Growth and Regression of the Germfree (Axenic) Thymus

Qualitatively, the growth patterns of the thymus of germfree and conventional mice are very similar, and the gross appearance is the same. In both groups of animals the period of thymus growth extends from birth to about 50 days, with accelerated growth during the first 2 weeks after birth, and peaks at about 15 days. Growth continues at a decelerated rate and regression of the thymus begins at about 50 days of age. A plateau is reached at about 200 days of age, but the thymus does not disappear completely. However, the thymus of conventional mice grows more rapidly and attains greater size than that of the germ-free mice.