Development of Immune System Organs

Abstract

Knowledge of the embryonic, fetal and postnatal histogenesis of immune system organs is important in the detection and interpretation of histopathologic alterations in nonclinical toxicology studies, particularly studies that are conducted in juvenile animals (exposed either in utero or postnatally). In addition to the direct, indirect, nutrition-, and stress-related histologic alterations that are encountered in immune system organs in nonjuvenile toxicology studies, investigators conducting studies involving juvenile animals must also be concerned with the normal embryonic, fetal and postnatal development and the potential for xenobiotic-associated influences on those normal processes. The histomorphology of the secondary immune system organs is heavily influenced by environmental factors, thus xenobiotic-related influences on nonimmune organs or functions, for example, the microbiome of the gastrointestinal tract, may have secondary effects on the development of immune system organs. Conversely, altered development of immune system organs may have secondary effects on the development of non-immune organs. Given the emphasis on histopathologic evaluation in detection of xenobiotic-associated influences, large sections of this article are focused on histomorphological rather than functional alterations. The laboratory rat is commonly used in nonclinical toxicology studies; thus, the rat is used as the major example of processes that occur during the development of immune system organs.