An electron microscopic study of the development of the thymus anlage of athymic (nu/nu) mice.

Using electron microscopy, we examined the anlage of dysgenetic embryo thymuses of nu/nu mice aged 14.5-15 days, 16 days and 18 days p.c., with thymus anlage of nu/+ mice serving as controls. On comparing both types, we noted differences at all intervals which increased in a time-related manner. Lymphoid cells were found only very occasionally in dysgenetic thymus anlage up to day 16 p.c. while there were no capillaries at all in any of the stages investigated. In epithelial reticular cells, undemarcated (membrane non limited) spaces of varying size and containing PAS+ material were found in the cytoplasm (glycogen-like particles seen in electron microscopy). Based on our findings, and on general assumptions, we believe that the primary process occurring in the dysgenetic thymus is abnormal epithelial stroma differentiation resulting not only in a lack of lymphoid cells but, also, in a limited or probably absent vascular bed.