Phenotypic characterization of the thymus microenvironment study of the human thymus architecture.

Phenotypic characterization of the human thymic microenvironment shows the heterogeneity of the epithelial component. Using monoclonal antibody L191, we define here a new antigen common to the flat epithelial cells lining up the thymic capsule and septum, stellate epithelial cells from the subcapsular area and the medulla (M), but lacking on stellate epithelial cells from the inner cortex. Thus, this cell population can be characterized by a series of antigens (TE-4 and P19--shared with HTLV--previously described by B. Haynes et al.) which reflect a common differentiation program--including neuroendocrine specialization--with remarkable differences in the expression of these antigens, occurring during ontogeny and after birth. Another Ab we obtained shows particularities in the cytoskeletal organization of this cell population. Finally the organization of the mesenchymal component was also investigated with a third, anticollagen, antibody. A striking observation we made, using these antibodies, was the relationship between the thymic septum and flat epithelial cells with medullary and Hassal's body (HB) epithelial cells: septae deeply penetrate the medulla so that flat epithelial cells come into direct contact with medullary epithelial cells; in addition, HB are in close contact with the deepest portion of the septae, often separated solely by a single stellate cell. This architectural organization might be related to the differentiation--regeneration process of the thymic epithelium.