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Development and Inheritance Muse spring 2440 lecture # 17 7/15/10 Development Differentiation Creation of different types of cells required in development Occurs through selective changes in genetic activity As development proceeds, some genes are turned off, others are turned on Fertilization Also called conception When development begins Development Embryological Development Occurs during first 2 months after fertilization Study of these events is called embryology Fetal Development Begins at start of ninth week Continues until birth Development Prenatal Development Embryological and fetal development stages Postnatal Development Commences at birth Continues to maturity when aging process begins Fertilization Fertilization Fusion of two haploid gametes, each containing 23 chromosomes Produces zygote containing 46 chromosomes Fertilization and the Preparation for Cleavage Fertilization Gamete Provides Cellular organelles (female) Inclusions Nourishment Genetic programming necessary to support development of embryo for a week Fertilization Fertilization Occurs in uterine tube within a day after ovulation Secondary oocyte travels a few centimeters Spermatozoa must cover distance between vagina and ampulla (30 + cm) Fertilization Hyaluronidase Enzyme breaks down bonds between adjacent follicle cells Allows spermatozoon to reach oocyte Acrosin Is a proteolytic enzyme Is required to reach oocyte Fertilization Acrosomal Caps Release hyaluronidase and acrosin Penetrate corona radiata, zona pellucida, toward oocyte surface Oocyte Activation Contact and fusion of cell membranes of sperm and oocyte Follows fertilization Oocyte completes meiosis II, becomes mature ovum Fertilization Polyspermy - would be bad Fertilization by more than one sperm Prevented by cortical reaction Cortical Reaction- initiated upon sperm penetration Releases enzymes that Inactivate sperm receptors Harden zona pellucida Lift fertilization envelope (vitelline layer) Fertilization Female Pronucleus Nuclear material remaining in ovum after oocyte activation Male Pronucleus Swollen nucleus of spermatozoon Migrates to center of cell Fertilization Amphimixis Fusion of female pronucleus and male pronucleus Moment of conception Cell becomes a zygote with 46 chromosomes Fertilization is complete Fertilization Cleavage Series of cell divisions Produces daughter cells Differentiation Involves changes in genetic activity of some cells but not others Fertilization Figure 29–1a Fertilization: An Oocyte and Numerous Sperm at Time of Fertilization. Fertilization Figure 29–1b Fertilization and the Preparations for Cleavage. Fertilization Figure 29–1b Fertilization and the Preparations for Cleavage. Fertilization Figure 29–1b Fertilization and the Preparations for Cleavage. Fertilization Figure 29–1b Fertilization and the Preparations for Cleavage. Gestation Induction Cells release chemical substances that affect differentiation of other embryonic cells Can control highly complex processes Gestation Time spent in prenatal development Consists of three integrated trimesters, each 3 months long Gestation First Trimester Period of embryological and early fetal development Rudiments of all major organ systems appear Second Trimester Development of organs and organ systems Body shape and proportions change By end, fetus looks distinctively human Third Trimester Rapid fetal growth and deposition of adipose tissue Most major organ systems are fully functional The First Trimester Cleavage Sequence of cell divisions begins immediately after fertilization Zygote becomes a pre-embryo, which develops into multicellular blastocyst Ends when blastocyst contacts uterine wall The First Trimester Implantation Begins with attachment of blastocyst to endometrium of uterus Sets stage for formation of vital embryonic structures Placentation Occurs as blood vessels form around periphery of blastocyst and placenta develops The First Trimester Placenta Complex organ permits exchange between maternal and embryonic circulatory systems Supports fetus in second and third trimesters Stops functioning and is ejected from uterus after birth Embryogenesis Formation of viable embryo Establishes foundations for all major organ systems The First Trimester Most dangerous period in prenatal life 40% of conceptions produce embryos that survive past first trimester The First Trimester Blastomeres Identical cells produced by cleavage divisions Morula Stage after 3 days of cleavage Pre-embryo is solid ball of cells resembling mulberry Reaches uterus on day 4 The First Trimester Figure 29–2 Cleavage and Blastocyst Formation. The First Trimester Blastocyst Formed by blastomeres Hollow ball with an inner cavity Known as blastocoele The First Trimester Trophoblast Outer layer of cells separate outside world from blastocoele Cells responsible for providing nutrients to developing embryo The First Trimester Inner Cell Mass Clustered at end of blastocyst Exposed to blastocoele Insulated from contact with outside environment by trophoblast Will later form embryo The First Trimester Figure 29–2 Cleavage and Blastocyst Formation. The First Trimester Implantation Occurs 7 days after fertilization Blastocyst adheres to uterine lining Trophoblast cells divide rapidly, creating several layers Stage of Implantation The First Trimester Cellular Trophoblast Cells closest to interior of blastocyst Syncytial Trophoblast Outer layer Erodes path through uterine epithelium by secreting hyaluronidase The First Trimester Figure 29–3 Stages in Implantation. The First Trimester Ectopic Pregnancy Implantation occurs outside of uterus Does not produce viable embryo Can be life threatening Lacunae Trophoblastic channels carrying maternal blood The First Trimester Villi Extend away from trophoblast into endometrium Increase in size and complexity until day 21 Amniotic Cavity A fluid-filled chamber Inner cell mass is organized into an oval sheet two layers thick Superficial layer faces amniotic cavity Deeper layer is exposed to fluid contents of blastocoele The First Trimester Gastrulation Formation of third layer of cells Cells in specific areas of surface move toward central line Known as primitive streak Gastrulation Week 3 - 15 days in The First Trimester Primitive Streak Migrating cells leave surface and move between two layers Creates 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 endoderm The First Trimester Ectoderm makes me nervous The First Trimester Mesoderm is myo favorite The First Trimester Endoderm gives me endogestion The First Trimester Embryonic Disc Oval, three-layered sheet Produced by gastrulation Will form body of embryo Rest of blastocyst will be involved in forming extraembryonic membranes The First Trimester Figure 29–4 The Inner Cell Mass and Gastrulation. The First Trimester Formation of the Extraembryonic Membranes Support embryological and fetal development Yolk sac Amnion Allantois Chorion The First Trimester Yolk Sac Begins as layer of cells spread out around outer edges of blastocoele to form complete pouch Important site of blood cell formation Amnion Combination of mesoderm and ectoderm Ectodermal layer enlarges and cells spread over inner surface of amniotic cavity Mesodermal cells create outer layer Continues to enlarge through development The First Trimester Amniotic Fluid Contained in amniotic cavity Surrounds and cushions developing embryo or fetus Allantois Sac of endoderm and mesoderm Base later gives rise to urinary bladder The First Trimester Chorion Combination of mesoderm and trophoblast Blood vessels develop within mesoderm Rapid-transit system for nutrients that links embryo with trophoblast First step in creation of functional placenta The First Trimester Chorionic Villi In contact with maternal tissues Create intricate network within endometrium carrying maternal blood Body Stalk Connection between embryo and chorion Contains distal portions of allantois and blood vessels that carry blood to and from placenta The First Trimester Yolk Stalk Narrow connection between endoderm of embryo and yolk sac Decidua Capsularis Thin portion of endometrium No longer participates in nutrient exchange and chorionic villi in region disappear The First Trimester Figure 29–5 Extraembryonic Membranes and Placenta Formation. The First Trimester Figure 29–5 Extraembryonic Membranes and Placenta Formation. The First Trimester Figure 29–5 Extraembryonic Membranes and Placenta Formation. The First Trimester Umbilical Cord Connects fetus and placenta Contains allantois, placental blood vessels, and yolk stalk Blood Flow to Placenta Through paired umbilical arteries Returns in single umbilical vein The First Trimester Figure 29–6 A Three-Dimensional View of Placental Structure. The First Trimester The Endocrine Placenta Synthesized by syncytial trophoblast, released into maternal bloodstream Human chorionic gonadotropin (hCG) Human placental lactogen (hPL) Placental prolactin Relaxin Progesterone Estrogens The First Trimester Human Chorionic Gonadotropin (hCG) Appears in maternal bloodstream soon after implantation made by trophoblast Provides reliable indication of pregnancy Pregnancy ends if absent The First Trimester Human Chorionic Gonadotropin (hCG) Helps prepare mammary glands for milk production Stimulatory effect on other tissues comparable to growth hormone (GH) The First Trimester Placental Prolactin Helps convert mammary glands to active status Relaxin A peptide hormone secreted by placenta and corpus luteum during pregnancy Increases flexibility of pubic symphysis, permitting pelvis to expand during delivery Causes dilation of cervix Suppresses release of oxytocin by hypothalamus and delays labor contractions The First Trimester Embryogenesis Body of embryo begins to separate from embryonic disc Body of embryo and internal organs start to form Folding, differential growth of embryonic disc produces bulge that projects into amniotic cavity Projections are head fold and tail fold Organogenesis Process of organ formation The First Trimester Figure 29–7a The First Trimester. The First Trimester Figure 29–7b The First Trimester. What will I be when I grow up? What will I be when I grow up? What will I be when I grow up? The First Trimester Figure 29–7c The First Trimester. The First Trimester Figure 29–7d The First Trimester. Summary of changes during embryonic and fetal development The Second and Third Trimesters Second Trimester Fetus grows faster than surrounding placenta Third Trimester Most of the organ systems become ready Growth rate starts to slow Largest weight gain Fetus and enlarged uterus displace many of mother’s abdominal organs The Second and Third Trimesters Figure 29–8a The Second and Third Trimesters: A Four-Month-Old Fetus As Seen through a Fiber-Optic Endoscope. The Second and Third Trimesters Figure 29–8b The Second and Third Trimesters: Head of a Six-MonthOld Fetus As Seen through Ultrasound. The Second and Third Trimesters Figure 29–9c, d Growth of the Uterus and Fetus. The Second and Third Trimesters Pregnancy and Maternal Systems Developing fetus is totally dependent on maternal organ systems for nourishment, respiration, and waste removal Maternal adaptations include increases in Respiratory rate and tidal volume Blood volume Nutrient and vitamin intake Glomerular filtration rate Uterus and mammary glands increase in size The Second and Third Trimesters Progesterone Released by placenta Has inhibitory effect on uterine smooth muscle Prevents extensive, powerful contractions Opposition to Progesterone Three major factors Rising estrogen levels Rising oxytocin levels Prostaglandin production The Second and Third Trimesters Parturition is forcible expulsion of fetus Contractions Begin near top of uterus, sweep in wave toward cervix Strong, occur at regular intervals, increase in force and frequency Change position of fetus, move it toward cervical canal Labor Dilation Stage Begins with onset of true labor Cervix dilates Fetus begins to shift toward cervical canal Highly variable in length, but typically lasts over 8 hours Frequency of contractions steadily increases Amniochorionic membrane ruptures (water breaks) Labor Figure 29–11 The Stages of Labor. Labor Expulsion Stage Begins as cervix completes dilation Contractions reach maximum intensity Continues until fetus has emerged from vagina Typically less than 2 hours Delivery Arrival of newborn infant into outside world Labor Figure 29–11 The Stages of Labor. Labor Placental Stage Muscle tension builds in walls of partially empty uterus Tears connections between endometrium and placenta Ends within an hour of delivery with ejection of placenta, or afterbirth Accompanied by a loss of blood Labor Figure 29–11 The Stages of Labor. Labor Immature Delivery Refers to fetuses born at 25–27 weeks of gestation Most die despite intensive neonatal care Survivors have high risk of developmental abnormalities Premature Delivery Refers to birth at 28–36 weeks Newborns have a good chance of surviving and developing normally Labor Forceps Delivery Needed when fetus faces mother’s pubis instead of sacrum Risks to infant and mother are reduced if forceps are used Forceps resemble large, curved salad tongs Used to grasp head of fetus Labor Breech Birth Legs or buttocks of fetus enter vaginal canal first instead of head Umbilical cord can become constricted, cutting off placental blood flow Cervix may not dilate enough to pass head Prolongs delivery Subjects fetus to severe distress and potential injury Labor Dizygotic Twins Also called fraternal twins Develop when two separate oocytes were ovulated and subsequently fertilized Genetic makeup not identical 70% of twins Labor Monozygotic Twins Identical twins Result either from Separation of blastomeres early in cleavage Splitting of inner cell mass before gastrulation Genetic makeup is identical because both formed from same pair of gametes Labor Rates of Multiple Births Twins in 1 of every 89 births Triplets in 1 of every 892 (7921) births Quadruplets in 1 of every 893 (704,969) births Octuplets = ridiculous Postnatal Life Figure 29–13 Growth and Changes in Body Form and Proportion. Inheritance Nucleated Somatic Cells Carry copies of original 46 chromosomes present in zygote Genotype Chromosomes and their component genes Contain unique instructions that determine anatomical and physiological characteristics Derived from genotypes of parents Phenotype Physical expression of genotype Anatomical and physiological characteristics Inheritance Homologous Chromosomes Members of each pair of chromosomes 23 pairs carried in every somatic cell At amphimixis, one member of each pair is contributed by spermatozoon, other by ovum Inheritance Autosomal Chromosomes 22 pairs of homologous chromosomes Most affect somatic characteristics Each chromosome in pair has same structure and carries genes that affect same traits Inheritance Sex Chromosomes Last pair of chromosomes Determine whether individual is genetically male or female Karyotype Entire set of chromosomes Locus Gene’s position on chromosome Inheritance Figure 29–14 A Human Karyotype. Inheritance Alleles are various forms of given gene Alternate forms determine precise effect of gene on phenotype Homozygous Both homologous chromosomes carry same allele of particular gene Simple Inheritance Phenotype determined by interactions between single pair of alleles Inheritance Heterozygous Homologous chromosomes carry different allele of particular gene Resulting phenotype depends on nature of interaction between alleles Strict Dominance Dominant allele expressed in phenotype, regardless of conflicting instructions carried by other allele Inheritance Recessive Allele Expressed in phenotype only if same allele is present on both chromosomes of homologous pair Incomplete Dominance Heterozygous alleles produce unique phenotype Codominance Exhibits both dominant and recessive phenotypes for traits Inheritance Penetrance Percentage of individuals with particular genotype that show “expected” phenotype Expressivity Extent to which particular allele is expressed Teratogens Factors that result in abnormal development Punnett Square Simple box diagram used to predict characteristics of offspring Mutation - change in normal form of gene Inheritance Figure 29–15 Predicting Phenotypic Characters by Using Punnett Squares. Inheritance Polygenic Inheritance Involves interactions among alleles on several genes Cannot predict phenotypic characteristics using Punnett square Linked to risks of developing several important adult disorders Suppression One gene suppresses other Second gene has no effect on phenotype Inheritance Inheritance Complementary Gene Action Dominant alleles on two genes interact to produce phenotype different from that seen when one gene contains recessive alleles Sources of Individual Variation During meiosis, maternal and paternal chromosomes are randomly distributed Each gamete has unique combination of maternal and paternal chromosomes Inheritance Genetic Recombination During meiosis, various changes can occur in chromosome structure, producing gametes with chromosomes that differ from those of each parent Greatly increases range of possible variation among gametes Can complicate tracing of inheritance of genetic disorders Inheritance Crossing Over Parts of chromosomes become rearranged during synapsis When tetrads form, adjacent chromatids may overlap Translocation Reshuffling process Chromatids may break, overlapping segments trade places Inheritance Figure 29–17 Crossing Over and Translocation. Inheritance Genomic Imprinting During recombination, portions of chromosomes may break away and be deleted Effects depend on whether abnormal gamete is produced through oogenesis or spermatogenesis Inheritance Chromosomal Abnormalities Damaged, broken, missing, or extra copies of chromosomes Few survive to full term Produce variety of serious clinical conditions Humans are poorly tolerant of changes in gene copy number (to few or too many = lethal or bad news) Mutation Changes in nucleotide sequence of allele Inheritance Spontaneous Mutations Result of random errors in DNA replication Errors relatively common, but in most cases error is detected and repaired by enzymes in nucleus Errors that go undetected and unrepaired have potential to change phenotype Can produce gametes that contain abnormal alleles Inheritance Carriers Individuals who are heterozygous for abnormal allele but do not show effects of mutation Inheritance Sex Chromosomes X Chromosome Considerably larger Have more genes than do Y chromosomes Carried by all oocytes Y Chromosome Includes dominant alleles specifying that the individual will be male Not present in females Autosomes, sex chromosomes and sex determination Karyotype shows 46 chromosomes arranged in pairs by size and centromere position 22 pairs are autosomes – same appearance in males and females 23rd pair are sex chromosomes XX = female XY = male Inheritance Sperm Carry either X or Y chromosome Because males have one of each, can pass along either 50% chance of each Inheritance X-Linked Genes that affect somatic structures Carried by X chromosome Inheritance does not follow pattern of alleles on autosomal chromosomes Sex determination Males produce sperm carrying an X or Y Females only produce eggs carrying an X Individual’s sex determined by father’s sperm carrying X or Y Male and female embryos develop identically until about 7 weeks Y initiates male pattern of development SRY on Y chromosome Absence of Y determines female pattern of development Inheritance Figure 29–18 Inheritance of an X-Linked Trait Inheritance of red-green color blindness Sex-linked inheritance Genes for these traits on the X but not the Y Genotype XCXC Red-green colorblindness Most common type of XCXc XcXc color blindness Red and green are seen as same color Males have only one X – They express XCY XcY Phenotype Normal Normal female female Color blind (carrier) female Normal male Color blind male Inheritance Human Genome Project Goal was to transcribe entire human genome Has mapped thousands of human genes Genome Full complement of genetic material Inheritance Figure 29–19 A Map of Human Chromosomes. Inheritance Passage of hereditary traits from one generation to the next Genotype and phenotype Nuclei of all human cells except gametes contain 23 pairs of chromosomes – diploid or 2n One chromosome from each pair came from father, other member from mother Each chromosome contains homologous genes for same traits Allele – alternative forms of a gene that code for the same trait Mutation – permanent heritable change in allele that produces a different variant Inheritance Phenylketonuria or PKU example Unable to manufacture enzyme phenylalanine hydroxylase Allele for function enzyme = P Allele that fails to produce functional enzyme = p Punnet square show possible combinations of alleles between 2 parents Genotype – different combinations of genes Phenotype – expression of genetic makeup PP – homozygous dominant – normal phenotype Pp – heterozygous – normal phenotype – 1 dominant allele codes for enough enzyme – Can pass recessive allele on to offspring – carrier pp - homozygous recessive – PKU – 2 recessive alleles make no functional enzyme Inheritance Alleles that code for normal traits are not always dominant Huntington disease caused by dominant allele Both homozygous dominant and heterozygous individuals get HD Nondisjunction Error in cell division resulting in abnormal number of chromosomes Aneuploid – chromosomes added or missing Monosomic cell missing 1 chromosome (2n-1) Trisomic cell has additional chromosome (2n +1) – Down Syndrome – trisomy 21 – 3 21st chromosomes Variations of Dominant-recessive inheritance Simple dominance-recessive Just described where dominant allele covers effect of recessive allele Incomplete dominance Neither allele dominant over other Heterozygote has intermediate phenotype Sickle-cell disease Sickle-cell disease Sickle-cell disease HbAHbA – normal hemoglobin HbSHbS – sickle-cell disease HbAHbS – ½ normal and ½ abnormal hemoglobin Minor problems, are carriers for disease Multiple-allele inheritance Phenotype Some genes have more than 2 alleles ABO blood group IA produces A antigen IB produces B antigen Genotype (blood IA IA or IA i type) A IB IB or IB i B IA IB AB Ii O i produces neither A and B are codominant – Both genes expressed equally in heterozygote Blood type inheritance Complex inheritance Polygenic inheritance – most inherited traits not controlled by one gene Complex inheritance – combined effects of many genes and environmental factors Skin color, hair color, height, metabolism rate, body build Even if a person inherits several genes for tallness, full height can only be reached with adequate nutrition Skin color is a complex trait Depends on environmental conditions like sun exposure and nutrition and several genes Additive effects of 3 genes plus environmental affect produces actual skin color