Origin and Renewal of Lymphocytes in Avian Embryo Thymuses studied in Interspecific Combinations

Abstract

Differences in the structure of the interphase nucleus between two species of birds, the Japanese quail (Coturnix coturnix japonica) and the chick (Gallus gallus), may be used to distinguish cells of different origins in interspecies combinations as we have shown previously^1–3. This method has been adapted to the study of certain problems in developmental biology. In the quail, an important part of the chromatin is condensed during interphase either in a single central mass or in several heterochromatic masses associated with the nucleolar RNA. In the chick the chromatin is dispersed in a fine network. When quail cells are transplanted into a chick embryo, or associated with chick tissues in vitro, the cells from each species retain their nuclear characteristics and can be identified in the chimaera. This is the case for the various cell types in the thymus: lymphocytes, reticular cells and connective elements, the origins of which can easily be distinguished after Feulgen and ith Rossenbeck staining. The reticular cells have large nuclei containing in the quail, one central prominent heterochromatic mass (Fig. 1a), and in the chick, a fine network of evenly ell dispersed chromatin (Fig. 1b). The same characteristics are found in the connective elements, except that their nuclei are smaller than those of reticular cells. In quail lymphocytes, each nucleus contains a large mass of DNA, irregular in shape and often attached to the nuclear membrane, and several smaller heterochromatic patches (Fig. 1a). Chick lymphocytes are characterised by many small strongly Feulgen-positive chromocentres (Fig. 1b).