Case Study Conclusion: Lead Danger
These Michigan National Guard Members are on a mission, but they are defending residents from a different type of threat than you might expect — lead in their drinking water. This picture was taken in January 2016 in Flint, Michigan, during what is widely known as the “Flint water crisis.” The crisis started when the city’s water source was switched to a new source that had highly corrosive water. The corrosive water caused lead in pipes to be leached into residents’ drinking water. As you learned at the beginning of this chapter, lead is highly toxic and particularly dangerous to young children, so this was (and continues to be) a major public health crisis. In order to help protect residents’ health, the National Guard was deployed to distribute clean water, water filters, and lead test kits door to door.
Contaminated water is just one source of possible lead exposure. There are many other sources, and one of the most common is lead-based paint. Lead-based paint was banned by the U.S. government for use in housing in 1978, but many older homes still have this paint under newer layers, as shown in the graph below. Lead paint and associated dust can be exposed when paint deteriorates, or when renovations occur. In fact, in 2014, the CDC estimated that 24 million housing units in the U.S. had deteriorating lead-based paint and associated dust.
Vanessa, Paul, and their 18-month-old son Lucas — who you learned about in the beginning of this chapter — live in a home built before 1978. Therefore, exposure to lead paint or lead-containing dust might be the source of Lucas’ elevated blood lead level. Lucas’ pediatrician Dr. Morrison recommended that they get their home inspected for lead. The results show that their home does have lead paint underneath other layers. Even though the paint is not currently cracking or peeling, it did when they first moved in, and they had sanded and painted over many areas. Dry sanding over lead paint can create large amounts of lead dust. The inspectors believe this was likely the source of Lucas’ lead exposure since their water supply and other possible sources were ruled out. Since Lucas is a toddler, he was crawling around, playing on the floor, and putting hands and objects in his mouth during and after the period of sanding — all of which are normal toddler behaviors, but ones that can make children at this age more susceptible to lead poisoning.
What about Vanessa, Lucas’ mother, who is three months pregnant? Fortunately, her blood lead level turned out to be relatively low — 3 µg/dL. Under 5 µg/dL is considered normal, although there is no known safe level. As you have learned, her developing offspring is considered a fetus at this stage. It has recently completed the embryonic stage, which is when the major events of gastrulation, neurulation, and organogenesis occur to create the three germ layers, the nervous system, and most of the organs of the human body. Additionally, limbs, facial features, and sex organs have started to develop by this time, as well as the extraembryonic structures (such as the placenta and amnion) that protect and nourish the developing embryo and fetus. Because so many major developmental events occur during the early stages of pregnancy, exposure to toxic substances during this period can cause severe damage or even death of the embryo or fetus. Lead exposure during pregnancy can cause impaired growth, impaired neural development, low birth weight, spontaneous abortion (miscarriage), and stillbirth.
This is why Vanessa, Paul, and Dr. Morrison were concerned that Vanessa might have been exposed to lead. Lead can cross the placenta from the blood of the mother to the fetus. Also, lead stored in the mother’s bones from prior exposure can become liberated during pregnancy as her body breaks down the bone for calcium to form the bones of the fetus. This effect can be minimized if the pregnant woman gets enough calcium from her diet instead. Finally, lead can be transmitted through breast milk, so it is very important that pregnant and lactating women be tested for lead if they are at risk of being exposed to it.
What about Paul, Lucas’ father? While lead exposure can cause a variety of adverse health effects in adults — including problems in memory and concentration; headache; high blood pressure; and abdominal, joint, and muscle pain — these symptoms typically only occur at higher lead levels than those that are dangerous for children. Because the bodies and brains of adults have generally completed their development by early adulthood, there is usually less of an impact of poisons like lead that affects developmental processes than there is for infants and young children who are still undergoing rapid development. However, lead exposure can cause lowered sperm count and increased the production of abnormal sperm in adult men, so if Paul wants to have more children, he may want to get his lead level checked.
Because Vanessa’s lead level is not high, it is likely that Paul’s level is not high either, although it still would be a good idea for him to get tested. In general, adults are less exposed to lead than children, because they are less likely to engage in behaviors such as putting unwashed hands or non-food objects into their mouths. Physiologically, adults also absorb less lead into their bloodstream than children, due to differences in their breathing rate and gut absorption. One exception is adults who have jobs that expose them to lead, such as lead smelter workers, automotive repair workers, construction workers, and plumbers. Adults in high-risk occupations should take precautions to limit their exposure, and they may need to get tested on a periodic basis — particularly if they are pregnant or plan to get pregnant. They should also take safety measures such as changing clothes and showering before coming home since people who work with lead can unwittingly bring lead dust home on their clothes, shoes, and skin, and expose children and other family members.
As you may recall, Lucas’ lead level was 10 µg/dL, which is a cause for concern. Now that Vanessa and Paul have identified the most likely source of exposure — paint dust generated during sanding — they can take steps to prevent future exposure. This includes frequent mopping and wet-wiping of flat surfaces (such as window sills) to remove dust; cleaning around doors and other areas where friction can generate paint dust; frequently inspecting paint for any damage or deterioration, and using only lead-safe certified professionals for any future painting or repairs. They will also make sure that Lucas washes his hands frequently and that he eats a balanced diet. Nutrients such as iron, calcium, and vitamin C can inhibit the absorption of lead or help protect the body from the damaging effects of lead, so it is particularly important that children eat a nutritious diet when there is lead in the home. There are many other steps you can take to prevent lead exposure, which you can find online from the U.S. EPA, the CDC, and other reliable sources.
Dr. Morrison will closely monitor Lucas to make sure his blood lead level goes down and that his development is occurring normally. Lead can cause developmental delays. As you have learned, as children grow, they go through typical stages of development, including the acquisition of specific motor, language, and cognitive skills around certain ages. When these milestones are not achieved within a typical timeframe, it is considered a developmental delay. By assessing Lucas’ developmental milestones, Dr. Morrison can help monitor whether Lucas’ early lead exposure has affected his development. Many of the impacts of lead exposure in infancy and toddlerhood are not obvious until later in life as the brain develops further and starts carrying out more complex tasks. Early lead exposure can lead to later learning disabilities, behavioral problems, and lower IQ. The effects on the nervous system are generally irreversible, although therapies such as speech or behavioral therapies may help the child function better. Lead can also cause other types of medical problems such as anemia, hearing problems, and delayed puberty. The image below illustrates some of the problems that lead exposure can cause in children.
Lead exposure in children is a serious public health problem. You can go online to read about the current status of the Flint water crisis, as well as the problem of childhood lead exposure more generally. Many experts agree that preventing lead exposure and more widespread blood lead level screening is critical to prevent permanent damage to children’s health. Infancy and early childhood is a wonderful time of tremendous growth and change in a person’s lifespan, but it is also a time that is highly vulnerable to damage — with potential lifelong consequences.
In this chapter, you learned about the growth and development of humans, from fertilization to old age. Specifically, you learned that:
- The germinal stage of development is the first and shortest of the stages of the human lifespan. It lasts between eight and nine days, beginning with fertilization and ending with implantation in the endometrium of the uterus, after which the developing organism is called an embryo.
- The germinal stage involves several different processes that change an egg and sperm first into a zygote, and then into an embryo. The processes include fertilization, cleavage, blastulation, and implantation.
- Fertilization takes place when a haploid sperm successfully enters a haploid egg and results in a single diploid cell called a zygote. This usually occurs in a Fallopian tube. Successful fertilization is enabled by the processes of chemotaxis, adhesion, and digestion.
- Cleavage refers to the first several mitotic cell divisions of the zygote. It takes place in the Fallopian tube and results in a solid ball of undifferentiated cells called a morula. The morula forms by about the fourth day after fertilization.
- Blastulation is the process in which the morula changes into a fluid-filled ball of differentiated cells called a blastocyst. It generally occurs during days 5 to 7 after fertilization. The major parts of the blastocyst are an internal cell mass called the embryoblast (which will develop into the embryo), an external layer of cells called the trophoblast (which will develop into the fetal part of the placenta and other extraembryonic structures), and a fluid-filled cavity called a blastocoele.
- Implantation is the process in which the blastocyst becomes embedded in the endometrium of the uterus. It occurs around day 8 or 9 after fertilization when trophoblast cells “hatch” from the zona pellucida and penetrate the endometrium.
- The embryonic stage of human development lasts from the time of implantation until the end of the eighth week after fertilization. Besides an increase in size, some of the changes that occur in the embryo include the formation of three cell layers, development of the nervous system, and the initial formation of most organs.
- During the second week after fertilization, the embryoblast differentiates into two groups of cells, called the epiblast and the hypoblast. Cell migration results in the formation of a two-layered (bilaminar) embryonic disc.
- By the end of the second week after fertilization, gastrulation occurs. In this process, the two-layered embryonic disc develops a third cell layer and a primitive gut. The three cell layers are germ layers that will give rise to different cells throughout the body. The endoderm (inner layer) will eventually develop into cells of most internal glands and organs; the mesoderm (middle layer) will develop into cells of organs such as the bones, muscles, and heart; and the ectoderm (outer layer) will later develop into the skin and nervous system cells.
- Neurulation begins in the third week after fertilization. In this process, which takes about two weeks, the embryo forms structures that will eventually become the nervous system. A structure called the neural tube forms that will later develop into the spinal cord and brain, and a structure called the neural crest forms that will later develop into peripheral nerves.
- Organogenesis, or the formation of organs, also begins during the third week after fertilization. It continues through the end of the embryonic stage, by which time most organs have at least started to develop. The heart is the first functional organ to develop in the embryo. The heart starts to beat and pump blood by the end of the third week, but it continues to develop for several more weeks.
- Other developments that occur in the embryo during the fifth through eighth weeks after fertilization include limb and digit formation; formation of ears, eyes, and other facial features; and the main prenatal development of the sex organs.
- The embryonic stage is a critical period of development. Genetic defects or harmful environmental exposures (such as alcohol or radiation) during this stage are likely to have devastating effects.
- Several extraembryonic structures form at the same time as the embryo and help the embryo grow and develop. They include the placenta, chorion, yolk sac, and amnion.
- The placenta is a temporary organ consisting of both fetal and maternal tissues that provides a connection between the embryo’s and mother’s blood for the exchange of substances.
- The chorion is a membrane that helps form the fetal portion of the placenta.
- The yolk sac provides nourishment to the early embryo until the placenta develops.
- The amnion is a membrane that forms a fluid-filled sac around the embryo and helps protect it.
- A fetus is a prenatal human being between the embryonic stage and birth. The fetal stage extends from the beginning of week 9 after fertilization to about 38 weeks after fertilization, which is the average time of birth.
- At the start of the fetal stage, the fetus is recognizable as a human being and possesses virtually all of the major body organs, although most of them are not yet fully developed and functional. The organs will continue to grow and develop during the fetal stage.
- Fetal developments that occur between weeks 9 and 15 after fertilization include differentiation of the reproductive organs. The thyroid, liver, pancreas, and kidneys also start functioning. The fetus is very active during this period, but the movements are mostly uncontrolled. Fine hair called lanugo starts to grow on the face and will eventually cover the body as well.
- Fetal developments that occur between weeks 16 and 26 after fertilization include the development of the senses of touch and hearing, the initial formation of alveoli in the lungs, beginning of ossification of the bones, and considerable muscle development. The bone marrow also starts producing blood cells, and waxy vernix develops to cover the fetus’s skin.
- Fetal developments that occur between weeks 27 and 38 include further development of the skeletal system, rapid body growth, and a substantial increase in body fat. Head hair grows thicker and coarser while the lanugo is shed. Vernix first increases and then disappears, usually before birth. The eyes develop to the point that the fetus can detect light.
- The heart and blood vessels are among the earliest organs to develop and function, but the circulation of blood in the fetus is different than the postnatal circulation will be because the lungs are not yet in use. Fetal hemoglobin is also different than adult hemoglobin. Fetal hemoglobin can bind with oxygen at lower pressures, which enables it to bind with oxygen from the mother’s blood in the placenta.
- The size of the fetus generally increases linearly during the fetal stage up until the last week or two before birth, when the rate of growth typically tapers off. Fetal growth deficit, called intrauterine growth restriction (IUGR), may occur because of maternal, fetal, or placental factors. Placental insufficiency is the leading cause of IUGR.
- The average weight of a full-term infant at birth is 3.4 kg (7.5 lb). Low birthweight is defined as a weight at birth of less than 2.5 kg (5.5 lb). Low birth weight is a major cause of mortality shortly after birth. It may occur because of IUGR or pre-term birth.
- Viability of the fetus refers to the point in fetal development at which the fetus is likely to survive outside the uterus. More than 90 percent of babies survive if they are born after 26 weeks and receive any necessary medical care. Babies born even a few weeks earlier have a much lower chance of surviving, mainly due to inadequately developed respiratory and nervous systems.
- Immediately after birth, an Apgar test is administered to determine whether the newborn needs urgent medical care. The baby is scored on five traits, including skin color and heart rate. The umbilical cord is also cut within seconds of birth, leaving a stub that will eventually dry out and fall off, forming the naval.
- Infancy refers to the first year of life after birth. An infant is defined as a human being between birth and the first birthday. A newborn baby is called a neonate up until the first four weeks after birth.
- Newborns may or may not have vernix or lanugo covering the skin, and they may or may not have head hair. Their body proportions are distinctive, and the head is very large relative to the body. Soft spots in the skull called fontanels — which allow the head to change shape slightly to fit through the birth canal — gradually ossify after birth.
- A well-nourished, full-term newborn averages about 51 cm (20 in.) in length and has an average birth weight of 3.4 kg (7.5 lb). A newborn typically loses a small amount of weight in the first week, but after that, a healthy neonate should start gaining weight rapidly.
- Newborns have well-developed senses of touch and hearing, and they can respond to different tastes and smells. However, their sense of vision is not yet fully developed, their visual acuity is poor, and they have limited color and depth perception.
- Infants are born with several reflexes that help them survive the first few months of life. They include crying for communication, suckling, and the rooting reflex, which helps them find a nipple. The Moro and grasping reflexes help them maintain close physical contact with the mother or another caregiver.
- Many important developments in motor, sensory, and cognitive abilities occur during infancy. There is variation among infants in the exact timing of these developments, but the sequence in which they occur is usually similar.
- The deciduous teeth generally start to emerge around six months of age. This is called teething, and it may cause discomfort and fussiness. Typically, all of the upper and lower incisors emerge during the first year.
- Infancy is the period of most rapid growth after birth. A healthy, well-nourished infant generally triples his birth weight and doubles his birth length by the first birthday. The head and trunk normally grow most rapidly, allowing rapid growth and development of the brain, heart, and lungs.
- Infancy is associated with a higher risk of death than any other life stage except old age. The infant mortality rate — defined as the number of infant deaths per one thousand live births — is an important measure of the level of health in a nation. It tends to be inversely correlated with a nation’s wealth.
- Legally, childhood is defined as the period of minority, from birth to the age of majority, or adulthood. Biologically, childhood is defined as the stage of a human organism between birth and adolescence. Following infancy, biological childhood includes early childhood, middle childhood, and pre-adolescence.
- Early childhood is the life stage between infancy and middle childhood. It is divided into toddlerhood (ages one to three) and the preschool years (ages three to five).
- Early childhood is a time of great physical, cognitive, and psychosocial development. Children go from just starting to walk and using a word or two at age one to riding a bike and using 1,500 words at age five. While the one-year-old toddler clings to her parents, the five-year-old preschooler runs off to play with her friends.
- Physical growth is slower in early childhood than it was in infancy, but still relatively rapid, especially in the toddler years. The remaining deciduous dentition also erupts during early childhood.
- Middle childhood is the life stage between early childhood and pre-adolescence. It covers the ages of six to ten when most children are in elementary school. During middle childhood, children are more independent and physically active than they were in the preschool years. They become more involved with friends and develop more sophisticated language and cognitive abilities.
- Pre-adolescence is the life stage between middle childhood and adolescence or ages 11 to 12 years. It coincides with the start of puberty, although few of the obvious physical changes of puberty have yet to occur. It is also a time of significant cognitive and psychosocial development. Pre-adolescents develop the ability to think abstractly, start to develop a sense of identity, and may experience “puppy love.”
- During both middle childhood and pre-adolescence, the deciduous teeth are lost and replaced by most of the permanent dentition. Physical growth and motor development are usually slow but steady during these two stages of childhood, but some children may begin the adolescent growth spurt during pre-adolescence.
- Adolescence is the period of transition between childhood and adulthood. It is generally considered to start with puberty, although in many children, puberty actually begins during pre-adolescence. Adolescence includes significant cognitive and psychosocial changes, some of which continue past the physical changes of puberty and into the late teens.
- Puberty is the period during which children become sexually mature. It includes maturation of the primary sex organs, the development of secondary sex characteristics, and the adolescent growth spurt.
- Puberty starts when the hypothalamus starts releasing pulses of gonadotropin-releasing hormone, which triggers the pituitary gland to secrete follicle stimulating hormone (FSH) and luteinizing hormone (LH). FSH and LH, in turn, stimulate the gonads to develop and release sex hormones. Sex hormones bring about most of the other changes of puberty.
- In boys, puberty generally begins between the ages of 11 and 12, when the external genitals and pubic hair start to develop. In girls, puberty generally begins between the ages of nine and ten, when the breasts and pubic hair start to develop.
- The adolescent growth spurt (AGS) is a period of rapid growth in height and weight. It also includes significant changes in body composition and shape. Girls start the growth spurt earlier than boys and typically have a somewhat lower peak growth rate. Sex differences in the AGS result in sexual dimorphism in adult size, body composition, and shape. Other body systems also undergo rapid growth and development during the AGS.
- Although the brain does not increase very much in size during adolescence, it does become more complex. The cerebral cortex becomes more folded, and unused pathways are eliminated. The most significant changes occur in the prefrontal cortex, which controls executive functions, such as decision making, abstract reasoning, and impulse control. By the age of 15 or so, many adolescents have basic thinking abilities comparable to those of adults.
- Psychosocial changes that occur during adolescence include the development of a stronger sense of personal identity and a personal system of moral and ethical values. Adolescents also become more emotionally separated from their parents, while peers and peer influence become more important to them. Many teens also become involved in romantic relationships for the first time.
- Adulthood is the stage of life between adolescence and death. The age at which death occurs varies considerably. The age at which adulthood starts varies depending on whether adulthood is defined biologically or legally. Adulthood is usually the longest stage of life, potentially lasting for many decades. It is generally divided into stages, such as the stages of early adulthood, middle adulthood, and old age.
- Early adulthood coincides more-or-less with the 20s and early 30s. During this stage, many people complete their education, start a career, and form intimate relationships. They may marry and start a family. Strength and physical performance typically reach their peak during early adulthood, as does fertility in females.
- Middle adulthood lasts from about the mid-30s to the mid-60s. Many adults raise a family and attain career goals during this stage. This is also the stage when most people start showing physical signs of aging and experience physical declines, including a decline in fertility. Some cognitive loss may also occur, and many people develop chronic diseases such as type 2 diabetes during middle adulthood.
- Old age begins in the mid-60s and lasts until death. Most people over the age of 65 have retired and have more free time. Some may have less social contact and experience loneliness, and many experience ageism. Physical declines that started in middle adulthood continue during old age. Most people become increasingly frail and have a greater risk of serious illnesses such as cancer. Cognitive impairment is common in old age and may become very serious in cases of disorders such as Alzheimer’s disease.
- The average age at death is given by the statistical measure called life expectancy, which varies by birth year and gender. For people born in the United States in 2015, life expectancy was 77 years for males and 82 years for females. Increasing numbers of Americans are living to be older than 100. The average maximum human lifespan is estimated to be 115 years, with an absolute upper limit of 125 years.
- The causes of aging (and ultimately death) are not known for certain. Factors that have been proposed fall into two general categories: programmed factors and damage-related factors. An example of the former is the shortening of telomeres; an example of the latter is mutations in DNA due to exposure to environmental mutagens.
In this chapter, you learned about the stages of human development and some of the factors — such as exposure to poisons like lead — that can damage the human organism. A disease is clearly a major factor that affects human health and lifespan. Read the next chapter to learn more about diseases in human populations, including causes, effects, treatments, and prevention of major types of diseases.
Chapter Summary Review
1. At which stages in the human lifespan do deciduous teeth, permanent teeth, and wisdom teeth generally emerge?
2. What is the shortest stage in the human lifespan? How long is it, and when does it occur?
3. What is generally the longest stage in the human lifespan? Why might this stage not be the longest for a given individual?
4. Which implants into the lining of the uterus?
5. Compare and contrast blastulation and gastrulation.
6. What is the term that refers to the first several mitotic divisions of the zygote?
7. At what point does the prenatal human start getting nutrients from its mother? How did it get nutrients before this point?
8. True or False: Blood does not normally mix between the mother and the fetus.
9. True or False: Adolescents have the highest rate of suicide of any age group.
10. True or False: The adolescent growth spurt is the most rapid period of growth that occurs after birth.
11. Hypothetically, what do you think would happen if a fetus had adult hemoglobin instead of fetal hemoglobin? Explain your answer.
12. For each of the stages or events below, identify approximately when they occur:
C. middle childhood
13. True or False: A fetus can usually survive if it is born at 26 weeks and without any medical intervention.
14. True or False: FSH and LH are involved in the onset of puberty in males.
15. Which cells in the blastocyst ultimately become muscle cells? Trace the development of these cells through gastrulation, naming each tissue they develop into along the way.
16. The chorion:
A. helps form the fetal portion of the placenta
B. is a sac that contains fluid that surrounds and protects the fetus
C. protects the developing zygote
D. is the lining of the uterus where implantation occurs
17. Pituitary tumors can affect the timing of the onset of puberty, causing it to occur abnormally early or late. Why do you think this might happen?
18. Why is it particularly important for a pregnant woman to avoid exposure to toxins during the embryonic stage of her pregnancy?
19. The neural tube develops into the:
A. peripheral nerves
B. spinal cord
D. B and C
20. What do you think would be more concerning — a five percent weight loss of an infant in the first week after birth, or a five percent weight loss between the infant's third and fourth weeks after birth? Explain your answer.
21. What factors would you need to take into consideration to determine whether a 16-year-old should be considered an adult?
22. Name a disease that is more common in old age than in other stages of life.
23. Describe some of the cognitive changes that occur between the ages of six and ten.
A. develop into extraembryonic tissues such as the placenta
B. are regions at the ends of chromosomes
C. are regions of bone growth during childhood and adolescence
D. turn into the primitive streak in the embryo
25. What are two characteristics of a newborn’s head that differ from that of adults?
26. The zona pellucida:
A. surrounds the cell membrane of the egg
B. surrounds the developing zygote
C. surrounds the developing fetus
D. A and B
27. Put the following events in order of when they typically occur during the human lifespan, from earliest to latest:
B. loss of deciduous teeth
C. loss of muscle mass
D. beginning of organogenesis
E. loss of grasping reflex