Development of Immune System Organs

Abstract

In addition to the direct, indirect, nutrition- and stress-related histologic alterations encountered in immune system organs of adult animals in toxicology studies, in juvenile toxicology studies the pathologist and toxicologist must also be concerned with the influence of normal postnatal development on the histologic appearance and functional attributes of immune system tissues. In addition to direct age-related differences in histomorphology, the postnatal development of the immune system is partially controlled by environmental factors, thus test article-related influences on non-immune organs or functions, e.g., the microbiome of the gastrointestinal tract, may have secondary effects on immune system organs. It is important that pathologists and toxicologists have a thorough understanding of the normal embryological and postnatal development of immune system organs in order to prevent misinterpretation of these normal changes as xenobiotic-associated. This chapter is focused on histomorphological rather than physiological alterations, though the intent certainly is not to dismiss the importance of traditional immunological assays in the detection of xenobiotic-associated influences. The laboratory rat is commonly used in nonclinical toxicology studies, thus the rat is used as the major example of the processes that occur during the postnatal development of immune system organs. An attempt is made to provide cursory information relating developmental and homeostatic changes in the rat as compared to those seen in other commonly used lab animals and humans, but a thorough presentation of these changes in humans and all animal species is beyond the practical scope of this Chapter.