-CD146 mAb. No labelling was observed in interdigitating cells. Interestingly, the CD146 molecule was strongly expressed on apical and lateral membranes of endothelial cells as was demonstrated electron microscopically. This selective CD146 labelling of capillary endothelium mainly localized at the cortico-medullary junction may be manifestations of lymphocyte transmembrane migration and lymphocyte homing. In conclusion, the present study suggests that CD146 is expressed by most elements of the microenvironment of normal human thymus. Therefore, it may be a pan-antigen which is essential for the maintenance of thymic architecture and function.

[Development of thymic postcapillary venules in human prenatal ontogenesis]

Postcapillary venules with high endotheliocytes as paths of lymphocyte migration differentiate in corticomedullary zone of thymus in human 4-5 months old foetuses. Development of postcapillary venules from blood capillaries is closely associated with zonal differentiation of the organ substance and intensification of lymphocyte recirculation in the fetal immune system.

Endoglin (CD 105) expression in human lymphoid organs and placenta

Endoglin (CD 105) is a cell surface antigen widely expressed on vascular endothelium, syncytiotrophoblast, some tissue macrophages, certain culture cells (including early leukemic B-lineage) and some endothelial cell lines. Though its relation to the transforming growth factor-beta (TGF-beta) receptor system is well documented, its function and detailed pattern of expression still remain to be clarified. We examined the differential tissue distribution of endoglin in human lymphoid organs and placenta with several anti-CD 105 monoclonal antibodies (mAbs) using an indirect immunoperoxidase technique, and performed semi-quantitative measurements using an image-analyzing system for comparison. Arterial, venous and capillary endothelia in these organs were reactive with anti-CD 105 mAbs at varying intensities. Interestingly, a distinctly stronger staining pattern was observed in the high endothelial venules (HEVs) which may indicate a special role for endoglin in lymphocyte trafficking. Syncytiotrophoblast expressed endoglin strongly on their apical cell membrane. Extravillous trophoblasts at certain locations selectively expressed endoglin on their cell membranes, suggesting a special role for this surface antigen during trophoblast differentiation.

Malignant mesenchymoma associated with an unusual vasoinvasive metastasis in a dog

A case of a malignant mesenchymoma with an unusual, vasoinvasive, metastatic behavior in a three-year-old, intact female basset hound is presented. Malignant mesenchymomas are rare neoplasms in humans and in dogs. No previous reports of a malignant mesenchymoma with vasoinvasive metastasis in the dog were found in the literature. The constituent neoplasms are discussed in relation to reports in the human and veterinary literature, and a potential etiology for this unique presentation is hypothesized.

Progesterone receptor in chicken bursa of Fabricius and thymus: evidence for expression in B-lymphocytes

In the present work constitutive progesterone receptor (PR) expression in the chicken bursa of Fabricius was detected in the stromal, smooth muscle and follicular medullary cells and smooth muscle cells of blood vessels. PR expression was increased during sexual maturation and after estrogen treatment. Bursal medullary PR-positive cells were further characterized to be B-lymphocytes by flow cytometric analysis. In addition, estrogen induced expression of PR in the bursal FAE-cells (follicle-associated epithelial cells). In the thymus PR was expressed constitutively in the connective tissue cells of the capsule and interfollicular septa, in a few medullary cells and in vascular smooth muscle. The PR-positive medullary cells consisted of epithelial cells, large polygonal cells resembling macrophages and plasma cells. T-lymphocytes were PR-negative. Estrogen up-regulated PR expression in the thymus. Immunoblotting studies revealed that both isoforms of PR, i.e. PR-A and PR-B, were expressed in the bursa of Fabricius and thymus with PR-B dominance. These results suggest that the chicken primary lymphoid organs bursa and thymus are under regulation of estrogen and progesterone. Expression of PR in B-lymphocytes, macrophages and plasma cells in the chicken is documented for the first time and suggests evidence for direct action of progesterone on immune responses.

Lipopolysaccharide induces disseminated endothelial apoptosis requiring ceramide generation

The endotoxic shock syndrome is characterized by systemic inflammation, multiple organ damage, circulatory collapse and death. Systemic release of tumor necrosis factor (TNF)-alpha and other cytokines purportedly mediates this process. However, the primary tissue target remains unidentified. The present studies provide evidence that endotoxic shock results from disseminated endothelial apoptosis. Injection of lipopolysaccharide (LPS), and its putative effector TNF-alpha, into C57BL/6 mice induced apoptosis in endothelium of intestine, lung, fat and thymus after 6 h, preceding nonendothelial tissue damage. LPS or TNF-alpha injection was followed within 1 h by tissue generation of the pro-apoptotic lipid ceramide. TNF-binding protein, which protects against LPS-induced death, blocked LPS-induced ceramide generation and endothelial apoptosis, suggesting systemic TNF is required for both responses. Acid sphingomyelinase knockout mice displayed a normal increase in serum TNF-alpha in response to LPS, yet were protected against endothelial apoptosis and animal death, defining a role for ceramide in mediating the endotoxic response. Furthermore, intravenous injection of basic fibroblast growth factor, which acts as an intravascular survival factor for endothelial cells, blocked LPS-induced ceramide elevation, endothelial apoptosis and animal death, but did not affect LPS-induced elevation of serum TNF-alpha. These investigations demonstrate that LPS induces a disseminated form of endothelial apoptosis, mediated sequentially by TNF and ceramide generation, and suggest that this cascade is mandatory for evolution of the endotoxic syndrome.

Thymic microvascular system

Morphological studies of the microcirculatory system in the thymus were reviewed in regards to methodology and structural organization of blood and lymphatic vessels. The blood capillaries and postcapillary venules (PCVs) in the thymus are characterized by a double-walled structure. These vessels are surrounded more or less by perivascular spaces (PVSs) containing many lymphocytes. This space is delimited on the one side by abluminal surface of the vascular endothelium and on the other side by cytoplasmic processes of epithelial reticular cells. There are interruptions or gaps on the outer epithelial reticular layer. The lymphatic vessels can be distinguished histochemically from blood vessels based on strong 5'-nucleotidase (5'-Nase) activity. The 5'-Nase-positive lymphatic vessels were seen predominantly in the capsule and interlobular connective tissue but sometimes in the immediate vicinity of the PVS around the PCV, when a discrete opening in the lymphatic wall next to the PVS was found. Thus, it may be regarded as an initial part of lymphatics closely associated with the PVS, suggesting a possible route for lymphocyte efflux into the lymphatic vessel from the PVS. The endothelial cells of lymphatic vessels as well as PCVs are often infiltrated by lymphocytes, particularly more heavily during acute involution of the thymus. These images represent the migration of lymphocytes into the blood or lymphatic microcirculation.

Comparative anatomical studies on the thyroid and thymic arteries. V. House musk shrew (Suncus murinus)

The thyroid and thymic arteries were investigated in 36 male and 36 female house musk shrews. The superior thyroid artery was constant. The artery arose, in the most instances, from the external carotid arteries. The artery, in about one fourth instances, sent both the superior and inferior branches supplying the gland and the former branch terminated in the superior and inferior laryngeal arteries. The latter branch was independent in the remaining instances. The superior laryngeal artery was independent in a few specimens. In some instances, the superior thyroid artery did not send the inferior laryngeal which was the terminal branch of the tracheo-esophageal artery arising mainly from the subclavian and costocervical trunk. The middle thyroid artery was rare only appearing in one male specimen. The inferior thyroid and the thyroid ima arteries were absent in both sexes. On the other hand, the independent thyroid arteries arising from the external carotid, occipital, common carotid and internal carotid arteries appeared in high frequency (89% per body side in an average) in that order of frequency. Most of them may be some separated component of the superior thyroid artery. The middle thymothyroid artery was absent. The inferior thymic artery arising from the costocervical trunk or the internal thoracic (49% in an average) and the middle thymic artery arising from the subclavian (52%) shared the arteries supplying the intra-thoracic thymus. The supreme thymic and the superior thymic arteries were very rare, occurring in only one specimen, respectively. These results show that both the thyroid and thymic arteries in house musk shrews are very simple and seemingly basic, although there are some features characteristic only to this animal.

Three-dimensional ultrastructure of the perivascular space in the rat thymus

The overall architecture and structure of the perivascular space in the rat thymus were studied by light microscopy using silver-impregnated sections and sections stained immunohistochemically with anti-cytokeratin antibody, and by transmission and scanning electron microscopy (TEM and SEM). In silver-impregnated sections, the perivascular space was delimited by a thin sheath of delicate argyrophilic fibers from the thymic parenchyma in the cortico-medullary region and medulla. This space was continuous with the septal connective tissue, indicating that this was the connective tissue compartment rather than with the epithelial compartment of the parenchyma. In the medulla, the perivascular space widened at places, where the argyrophilic sheath was often discontinuous and the boundary between the perivascular space and parenchyma was indistinct. Lymphatics were located in the perivascular space of the corticomedullary region and sometimes in the wide perivascular space of the medulla. The presence of a thymic epithelial sheath surrounding the perivascular space was confirmed by light microscopy of anti-cytokeratin antibody immunostained sections and by TEM. SEM observations revealed three-dimensionally that the epithelial sheath lined by collagen fibrillar (i.e., argyrophilic) layer form a rather continuous tubular structure in the cortico-medullary region, while it often interrupted in the medulla. These findings indicated that the perivascular space (i.e., the connective tissue compartment) is extensively open to the parenchyma (i.e., the epithelial compartment) in some portions of the medulla, where medullary lymphocytes are probably freely exposed to blood borne substances similar to the peripheral lymphoid tissues.

Vanin-1, a novel GPI-linked perivascular molecule involved in thymus homing

Migration of hematopoietic precursor cells to the thymus is shown to depend upon a novel molecule called Vanin-1 expressed by perivascular thymic stromal cells. An anti-Vanin-1 antibody blocks the binding of pro-T cells to thymic sections in vitro, the in vivo accumulation of bone marrow cells around cortical thymic vessels, and long-term thymic regeneration. Thus, it interferes with the entry, and not the differentiation, of hematopoietic precursor cells. The Vanin-1 gene codes for a GPI-anchored 70 kDa protein that shows homology only with human biotinidase. Transfection of thymic stromal cells with the Vanin-1 cDNA enhances thymocyte adhesion in vitro. These data suggest that Vanin-1 regulates late adhesion steps of thymus homing under physiological, noninflammatory conditions.

Day 11 mouse fetal thymus: phenotype and search for the point of commitment

This phenotype study of uncommitted hematopoietic cells (UHC) and early T cell precursors (ETCP) in the thymus provides an understanding of the commitment process from UHC to ETCP. The study of genes that are involved in this process depends on the proper identification of these early cells. Nonetheless, most current phenotype studies of these cells are based on the observations from the late stages of fetal or adult thymocytes. Though conclusions drawn from these studies are insightful, cell maturation occurs so fast during thymus development that some important phenotypic nuances go unnoticed if one only looks at late-stage cells. Furthermore, even though early-stage thymocytes are phenotypically similar to those of late stage, they may have different properties. In order to study those thymus populations at the very beginning of their differentiation and commitment, 11-day mouse fetal thymuses were tested. This shows, using day-12 CFU-S as a measure, that at this stage of development there are UHC present. Thymocytes at this very initial stage of development are isolated for the first time, and their phenotypes as well as their differentiation potentials are analyzed. The result shows that UHC are detected only in the Mac-1- C-kit+ subset, which comprises 2% of the total thymus population. Surprisingly, 93% of the C-kit+ population is Mac-1+, which generates T cells with a frequency of 1/72. This indicates that the Mac-1 molecule is a differentiation marker rather than a lineage-specific marker. In addition, C-kit+ thymocytes at this stage do not express any T cell markers, such as Thy1.2, IL-2R alpha, CD2 and Mel-14. These thymocytes are very akin to UHC, since they express Sca-1, Wga, and CD34, and are likewise similar to ETCP, as they express Pgphigh, Mhc-Ihigh, Hsalow, and FcRlow. Moreover, they express a high level of adhesion molecules such as Lfa-1, Icam-1 and Lpam-1. As expected, these C-kit+ all contain the hematopoietic cell marker CD45. Low expression of CD4 (the typical marker associated to the earliest T cell precursor in adult thymuses) is also found in 10% of the C-kit+ population. While the C-kit+ population at this stage is more homogeneous than at any other stage of fetal thymus development, there are still markers (B220, CD5, Tsa, Mac-1, CD4, and Sca-1) that can split this population into other subsets. However, the majority of these markers are present in the Mac-1+ C-kit+ population, indicating that the C-kit+ population is essentially made of two populations (Mac-1+ and Mac-1- subsets). Interestingly, two major single-positive populations appear to emerge from this C-kit+ population one day later, namely Thy1.2+ IL-2R alpha-and Thy1.2- IL-2R alpha +. These two major single-positive populations seem to be derived directly from the Mac-1+ rather than from the Mac-1- subset of the C-kit+ population. Thus, these data suggest that important phenotypes are present during the early differentiation process. These phenotypes have never been shown before. Hopefully, this study will open up a new avenue for the study of very early stage T cell sublineages and their relationship to uncommitted thymic hematopoietic cells.

The natural history of disease expression in CD4 and CD8 gene-deleted New Zealand black (NZB) mice

CD4 and CD8 gene-deleted New Zealand black (NZB) mice and, as controls, B6.CD4 -/-, B6.CD8 -/-, NZB, and B6 wild-type (wt) mice were studied for phenotypic and immunologic parameters to determine the contribution of CD4 and CD8 cell lineages in NZB mice. These studies suggest surprisingly that a number of abnormalities are not due to either CD4+ or CD8+ cell lineages but rather are most likely due to non-CD4+ and -CD8+ cell lineages and/or background genes. Such abnormalities include altered thymic architecture, decreased staining of MITS 33+ medullary thymocytes, and an increased frequency of splenic IgM secretory cells. In contrast, deletion of either CD4+ or CD8+ cells appears to differentially influence immunologic function. Deletion of CD8+ cells did not influence titers of spontaneously occurring anti-erythrocyte or anti-DNA autoantibodies. interestingly, 50% of NZB.CD4 -/- mice contained levels of anti-erythrocyte IgG and anti-ssDNA IgM autoantibodies; even without detectable CD4+ cells. Such deletion of CD4+ cells, while leading to marked decreases in the prototype cytokines that characterize Th1 and Th2 subsets in B6 mice, led to a marked increase in IFN-gamma and a moderate increase in IL-4 mRNA levels in NZB.CD4 -/- mice. These data suggest that whereas non-CD4+ and -CD8+ cell lineages and NZB background genes have a marked influence in the development of autoimmune abnormalities, CD4+ cells appear to play a major role in influencing the cytokine environment, whereas CD8+ cells appear to play a minor role.

Reticular fibers in the stroma of the thymus

There were investigated thymic biopsies taken from 14 children deceased of intercurrent diseases. After routine paraffin procedures, slides were stained with hematoxylin-eosin, Gordon-Sweet, Wilder and Laidlaw methods. Reticular fibers are described in the stroma and parenchyma of the thymus. Their number and thickness are higher in connective septa, perivascular sheaths, at the cortico-medullary junction and around Hassall's bodies. Their involvement in a basal membrane like structure located between epithelio-reticular cells and thymocytes is discussed.

Comparative anatomical studies on the thyroid and thymic arteries. IV. Rabbit (Oryctolagus cuniculus)

The thyroid and thymic arteries were investigated in 30 male and 30 female Japanese white strain rabbits. Because of the high frequency (87% on average) of an independent superior laryngeal artery, the superior thyroid artery which arose either from the external or the common carotid artery was less common (13%). On the other hand, the middle thyroid artery was constant (99%); it always arose from the common carotid and penetrated the gland, supplying it, and terminated in the inferior laryngeal artery. The inferior thyroid artery was observed only in 1 female specimen; it arose from the common carotid, reached the gland caudally, and also terminated in the inferior laryngeal. The thyroid ima, arising from the brachiocephalic trunk, was found only in 1 male specimen. There were some independent thyroid arteries in about one fourth of body sides; they arose from the common carotid artery singly or together with the tracheooesophageal. A middle thymothyroid artery occurred in 1 male and 1 female specimen. The inferior thymic artery, arising from the internal thoracic or rarely from the costocervical trunk, was the most constant (92%). The middle thymic artery was also frequent (55%), arising from the common carotid, brachiocephalic trunk, aortic arch or the subclavian artery, in that order of frequency. The supreme thymic artery was present in 17%, on average, originating from the middle thyroid or directly from the common carotid artery. The superior thymic artery arising from the superficial cervical artery or the thyrocervical trunk was rare (5%). These results show that both the thyroid and thymic arteries in rabbits are more complex than in polyprotodont marsupials, rats and man, and that the thyroid arteries are simpler than in guinea pigs.