HISTOGENESIS OF THE EPITHELIAL COMPONENT OF RAT THYMUS - AN ULTRASTRUCTURAL AND IMMUNOHISTOLOGICAL ANALYSIS

Background: Despite the assumed importance of thymic cell microenviron ments for governing T-cell maturation, little is known about the ontog eny of their cell components. A few studies have analyzed previously t he 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., 1 5:443-450) and recently we have reported the development of both inter digitating/dendritic cells and macrophages (Vicente et al., 1994 Immun ology, 82:75-81, 1995 Immunology, 85:99-105). Methods: In the present work we analyze in situ ultrastructural, immunohistochemical, and hist oenzymatically the appearance and development of the thymic epithelial cell component in both embryonic and neonatal Wistar rats with specia l emphasis on the origin of the different epithelial cell types, the: occurrence or absence of a common precursor for these, and the express ion of MHC molecules. Results: The thymic primordium of 13-day-old emb ryos is formed by a homogeneous population of primitive epithelial cel ls 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, keratini zed, and vacuolated epithelial cells occur in the thymic medulla. Thes e last two cell types differentiate subsequently into Hassall's bodies and hypertrophied cells. Lympho-epithelial cell complexes are identif ied in the deep cortex around birth, when the cortical parenchyma hous es a transitional erythropoiesis. mAbs (His-39, RMC-20) which recogniz e medullary epithelial cells in the adult thymus stain positively cell s of the thymic primordium as early as day 16 of embryonic life. Corti cal 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 epitheli al cells in the thymus of 15-day-old embryonic rats although they reac h the highest expression around birth. Conclusions: Our results confir m the heterogeneity of the thymic epithelial component, the persistenc e of primitive, non-differentiated epithelial cells morpho:logically s imilar to those occurring in the early thymic primordium in adult thym us, and the mutual relevance of epithelial cells and thymocytes for an adequate development of rat thymus gland. (C) 1996 Wiley-Liss, Inc.