Download First Trimester

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