Development of sheep thymus in relation to in utero thymectomy experiments.

The developing thymus of sheep embryos has been examined from 20 days of gestation onwards. The initial relationships of the thymus to the parathyroid III and the pericardium are maintained so that the thymus develops as an extensive and complex structure consisting of paired cords of tissues on either side of the neck continuous with a single, fused structure in the anterior mediastinum. Histological examination of the developing thymus revealed that thymic lymphopoiesis commences at 36 days of gestation (term = 150 days). The results indicate that conclusions drawn from sheep thymectomy, usually performed after 60 days of gestation, must be interpreted with caution.