3Embryonic period i.e. first 8 weeks First week of developmentFertilizationGenetic material from haploid sperm and haploid secondary oocyte merges into single diploid zygoteNormally occurs in uterine (fallopian) tubesSperm undergo capacitation – series of functional changes that prepare its plasma membrane to fuse with oocyte’sSperm must penetrate coronoa radiata (granulosa cells) and zona pellucida (clear glycoprotein layer between corona radiate and oocyte plasma membrane)Acrosomal enzymes and strong movements help with penetrationCopyright 2009, John Wiley & Sons, Inc.
5First week of development (cont.) Fusion of sperm cell and oocyte sets in motion events to block polyspermy – fertilization by more than one spermFast block to polyspermy – oocyte cell membrane depolarizes so another sperm cannot fuseAlso triggers exocytosis of secretory vesiclesSlow block to polyspermy – molecules released in exocytosis harden entire zona pellucidaOocyte must complete meiosisDivides into ovum and polar body (disintegrates)Copyright 2009, John Wiley & Sons, Inc.
6First week of development (cont.) Male pronucleus and female pronucleus fuse to form single diploid (2n) zygote with 46 chromosomes (23 pairs)Dizygotic (fraternal) twins are produced by the release of 2 secondary oocytes and fertilization by separate spermAs genetically dissimilar as any other siblingsMonozygotic (identical) twins develop form a single fertilized ovum – they have exactly the same DNALate separation results in conjoined twinsCopyright 2009, John Wiley & Sons, Inc.
7First week of development (cont.) Cleavage of zygoteRapid mitotic cell divisions after zygote formsFirst division begins 24 hours after fertilization and takes 6 hoursEach succeeding division takes less timeBlastomeres – progressively smaller cells produced by cleavageMorula – solid sphere of cellsStill surrounded by zona pellucidaAbout same size as original zygoteCopyright 2009, John Wiley & Sons, Inc.
8First week of development (cont.) Blastocyst formationMorula moves through uterine tubes toward uterusDay 4 or 5 reaches uterusUterine milk – glycogen-rich secretions of endometrial glands nourishes morulaBlastocyst – at 32-cell stage, fluid collects and forms blastocyst cavity or blastocoel2 distinct cell populationsEmbryoblast or inner cell mass – develops into embryoTrophoblast – outer layer that forms wall and will ultimately develop into outer chorionic sac surrounding fetus and fetal portion of placentaDay 5 “hatches” from zona pellucidaCopyright 2009, John Wiley & Sons, Inc.
9Cleavage and the formation of the morula and blastocyst Copyright 2009, John Wiley & Sons, Inc.
10Copyright 2009, John Wiley & Sons, Inc. ImplantationAbout 6 days after fertilization attaches to endometriumOrients inner cell mass toward endometrium7 days after fertilization attaches more firmly and burrows inEndometrium becomes more vascularized and glands enlargeDecidua – modified portion of endometrium after implantationRegions named relative to site of implantationCopyright 2009, John Wiley & Sons, Inc.
11Copyright 2009, John Wiley & Sons, Inc. Relationship of a blastocyst to the endometium of the uterus at implantationCopyright 2009, John Wiley & Sons, Inc.
12Summary of events associated with the first week of development Copyright 2009, John Wiley & Sons, Inc.
13Second week of development Development of trophoblastAbout 8 days after fertilization, trophoblast develops into 2 layers in region of contact between blastocyst and endometriumBecome part of chorionBlastocyst becomes buried in endometrium and inner 1/3 of myometriumSecretes human chorionic gondadotropin (hCG) that maintains corpus luteum so it continues to secrete estrogens and progesteroneMaintains uterine liningCopyright 2009, John Wiley & Sons, Inc.
14Second week of development (cont.) Development of bilaminar embryonic discCells of embryoblast also differentiate into 2 layers around 8 days after fertilizationHypoblast (primitive endoderm)Mesoderm?Epiblast (primitive ectoderm)Small cavity enlarges to form amniotic cavityDevelopment of amnionAmnion forms roof of amniotic cavity and epiblast forms floorAmnion eventually surrounds entire embryoAmniotic cavity filled with amniotic fluidFluid derived from maternal blood and later fetal urineCopyright 2009, John Wiley & Sons, Inc.
15Principal events in the second week of development
18Second week of development (cont.) Development of yolk sacAlso on 8th day after fertilization, cells at edge of hypoblast migrate to cover inner surface of blastocyst wallForm exocoelomic membraneYolk sac – hypoblast and exocoelomic membraneRelatively small and empty since nutrition derived from endometriumSeveral important functions – supplies early nutrients, source of blood cells, contains primordial germ cells that migrate to gonads to form gametes, forms part of gut, functions as shock absorber, prevents desiccation
19Second week of development (cont.) Development of sinusoids9th day after fertilization, blastocyst completely embedded in endometriumSyncytiotrophoblast expands and spaces (lacunae) develop12th day – lacunae fuse to form lacunar networksEndometrial capillaries dilate to form maternal sinusoidsEmbryonic/maternal exchangeDevelopment of extraembryonic coleom - about 12th day after fertilizationFuse to form single large cavityDevelopment of chorionFormed by extraembryonic mesoderm and 2 layers or trophoblastBecomes principal embryonic part of placentaProtect embryo from immune responses of motherProduces hCGConnecting (body) stalk connects bilaminar embryonic disc to trophoblast – will become umbilical cord
20Third week of development Begins 6 week period of rapid development and differentiationGastrulation1st major event of 3rd week – about 15 daysBilaminar embryonic disc transforms into trilaminar embryonic disc (Table 29.1 page 1190)Ectoderm (skin and nervous system), mesoderm (muscle, bones, connective tissues, peritoneum), and endoderm (epithelial lining of GI tract, respiratory tract, and several other organs)Involves rearrangement and migration of epiblast cellsPrimitive streak establishes head (primitive node) and tail endsCopyright 2009, John Wiley & Sons, Inc.
21Copyright 2009, John Wiley & Sons, Inc. GastrulationCopyright 2009, John Wiley & Sons, Inc.
22Third week of development (cont.) Gastrulation (cont.)16 days after fertilization notochord forms – induces tissue to become vertebral bodies (via induction)2 depressions formOropharyngeal membrane will later break down to connect mouth to pharynx and GI tractCloacal membrane will later degenerate to form openings of anus, urinary and reproductive tractsWhen cloacal membrane appears, wall of yolk sac forms allantoisExtends into connecting stalkIn most other mammals used for gas exchange and waste removal – human placenta does this insteadDoes function in early formation of blood and blood vessels and urinary bladderCopyright 2009, John Wiley & Sons, Inc.
23Development of the notochordal process Copyright 2009, John Wiley & Sons, Inc.
24Third week of development (cont.) NeurulationNotochord also induces formation of neural plateEdges of plate elevate to form neural foldNeural folds fuse to form neural tubeDevelop into brain and spinal cordNeural crest cells give rise to spinal and cranial nerves and ganglia, autonomic nervous system ganglia, CNS meninges, adrenal medullae and several skeletal and muscular components of headHead end of neural tube develops into 3 primary brain vesiclesProsencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain)Copyright 2009, John Wiley & Sons, Inc.
25Neurulation and the development of somites Copyright 2009, John Wiley & Sons, Inc.
27Third week of development (cont.) Development of somites pairsMesoderm adjacent to notochord and neural tube forms paired longitudinal columns of paraxial mesodermSegment into paired, cube-shaped somitesNumber of somites can be correlated to age of embryoEach somite has 3 regionsMyotome – develops into skeletal muscles of neck, trunk and limbsDermatome – develops into connective tissue and dermisSclerotome - develops into vertebrae and ribsDevelopment of intraembryonic coelomSplits lateral plate mesoderm intoSplanchnic mesoderm – forms heart, blood vessels, smooth muscle and connective tissues of respiratory and digestive systemsSomatic mesoderm – gives rise to bones, ligaments, dermis of skinCopyright 2009, John Wiley & Sons, Inc.
28Neurulation and the development of somites Copyright 2009, John Wiley & Sons, Inc.
29Third week of development (cont.) Development of cardiovascular systemAngiogenesis – formation of blood vesselsSpaces develop in blood islands to form lumens of blood vesselsPluripotent stem cells form blood cellsBy end of 3rd week, heart forms and begins to beatDevelopment of chorionic villi and placentaChorionic villi – fingerlike projections of chorion projecting into endometriumBlood vessels in chorionic villi connect to embryonic heart through body stalk (becomes umbilical cord)Maternal and fetal blood do not mix – diffusion onlyCopyright 2009, John Wiley & Sons, Inc.
30Development of chorionic villi Copyright 2009, John Wiley & Sons, Inc.
31Copyright 2009, John Wiley & Sons, Inc. PlacentationProcess of forming placentaBy beginning of 12th week has 2 partsFetal portion formed by chorionic villi of chorionMaternal portion formed by decidua basalis of endometriumFunctionally allows oxygen and nutrients to diffuse from maternal to fetal blood while carbon dioxide and wastes diffuse from fetal to maternal bloodNot a protective barrier – allows microorganisms (HIV), drugs, alcohol to passConnection between embryo and placenta through umbilical cord2 umbilical arteries carry deoxygenated fetal blood to placenta1 umbilical veins carries oxygenated blood away from placentaAfterbirth – placenta detaches from uterusCopyright 2009, John Wiley & Sons, Inc.
32Placenta and umbilical cord Copyright 2009, John Wiley & Sons, Inc.
33Fourth week of development 4th -8th week - all major organs developOrganogenesis – formation of body organs and systemsEmbryo triples in size this weekConverted from flat disc to 3D cylinder through embryonic foldingMain force is different rates of growth for different partsHead fold brings heart and mouth into eventual adult positionTail fold brings anus into eventual adult positionLateral folds for primitive gut – forerunner of GI tractCopyright 2009, John Wiley & Sons, Inc.
34Copyright 2009, John Wiley & Sons, Inc. Embryonic foldingCopyright 2009, John Wiley & Sons, Inc.
35Copyright 2009, John Wiley & Sons, Inc. Embryonic foldingCopyright 2009, John Wiley & Sons, Inc.
36Copyright 2009, John Wiley & Sons, Inc. 4th week (cont.)Somite and neural tube developmentPharyngeal (branchial) arches (5), clefts and pouches give rise to specific structures in head and neck1st pharyngeal arch forms jawOtic placode – future internal earUpper and lower limb buds appear – distinct tailCopyright 2009, John Wiley & Sons, Inc.
375th – 8th weeks of development During 5th week brain develops rapidly so head growth considerableLimbs show substantial development by end of 6th weekHeart now 4-chambered8th weekDigits of hands are short and webbed – by the end of the week the webbing dies (apoptosis)Tail shorter and disappears by end of weekEyes open – eyelids come together and may fuseAuricles of ear visibleExternal genitals begin to differentiateCopyright 2009, John Wiley & Sons, Inc.
38Copyright 2009, John Wiley & Sons, Inc. Fetal periodDuring this period, tissues and organs that developed during embryonic period grow and differentiateVery few new structures appearRate of body growth remarkableFetus less vulnerable to damaging effect of drugs, radiation, and microbesCopyright 2009, John Wiley & Sons, Inc.
39Summary of changes during embryonic and fetal development Table 29.2 Copyright 2009, John Wiley & Sons, Inc.
40Copyright 2009, John Wiley & Sons, Inc. Summary of changes during embryonic and fetal development Table 29.2Copyright 2009, John Wiley & Sons, Inc.
41Summary of changes during embryonic and fetal development Copyright 2009, John Wiley & Sons, Inc.
42Summary of events of the embryonic and fetal periods Copyright 2009, John Wiley & Sons, Inc.
43Hormones during pregnancy Human chorionic somatomammotropin (hCS) or human placental lactogen (hPL) produced by chorionHelps prepare mammary glands for lactationRegulates certain aspects of fetal and maternal metabolismCorticotropin-releasing hormone (CRH) produced by placentaIn nonpregnant people secreted only by hypothalamusThough to be part of “clock” establishing timing of birthIncreases secretion of cortisol needed for maturation of fetal lungs and production of surfactantCopyright 2009, John Wiley & Sons, Inc.
46Copyright 2009, John Wiley & Sons, Inc. Human OvaryCopyright 2009, John Wiley & Sons, Inc.
47Copyright 2009, John Wiley & Sons, Inc. Cat OvaryCopyright 2009, John Wiley & Sons, Inc.
48Copyright 2009, John Wiley & Sons, Inc. Human TestisCopyright 2009, John Wiley & Sons, Inc.
49Copyright 2009, John Wiley & Sons, Inc. Human SpermCopyright 2009, John Wiley & Sons, Inc.
50Copyright 2009, John Wiley & Sons, Inc. Human ChromosomesCopyright 2009, John Wiley & Sons, Inc.
51Changes during pregnancy By the end of a full-term pregnancy, uterus fills nearly the entire abdominal cavityPhysiological changesWeight gain due to fetus, amniotic fluidIncreased storage of proteins, triglycerides and mineralsMarked breast enlargementLower back pain – lordosisChanges in cardiovascular (30% SV) system due to increased maternal blood flow to placenta and increased metabolismRespiratory functions change to meet added oxygen demands of fetusDigestive system – increased appetite to meet energy demands of fetusUrinary system – pressure on bladder can cause incontinenceIncreased renal filtering to eliminate wastes from fetusCopyright 2009, John Wiley & Sons, Inc.
52Normal fetal location and position at the end of a full-term pregnancy
53Copyright 2009, John Wiley & Sons, Inc. Labor or parturitionProcess by which fetus expelled from uterus through vaginaOnset determined by interactions between several placental and fetal hormonesLevels of estrogen must rise to overcome inhibiting effect of progesterone on uterine contractionsHigh levels of estrogens increase number of receptors for oxytocin on uterine muscle fibersOxytocin stimulates contractionsRelaxin increases flexibility of pubic symphysis and dilates cervixControl of labor through positive feedback cycleContraction force fetal head into cervix which stretchesStimulated stretch receptors cause release of more oxytocinMore oxytocin, more stretchingCycle broken when stretching stops as baby exitsCopyright 2009, John Wiley & Sons, Inc.
54Copyright 2009, John Wiley & Sons, Inc. Stages of true laborTrue labor begins when uterine contractions occur at regular intervalsAs interval shortens, contractions intensify3 stagesCopyright 2009, John Wiley & Sons, Inc.
55Adjustments of infant after birth Respiratory adjustmentsFetal lungs collapsed or partially filled with amniotic fluidRespiratory system fairly well developed at least 2 months before birthRising CO2 level in blood after delivery stimulates respiratory center in medulla oblongata causes respiratory muscle to contractFirst inspiration is unusually deep with vigorous exhalation and cryingCardiovascular adjustmentsClosure of foramen ovale between atria of fetal heart occurs at moment of birthDiverts blood to lungs for the first timeRemnant called fossa ovalisDuctus arteriosus constricts and becomes ligamentum arteriosumGenerally does not close completely for 3 monthsUmbilical arteries become medial umbilical ligamentsUmbilical vein becomes round ligament of the liverCopyright 2009, John Wiley & Sons, Inc.
56Physiology of lactation Secretion and ejection of milk from mammary glandsProlactin – principal hormone promoting milk synthesis and secretionSecreted by anterior pituitaryProlactin levels rise during pregnancy but progesterone inhibits effects of prolactinAfter delivery, inhibition removed as estrogen and progesterone levels fallPrincipal stimulus maintaining prolactin secretion is sucking action of infantImpulses from stretch receptors decrease release of prolactin-inhibiting hormone (PIH) and increases release of prolactin-releasing hormone (PRH) from hypothalamusCopyright 2009, John Wiley & Sons, Inc.
57The milk ejection reflex Oxytocin causes milk ejection reflexSuckling, hearing baby cry, touching mother’s genitals can initiateColostrum – before appearance of true milk on 4th dayContain important antibodiesLactation often blocks ovarian cycles for few months after deliveryPrimary benefit of breast-feeding is nutritionalOther benefits alsoCopyright 2009, John Wiley & Sons, Inc.
58Copyright 2009, John Wiley & Sons, Inc. InheritanceCopyright 2009, John Wiley & Sons, Inc.
59Copyright 2009, John Wiley & Sons, Inc. InheritancePassage of hereditary traits from one generation to the nextGenotype and phenotypeNuclei of all human cells except gametes contain 23 pairs of chromosomes – diploid or 2nOne chromosome from each pair came from father, other member from motherEach chromosome contains homologous genes for same traitsAllele – alternative forms of a gene that code for the same traitMutation – permanent heritable change in allele that produces a different variantCopyright 2009, John Wiley & Sons, Inc.
60Phenylketonuria or PKU example Unable to manufacture enzyme phenylalanine hydroxylaseAllele for function enzyme = PAllele that fails to produce functional enzyme = pPunnet square show possible combinations of alleles between 2 parentsGenotype – different combinations of genesPhenotype – expression of genetic makeupPP – homozygous dominant – normal phenotypePp – heterozygous – normal phenotype1 dominant allele codes for enough enzymeCan pass recessive allele on to offspring – carrierpp - homozygous recessive – PKU2 recessive alleles make no functional enzymeCopyright 2009, John Wiley & Sons, Inc.
61Copyright 2009, John Wiley & Sons, Inc. InheritanceCopyright 2009, John Wiley & Sons, Inc.
63Copyright 2009, John Wiley & Sons, Inc. InheritanceAlleles that code for normal traits are not always dominantHuntington disease caused by dominant alleleBoth homozygous dominant and heterozygous individuals get HDNondisjunctionError in cell division resulting in abnormal number of chromosomesAneuploid – chromosomes added or missingMonosomic cell missing 1 chromosome (2n-1)Trisomic cell has additional chromosome (2n +1)Down Syndrome – trisomy 21 – 3 21st chromosomesCopyright 2009, John Wiley & Sons, Inc.
64Variations of Dominant-recessive inheritance Simple dominance-recessiveJust described where dominant allele covers effect of recessive alleleIncomplete dominanceNeither allele dominant over otherHeterozygote has intermediate phenotypeSickle-cell diseaseCopyright 2009, John Wiley & Sons, Inc.
65Copyright 2009, John Wiley & Sons, Inc. Sickle-cell diseaseSickle-cell diseaseHbAHbA – normal hemoglobinHbSHbS – sickle-cell diseaseHbAHbS – ½ normal and ½ abnormal hemoglobinMinor problems, are carriers for diseaseCopyright 2009, John Wiley & Sons, Inc.
66Multiple-allele inheritance GenotypePhenotype(blood type)IA IA or IA iAIB IB or IB iBIA IBABIiOSome genes have more than 2 allelesABO blood groupIA produces A antigenIB produces B antigeni produces neitherA and B are codominantBoth genes expressed equally in heterozygoteCopyright 2009, John Wiley & Sons, Inc.
67Blood type inheritance Copyright 2009, John Wiley & Sons, Inc.
68Copyright 2009, John Wiley & Sons, Inc. Complex inheritancePolygenic inheritance – most inherited traits not controlled by one geneComplex inheritance – combined effects of many genes and environmental factorsSkin color, hair color, height, metabolism rate, body buildEven if a person inherits several genes for tallness, full height can only be reached with adequate nutritionNeural tube deficits are more common if the mother lacks adequate folic acid in the diet – environmental effectCopyright 2009, John Wiley & Sons, Inc.
69Skin color is a complex trait Depends on environmental conditions like sun exposure and nutrition and several genesAdditive effects of 3 genes plus environmental affect produces actual skin color
70Autosomes, sex chromosomes and sex determination Karyotype shows 46 chromosomes arranged in pairs by size and centromere position22 pairs are autosomes – same appearance in males and females23rd pair are sex chromosomesXX = femaleXY = maleCopyright 2009, John Wiley & Sons, Inc.
71Copyright 2009, John Wiley & Sons, Inc. Sex determinationMales produce sperm carrying an X or YFemales only produce eggs carrying an XIndividual’s sex determined by father’s sperm carrying X or YMale and female embryos develop identically until about 7 weeksY initiates male pattern of developmentSRY on Y chromosomeAbsence of Y determines female pattern of developmentCopyright 2009, John Wiley & Sons, Inc.
72Sex-linked inheritance GenotypePhenotypeXCXCNormal femaleXCXcNormal female (carrier)XcXcColor blind femaleXCYNormal maleXcYColor blind maleGenes for these traits on the X but not the YRed-green colorblindnessMost common type of color blindnessRed and green are seen as same colorMales have only one XThey express whatever they inherit from their motherCopyright 2009, John Wiley & Sons, Inc.