Dark reticular cells in the thymus of mice

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.

Heterogeneity of epithelial cells in the human thymus. An ultrastructural study

To evaluate interrelationships among epithelial cells, and between morphology and function in the microenvironment, we studied the ultrastructural morphology of epithelial cells in sections of human thymus from donors aged 2 months to 31 years. Six types of epithelial cells were observed: "subcapsular-perivascular" (type 1); "pale" (type 2); "intermediate" (type 3); "dark" (type 4); "undifferentiated" (type 5); and "large-medullary" (type 6). Cells of types 2, 3 and 4 were found throughout the organ. The type-2 to -4 epithelial cells may represent various stages in a differentiation process. In this, type-2 cells are very active and type-4 cells are possibly degenerating elements. Type-4 cells can also contribute to Hassall's corpuscles. Type-5 cells were located mainly in the cortico-medullary region and showed the morphological characteristics of undifferentiated elements. Type-6 cells were located exclusively in the medulla and displayed characteristics of cellular activity. Small Hassall's corpuscles consisted of type-6 epithelial cells; in larger corpuscles many nuclei of type-6 cells were found. Cells of types 2 and 6 contained tubular structures (diameter approximately 20 nm). Concerning the function of thymus epithelial cells, the features associated with protein synthesis observed in cellular types 2 and 6 make them likely candidates for humoral factor-producing and/or secreting elements. In addition, type-2 and -3 cells in the cortex appear to contribute to a special pattern of epithelium-lymphocyte interaction ("thymic nurse cells"), as demonstrated by the intracytoplasmic location of lymphocytes in the epithelial cells. The various steps in intrathymic T-cell maturation occur at locations in a microenvironment composed of morphologically distinct epithelial cells.

Ultrastructural studies on the thymus gland after the administration of phenylhydrazine to bank voles (Clethrionomys glareolus)

The thymus glands of wild and laboratory reared bank voles (Clethrionomys glareolus) were examined for ultrastructural changes before and after the administration of low (20 mg/kg) and high (60 mg/kg) doses of phenylhydrazine (PhZ) given intraperitoneally on days 0, 1 and 3 of a dosing regime. Erythrocytes developed in focal aggregations in the subcapsular and outer cortical zones. There was a marked depletion of the subcapsular cortex following drug treatment that was reversed with time. A large number of granulocytes were visible in the cortex after PhZ, and mast cells developed in situ.

Ultrastructural studies on erythropoiesis in the avian thymus. I. Description of cell types

Thymus lobes from three species of birds, Quelea quelea, Passer domesticus and Sturnus vulgaris, have been examined ultrastructurally. The component cell types are compared with their counterparts in mammalian thymus glands, and found to be similar. Greater differences between small, intermediate and enlarged lobes of one species than exist between species. Developing erythroid cells are present in most enlarging and some enlarged glands. They appear to be developing at the expense of lymphoid cells in some birds. The origin of these cells is discussed. Cells that are possible candidates for the production of some thymic hormones are also described.

Fine structure of epithelial thymus cysts in dogs

Epithelial thymus cysts in dogs were investigated by electron microscopy. Epithelial cells from one and the same cyst showed morphological differences. Regular columnar or cuboidal epithelial cells predominated. These had an elongated nucleus, poor in chromatin but with abundant cytoplasmic organelles, e.g. mitochondria, and a Golgi region with sacs and vacuoles. Other cells possesed a narrower, indented nucleus richer in chromatin, a cytoplasm with numerous tonofilaments and cell membranes showing interdigitations with other cell membranes. Some cysts displayed a pseudostratified epithelium. In these cells the same organelles were present as were seen in the cuboidal cells. Abundant cilia and microvilli were present in the lumenal cells. All these different morphological characteristics were considered to be different functional aspects of the same kind of thymus epithelial cell. The cells had a structure indicating a secretory-activity.