Thymus rudiments with their surrounding connective tissues were removed from 4-day-old larvae (stages 45-46) of Xenopus laevis, placed in shallow depression slides, and observed microscopically for 3 hours. In mesenchyme surrounding the rudiment, there were many cells frequently observed possessing conspicuous blebs or lobopodia and occasional constrictions. These cells consistently migrated toward the rudiment at the rate of approximately 10-35 μm/10 min. (60-210 μm/hr.). On attaching to the surface of thymus rudiments these cells became cone-shaped and then entered the rudiment. The migratory cells had histological characteristics of lymphoid stem cells: high nucleo-cytoplasmic ratio, prominent nucleolus, and basophilic cytoplasm. Scanning electron microscopy confirmed the occurrence of migratory cells with the above characteristics in the meshwork of fine mesenchymal fibers. These cells were no longer observable in 5-day larvae (stages 46-47), supporting earlier experimental results that immigration of thymis lymphocyte stem cells occurs only in 3-4-day-old larvae. © 1980 Pergamon Press Ltd. All rights reserved.
Tochinai, S. (1980). Direct observation of cell migration into Xenopus thymus rudiments through mesenchyme. Developmental and Comparative Immunology, 4(C), 273–282. https://doi.org/10.1016/S0145-305X(80)80031-0