Fatty infiltration of the thymus in response to illness in the pediatric population.

Under physiologic stress, glucocorticoids contribute to thymic involution. While steroids enhance fatty infiltration, this change has not been well studied in the pediatric thymus during illness. Evaluation of 130 thymuses from fetuses, infants and children determined the frequency of thymic fatty infiltration to be low (25%), particularly in fetal thymus (4%). In most cases, fatty infiltration was focal. There was a significant correlation with duration of illness, but not with patient age, gestational age, or underlying disease. There was significantly less fatty infiltration in fetal thymus compared to post-natal thymus, for the same degree of thymic involution. Only seven cases showed diffuse thymic fatty infiltration; all were post-natal associated with an infectious etiology. In contrast, fetal cases of chorioamnionitis seldom showed fatty infiltration and only focally, implying the stress response of fetal thymus differs post-natal, possibly related to the timing of adipose tissue development and fetal glucocorticoid response to stress.