Heterogeneity of epithelial cells in the human thymus

Abstract

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.

The thymus in “bare lymphocyte” syndrome: Significance of expression of major histocompatibility complex antigens on thymic epithelial cells in intrathymic T-cell maturation

“…Epithelial cells with an irregular nucleus with a large prominent nucleolus were most abundant. In light-microscopic histology, as well as in ultrastructural cytomorphology, both thymic biopsies did not differ from the normal appearance for age [19].…”

“…Sections were stained with hematoxylin and eosin. For electron microscopy, pieces of about 1 mm ~ were processed according to standard procedures [ 18,19].…”

Thymoma: Update for the New Millenium

“…Although the various histological subtypes of thymoma may be associated with different degrees of metastatic and invasive potential and, even in some cases, with various systemic syndromes, it is generally agreed that the cell of origin is epithelial and not lymphocytic [12,14]. In addition, overall prognosis of patients with thymoma has more to do with the degree of tumor invasiveness than the tumor's cytologic or histologic classification [13,15,16].…”

Parvalbumin in cortical epithelial cells of the pigeon thymus

“…The dark epithelial cells show high electron density both in the nucleus and cytoplasm and they constitute only a small proportion of the thymic cortical epithelial cells. Dark cells in mammalian thymic cortex appear to be degenerated as suggested by van de Wijngaert et al (1984). Dark and pale cortical epithelial cells are also seen in the thymus of the quelea, sparrow, and starling (Kendall & Frazier, 1979).…”