中华鳖胸腺显微和亚显微结构及其在进化上的意义

应用透射电子显微镜观察了中华鳖胸腺的显微和亚显微结构。发现中华鳖胸腺从结构上可分为皮质和髓质,皮质富含淋巴细胞,髓质富含上皮性网状细胞。在各发育期中华鳖胸腺皮质、髓质交界处和髓质区有明显的囊状胸腺小体和交错突细胞。在２００ｇ以上的成年鳖胸腺内发现有同心圆状胸腺小体。电镜下,胸腺内淋巴细胞分为大、中、小３型。上皮性网状细胞从亚显微结构上分为支持型和分泌型。胸腺囊包括细胞内囊和细胞间囊,以细胞内囊居多     

斜带石斑鱼胸腺的显微和超微结构

本文通过解剖及组织切片技术、光学显微镜、透射和扫描电子显微镜技术,对斜带石斑鱼(Epinephelus coioides)胸腺器官组织进行了观察研究。结果表明:斜带石斑鱼胸腺实质主要由胸腺细胞(淋巴细胞)和网状上皮细胞构成。鱼体从Ⅰ龄之后,其胸腺发生明显的变化,与幼鱼有所不同,主要是胸腺可明显区分为三个区域:胸腺外皮质区、内皮质区和髓质区。外皮质区主要由网状上皮细胞、黏液细胞、成纤维细胞和少量淋巴细胞构成,细胞排列疏松;内皮质区主要由密集的淋巴细胞和网状上皮细胞组成,以含有大量的淋巴细胞为特征;髓质区主要由淋巴细胞和较多的网状上皮细胞构成,总体特征是淋巴细胞数量比内皮质区的少,且细胞排列较疏松。外皮质区、内皮质区相当于高等脊椎动物的皮质;髓质区相当于高等脊椎动物的髓质。髓质区之下有结缔组织,在Ⅱ龄以上的成体出现胸腺小体(Hassall's corpuscles)或类似胸腺小体的结构,而且随着年龄的增加,胸腺外皮质区增厚,结缔组织增加,还表现在内皮质区和髓质区组织逐渐萎缩变薄,胸腺的细胞组成类型和淋巴细胞数量上有所变化等等。这些现象在Ⅱ龄鱼开始出现,即胸腺呈现退化迹象,在Ⅲ龄以上鱼体呈现明显的退化和萎缩。胸腺表面扫描电镜结果表明:其上皮细胞表面具有微嵴以及由微嵴组成的指纹状结构,有一些微孔分布。透射和断面扫描电镜的结果进一步表明:胸腺组织内的细胞成分复杂,除了淋巴细胞和网状上皮细胞外,还具有巨噬细胞、肥大细胞、肌样细胞、浆细胞、指状镶嵌细胞和纤维细胞等。     

Ultrastructural changes in the thymus of the turtle Mauremys caspica in relation to the seasonal cycle

Summary Changes in the ultrastructure of the thymus of the turtle Mauremys caspica, with special reference to its non-lymphoid components, were studied in relation to the seasonal cycle. The thymic cortex contains framework-forming epithelial-reticular cells and free macrophages, while the medulla includes, in addition, mature and presumptive pro-interdigitating cells. The ultrastructural features of these cells are generally similar to those described for non-lymphoid components of the mammalian thymus. The turtle thymus undergoes cortical involution in spring, with recovery periods in May–June and during autumn. A moderate involution occurs in winter. At the beginning of spring, cortical (but not medullary) epithelial-reticular cells show degenerative changes, probably related to high levels of circulating testosterone. In spring and autumn, mature interdigitating cells are absent, but macrophages, monocytes, and pro-interdigitating cells are found. During May–June, the cortical epithelial-reticular population recovers and macrophages, monocytes, and interdigitating cells are actively phagocytic. In summer, the epithelial-reticular cells in both cortex and medulla display normal ultrastructural features; mature and immature interdigitating cells are absent and some macrophages are detected occasionally. The results suggest that non-lymphoid components of the reptilian thymus can play a role in governing T-lymphocyte differentiation, and that the thymic cortex and medulla exhibit different cycles of seasonal activity.     

A scanning electron-microscopic study of the rat thymus with special reference to cell types and migration of lymphocytes into the general circulation

Summary The three-dimensional structure of the rat thymus was studied by combined scanning and transmission electron microscopy. The thymus consists mainly of four types of cells: epithelial cells, lymphocytes, macrophages, and interdigitating cells (IDCs).The epithelial cells form a meshwork in the thymus parenchyma. Cortical epithelial cells are stellate in shape, while the medullary cells comprise two types: stellate and large vacuolated elements. A continuous single layer of epithelial cells separates the parenchyma from connective tissue formations of the capsule, septa and vessels. Surrounding the blood vessels, this epithelial sheath is continuous in the cortex, while it is partly interrupted in the medulla, suggesting that the blood-thymus barrier might function more completely in the cortex.Cortical lymphocytes are round and vary in size, whereas medullary lymphocytes are mainly small, although they vary considerably in surface morphology.Two types of large wandering cells, macrophages and IDCs, could be distinguished, as well as intermediate forms. IDCs sometimes embraced or contacted lymphocytes, suggesting their role in the differentiation of the latter cells.Perivascular channels were present around venules and some arterioles in the cortico-medullary region and in the medulla. A few lymphatic vessels were present in extended perivascular spaces.The present study suggests the possible existence of two routes of passage of lymphocytes into the general circulation. One is via the lymphatics, while the other is through the postcapillary venules into the blood circulation. Our SEM images give evidence that lymphocytes use an intracellular route, i.e., the endothelium of venules.     

Ultrastructure and function of interdigitating cells in the guinea pig thymus.

Electron micrsocopic investigation of the thymus of normal guinea pigs of both sexes, approximately 3 months of age, have shown that the organ contains at least two types of non-lymphoid cells: epithelial reticulum cells and so-called interdigitating cells. The latter have been observed in the inner part of the cortex and are morphologically characterized by an irregularly shaped or kidney-shaped nucleus, many tubulovesicular cytoplasmic structures accumulated mostly within the cytocentre as well as by finger-like protrusions of the cytoplasm. These interdigitating cells are in close contant with neighbouring lymphocytes with partly destroyed pycnotic nucleus.     

Ontogeny and organization of the stationary non-lymphoid cells in the human thymus

Summary Ontogenetic differentiation of the human thymus was investigated in 50 embryos by means of light and electron microscopic methods in an attempt to clarify the morphogenesis of the complicated microecology of thymic tissue. At the 8th gestational week (g.w.), the primordium of the thymus contains almost exclusively undifferentiated epithelial cells. At the 10th g.w., the epithelial cells in the central part are spindle-shaped. During the subsequent weeks the cortical region of the thymus becomes separated into lobes by mesenchymal septa containing hemopoietic precursor cells and large electronlucent cells with irregularly shaped nuclei. The latter cells are also found in the deeper presumptive medullary regions of the thymus; they differentiate into interdigitating reticulum cells (IDC). The permeation of the medulla of the thymus by non-epithelial IDC occurs concurrently with the formation of cortical and medullary epithelial cells. Between the 12th and 14th g.w. the cortical and medullary differentiation is completed. At this time-stage cortical small lymphocytes differ in morphological shape from medullary lymphocytes, the latter acquiring the appearance of immunocompetent T cells and establishing intimate contact with the IDC.These findings indicate that the thymic cortex and medulla contain different epithelial cells. In addition, the thymic medulla displays cells characterized by the morphology of typical interdigitating reticulum cells of peripheral lymphoid tissue. The structural pattern of the thymus is correlated to morphologically differing lymphoid cell populations in the cortical and medullary regions.This investigation was supported by grants from the Deutsche Forschungsgemeinschaft and by the Sonderforschungsbereich 111The authors dedicate this paper to Professor Helmut Leonhardt on the occasion of his 60th birthday. The authors also appreciate the excellent technical assistance of Mrs. I. Knauer, Mrs. H. Waluk and Mrs. H. Siebke     

Sites of action of soltriol (vitamin D) in hamster spleen, thymus, and lymph node, studied by autoradiography

Siberian hamsters (Photopus sungorus) were injected with 3H dihydroxycholecalciferol (vitamin D, soltriol). Autoradiograms of spleen, thymus, and lymph nodes revealed nuclear concentration of the hormone in a select population of cells in all of these organs. In the spleen, labeled cells were abundant in the red pulp, but sparse in the white pulp. In the periarterial lymphatic sheath (PALS) labeled cells were found predominantly at the outer rim, with a few scattered labeled cells in the inner PALS and in the marginal zone. Lymphocytes, including pyronin-positive plasma cells, did not display nuclear labeling. In the red pulp, some of the labeled cells contained pigmented inclusions in the cytoplasm, while most of the labeled cells did not appear phagocytic under the conditions of the experiment. In the thymus, labeled cells were most numerous in the medulla, but sparse in the cortex. Many of the thymic target cells were larger than the unlabeled lymphocytes, with a large and pale nucleus, sometimes containing a distinct nucleous, and with large and dendritic cytoplasm, having the appearance and distribution of epithelio-reticular cells. In lymph nodes, scattered labeled cells were conspicuous in or near the subcapsular sinus, while other cells did not concentrate radioactivity in their nuclei. The results indicate that nuclear receptors and direct genomic actions for soltriol exist in certain cell populations of lymphatic tissues that probably include reticular cells and a subpopulation of macrophages. These target cells may mediate effects of the steroid on lymphocytes that appear to have no or only very low numbers of nuclear receptors.     

Sites of action of soltriol (vitamin D) in hamster spleen,thymus, and lymph node,studied by autoradiography

Summary Sibirian hamsters (Photopus sungorus) were injected with³H dihydroxycholecalciferol (vitamin D, soltriol). Autoradiograms of spleen, thymus, and lymph nodes revealed nuclear concentration of the hormone in a select population of cells in all of these organs. In the spleen, labeled cells were abundant in the red pulp, but sparse in the white pulp. In the periarterial lymphatic sheath (PALS) labeled cells were found predominatly at the outer rim, with a few scattered labeled cells in the inner PALS and in the marginal zone. Lymphocytes, including pyronin-positive plasma cells, did not display nuclear labeling. In the red pulp, some of the labeled cells contained pigmented inclusions in the cytoplsm, while most of the labeled cells did not appear phagocytic under the conditions of the experiment. In the thymus, labeled cells were most numerous in the medulla, but sparse in the cortex. Many of the thymic target cells were larger than the unlabeled lymphocytes, with a large and pale nucleus, sometimes containing a distinct nucleolus, and with large and dendritic cytoplasm, having the appearance and distribution of epithelio-reticular cells. In lymph nodes, scattered labeled cells were conspicuous in or near the subcapsular sinus, while other cells did not concentrate radioactivity in their nuclei. The results indicate that nuclear receptors and direct genomic actions for soltriol exist in certain cell populations of lymphatic tissues that probably include reticular cells and a subpopulation of macrophages. These target cells may mediate effects of the steroid on lymphocytes that appear to have no or only very low numbers of nuclear receptors.     

Distribution and ultrastructure of S-100-immunoreactive cells in the human thymus

Summary The present study deals with the localization and ultrastructure of S-100-immunoreactive cells in the human thymus. These immunoreactive cells are distributed mainly in the medulla with some scattered elements in the cortex. Electron-microscopic observation revealed that the cells are characterized by an irregularly shaped nucleus, tubulovesicular structures in the cytoplasm and characteristic interdigitations of the plasma membrane. The cells often embrace lymphocytes with their branched processes. On the basis of these morphological features, the immunostained elements were identified as interdigitating cells (IDCs). The immunocytochemistry for S-100 visualizes the precise distribution and extension of the IDCs under the light microscope and indicates that the IDCs form no structural networks such as those established by the thymic epithelial cells. Since the IDCs in human lymph nodes have also been reported to contain S-100-like immunoreactivity, S-100 protein can be regarded as a useful marker for identifying the IDCs in the human thymus and other lymphoid organs.     

Histological changes of the mouse thymus during involution and regeneration following administration of hydrocortisone

Summary A histological study has been made of the thymus in mice during acute involution and regeneration following administration of hydrocortisone. The cortex undergoes remarkable changes in the microscopic structure during involution and regeneration. During involution the lymphocytes in the cortex rapidly decrease and are removed. Then a rapid replacement of lymphocytes occurs during regeneration. On the basis of formation and repopulation of lymphocytes the regenerative process of the cortex is divided into seven phases. The reconstitution of the cortex proceeds more rapidly in females than in males. Newly formed lymphocytes take origin from the mesenchymal cells in the cortex. Such mesenchymal cells become distinguishable from epithelial reticular cells during involution. They appear to engulf destroyed lymphocytes and debris during involution and then transform into immature lymphoid cells during early regeneration. The findings may support the recent reutilization concept that destroyed lymphocytes are phagocytized and reutilized by reticular cells in heteroplastic differentiation into immature lymphoid cells. In the cortex PAS-positive sudanophilic cells which are derived from the perivascular and subcapsular connective tissue appear with involutionary changes. They become gradually reduced again with progress of the regeneration of the cortex. During involution the medulla are temporarily filled with lymphocytes migrated from the cortex. The epithelial reticular cells in the medulla are found grouped in cords or clumps in the severely involuted thymus. In the medulla there are two types of PAS-positive epithelial reticular cells; one contains a large, colloid-like, PAS-positive inclusion within the cytoplasm and the other has cytoplasm diffusely filled with PAS-positive substance. During involution and early regeneration, the former type increases while the other shows almost no significant changes. Hassall's corpuscles somewhat increase in frequency during involution and early regeneration.     

Morphology of rat kidney and thymus after native and antibody-coupled cyclosporin A application (Reduced toxicity of targeted drug)

This study compares the toxic effects of native cyclosporin A (CyA) with those of targeted CyA that is conjugated with the anti-rat-thymocyte antibody of rabbit originvia the N-(2-hydroxypropyl)methacrylamide (HPMA) carrier bearing digestible, reactive oligopeptide side chains. Ten toxic doses of native CyA (50 mg/kg i.p.) given to young adult rats in the course of 14 d produced a severe renal lesion—diffuse microvacuolization of the proximal tubules in the deep cortex, and hypergranulation of juxtaglomerular regions. Severe atrophy of the thymic medulla was documented by morphometry. In the cortex the epithelial reticular (but not deep interdigitating) cells showed ultrastructural signs of severe degeneration and lysis. The immature CD4⁺8⁺ double-positive cortical lymphocytes were preserved whereas, the single-positive medullary thymocytes were greatly depleted; there was also a restriction of MHC class II antigen expression in the medulla. The number of medullary B cells was increased. The cytokeratin net was focally shrunken in the cortex and almost negative in the medulla, with loss of Hassall's corpuscles. After ten corresponding doses of antibody-targeted conjugated CyA no damage to the renal tubules and arterioles appeared and the antiGBM or immune-complex deposition was absent. The thymus had a normal medulla with numerous mature thymocytes and the cortical epithelial reticulum remained well preserved. Thus, the main toxic effects of CyA could be eliminated by targeting. The T-cell-targeted drug was tested for preserved immunosuppressive properties and non-toxic character of HPMA copolymer carrier.     

Intralobular lymphatic vessels and their relationship to blood vessels in the mouse thymus

Summary The spatial distribution and fine structure of the lymphatic vessels within the thymic lobules of normal and hydrocortisone-injected mice were studied by light- and electron microscopy. The lymphatic vessels of the cortex and medulla of normal thymus are irregularly shaped spaces closely associated with branches of the intralobular artery and vein. The overall distribution of these vessels in the greatly involuted thymus of hydrocortisone-treated mice is essentially the same as in the normal thymus. The wall of the lymphatic vessels consists of only a layer of endothelial cells supported by underlying reticular cells. The luminal surface of the endothelial cell is smooth, but trabecular processes are often seen. There are three morphological types of intercellular contacts between contiguous cells, namely, end-to-end, overlapping and interdigitating. The lymphatic vessel has anchoring filaments and collagen fibrils, but a basal lamina is either absent, or if present, is discontinuous. This is in contrast to the continuous basal lamina of the venule. The perivascular space surrounding the postcapillary venule opens into a terminal lymphatic vessel at the cortico-medullary junction and in the medulla. Lymphocytes are seen penetrating the lymphatic endothelium, particularly in acutely involuted thymuses. These findings suggest that the intralobular lymphatic vessels may originate from the vacuities that surround the postcapillary venules, and the lymphatic system may function as a pathway for the migration of lymphocytes into or out of the lymphatic circulation.     

Hemopoietic tissue in the adult newt,Notopthalmus viridescens

The presence of developmental stages of lymphocytes and their precurors, as revealed by serial and thin sections of hemopoietic organs of normal adult newts (Notopthalmus viridescens) suggests that lymphopoiesis is limited to the thymus, medulla of the spleen and, to a lesser degree, the intestine. Stromal cells, small lymphocytes, granulocytes, mature erythrocytes and melanocytes were observed either within or near the parenchyma of the thymus. The urodele thymus differs from the thymus of anurans and higher vertebrates in that it lacks a cortex and a medulla, myoid cells and Hassall's corpuscles.     

Immunohistology of lymphoid and non-lymphoid cells in the thymus in relation to T lymphocyte differentiation

The present paper reports the distribution of lymphoid and non-lymphoid cell types in the thymus of mice. To this purpose, we employed scanning electron microscopy and immunohistology. For immunohistology we used the immunoperoxidase method and incubated frozen sections of the thymus with 1) monoclonal antibodies detecting cell-surface-differentiation antigens on lymphoid cells, such as Thy-1, T-200, Lyt-1, Lyt-2, and MEL-14; 2) monoclonal antibodies detecting the major histocompatibility (MHC) antigens, H-2K, I-A, I-E, and H-2D; and 3) monoclonal antibodies directed against cell-surface antigens associated with cells of the mononuclear phagocyte system, such as Mac-1, Mac-2, and Mac-3. The results of this study indicate that subsets of T lymphocytes are not randomly distributed throughout the thymic parenchyma; rather they are localized in discrete domains. Two major and four minor subpopulations of thymocytes can be detected in frozen sections of the thymus: 1) the majority of cortical thymocytes are strongly Thy-1+ (positive), strongly T-200+, variable in Lyt-1 expression, and strongly Lyt-2+; 2) the majority of medullary thymocytes are weakly Thy-1+, strongly T-200+, strongly Lyt-1+, and Lyt-2- (negative); 3) a minority of medullary cells are weakly Thy-1+, T-200+, strongly Lyt-1+, and strongly Lyt-2+; 4) a small subpopulation of subcapsular lymphoblasts is Thy-1+, T-200+, and negative for the expression of Lyt-1 and Lyt-2 antigens; 5) a small subpopulation of subcapsular lymphoblasts is only Thy-1+ but T-200- and Lyt-; and 6) a small subpopulation of subcapsular lymphoblasts is negative for all antisera tested. Surprisingly, a few individual cells in the thymic cortex, but not in the medulla, react with antibodies directed to MEL-14, a receptor involved in the homing of lymphocytes in peripheral lymphoid organs. MHC antigens (I-A, I-E, H-2K) are mainly expressed on stromal cells in the thymus, as well as on medullary thymocytes. H-2D is also expressed at a low density on cortical thymocytes. In general, anti-MHC antibodies reveal epithelial-reticular cells in the thymic cortex, in a fine dendritic staining pattern. In the medulla, the labeling pattern is more confluent and most probably associated with bone-marrow-derived interdigitating reticular cells and medullary thymocytes. We discuss the distribution of the various lymphoid and non-lymphoid subpopulations within the thymic parenchyma in relation to recently published data on the differentiation of T lymphocytes.     

Structure of the non-lymphoid cells during the postnatal development of the rat lymph nodes

This study describes the postnatal development of the nonlymphoid cells with special reference to the fibroblastic reticulum cells (FRCs) and interdigitating cells (IDCs). The first lymphocytes of the neonatal lymph nodes are located in the developing deep cortex units (DCUs) identified by the Gomori's technique for reticulin fibres. Ultrastructural studies demonstrate that FRCs form the stroma of the DCUs. By light and electron microscopy, it is demonstrated that FRCs occupy the outer cortex in the following stages of development of the lymph nodes. Thus, FRCs form the stroma of the primary follicles and, later, are transformed in follicular dendritic cells (FDCs) of the germinal centres. Immature or pro-IDCs appear as migrating elements in the deep cortex of lymph nodes of the neonatal rats. The ultrastructure of the pro-IDCs resembles that of the mature IDCs but not that of the phagocytic cells. Pro-IDCs are transformed into mature IDCs whose cytoplasmic expansions contact lymphocytes via tight junctions. Some of these lymphocytes are likely apposed to FRCs of the DCUs. No cells containing Birbeck granules were found in the parenchyma of the lymph nodes during the postnatal development. The role of these nonlymphoid cells is discussed with respect to the immunologic function of mammalian lymph nodes.     

The pattern of lymphocytes in the thymus and spleen after labeling with 3H-thymidine and 3H-deoxycytidine.

Adult male untreated mice (NMRI) were investigated after radioactive labeling with 3H-thymidine and 3H-deoxycytidine to find out whether the lymphocytes in the cortex and medulla of the thymus as well as in the perifollicular and periarteriolar regions of the spleen show a labeling pattern which allows a classification into T- and B-lymphocytes. The percentages of radioactively labeled small lymphocytes and their mean grain counts were determined. The percentages of radioactively labeled small lymphocytes after 3H-TdR and 3H-CdR showed no significant differences in both splenic zones. The grain counts over the lymphocyte nuclei in the periarteriolar zone showed lower values after 3H-TdR than after 3H-CdR. The lymphocytes in the perifollicular zone were strongly labeled with 3H-TdR and weakly labeled with 3H-CdR. In the thymus medulla, lymphocytes were weakly labeled with 3H-thymidine and strongly labeled with 3H-CdR. In the cortex no significant differences were observed. 75 to 80% of the small lymphocytes in the peripheral blood were weakly and 20-25% strongly labeled after 3H-TdR. Therefore there are similarities in the radioactive labeling pattern of thymic medulla lymphocytes and that of small lymphocytes of the periarteriolar zone of the spleen by both DNA precursors. The small lymphocytes in the peripheral T-dependent tissue zones, for example in the spleen, as well as in the mixed lymphocyte population of the peripheral blood can be differentiated from the B-lymphocytes through the difference in the amount of incorporation of 3H-thymidine and 3H-deoxycytidine.     

SEM and TEM study of caprine superficial lymph nodes

The splenic capsule was characteristic, having dense connective tissue. Smooth muscle cells and unmyelinated nerve fibers were observed. Smooth muscle cells were found to be independent of blood vessels in both the capsule and trabeculae. Littoral cells separated the capsule from the subcapsular sinus. Highly branched reticular cells were associated with the sinuses. The cellular components (large and small lymphocytes, plasma and mast cells, and macrophages) of the cortex and medulla were observed and described. No Golgi apparatus was observed in small lymphocytes and two surface types (rough and smooth) were observed on lymphocytes. Russell bodies were not observed in plasma cells. The paracortical postcapillary venule had cuboidal endothelium with microvilli. Two shapes of lymphocytes were seen associated with the endothelium of postcapillary venules.     

Cellular distribution of prothymosin alpha and parathymosin in rat thymus and spleen

By means of immunohistochemical methods, we have investigated the cellular distribution of prothymosin alpha and parathymosin in rat thymus and spleen, using specific antibodies raised against thymosin alpha-1 and against parathymosin. We observed prothymosin alpha immunoreactivity in lymphoid cells both in thymus and spleen. In the thymus, prothymosin alpha staining was more marked in cortex than in medulla. In the spleen, prothymosin alpha was found in lymphocytes of the periarteriolar lymphatic sheaths and was especially prominent in the germinal centers. Parathymosin immunoreactivity in the thymus was mainly localized in the medulla; positive cells were reticuloepithelial cells from the thymic reticulum and the blood barrier. Thymocytes were negative. In spleen, parathymosin was found in reticular cells arranged in a ring between the periarteriolar lymphatic sheath and the marginal zone. Our results do not support an exclusive role for these peptides as immune system hormones or cytokines.     

B lymphocytes in the guinea pig thymus are specifically localized in the central area.

Since the central area is an integral part of the guinea pig thymus, the cells in this area were compared with those in the thymic cortex and medulla in cryostat-sections by using methods for demonstration of E-, EA-, EAC-adherence and surface membrane immunoglobulins. In the extra cortical central area (ECCA) 15 to 25% of the lymphocytes showed EAC-adherence and 5 to 10% appeared to bear surface membrane immunoglobulins (SIg). In the lymph sinuses up to 70% of the lymphocytes were EAC- and SIg-positive. A small amount of EAC-adhering cells was present in the medulla of the central area. Cortical lymphocytes were EAC- and SIg-negative. From these results we conclude that in the guinea pig thymus B lymphocytes are specifically localized in the central area.     

Macrophages and epithelial cells of the thymus gland. An ultrastructural study in the natterjack, Bufo calamita

In the present study, the ultrastructure of the stromal components, basically epithelial elements and macrophages, of the thymus of adult natterjacks, Bufo calamita has been analyzed. A network of stellate epithelial-reticular cells joined together by desmosomes, constitutes the main component of the thymic parenchyma in both cortex and medulla. In the medulla pale, electron-lucent epithelial cells, sometimes showing surface interdigitations, are striking elements. Moreover, uni- and multicellular epithelial cysts appear in the thymic medulla as well as granulated cells of possible endocrine significance. Remarkably, isolated or grouped gland cells whose morphology and cytoplasmic content resemble that of the skin glands, were occasionally found. Finally, macrophages, multinucleated giant cells and dendritic-like cells, the latter intimately associated to lymphocytes, occur in the thymus of Bufo calamita. The most remarkable morphologic characteristics of all those non-lymphoid cell types, as well as their possible functional significance are comparatively discussed with available information on the amphibian and higher vertebrate thymic cytoarchitecture.