Printed in U.S.A. Morphometric Assessment of Thymic Size Variation in Laboratory Rabbits*

Sections of thymus from New Zealand white rabbits used as controls in 28-day and 91-day percutaneous toxicity studies conducted at different laboratories were morphometrically assessed. Measurements of total thymic area, medullary area, and cortical area were greater for 28-day vs 91-day studies conducted at a given laboratory, but varied from one laboratory to another. Relative thymic measurements (percent medulla, percent cortex, and medu1la:cortex ratio) were similar for studies conducted at each laboratory and froam one laboratory to another regardless of study duration. A decrease in thymic size occurred between approx-imately 16 and 25 weeks of age (i.e., 28-day studies vs 91-day studies) due to proportionally equivalent decreases in both the cortical and medullary areas. The consistency of the relative measurements can be used to assist in distinguishing changes in thymic size due to aging from changes in size due to stress or toxicity which would be expected to differentially affect the conical and medullary areas. Appreciation of the normal variation in thymic size is needed when evaluating results of toxicity testing in rabbits. Data are provided as to the degree of normal variation of rabbit thymic size expected within and across percutaneous toxicity studies, with considerations for interpreting such data