Hockenbury Discovering Psychology 5th txtbk

374 CHAPTER 9 Lifespan Developmentwater and carries away carbon dioxide and other wastes. The placenta is actuallya disk-shaped, vascular organ that prevents the mother’s blood from directlymingling with that of the developing embryo. Acting as a filter, the placentaprevents many harmful substances that might be present in the mother’sblood from reaching the embryo.The placenta cannot, however, filter out all harmful agents from themother’s blood. Harmful agents or substances that can cause abnormal developmentor birth defects are called teratogens. Generally, the greatest vulnerabilityto teratogens occurs during the embryonic stage, when major body systemsare forming. Known teratogens include:• Exposure to radiation• Toxic chemicals and metals, such as mercury, PCBs, and lead• Viruses and bacteria, such as German measles (rubella), syphilis, genital herpes,and human immunodeficiency virus (HIV)• Drugs taken by the mother, such as alcohol, cocaine, and heroinBy the end of the embryonic period, the embryo has grown from a clusterof a few hundred cells no bigger than the head of a pin to over an inch inlength. Now weighing about an ounce, the embryo looks distinctly human,even though its head accounts for about half its body size.Prenatal Development Although it is lessthan an inch long, the beginnings of arms,legs, and fingers can already be distinguishedin the 7-week-old embryo (topleft). The amniotic sac can be clearly seenin this photograph. The fetus at 4 months(top right) measures 6 to 10 inches long,and the mother may be able to feel thefetus’s movements. Notice the well-formedumbilical cord. Near full term (bottom),the 8-month-old fetus gains body fat tohelp the newborn survive outside themother’s uterus.teratogensHarmful agents or substances that cancause malformations or defects in anembryo or fetus.stem cellsUndifferentiated cells that can divide andgive rise to cells that can develop into anyone of the body’s different cell types.fetal periodThe third and longest period of prenataldevelopment, extending from the ninthweek until birth.Prenatal Brain DevelopmentBy three weeks after conception, a sheet of primitive neural cells has formed.Just as you might roll a piece of paper to make a tube, this sheet of neural cellscurls to form the hollow neural tube. The neural tube is lined with stem cells. Stemcells are cells that can divide indefinitely, renew themselves, and give rise to a varietyof other types of cells. At four weeks, this structure is not much bigger than agrain of salt (MacDonald, 2007).The neural stem cells divide and multiply, producing other specialized cells thateventually give rise to neurons and glial cells. Gradually, the top of the neural tubethickens into three bulges that will eventually form the three main regions of thebrain: the hindbrain, midbrain, and forebrain (see Figure 9.2). As the neural tubeexpands, it develops the cavities, called ventricles, that are found at the core of thefully developed brain. The ventricles are filled with cerebrospinal fluid, which cushionsand provides nutrients for the brain and spinal cord.During peak periods of brain development, new neurons are being generated atthe rate of 250,000 per minute (MacDonald, 2007). The developing brain cellsmultiply, differentiate, and begin their migration to their final destination. Guidedby the fibers of a special type of glial cell, the newly born neurons travel to specificlocations (Nadarajah & Parnavelas, 2002). They join with other developing neuronsand begin forming the structures of the developing nervous system.The Fetal PeriodThe third month heralds the beginning of the fetal period—the final and longeststage of prenatal development. The main task during the next seven months is forbody systems to grow and reach maturity in preparation for life outside the mother’sbody. By the end of the third month, the fetus can move its arms, legs, mouth, andhead. The fetus becomes capable of reflexive responses, such as fanning its toes ifthe sole of the foot is stroked and squinting if its eyelids are touched. During thefourth month, the mother experiences quickening—she can feel the fetus moving.The fetal brain is constantly changing, forming as many as 2 million synaptic connectionsper second. Connections that are used are strengthened, while connectionsthat remain unused are eventually pruned or eliminated. By the fifth month, all thebrain cells the person will have at birth are present. The fetus now has distinctsleep–wake cycles and periods of activity (Mirmiran & others, 2003). During thesixth month, the fetus’s brain activity becomes similar to that of a newborn baby.