Hockenbury Discovering Psychology 5th txtbk

Prenatal Development373series of experiments conducted by teams led by Canadian neuroscientist MichaelMeaney (2001). He showed that newborn rats that were genetically predisposed tobe skittish, nervous, and high-strung would develop into calm, exploratory, andstress-resistant adult rats when raised by genetically unrelated, attentive mother rats.Conversely, newborn rats that were genetically predisposed to be calm and stressresistantgrew up to be nervous, high-strung, and easily stressed out when they wereraised by inattentive, genetically unrelated mother rats.The important point is that although the rats’ DNA did not change, the chemicalsthat control gene expression did change. The rats’ upbringing set in motiona cascade of epigenetic changes that changed their brain chemistry and literally“reprogrammed” their behavior (see Hyman, 2009; Sapolsky, 2004). Even morefascinating, the influence of their upbringing extended to the next generation: Thecalm, stress-resistant rats grew up to be attentive mothers themselves.Interactions among genes, and between the genotype and environmental influences,are two of the critical factors in considering the relationship between genotypeand phenotype. Beyond these factors, genes can also mutate, or spontaneouslychange, from one generation to the next. Further, DNA itself can be damaged byenvironmental factors, such as exposure to ultraviolet light, radiation, or chemicaltoxins. Just as a typographical error in a complex recipe can ruin a favorite dish, errorsin the genetic code can disrupt the production of the correct proteins and leadto birth defects or genetic disorders.The old “nature versus nurture” debatehas long since receded into scientificirrelevance. Instead, the frontier lies inunderstanding the mechanisms bywhich environmental factors—whetherexperiential, metabolic, microbiological,or pharmacologic—interact with thegenome to influence braindevelopment and to produce diverseforms of neural plasticity over thelifetime.STEVEN E. HYMAN (2009)Prenatal DevelopmentKey Theme• During the prenatal stage, the single-celled zygote develops into afull-term fetus.Key Questions• What are the three stages of prenatal development?• How does the brain develop?• What are teratogens?At conception, chromosomes from the biological mother and father combine toform a single cell—the fertilized egg, or zygote. Over the relatively brief span of ninemonths, that single cell develops into the estimated trillion cells that make up a newbornbaby. This prenatal stage has three distinct phases: the germinal period, theembryonic period, and the fetal period (see photos on page 374).The Germinal and Embryonic PeriodsThe germinal period, also called the zygotic period, represents the first two weeks ofprenatal development. During this time, the zygote undergoes rapid cell division beforebecoming implanted on the wall of the mother’s uterus. Some of the zygote’s cellswill eventually form the structures that house and protect the developing fetus and willprovide nourishment from the mother. By the end of the two-week germinal period,the single-celled zygote has developed into a cluster of cells called the embryo.The embryonic period begins with week 3 and extends through week 8. Duringthis time of rapid growth and intensive cell differentiation, the organs andmajor systems of the body form. Genes on the sex chromosomes and hormonalinfluences also trigger the initial development of the sex organs.Protectively housed in the fluid-filled amniotic sac, the embryo’s lifeline is theumbilical cord. Extending from the placenta on the mother’s uterine wall to theembryo’s abdominal area, the umbilical cord delivers nourishment, oxygen, andepigeneticsStudy of the cellular mechanisms that controlgene expression and of the ways thatgene expression impacts health and behaviorprenatal stageThe stage of development before birth;divided into the germinal, embryonic, andfetal periods.germinal periodThe first two weeks of prenatal development.embryonic periodThe second period of prenatal development,extending from the third week through theeighth week.