Chapter 29



Chapter 29Development and InheritanceAn Introduction to Development and InheritanceLearning Outcomes29-1 Explain the relationship between differentiation and development, and specify the various stages of development.29-2Describe the process of fertilization, and explain how developmental processes are regulated.29-3 List the three stages of prenatal development, and describe the major events of each.An Introduction to Development and InheritanceLearning Outcomes29-4Explain how the three germ layers are involved in forming the extraembryonic membranes, and discuss the importance of the placenta as an endocrine organ.29-5Describe the interplay between the maternal organ systems and the developing fetus, and discuss the structural and functional changes in the uterus during gestation.An Introduction to Development and InheritanceLearning Outcomes29-6 List and discuss the events that occur during labor and delivery.29-7Identify the features and physiological changes of the postnatal stages of life.29-8Relate basic principles of genetics to the inheritance of human traits. An Introduction to Development and InheritanceDevelopmentGradual modification of anatomical structures and physiological characteristics from fertilization to maturity InheritanceTransfer of genetic material from generation to generation29-1 DevelopmentDifferentiation Creation of different types of cells required in developmentOccurs through selective changes in genetic activityAs development proceeds, some genes are turned off, others are turned onFertilization Also called conceptionWhen development begins29-1 DevelopmentEmbryonic Development Occurs during first two months after fertilizationStudy of these events is called embryologyFetal DevelopmentBegins at start of ninth weekContinues until birth29-1 DevelopmentPrenatal Development Embryonic and fetal development stagesPostnatal DevelopmentCommences at birthContinues to maturity, the state of full development or completed growth29-1 DevelopmentInheritanceTransfer of genetically determined characteristics from generation to generation GeneticsStudy of mechanisms responsible for inheritance29-2 FertilizationFertilization Fusion of two haploid gametes, each containing 23 chromosomesProduces zygote containing 46 chromosomesSpermatozoon Delivers paternal chromosomes to fertilization siteTravels relatively large distance Is small, efficient, and highly streamlined29-2 FertilizationGamete Provides: Cellular organellesInclusionsNourishmentGenetic programming necessary to support development of embryo for a week29-2 FertilizationFertilization Occurs in uterine tube within a day after ovulationSecondary oocyte travels a few centimetersSpermatozoa must cover distance between vagina and ampullaCapacitation Must occur before spermatozoa can fertilize secondary oocyteContact with secretions of seminal glandsExposure to conditions in female reproductive tract29-2 FertilizationHyaluronidase Enzyme breaks down bonds between adjacent follicle cellsAllows spermatozoon to reach oocyteAcrosinIs a proteolytic enzymeIs required to reach oocyte29-2 FertilizationAcrosomes Release hyaluronidase and acrosinPenetrate corona radiata, zona pellucida, toward oocyte surfaceOocyte ActivationContact and fusion of cell membranes of sperm and oocyteFollows fertilizationOocyte completes meiosis II, becomes mature ovum29-2 FertilizationPolyspermy Fertilization by more than one spermPrevented by cortical reactionCortical Reaction Releases enzymes that: Inactivate sperm receptorsHarden zona pellucida29-2 FertilizationFemale Pronucleus Nuclear material remaining in ovum after oocyte activationMale PronucleusSwollen nucleus of spermatozoonMigrates to center of cell29-2 FertilizationAmphimixis Fusion of female pronucleus and male pronucleusMoment of conceptionCell becomes a zygote with 46 chromosomesFertilization is complete29-2 FertilizationCleavageSeries of cell divisionsProduces daughter cellsDifferentiation Involves changes in genetic activity of some cells but not others29-3 GestationInductionCells release chemical substances that affect differentiation of other embryonic cellsCan control highly complex processesGestationTime spent in prenatal developmentConsists of three integrated trimesters, each three months long29-3 Gestation First TrimesterPeriod of embryonic and early fetal developmentRudiments of all major organ systems appear Second TrimesterDevelopment of organs and organ systemsBody shape and proportions change Third TrimesterRapid fetal growth and deposition of adipose tissueMost major organ systems are fully functional29-4 The First TrimesterFirst TrimesterIncludes four major stages Cleavage Implantation Placentation Embryogenesis29-4 The First TrimesterCleavage Sequence of cell divisions begins immediately after fertilizationZygote becomes a pre-embryo, which develops into multicellular blastocystEnds when blastocyst contacts uterine wallImplantationBegins with attachment of blastocyst to endometrium of uterus Sets stage for formation of vital embryonic structures29-4 The First TrimesterPlacentationOccurs as blood vessels form around periphery of blastocyst and placenta developsEmbryogenesisFormation of viable embryoEstablishes foundations for all major organ systems29-4 The First TrimesterThe First TrimesterMost dangerous period in prenatal life40 percent of conceptions produce embryos that survive past first trimester29-4 The First TrimesterCleavage and Blastocyst FormationBlastomeres Identical cells produced by cleavage divisionsMorulaStage after three days of cleavagePre-embryo is solid ball of cells resembling mulberryReaches uterus on day 429-4 The First TrimesterCleavage and Blastocyst FormationBlastocyst Formed by blastomeresHollow ball with an inner cavityKnown as blastocoele29-4 The First TrimesterCleavage and Blastocyst FormationTrophoblast Outer layer of cells separate outside world from blastocoeleCells responsible for providing nutrients to developing embryo29-4 The First TrimesterCleavage and Blastocyst FormationInner cell mass Clustered at end of blastocystExposed to blastocoele Insulated from contact with outside environment by trophoblastWill later form embryo29-4 The First TrimesterImplantation Occurs seven days after fertilizationBlastocyst adheres to uterine liningTrophoblast cells divide rapidly, creating several layers29-4 The First TrimesterImplantation Cellular trophoblast Cells closest to interior of blastocyst Syncytial trophoblast Outer layerErodes path through uterine epithelium by secreting hyaluronidase29-4 The First TrimesterEctopic Pregnancy Implantation occurs outside uterusDoes not produce viable embryoCan be life threateningLacunaeTrophoblastic channels carrying maternal blood29-4 The First TrimesterFormation of the Amniotic CavityVilli extend away from trophoblast into endometriumIncrease in size and complexity until day 21Amniotic CavityA fluid-filled chamberInner cell mass is organized into an oval sheet two layers thick Superficial layer faces amniotic cavityDeeper layer is exposed to fluid contents of blastocoele29-4 The First TrimesterGastrulation and Germ Layer FormationFormation of third layer of cellsCells in specific areas of surface move toward central line Known as primitive streak29-4 The First TrimesterPrimitive StreakMigrating cells leave surface and move between two layersCreates three distinct embryonic layers, or germ layers Ectoderm: consists of the superficial cells that did not migrate into interior of inner cell mass Endoderm: consists of cells that face blastocoele Mesoderm: consists of poorly organized layer of migrating cells between ectoderm and endoderm29-4 The First TrimesterEctodermal ContributionsIntegumentary system: Epidermis, hair follicles and hairs, nails, and glands communicating with the skin (sweat glands, mammary glands, and sebaceous glands)Skeletal system: Pharyngeal cartilages and their derivatives in adults (portion of sphenoid, the auditory ossicles, the styloid processes of the temporal bones, the cornu and superior rim of the hyoid bone)Nervous system: All neural tissue, including brain and spinal cord29-4 The First TrimesterEctodermal ContributionsEndocrine system: Pituitary gland and adrenal medullaeRespiratory system: Mucous epithelium of nasal passagewaysDigestive system: Mucous epithelium of mouth and anus, salivary glands29-4 The First TrimesterMesodermal ContributionsIntegumentary system: Dermis and hypodermisSkeletal system: All components except some pharyngeal derivativesMuscular system: All components29-4 The First TrimesterMesodermal ContributionsEndocrine system: Adrenal cortex, endocrine tissues of heart, kidneys, and gonadsCardiovascular system: All components29-4 The First TrimesterMesodermal ContributionsLymphatic system: All componentsUrinary system: The kidneys, including the nephrons and the initial portions of the collecting systemReproductive system: The gonads and the adjacent portions of the duct systemsMiscellaneous: The lining of the body cavities (pleural, pericardial, and peritoneal) and the connective tissues that support all organ systems29-4 The First TrimesterEndodermal ContributionsEndocrine system: Thymus, thyroid gland, and pancreasRespiratory system: Respiratory epithelium (except nasal passageways) and associated mucous glandsDigestive system: Mucous epithelium (except mouth and anus), exocrine glands (except salivary glands), liver, and pancreas29-4 The First TrimesterEndodermal ContributionsUrinary system: Urinary bladder and distal portions of the duct systemReproductive system: Distal portions of the duct system, stem cells that produce gametes29-4 The First TrimesterEmbryonic Disc Oval, three-layered sheetProduced by gastrulationWill form body of embryoRest of blastocyst will be involved in forming extraembryonic membranes29-4 The First TrimesterFormation of the Extraembryonic Membranes Support embryonic and fetal developmentYolk sacAmnionAllantoisChorion29-4 The First TrimesterThe Yolk Sac Begins as layer of cells spread out around outer edges of blastocoele to form complete pouchImportant site of blood cell formation29-4 The First TrimesterThe AmnionCombination of mesoderm and ectodermEctodermal layer enlarges and cells spread over inner surface of amniotic cavityMesodermal cells create outer layerContinues to enlarge through developmentAmniotic fluid Surrounds and cushions developing embryo or fetus29-4 The First TrimesterThe Allantois Sac of endoderm and mesodermBase later gives rise to urinary bladderThe Chorion Combination of mesoderm and trophoblastBlood vessels develop within mesodermRapid-transit system for nutrients that links embryo with trophoblastFirst step in creation of functional placenta29-4 The First TrimesterChorionic Villi In contact with maternal tissuesCreate intricate network within endometrium carrying maternal blood29-4 The First TrimesterPlacentationBody stalk Connection between embryo and chorionContains distal portions of allantois and blood vessels that carry blood to and from placentaYolk stalk Narrow connection between endoderm of embryo and yolk sac29-4 The First TrimesterDecidua Capsularis Thin portion of endometriumNo longer participates in nutrient exchange and chorionic villi in region disappearDecidua Basalis Disc-shaped area in deepest portion of endometriumWhere placental functions are concentratedDecidua Parietalis Rest of the uterine endometriumNo contact with chorion29-4 The First TrimesterUmbilical Cord Connects fetus and placentaContains allantois, placental blood vessels, and yolk stalkPlacental Circulation Through paired umbilical arteries Returns in single umbilical vein29-4 The First TrimesterThe Endocrine Placenta Synthesized by syncytial trophoblast, released into maternal bloodstreamHuman chorionic gonadotropin (hCG)Human placental lactogen (hPL)Placental prolactinRelaxinProgesteroneEstrogens29-4 The First TrimesterHuman Chorionic Gonadotropin (hCG) Appears in maternal bloodstream soon after implantationProvides reliable indication of pregnancyPregnancy ends if absent29-4 The First TrimesterHuman Placental Lactogen (hPL) Prepares mammary glands for milk productionSynergistic with growth hormone at other tissuesEnsures adequate glucose and protein is available for the fetusPlacental Prolactin Helps convert mammary glands to active status29-4 The First TrimesterRelaxin A peptide hormone secreted by placenta and corpus luteum during pregnancyIncreases flexibility of pubic symphysis, permitting pelvis to expand during deliveryCauses dilation of cervixSuppresses release of oxytocin by hypothalamus and delays labor contractions29-4 The First TrimesterEmbryogenesis Body of embryo begins to separate from embryonic discBody of embryo and internal organs start to formFolding and differential growth of embryonic disc produce bulge that projects into amniotic cavityProjections are head fold and tail foldOrganogenesis Process of organ formation29-5 The Second and Third TrimestersSecond Trimester Fetus grows faster than surrounding placentaThird Trimester Most of the organ systems become readyGrowth rate starts to slowLargest weight gainFetus and enlarged uterus displace many of mother’s abdominal organs29-5 The Second and Third TrimestersPregnancy and Maternal SystemsDeveloping fetus is totally dependent on maternal organ systems for nourishment, respiration, and waste removalMaternal adaptations include increases in:Respiratory rate and tidal volumeBlood volumeNutrient and vitamin intakeGlomerular filtration rateSize of uterus and mammary glands29-5 The Second and Third TrimestersProgesterone Released by placenta Has inhibitory effect on uterine smooth musclePrevents extensive, powerful contractionsOpposition to Progesterone Three major factorsRising estrogen levelsRising oxytocin levelsProstaglandin production29-5 The Second and Third TrimestersStructural and Functional Changes in the UterusFalse labor Occasional spasms in uterine musculatureContractions not regular or persistentTrue labor Results from biochemical and mechanical factorsContinues due to positive feedbackLabor contractions Begin in myometrium29-6 LaborParturition Is forcible expulsion of fetus Contractions Begin near top of uterus, sweep in wave toward cervixStrong, occur at regular intervals, increase in force and frequencyChange position of fetus, move it toward cervical canal29-6 LaborStages of Labor Dilation stage Expulsion stage Placental stage29-6 LaborDilation Stage Begins with onset of true laborCervix dilates Fetus begins to shift toward cervical canalHighly variable in length, but typically lasts over eight hoursFrequency of contractions steadily increasesAmniochorionic membrane ruptures (water breaks)29-6 LaborExpulsion Stage Begins as cervix completes dilationContractions reach maximum intensityContinues until fetus has emerged from vaginaTypically less than two hoursDelivery Arrival of newborn infant into outside world29-6 LaborEpisiotomy Incision through perineal musculatureNeeded if vaginal canal is too small to pass fetusRepaired with sutures after delivery29-6 LaborCesarean Section (C-section)Removal of infant by incision made through abdominal wallOpens uterus just enough to pass infant’s headNeeded if complications arise during dilation or expulsion stages29-6 LaborPlacental Stage Muscle tension builds in walls of partially empty uterusTears connections between endometrium and placentaEnds within an hour of delivery with ejection of placenta, or afterbirthAccompanied by a loss of blood29-6 LaborPremature Labor Occurs when true labor begins before fetus has completed normal developmentNewborn’s chances of surviving are directly related to body weight at delivery29-6 LaborImmature Delivery Refers to fetuses born at 25–27 weeks of gestationMost die despite intensive neonatal careSurvivors have high risk of developmental abnormalitiesPremature Delivery Refers to birth at 28–36 weeksNewborns have a good chance of surviving and developing normally29-6 LaborDifficult DeliveriesForceps delivery Needed when fetus faces mother’s pubis instead of sacrumRisks to infant and mother are reduced if forceps are usedForceps resemble large, curved salad tongsUsed to grasp head of fetus29-6 LaborDifficult DeliveriesBreech birth Legs or buttocks of fetus enter vaginal canal first instead of headUmbilical cord can become constricted, cutting off placental blood flowCervix may not dilate enough to pass headProlongs deliverySubjects fetus to severe distress and potential injury29-6 LaborMultiple BirthsDizygotic twins Also called “fraternal” twinsDevelop when two separate oocytes were ovulated and subsequently fertilizedGenetic makeup not identical70 percent of twins29-6 LaborMultiple BirthsMonozygotic twins Also called “identical” twinsResult either from:Separation of blastomeres early in cleavageSplitting of inner cell mass before gastrulationGenetic makeup is identical because both formed from same pair of gametes29-6 LaborMultiple BirthsConjoined twins Siamese twinsGenetically identical twinsOccurs when splitting of blastomeres or of embryonic disc is not completed29-6 LaborRates of Multiple BirthsTwins in 1 of every 89 birthsTriplets in 1 of every 892 (7921) birthsQuadruplets in 1 of every 893 (704,969) births29-7 Postnatal LifeFive Life Stages Neonatal period Infancy Childhood Adolescence Maturity29-7 Postnatal LifeDuration of Life StagesNeonatal period: extends from birth to 1 monthInfancy: 1 month to 2 years of ageChildhood: 2 years until adolescenceAdolescence: period of sexual and physical maturationSenescence: process of aging that begins at end of development (maturity)29-7 Postnatal LifeThe Neonatal Period, Infancy, and ChildhoodTwo major events occurOrgan systems become fully operationalIndividual grows rapidly and body proportions change significantlyPediatrics Medical specialty focusing on postnatal development from infancy to adolescence29-7 Postnatal LifeThe Neonatal Period Transition from fetus to neonateNeonate NewbornSystems begin functioning independentlyRespiratoryCirculatoryDigestiveUrinary29-7 Postnatal LifeLactation and the Mammary GlandsColostrum Secretion from mammary glandsIngested by infant during first two to three daysContains more proteins and less fat than breast milkMany proteins are antibodies that help ward off infections until immune system is functionalMucins present that inhibit replication of rotavirusesAs production drops, mammary glands convert to milk production29-7 Postnatal LifeBreast Milk Consists of water, proteins, amino acids, lipids, sugars, and saltsAlso contains large quantities of lysozymes—enzymes with antibiotic propertiesMilk let-down reflex Mammary gland secretion triggered when infant sucks on nippleContinues to function until weaning, typically one to two years29-7 Postnatal LifeInfancy and Childhood Growth occurs under direction of circulating hormonesGrowth hormoneAdrenal steroidsThyroid hormonesGrowth does not occur uniformlyBody proportions gradually change29-7 Postnatal LifeAdolescence and MaturityPuberty is a period of sexual maturation and marks the beginning of adolescenceGenerally starts at age 12 in boys, age 11 in girlsThree major hormonal events interactHypothalamus increases production of GnRHCirculating levels of FSH and LH rise rapidlyOvarian or testicular cells become more sensitive to FSH and LHHormonal changes produce sex-specific differences in structure and function of many systems29-7 Postnatal LifeAdolescence Begins at pubertyContinues until growth is completedMaturity (Senescence)AgingReduces functional capabilities of individualAffects homeostatic mechanismsSex hormone levels decline at menopause or male climacteric29-7 Postnatal LifeGeriatrics Medical specialty dealing with problems associated with agingTrained physicians, or geriatricians29-7 Postnatal LifeEffects of Aging on Organ SystemsThe characteristic physical and functional changes that are part of the aging process affect all organ systemsExamples discussed in previous chapters include the following:A loss of elasticity in the skin that produces sagging and wrinkling (pp. 172–173)A decline in the rate of bone deposition, leading to weak bones, and degenerative changes in joints that make them less mobile (pp. 199, 283)Reductions in muscular strength and ability (p. 377)29-7 Postnatal LifeEffects of Aging on Organ SystemsExamples discussed in previous chapters include the following:Impairment of coordination, memory, and intellectual function (p. 557)Reductions in the production of, and sensitivity to, circulating hormones (p. 644–646)Appearance of cardiovascular problems and a reduction in peripheral blood flow that can affect a variety of vital organs (p. 775)Reduced sensitivity and responsiveness of the immune system, leading to infection, cancer, or both (p. 825)29-7 Postnatal LifeEffects of Aging on Organ SystemsExamples discussed in previous chapters include the following:Reduced elasticity in the lungs, leading to decreased respiratory function (p. 873)Decreased peristalsis and muscle tone along the digestive tract (p. 928)Decreased peristalsis and muscle tone in the urinary system, coupled with a reduction in the glomerular filtration rate (p. 928)Functional impairment of the reproductive system, which eventually becomes inactive when menopause or the male climacteric occurs (pp. 1088–1089)29-8 InheritanceNucleated Somatic Cells Carry copies of original 46 chromosomes present in zygoteGenotype Chromosomes and their component genesContain unique instructions that determine anatomical and physiological characteristicsDerived from genotypes of parentsPhenotype Physical expression of genotypeAnatomical and physiological characteristics29-8 InheritancePatterns of InheritanceHomologous chromosomes Members of each pair of chromosomes23 pairs carried in every somatic cellAt amphimixis, one member of each pair is contributed by spermatozoon, other by ovum29-8 InheritancePatterns of InheritanceAutosomal chromosomes 22 pairs of homologous chromosomesMost affect somatic characteristicsEach chromosome in pair has same structure and carries genes that affect same traits29-8 InheritancePatterns of InheritanceSex chromosomes Last pair of chromosomesDetermine whether individual is genetically male or femaleKaryotype Entire set of chromosomesLocus Gene’s position on chromosome29-8 InheritancePatterns of InheritanceAlleles are various forms of given geneAlternate forms determine precise effect of gene on phenotypeHomozygous Both homologous chromosomes carry same allele of particular geneSimple inheritance Phenotype determined by interactions between single pair of alleles29-8 InheritanceInteractions between AllelesHeterozygous Homologous chromosomes carry different allele of particular geneResulting phenotype depends on nature of interaction between allelesStrict dominance Dominant allele expressed in phenotype, regardless of conflicting instructions carried by other allele29-8 InheritanceInteractions between AllelesRecessive allele Expressed in phenotype only if same allele is present on both chromosomes of homologous pairIncomplete dominance Heterozygous alleles produce unique phenotypeCodominance Exhibits both dominant and recessive phenotypes for traits29-8 InheritancePenetrance and Expressivity Penetrance Percentage of individuals with particular genotype that show “expected” phenotypeExpressivity Extent to which particular allele is expressedTeratogens Factors that result in abnormal development29-8 InheritancePredicting InheritancePunnett square Simple box diagram used to predict characteristics of offspringPolygenic inheritance Involves interactions among alleles on several genesCannot predict phenotypic characteristics using Punnett squareLinked to risks of developing several important adult disorders29-8 InheritancePredicting InheritanceSuppression One gene suppresses otherSecond gene has no effect on phenotypeComplementary gene action Dominant alleles on two genes interact to produce phenotype different from that seen when one gene contains recessive alleles29-8 InheritanceSources of Individual Variation During meiosis, maternal and paternal chromosomes are randomly distributedEach gamete has unique combination of maternal and paternal chromosomes29-8 InheritanceGenetic Recombination During meiosis, various changes can occur in chromosome structure, producing gametes with chromosomes that differ from those of each parentGreatly increases range of possible variation among gametesCan complicate tracing of inheritance of genetic disorders29-8 InheritanceGenetic Recombination Crossing over Parts of chromosomes become rearranged during synapsis When tetrads form, adjacent chromatids may overlapTranslocation Reshuffling processChromatids may break, overlapping segments trade places29-8 InheritanceGenetic Recombination Genomic imprinting During recombination, portions of chromosomes may break away and be deletedEffects depend on whether abnormal gamete is produced through oogenesis or spermatogenesis29-8 InheritanceGenetic Recombination Chromosomal abnormalities Damaged, broken, missing, or extra copies of chromosomesFew survive to full termProduce variety of serious clinical conditions29-8 InheritanceMutation Changes in nucleotide sequence of alleleSpontaneous mutations Result of random errors in DNA replicationErrors relatively common, but in most cases error is detected and repaired by enzymes in nucleusErrors that go undetected and unrepaired have potential to change phenotypeCan produce gametes that contain abnormal alleles29-8 InheritanceCarriers Individuals who are heterozygous for abnormal allele but do not show effects of mutation29-8 InheritanceSex-Linked InheritanceSex Chromosomes X Chromosome Considerably larger than YHas more genes than does Y chromosomeCarried by all oocytesY Chromosome Includes dominant alleles specifying that the individual will be maleNot present in females29-8 InheritanceSperm Carry either X or Y chromosomeBecause males have one of each, can pass along eitherX-Linked Genes that affect somatic structuresCarried by X chromosomeInheritance does not follow pattern of alleles on autosomal chromosomes29-8 InheritanceThe Human Genome Project and BeyondGoal was to transcribe entire human genomeHas mapped thousands of human genesGenome Full complement of genetic materialKaryotyping Determination of individual’s complete chromosomal complement ................
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