Ontogeny of the thymus in a teleost fish, Cyprinus carpio L.: developing thymocytes in the epithelial microenvironment

A monoclonal antibody, WCL9, specific for membrane molecules of a thymocyte subpopulation was used to detect these cells in situ during the ontogeny o f thymus. Cryo-sections revealed WCL9(+) cells in the rudiment of the thymu s (day 4 post fertilization); thereafter, the positive cells were observed exclusively in the cortex from the first appearance of thymic regionalizati on (week 3 post fertilization) until adult age. Whole-mount immunostaining of the thymus with WCL9 revealed the three-dimensional structure of the cor tex by specific staining. The presence and distribution of apoptotic cells during thymus development was studied by in situ end-labelling of fragmente d DNA. From week 4 post fertilization onwards, apoptotic cells were more fr equently detected in the cortex than medulla, suggesting a continuous selec tion of thymocytes in the cortex. Ultrastructural studies confirmed the pre sence of numerous cortical apoptotic cells inside macrophages. Electron microscopy provided evidence for the existence of epithelial heter ogeneity in the thymus. During the ontogeny, the differentiation of epithel ial cells was followed from the first weeks until the juvenile age. Cell ty pes were classified on the basis of their localization and cytological char acteristics as: i) limiting epithelial cells located in subcapsular. periva scular and peritrabecular zones; ii) reticular epithelial cells situated in medullary and cortical zones; iii) nurse-like cells at the border between the cortex and medulla, iiii) Hassall's body-like structures localized in t he medulla. This study could suggest the occurrence of a wide range of lymp ho-epithelial interactions throughout thymocytes differentiation. (C) 1999 Elsevier Science Ltd. All rights reserved.