Thymic barriers to antigen entry during the post-hatching development of the thymus of rainbow trout,Oncorhynchus mykiss

Abstract

The permeability of the pharyngeal epithelium was analysed, using ruthenium red and the entry of water-borne ferritin, into the thymus and gills of developing rainbow trout. In post-hatching trout, staining with ruthenium red did not reveal any gaps between the cells of the pharyngeal epithelium covering the thymus, the intercellular spaces being sealed by tight junctions. Developing trout (from hatching to 30 days old) were bathed in ferritin aqueous solutions, thymi and gills being sampled at different times thereafter. For most treatments, ferritin was not observed in the thymus. Only in 4-day-old fry, exposed for 30 min to ferritin, was this marker found: firstly in the outer surface of the thymic connective tissue capsule, on the thymic basement membrane, and later occasionally in the thymic parenchyma. In gills, however, ferritin was detected in fry of all ages after a 5 min bath. The amount of ferritin in the gills increased proportionally to the length of the bath, although it showed a similar distribution pattern in all cases. Shortly after bathing, it was found in the lateral and basal intercellular spaces of the epithelium of the secondary lamellae, on the basement membrane and in the blood capillaries. Later on, ferritin was also present in the pillar cells. Decreasing amounts remained in the gills 2 days after the bath. These results suggest that while the gills are involved in ferritin uptake from hatching onward, the pharyngeal epithelium covering the thymus constitutes an effective barrier for the entry of both antigenic and non-antigenic materials from the pharyngeal cavity into the thymic parenchyma. A small amount of ferritin reached the thymic parenchyma in 4-day-old trout, but through the connective capsule, rather than via the pharyngeal epithelium, reminiscent of the so-called transcapsular route in mammals.