Histogenesis of the epithelial component of rat thymus: an ultrastructural and immunohistological analysis

Despite the assumed importance of thymic cell microenvironments for governing T-cell maturation, little is known about the ontogeny of their cell components. A few studies have analyzed previously the ontogenetical development of rat thymic epithelium (Bogojevic et al. 1990. Period. Biol., 92:126; Kampinga and Aspinall 1990 Harwood Acad. Pub., London, pp. 149-186; Micic et al., 1991 Dev. Comp. Immunol., 15:443-450) and recently we have reported the development of both interdigitating/dendritic cells and macrophages (Vicente et al., 1994 Immunology, 82:75-81, 1995 Immunology, 85:99-105).

In the present work we analyze in situ ultrastructural, immunohistochemical, and histoenzymatically the appearance and development of the thymic epithelial cell component in both embryonic and neonatal Wistar rats with special emphasis on the origin of the different epithelial cell types, the occurrence or absence of a common precursor for these, and the expression of MHC molecules.

The thymic primordium of 13-day-old embryos is formed by a homogeneous population of primitive epithelial cells differentiating gradually into various epithelial cell subtypes of both the cortex and the medulla. In the cortex, subcapsular and stroma-supporting epithelial cells appear at days 14-15 as two structurally different cell entities. At the same time, stroma-supporting, keratinized, and vacuolated epithelial cells occur in the thymic medulla. These last two cell types differentiate subsequently into Hassall's bodies and hypertrophied cells. Lympho-epithelial cell complexes are identified in the deep cortex around birth, when the cortical parenchyma houses a transitional erythropoiesis. mAbs (His-39, RMC-20) which recognize medullary epithelial cells in the adult thymus stain positively cells of the thymic primordium as early as day 16 of embryonic life. Cortical epithelial cell markers (His-37, RMC-17) appear, however, slightly later and the subcapsulary region is not established until postnatal life. MHC class I and class II molecules can be identified on epithelial cells in the thymus of 15-day-old embryonic rats although they reach the highest expression around birth.

Our results confirm the heterogeneity of the thymic epithelial component, the persistence of primitive, non-differentiated epithelial cells morphologically similar to those occurring in the early thymic primordium in adult thymus, and the mutual relevance of epithelial cells and thymocytes for an adequate development of rat thymus gland.