Hematopoietic and immune system development in rodents can be segmented into five critical windows based on organogenesis, cell migration, and maturation of immune function. This well-documented series of developmental events has resulted in a working hypothesis that critical windows of vulnerability to chemical exposure exist for the developing immune system, and these critical windows may be differentially susceptible to chemical exposure (Dietert et al. 2000). This hypothesis is currently being tested and will need to be considered in designing experimental protocols to determine acute and persistent immunotoxic effects of environmentally applied chemicals. It is also important to realize that renewal of immunocompetent cells in postnatal animals continues to depend on differentiation of stem cells and expansion of progenitor cells in the bone marrow. This process is fundamentally different in neonatal, young, and older animals. For that reason, developmental windows of vulnerability to chemical exposure are not restricted to the period of prenatal development.
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