Expression of major histocompatibility complex class II antigens on normal porcine intestinal endothelium

A novel monoclonal antibody (MIL 11) specific for an antigen expressed on porcine endothelial cells is described. The antigen recognized by MIL 11 is most strongly expressed in the intestine but is also expressed on the capillary endothelium of a wide range of tissues. Using two- and three-colour immunofluorescence microscopy we demonstrated the extensive coexpression of MIL 11 and major histocompatibility complex (MHC) class II antigens on normal porcine capillary endothelium in the intestine, trachea, thymus and small veins, while endothelium of large vessels and the heart were negative for MHC class II. In contrast to humans and rodents, available reagents do not detect MHC class II on the intestinal epithelium of pigs. However, porcine intestinal endothelium expressed both DR and DQ antigens. A population of strongly class II-positive cells was also detected immediately adjacent to the endothelium in the lamina propria. Three-colour immunofluorescence microscopy highlighted the close association between endothelium and intestinal CD4+ T cells. Lamina propria T cells were mainly MHC class II positive, whereas those in the epithelial compartment were MHC class II negative.

Season-specific thymic architecture in the frog, Rana temporaria: SEM studies

It is already known that the thymus of the adult common frog, Rana temporaria, undergoes conspicuous annual cyclic changes. Light microscopic (LM) observations are at present confirmed by three-dimensional scanning electron microscopy (SEM) pictures showing the presence of a characteristic cortico-medullary division of summer thymuses and cystic appearance of winter organs. Summer thymuses are larger, much heavier and populated by a significantly higher number of cells than winter organs. The season-specific thymic size, cell content and distribution are reflected by its angioarchitecture. The microcorrosion casts of vascular system of winter thymuses are much smaller and more compact than those of the summer organs in which vessels are arranged at larger spatial intervals. It seems that thymic enlargement following the winter atrophy results from vernal repopulation of the thymic cortex by thymocytes which in consequence induces a reshaping of the existing capillary network.

L-selectin and alpha 4 beta 7 integrin homing receptor pathways mediate peripheral lymphocyte traffic to AKR mouse hyperplastic thymus

Before the development of thymic lymphoma, AKR mice undergo a striking lymphoid hyperplasia of the thymic medulla. We have previously shown that there is a marked increase in traffic of B and T lymphocytes from the periphery into the preneoplastic, hyperplastic thymuses of these mice, in contrast to the scant traffic of such cells to normal thymuses. The traffic of lymphocytes to lymph nodes and Peyer's patches is controlled in part by the interaction of lymphocyte adhesion molecules called homing receptors with their tissue-selective endothelial ligands known as vascular addressins. We have investigated the roles of homing receptors and vascular addressins in the traffic of lymphocytes to the AKR hyperplastic thymus. We demonstrate that development of hyperplasia is accompanied by an increase in the number of thymic medullary blood vessels with high endothelial venule morphology and expression of the peripheral node addressin (PNAd) and the mucosal addressin (MAdCAM-1). In vitro and in vivo functional assays show that the addressin/homing receptor pairs PNAd/L-selectin and MAdCAM-1/alpha 4 beta 7 are involved in lymphocyte traffic to the hyperplastic thymus. These results indicate that molecular adhesion mechanisms involved in tissue-selective migration of lymphocytes to peripheral lymph node and to mucosal lymphoid tissues play a role in the recruitment of B and T lymphocytes to the AKR thymus and thus in the pathogenesis of thymic hyperplasia.

[The morphological characteristics of the microcirculatory bed of the thymic capsule in the dynamics of the posttraumatic period]

Describes the status of microvessels of thymic capsule in mechanical injury. Features the changes occurring in the capsular microvessels in the course of the posttraumatic period and the morphology of all the components involved.

Thymic reticulum of mice. III. The connective compartment (innervation, vascularisation, fibrous tissues and myoid cells)

T lymphocytes interact at various levels of differentiation, with cells of the thymic reticulum, forming a peculiar and complex microenvironment. Following earlier descriptions by electron microscopy of three types of epithelial cells and two types of non-epithelial cells (macrophages and interdigitated cells) forming the thymic microenvironment, we report a study on a third compartment, the connective tissue, whose elements occur throughout the organ. The components of the capsule and trabeculae, the vascularisation and the innervation of the thymus and the presence of a few myoid cells are described. This is very rarely studied in ultrastructure. All these cells are completely imbricated and form a network trapping the lymphocytes, playing an essential role in the differentiation, maturation and selection of T cells.

Comparative anatomical studies on the thyroid and thymic arteries. III. Guinea pig (Cavia cobaya)

The thyroid and thymic arteries were investigated in 30 male and 30 female Hartley guinea pigs. The superior thyroid artery was the most common (73% of males, 55% of females). It arose from the external carotid artery and gave rise to the superior and inferior laryngeal arteries in addition to glandular rami. In the remaining instances, the independence of the superior laryngeal resulted in the presence of middle and inferior thyroid arteries. The middle thyroid artery, arising from the external carotid or distal half of the common carotid artery, passed the thyroid gland cranially or penetrated it to terminate in the inferior laryngeal artery. The inferior thyroid artery, arising from the distal or proximal half of the common carotid, reached the gland caudally also to end in the inferior laryngeal. A thyroid ima artery was rare. There were many 'independent thyroid arteries'. Some of them arose from the lingual and the ascending pharyngeal arteries, suggesting that they were the original thyroid arteries. Thus the thyroid arteries were much more abundant and variable in the guinea pig than in man. The arteries supplying the 'superficial cervical thymus', which is characteristic of this animal, were the superior and inferior superficial cervical thymic arteries. The latter artery, arising from the superficial cervical, appeared in about 30% of the specimens. The former was always present and arose from the lingual (75% of males, 67% of females); superior laryngeal, and superior thyroid or common carotid arteries. The cranial extension of the thymus and the superior superficial cervical thymic artery had complex relationships with the hypoglossal nerve.

Intrathoracic and subconjunctival petechiae in sudden infant death syndrome (SIDS)

The frequency and density of intrathoracic and subconjunctival petechiae was studied in 250 cases of SIDS and 69 controls. The control group included 37 infants with natural and 32 infants with traumatic causes of death. Intrathoracic petechiae were found significantly more frequently in the SIDS group (91.2% SIDS; 42% controls; p < 0.001) and were present at a higher density (p < 0.001). Subepicardial and thymic petechiae were detected at high density in older SIDS infants. Subconjunctival petechiae were low in density and found only in 2.4% of the SIDS group but they were detected in 8.1% of the natural death group and 21.9% (p < 0.05) of the lethal trauma group. Subconjunctival petechiae were found at highest density in strangulation. Intrathoracic petechiae are commonly found in SIDS but are not specific for SIDS. Subconjunctival petechiae are typical but not specific for strangulation. In SIDS, subconjunctival petechiae are rare and appear at low density.

Changes in the blood-thymus barrier of adult rats after estradiol-treatment

The accessibility of the thymus parenchyma for relatively large Mw (+/- 150 Kd) proteins has been studied by the intravenous injection of monoclonal antibodies (mAbs) specific either for all T cells (His-17) or MHC class II molecules (His-19) in control and estradiol benzoate (EB)-treated adult Wistar rats. In controls, the transcapsular route rather than cortical capillaries seems to be involved in the entry of molecules into the thymus. By contrast, a specific staining for either T cells (His-17) or MHC class II molecules (His-19 positive cells) disappears almost completely from the thymic cortex of EB-treated rats except in the immediate subcapsular epithelial cell layer. In these rats, T cells and epithelial cells intimately associated to blood vessels from both inner cortex and corticomedullary border showed additional staining with the respective mAbs confirmed by electron microscopy. The disappearance of the transcapsular route together with the increased vascular permeability of cortical blood vessels would be related to the reinforcement of the subcapsular epithelial cell layer and to direct effects of EB on vascular endothelia, respectively. These results are discussed in relationship to the cell migration into and out of adult thymus, as suggested by the changes in intrathymic T cell subsets evaluated by flow cytometry.

Transport of transferrin across the blood-thymus barrier in young rats

The interaction of transferrin-peroxidase (Tf-HRP) with the capillary endothelium of the rat thymus was analyzed by diaminobenzidine (DAB) cytochemistry and electron microscopy. The thoracic aortas of young rats were cannulated and the upper bodies perfused with the Tf-HRP conjugate. In the thymus, plasmalemmal vesicles of the endothelium mediated the endocytosis and transport of Tf-HRP. Transcytosis of Tf-HRP appeared to occur by micropinocytosis, without morphological evidence of involvement by endothelial endosomes. DAB reaction product was commonly present in the subendothelial interstitial space and, in addition, was often localized in clathrin coated pits and vesicles in epithelial reticular cells that surround the thymic capillaries. In perfusions done at 4 degrees C, no binding of Tf-HRP to the lumenal membranes of capillaries was detected. The transport of Tf-HRP across the capillary endothelium in the thymus was not inhibited by competition with excess native transferrin; however, the uptake of Tf-HRP by epithelial reticular cells was completely inhibited by excess transferrin. Transferrin receptors were localized in the thymus by indirect immunocytochemistry using OX-26, a mouse anti-rat transferrin receptor monoclonal antibody. No transferrin receptors were detected on the capillary endothelium but diffuse reaction product was localized on the subjacent epithelial reticular cells. These results indicate that transport of Tf-HRP across the thymus capillary wall is independent of transferrin receptors at the level of the endothelial cells but that subsequent uptake of Tf-HRP by epithelial reticular cells is a transferrin receptor dependent process.

Multiple nitric oxide synthase systems in adult rat thymus revealed using NADPH diaphorase histochemistry

Nitric oxide (NO) has become recognized as a multifunctional mediator, with roles in vascular physiology, neurotransmission and non-specific immune defense. The histochemical marker associated with the neural and endothelial form of NO synthase (NOS), reduced nicotinamide adenine dinucleotide diaphorase (NADPHd), has enabled the indirect localization of potential sites of NO production. Innervation of the thymus and its immunological functions made this tissue a candidate for utilization of various NO systems. In the present study on adult rat thymus, multiple cellular sites expressing NADPHd activity, thereby implicated as sites of NOS activity, have been identified using morphological criteria alone: blood vessel endothelium, dendritic cells, deep cortical or medullary stromal cells, intrinsic neuron-like profiles, granulocytes (possibly neutrophils) and fat cells. In addition, the availability to the thymic microenvironment of another form of NOS in macrophages, which is not stained by the diaphorase technique, was supported by the observation of these cells at corticomedullary and cortical locations. These results indicate that a wide variety of possible immunomodulatory roles can be expected for NO in the thymus including the induction of tolerance, major histocompatibility complex (MHC) restriction, lymphocyte trafficking and regulation of thymic endocrine output.

Prospective evaluation of selective venous sampling for parathyroid hormone concentration in patients undergoing reoperations for primary hyperparathyroidism

BACKGROUND

The utility of standard radiologic imaging studies in guiding reoperative parathyroid surgery for primary hyperparathyroidism is widely known and accepted. The additional information gained by selective venous sampling in that patient population has not been well defined. We report the results of our experience with this method.

METHODS

Between 1982 and 1992, 223 consecutive patients underwent reoperations for persistent or recurrent primary hyperparathyroidism after a prospectively determined series of imaging studies. Patients underwent noninvasive testing consisting of ultrasonography, computed tomography, technetium thallium scanning, and magnetic resonance imaging. Patients with negative, equivocal, or discordant results on the noninvasive studies proceeded to angiography. If angiography was negative, selective venous sampling was performed.

RESULTS

Eighty-six patients (39%) with negative or equivocal noninvasive test and angiogram results underwent selective venous sampling. Seventy-six patients (88%) had a significant gradient in levels of parathyroid hormone from veins draining the left side of the neck (n = 25), the right side of the neck (n = 33), both sides of the neck (n = 7), and the thymus (n = 11). Correlation of these findings with operative findings revealed a sensitivity of 88% and a specificity of 86%. In the subgroup of patients who underwent venous sampling and had completely negative results of standard radiologic studies (35 of 86; 40%), 28 patients (80%) had venous gradients and seven patients (20%) had no gradient. Of those 28 patients in whom the venous sampling gradients were the only positive localization study, the venous samplings were helpful in 23 patients (true positive gradients), and the operative success rate was 93%. In the seven patients with no positive preoperative localizing studies including venous sampling, there were two operative failures (operative success, 71%).

CONCLUSIONS

Our results show that selective venous sampling is a highly sensitive and specific method to regionally localize abnormal parathyroid glands not imaged by standard noninvasive and invasive radiologic techniques. Venous sampling is the study of choice in guiding reoperative procedures for occult abnormal parathyroid glands that are undetected despite the use of all available imaging studies.

Comparative anatomical studies on the thyroid and thymic arteries. II. Polyprotodont marsupials

The thyroid and thymic arteries were investigated in 27 specimens from 9 species belonging to the Australian Polyprotodont marsupials, which are subdivided into 2 superfamilies, Dasyuroidea and Perameloidea. The results were compared with those in rats and humans. The site of origin of the superior thyroid artery ranged from the external carotid artery to the common carotid in Dasyuroids, and converged on the external carotid and the bifurcation of the common carotid in Perameloids. The course of the superior thyroid artery suggested that it was originally the pharyngeal artery rather than an artery of the thyroid gland. Because the so-called inferior thyroid artery in Polyprotodonts has fewer relationships with the gland than in rats and humans, it is preferable to refer to it as the tracheo-oesophageal artery. The inferior thymic artery was constantly present in both groups. The middle thymic artery was rare, only appearing in 2 specimens of Dasyuroids. The supreme thymic, superior thymic (both abundant in rats and humans) and the middle thymothyroid (abundant in humans) arteries were absent. These results suggest that the arterial pattern of both organs in Perameloids is simpler than in Dasyuroids and that the pattern of the Polyprotodonts, as a whole, is simpler than in the rat or man.

Tissue distribution of very late activation antigens-1/6 and very late activation antigen ligands in the normal thymus and in thymoma

The expression of very late activation antigens (VLAs)-1/6 was correlated with that of the VLA ligands fibronectin, laminin, collagen, and vascular cell adhesion molecule-1 in sections of normal thymus, in thymocyte suspensions, and in 10 cases of thymoma. Capsular epithelial cells are VLA-2+, VLA-3+, and VLA-6+ and face the thymic basement membrane, which is rich in fibronectin, laminin, and collagen type IV. Cortical epithelial cells are VLA-2+ and are embedded in a reticular meshwork of nonorganized extracellular matrix (ECM) that is rich in fibronectin. Cortical thymocytes, identified as CD3dim cells by using immunofluorescence in suspension, are highly positive for VLA-4, a fibronectin ligand. Most cortical macrophages are positive for vascular cell adhesion molecule-1, a molecule recognized by VLA-4. Medullary epithelial cells are VLA-2+/VLA-3+ and are codistributed with fibrous strands of organized ECM that are positive for fibronectin, collagen, and laminin. Medullary thymocytes, identified as CD3bright cells, are positive for VLA-4 and VLA-6, a ligand for laminin. Our findings suggest that intrathymic thymocyte maturation is associated with changes in expression of VLA molecules, which are apparently correlated with the presence of VLA ligands in the tissue microenvironment. Thymomas were classified as cortical (three), common (five), or medullary (two) type. Expression of VLA molecules and distribution of ECM in the three histological subtypes were reminiscent of those observed in the respective regions of the normal thymus. All cases of thymoma were characterized by overexpression of VLA molecules on neoplastic cells, which was associated with increased deposition of organized ECM rich in fibronectin, laminin, and collagen.

Reptilian thymus gland: an ultrastructural overview

Like in higher vertebrates, the thymus gland of reptiles consists of lymphoid cells within epithelial framework and characteristic myoid cells. Mammalian-like Hassall's corpuscles are absent. Secretory cells, secretory and degenerative cysts as well as phagocytic cells, and plasma cells can be observed. Interdigitating cells and some characteristic features of thymic innervation and vascular system are also described in the reptilian thymus gland.

[A case of invasive thymoma which had grown in the SVC via left innominate vein through thymic vein]

A 66-year-old male was referred to our department with mediastinal tumor. The chest X-ray and the chest CT showed a large tumor adjacent to pulmonary artery and descending aorta. Cavogram revealed a filling defect of the SVC. He underwent complete resection of the tumor, resection of the SVC and reconstruction of the SVC with ringed PTFE. The main tumor was 12 x 7 cm in size and the tumor in the SVC was 20 x 15 mm in size. Histopathological findings showed a lymphocytic thymoma. Compact invaded nest of tumor cell was found in the innominate vein. The tumor in the SVC was invasive to the intima of the SVC, but its media and adventure were intact. This case is rare among patients with invasive thymoma as far as the growth is concerned.

Pericapillary collagen in the human thymus: implications for the concept of the 'blood-thymus' barrier

The deposition of collagen in the walls of capillary-size blood vessels was studied in 95 human thymuses with respect to the site of deposition, extent of the change and relation to age and degree of involution. When examined by electron microscopy the collagen was found to be situated between the 2 basement membranes of the so-called 'double-layered' capillaries characteristic of the thymus of many species. This results in the formation of substantial 'collars' of collagen around a proportion of the blood vessels examined. Few such collars are seen before birth, but their number and thickness increase markedly during the 1st year of life. The relationship of these changes to the degree of involution is less apparent. The significance of these changes to thymic structure and function is discussed in relation to cell traffic through the thymus and the postulated 'blood-thymus barrier', the existence of which is seen to be in some doubt.

Corticotropin-secreting carcinoid tumors of the thymus: diagnostic unreliability of thymic venous sampling

Three patients with Cushing syndrome due to ectopic production of corticotropin underwent total thymectomy on the basis of elevated concentrations of corticotropin in selective samples from thymic veins but in the absence of a radiographically detectible thymic mass. In one patient, radiologic examination demonstrated hyperplasia of neuroendocrine cells staining positively for corticotropin throughout the thymus but no discrete mass. This patient had complete remission after total thymectomy. The other two patients had no evidence of an intrathymic source of corticotropin, and both had persistent Cushing syndrome. Elevated levels of corticotropin in thymic vein samples may reflect corticotropin production by pulmonary bronchial carcinoid tumors, mediastinal metastases, thymic carcinoids, or diffuse hyperplasia of intrathymic neuroendocrine elements. In the absence of a demonstrable intrathymic mass, corticotropin gradients in thymic veins do not reliably indicate a thymic source of corticotropin and should not necessarily be used as a basis for exploratory thoracotomy or blind thymectomy.

A study of mast cells in autoimmune NZB/W F1 mice: possible relationship between mast cells and increased vascular permeability in the thymus of NZB/W F1 mice

We examined the possible relationship between thymic mast cells and increased vascular permeability in the thymus of autoimmune NZB/W F1 mice. Light-microscopic observation of tissue sections showed that non-autoimmune BDF1 mast cells increased with age. In contrast, autoimmune NZB/W F1 mast cells did not increase in the thymic parenchyma at the age of 9 weeks. However, NZB/W F1 mast cells resumed the age-associated increase from the age of 12 weeks and exceeded the number of BDF1 mast cells at the age of 30 weeks. Blood histamine levels of 9-week-old NZB/W F1 mice were higher than those of BDF1 mice of comparable age. Furthermore, peritoneal mast cells of NZB/W F1 mice were more sensitive to compound 48/80 than those of BDF1 mice. Increased blood histamine levels of NZB/W F1 mice seem to be due to the enhanced histamine release from mast cells. These results suggest a possible correlation between the high histamine levels by degranulation of mast cells and increased vascular permeability in the thymus of NZB/W F1 mice.

Three-dimensional architecture of pericytes with special reference to their topographical relationship to microvascular beds

Microvascular beds and pericytes in the submandibular gland, thyroid gland and heart were studied by combined scanning electron microscopy and chemical digestion. The submandibular gland had a relatively loose network of blood capillaries, the thyroid gland possessed a close-meshed network of capillaries, and those in the myocardium ran parallel to the long axis of myocardial cells. The thyroid gland exhibited the largest numbers of pericytes. Three types of pericytes could be distinguished by their shape and localization. Type I pericytes, which were confined to true capillaries, had a fusiform or polygonal cell body, a few long, slender longitudinal processes (primary ones) and short, fine circumferential processes (secondary ones). Type II pericytes, which were found in the arterial side of myocardial capillaries, were characterized by large, circumferential band-like processes completely encircling the vessel. Type III pericytes, which were seen on the venous side of thyroid and myocardial capillaries, had a flattened cell body and short, irregular processes. Type II and III pericytes appear to show an intermediate or transitional form between smooth muscle cells and typical pericytes (Type I). Judging from the configuration of pericytes suggests that these are related to functions such as contraction as well as mechanical support.

Increased vascular permeability of Brucella abortus bacilli in the thymus of NZB/W F1 mice

Various amounts of the bacterium, Brucella abortus (BA) were injected intravenously into autoimmune NZB/W F1 mice and non-autoimmune BDF1 mice and then the localization of BA in the thymus was traced using an immunohistochemical method at 30 min and 3 h after injection. The results showed that a greater amount of BA became consistently localized in the thymic parenchyma in a free form or in a phagocytized form in NZB/W F1 mice in comparison with BDF1 mice, indicating a marked increase of vascular permeability in the thymus of NZB/W F1 mice. The extravascular leakage of BA was clearly dose-dependent. The significance of invasion of bacterial antigens from the general circulation into the thymic parenchyma is discussed in relation to autoimmune states.

Vascular thymus transplantation in rats. Technique, morphology, and function

A new method of thymus transplantation is introduced, in which the graft is directly connected with the recipient's vascular system. This procedure was used both in euthymic rats and congenitally athymic nude rats. At all tested intervals after transplantation thymus grafts hardly differed from the recipient's own thymus in immunohistology and lymphocyte yield. In athymic nude rats, T cell-dependent immunity, tested by mitogen- and alloantigen-induced T cell responses, as well as by antibody production and delayed-type hypersensitivity after ovalbumin administration, showed that vascular thymus grafts could generate T cell functions to euthymic control levels. We conclude that the technique of vascular thymus transplantation represents a valuable tool, either in fundamental research on thymus function, or for the purpose of immune (re)constitution.

Immunological tolerance to many self epitopes may be unnecessary

The demonstration of the existence of antigenic mimicry suggests that immunological tolerance to self antigens may be an insufficient basis for distinguishing self from non-self. However, some data suggest that even in the absence of tolerance most self epitopes would not immunize the host and that tolerance to all self epitopes may, therefore, not be necessary for the prevention of autoimmune disease.

Characterization of lymphatic and venous emigrants from the thymus

Lymphocytes leaving the thymus via two different pathways were examined as to their morphology and phenotype. Cells leaving the thymus via lymphatics were obtained by a direct cannulation of thymic lymphatics and those leaving via the thymic vein were labelled within the thymus using an extracorporeal perfusion system and identified subsequently as fluorescent cells in the draining vein. In both cases the cervical thymus in lambs was used, since it is located in the neck region and ensures easy access to both blood and lymphatic vessels of the thymus without subjecting the animal to a major trauma or stress. Cells obtained from the thymic lymphatics or lymphatic emigrants were found to have distinct features different from peripheral T cells in terms of their surface morphology and expression of the MHC antigens. Venous emigrants were also slightly different from peripheral T cells in MHC expression. Estimation on the rate of thymocyte emigration into the periphery suggested that neither venous nor lymphatic emigrants represent a major fraction of de novo synthesized cells in the thymus of this animal species, as has been suggested in the mouse.

Comparative anatomical studies of thyroid and thymic arteries: I. Rat (Rattus norvegicus albinus)

The thyroid and thymic arteries were investigated in 50 male and 50 female rats. In more than 70% of the animals, on both sides the cranial thyroid artery forms a common trunk with the ascending pharyngeal artery. The caudal thyroid artery arises not from the deep cervical but from the pericardiacophrenic artery. It may be replaced, however, by a branch of some other artery, such as the brachiocephalic, subclavian, vertebral, or ascending cervical, suggesting a shift of its origin from the internal thoracic artery to the thyrocervical trunk as in man. All the thoracic lobes of the thymus are supplied directly by a thymic branch of the internal thoracic artery or indirectly by a branch of the pericardiacophrenic artery. More than half of the specimens have a cervical thymic lobe of variable size, which is supplied by a branch of the cranial thyroid, external carotid, and/or occipital arteries. Some of these thymic arteries, except those from the external carotid and occipital arteries, reach the thoracic lobe. The thoracic lobes lacking a cervical lobe may be supplied by the thymic branch arising only from the cranial thyroid artery. Other anomalous arteries supplying the thoracic lobe are derived from the superficial cervical and/or the right common carotid arteries. These results show that the thymic arteries of rats are basically similar to those of man, although they display a clear difference in their frequency and origin.

Interposition of the thymus as a pedicled flap in tracheal reconstructive surgery

Thymus was interposed in six patients following tracheal reconstruction: three tracheo-innominate fistula repairs, two resections for stenosis, and one repair of a membranous tear during esophagectomy. The two resections for stenosis were elective; the others were emergent repairs. This gland, readily accessible in the upper mediastinum, is easily dissected from the pleura and pericardium and separated into two lobes. Pedicles based at the thoracic inlet measure between 15 and 20 cm by 4 cm. Each lobe receives independent arterial blood supply and venous drainage which remains intact even if the innominate vein must be divided to effect tracheal repair. All patients survived the perioperative period. The patient with the tracheo-innominate fistula died 1 month later. No patient had evidence of further vascular complications or tracheal anastomotic leak even though one patient developed a long segment of necrotic trachea. Reinforcement following tracheal reconstruction is important in preventing complications from anastomotic leak or vascular erosion. The pedicled thymus gland is an excellent, readily available interposition flap for emergent or elective tracheal reconstruction.