Susceptible periods during embryogenesis of the heart and endocrine glands

One of the original principles of teratology states that, "Susceptibility t o teratogenesis Varies with the developmental stage at the time of exposure to an adverse influence" [Wilson JG. Environment and Birth Defects. New Yo rk:Academic Press, 1973]. The time of greatest sensitivity encompasses the period of organ formation during weeks 3-8 following fertilization in human gestation. At this time, stem cell populations for each organ's morphogene sis are established and inductive events for the initiation of differentiat ion occur. Structural defects of the heart and endocrine system are no exce ption to this axiom and have their origins during this time frame. Although the function and maturation of these organs may be affected at later stage s, structural defects and loss of cell types usually occur during these ear ly phases of development. Thus, to determine critical windows for studying mechanisms of teratogenesis, it is essential to understand the developmenta l processes that establish these organs.