Transcript Development and Inheritance

Development and Inheritance
• From fertilization to birth
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fertilization
implantation
placental development
fetal development
gestation
labor
parturition (birth)
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Terminology of Development
• Gestation period
– time span from fertilization to birth (38 weeks)
• Prenatal period (before birth)
– embryological development
• developing human for first 2 months after fertilization is known as an
embryo
• all principal adult organs are present
– fetal development
• from 9 weeks until birth is known as a fetus
• by end of 3rd month, placenta is functioning
• Neonatal period is first 42 days after birth
– obstetrics is branch of medicine that deals with the neonatal
period, pregnancy and labor
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From Fertilization to Implantation
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Events Before Fertilization
• Haploid sperm nucleus & haploid secondary oocyte
nucleus merge to form a single diploid nucleus
• Occurs in uterine tube within 24 hours after ovulation
(oocyte dies in 24 hours)
• Events occurring before fertilization
– peristalsis of uterine tube & movement of cilia transport the
oocyte towards the uterus
• oocyte releases chemical attractants
– sperm swim towards oocyte by means of flagella
• prostaglandins within the semen stimulate uterine contractions that
help move sperm towards the oocyte
– capacitation or final maturation of the sperm occurs within
female
• acrosomal membrane becomes fragile
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Sperm Contact during Fertilization
• Sperm penetrates the granulosa cells
around the oocyte (corona radiata)
• Sperm digests its way through
the zona pellucida
– when ZP3 glycoprotein binds to sperm
head, it triggers the acrosomal reaction
(enzyme release)
• First sperm to fuse with oocyte membrane triggers the
slow & the fast block to polyspermy
– 1-3 seconds after contact, oocyte membrane depolarizes & other
cells can not fuse with it = fast block to polyspermy
– depolarization triggers the intracellular release of Ca+2 causing
the exocytosis of molecules hardening the entire zona pellucida
= slow block to polyspermy
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Events Within the Egg
• Sperm entry, triggers oocyte to complete
meiosis II and dump second polar body
• Once inside the oocyte, the sperm loses its
tail & becomes a male pronucleus
• Fusion of male & female haploid pronuclei
is the true moment of fertilization
• Fertilized ovum (2n) is called a zygote
– zona pellucida still surrounds it
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Nature of Twins
• Fraternal twins (dizygotic)
– independent release of 2 oocytes fertilized by 2
separate sperm
– as different as any 2 siblings
• Identical twins (monozygotic)
– 2 individuals that develop from a single
fertilized ovum
– genetically identical & always the same sex
– if ovum does not completely separate,
conjoined twins (share some body structures)
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Formation of the Morula
• Rapid mitotic cell division of
embryo is called cleavage
• 1st cleavage in 30 hours produces
2 blastomeres
• 2nd cleavage on 2nd day
• By 3rd day has 16 cells
• By day 4 has formed a solid
ball of cells called a morula
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Development of the Blastocyst
• A blastocyst is a hollow ball of cells that enters the
uterine cavity by day 5
– outer covering of cells
called the trophoblast
– inner cell mass
– fluid-filled cavity called
the blastocele
• Trophoblast & part of inner
cell mass will develop into fetal portion of placenta
• Most of inner cell mass will become embryo
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Implantation
• Attachment of blastocyst to endometrium
– occurs 6 days after fertilization
– implants with inner cell mass in contact with the
endometrium
• Trophoblast develops 2 distinct layers
– syncytiotrophoblast secretes enzymes that digest the
endometrial cells
– cytotrophoblast is distinct layer of cells that defines
the original shape of the embryo
• Trophoblast secretes human chorionic
gonadotropin (hCG) that helps the corpus luteum
maintain the uterine lining
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Implantation
Notice: distinct
syncytiotrophoblast and
cytotrophoblast layers.
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Ectopic Pregnancy
• Development of an embryo outside the uterus
• Most often in uterine tube
– common causes are blockages of uterine tube such
as tumors or scars from pelvic inflammatory disease
– symptoms are missed menstrual cycles, bleeding &
acute pain
• Twice as common in smokers because nicotine
paralyzes the cilia
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Beginnings of Organ Systems(Gastrulation)
• Day 8
– cytotrophoblast forms amnion & amnionic cavity
• cells of inner cell mass on amnionic cavity form ectoderm
• cells bordering on blastocele form endoderm
– ectoderm & endoderm together form embryonic disk
• Day 12
– endodermal cells divide
to form a hollow sphere
(yolk sac)
– cytotrophoblast cells
divide to fill the spaces
surrounding the yolk sac with extraembryonic mesoderm
• spaces develop in that layer to form future ventral body cavity
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Primary Germ Layers
• Day 14 --cells of embryonic disc produce 3
distinct layers
• endoderm forms epithelial lining of GI & respiratory
• mesoderm forms muscle, bone & other connective tissues
• ectoderm develops into epidermis of skin & nervous system
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Formation of Embryonic Membranes
• Yolk sac
– site of early blood formation
– gives rise to gonadal stem cells (spermatogonia & oogonia)
• Amnion
– surrounds embryo with fluid: shock absorber, regulates body temperature
& prevents adhesions
– fluid is filtrate of mother’s blood + fetal urine
– examine a sample of it for embryonic cells (amniocentesis)
• Chorion
– becomes the embryonic contribution to the placenta
– derived from trophoblast & mesoderm lining it
– gives rise to human chorionic gonadotropin (hCG)
• Allantois
– outpocketing off yolk sac that becomes umbilical cord
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Amnion, Yolk sac, Chorion, allantois
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Placenta & Umbilical Cord
• Placenta forms during 3rd month
– chorion of embryo & stratum functionalis layer of
uterus
• Chorionic villi extend into maternal blood filled
intervillous spaces --- maternal & fetal blood
vessels do not join & blood does not mix
– diffusion of O2, nutrients, wastes
– stores nutrients & produces hormones
– barrier to microorganisms, except some viruses
• AIDS, measles, chickenpox, poliomyelitis, encephalitis
– not a barrier to drugs such as alcohol
• Placenta detaches from the uterus (afterbirth)
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Parts of Endometrial Lining
• Decidua = all of endometrium lost as placenta
– equals all of the endometrium, except stratum basalis
• Decidua basalis---portion of
endometrium deep to chorion
• Decidua capsularis---part of
endometrial wall that covers
implanted embryo
• Decidua parietalis---part of
endometrial wall not modified
by embryo until embryo bumps into it as it enlarges
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• Decidua
capsularis
Umbilical Cord
• Contents
– 2 arteries that carry
blood to the placenta
– 1 umbilical vein that
carries oxygenated
blood to the fetus
– primitive connective
tissue
• Stub drops off in 2
weeks leaving a scar
called the umbilicus
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Placenta Previa
• Placenta is implanted near or covering os of
cervix
– occurs in 1 to 250 live births
• May lead to spontaneous abortion, premature
birth or increased maternal mortality
• Major symptom is sudden, painless bright red
vaginal bleeding in the 3rd trimester
• Cesarean section is preferred delivery method
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Fetal Ultrasonography
• Transducer emits high-frequency sound waves
– reflected sound waves converted to on-screen
image called sonogram
– patient needs full bladder
• Used to determine fetal age, viability, growth,
position, twins and maternal abnormalities
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Amniocentesis
• Fetal cells from 10 ml sample
of amniotic fluid examined
for genetic defects
• Test at 14-16 weeks
• Results back in one month
• Needle through abdominal
wall & uterus
• Chance of spontaneous
abortion is 0.5%
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Chorionic Villi Sampling
• As early as 8 weeks
• Results in few days
• Chance of spontaneous
abortion 1-2%
• 30 mg of placenta
removed by suction
through cervix or with
needle through abdomen
• Chromosomal analysis
reveals same results as
amniocentesis
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Hormones of Pregnancy
• Chorion
– from day 8 until 4 months secretes hCG which keeps corpus
luteum active
– corpus luteum produces progesterone & estrogen to maintain
lining of uterus
• Placenta
– by 4th month produces enough progesterone & estrogen that
corpus luteum is no longer important
– relaxin which relaxes CT of pelvis and cervix
– human chorionic somatomammotropoin (hCS) or human placental
lactogen (hPL)
• maximum amount by 32 weeks
• helps prepare mammary glands for lactation
– corticotropin-releasing hormone (CRH) increases secretion of fetal
cortisol (lung maturation) & acts to establish timing of birth
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Hormone Blood Levels
• Human chorionic gonadotropin (hCG) produced by the chorion is less
important after 4 months, because the placenta takes over the
hormonal secretion of the corpus luteum.
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Hormonal Secretion by the Placenta
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Early Pregnancy Tests
• Detect human chorionic gonadotropin (hCG)
in the urine as soon as 8 days after fertilization
– color change hen reaction between urine &
antibodies in kit
• False-negatives & false-positives do occur
– excess protein or blood in urine
– rare type of uterine cancer
– steroid, diuretics, hormones and thyroid drugs alter
test results
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Developmental Changes
• Read Table 29.2 to get a full description of the
timing of fetal events during development
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Maternal Changes During Pregnancy
• Uterus nearly fills the
abdominal cavity
• GI tract compressed causing
heartburn & constipation
• Pressure on bladder causing
changes in frequency &
urgency
• Compression of vena cava
causing varicose veins &
edema in the legs
• Compression of renal vessels
causing renal hypertension
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Changes During Pregnancy
• Cardiovascular changes to meet needs of fetus
– rise in cardiac output of 20-30% due to placenta
– increase in heart rate 15% & increase in blood volume 30-50%
• Respiratory changes
– increase in tidal volume 30%
– decrease in expiratory reserve volume & airway resistance
– minute respiratory volume increases as O2 needs increase
• Reproductive system changes
– uterus increases in size from 80 g to 1200g
– hyperplasia and hypertrophy
• Urinary system changes
– increase in glomerular filtration rate of 40%
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Pregnancy-Induced Hypertension
• Elevated blood pressure
• Major cause is preeclampsia
– sudden hypertension
– large amounts of protein in the urine
– generalized edema, blurred vision & headaches
• Autoimmune or allergic reaction to presence of
fetus
• Eclampsia = convulsions & coma in mother
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Exercise and Pregnancy
• In early pregnancy
– avoid excessive exercise & heat buildup
– linked to neural tube defects
• Moderate exercise has beneficial effects
– no evidence of inadequate blood flow to the
placenta
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Labor and Parturition
• Parturition means giving birth; labor is the
process of expelling the fetus
• Labor begins when progesterone’s inhibition is
overcome by an increase in the levels of
estrogen
– progesterone inhibits uterine contraction
– placenta stimulates fetal anterior pituitary which
causes fetal adrenal gland to secrete DHEA
– placenta converts DHEA to estrogen
– estrogen overcomes progesterone and labor begins
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Positive Feedback during Labor
• Uterine contraction forces fetal head into
cervix (stretch)
• Nerve impulses reach hypothalamus
causing release of oxytocin
• Oxytocin causes more contractions
producing more stretch of cervix & more
nerve impulses
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True Versus False Labor
• True labor begins when contractions occur
at regular intervals
– produces pain
• back pain increases with walking
– dilation of cervix with a discharge of bloodcontaining mucus in the cervical canal
• False labor produces pain at irregular
intervals but there is no cervical dilation
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Stages of Labor
• Dilation
– 6 to 12 hours
– rupture of amniotic sac &
dilation of cervix
• Expulsion
– 10 minutes to several hours
– baby moves through birth canal
• Placental
– 30 minutes
– afterbirth is expelled by
muscular contractions
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Dystocia & Cesarean Section
• Dystocia = difficult labor
– due to fetal position or size
– breech presentation is butt or feet first in birth canal
• Cesarean section (C-section)
– horizontal incision through lower abdominal wall
and uterus
– a history of multiple cesarean sections does not
preclude a vaginal birth
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Adjustments of the Infant at Birth
• Respiratory System
– after cord is cut, increased CO2 levels in blood cause
muscular contractions and first breath
– breathing rate begins at 45/minute for the first 2 weeks
& declines to reach normal rate
• Cardiovascular System
– foramen ovale closes at moment of birth
– ductus arteriosus & umbilical vein close down by
muscle contractions & become ligaments
– pulse rate slows down (120 to 160 at birth)
– increase in rate of RBC & hemoglobin formation
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Premature Infants
• Preemie is any baby weighs less than 5lb.
8oz at birth
• Causes
– poor prenatal care
– drug abuse
– young or old mother (below 16 or above 35)
• Below 36 weeks
– respiratory distress syndrome due to
insufficient surfactant is major problem
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Physiology of Lactation
• Lactation = production & release of milk
• Prolactin from anterior pituitary increases during
pregnancy, but progesterone inhibits effects of
prolactin until after delivery
• After delivery, progesterone levels drop &
suckling increases the release of prolactin &
oxytocin (milk ejection reflex)
• Colostrum = cloudy fluid released for few days
• True milk produced by 4th day
• Tortora
If suckling
stops, milk secretion stops
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Milk Ejection Reflex
• Oxytocin cause release of milk
into mammary ducts
• Stimulation of touching nipple
causes hypothalamus to release
oxytocin
• Oxytocin causes contraction of
myoepithelial cells
• Milk moved from alveoli into
mammary ducts
• Oxytocin release by other stimuli
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– hearing a baby’s cry or touching the
genitals
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Benefits of Breast-feeding
• Faster & better absorption of the “right” nutrients
• Beneficial cells
– functional white blood cells
• neutrophils help ingest bacteria in baby’s gut
• macrophages produce lysozymes
• plasma cells provides antibodies prevent gastroenteritis
• Decreased incidence of diseases later in life
– reduction in allergies, respiratory & GI infections, ear
infections & diarrhea
• Parent-child bonding
• Infant in control of intake
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Nursing and Childbirth
• Nursing of first-born twin speeds birth of
second child
– stimulates release of oxytocin
• Nursing of only child
– promotes expulsion of the placenta
– helps control hemorrhage after birth
– helps uterus return to normal size
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Inheritance
• Passing of hereditary traits from one generation to
the next
• Genotype
– all human cells contain 23 pairs of chromosomes
• one chromosome in each pair came from the mother and the
other came from the father
• similar locations on each pair of chromosomes code for the
same trait (alleles)
– if one allele controls the express of a trait, it is the dominant allele
– if the other allele is completely masked it is the recessive allele
• a person with the same alleles on both chromosomes is said to
be homozygous for the trait----heterozygous for the trait is
having different alleles on homologous chromosomes
– heterozygous individuals are carriers of a recessive gene
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Genotype & Phenotype
• Genotype = your genetic
makeup
• Phenotype = what you look
like (outward expression of
your genes)
• Punnett square
– method of showing 4
possible genetic
combinations in offspring
of 2 individuals
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Genetic Problems
• Error in meiosis called nondisjunction
– chromosomes fail to separate properly
– cell with one or more extra or missing
chromosomes is called an aneuploid
• (2n-1) is missing a chromosome
• (2n+1) has an extra chromosome
• Error in meiosis called translocation
– location of chromosome segment is moved
• crossing-over between 2 nonhomologous chromosomes
• Down syndrome results from a portion of chromosome
21 becoming part of another chromosome
– individuals have 3 copies of that part of chromosome 21
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Incomplete Dominance
• Neither member of an allelic pair is dominant
over the other --- resulting phenotype is
intermediate
• Sickle-cell trait individuals have both HbA & HbS
– suffer from only minor problems with anemia since
have both normal & sickle-cell hemoglobin
• Sickle-cell anemic individuals have 2HbS alleles
– produce sickle-cell hemoglobin
– suffer from severe anemia
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Sickle-Cell Inheritance
• 1 normal
• 2 embryos will be
sickle-cell trait
• 1 sickle-cell anemia
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Multiple-Allele Inheritance
– Genes with more than two
alternate forms
• 3 different alleles of the I gene
• IA, IB, or i
– A and B alleles are codominant
since both genes are expressed
equally
– 6 possible genotypes produce 4
blood types
• 4 phenotypes of the ABO blood
groups are (A, B, AB & O)
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Polygenic Inheritance
• Traits controlled by many genes
– continuous gradations of
small differences
– body build, height and
skin, hair & eye color
• Skin color controlled by
3 genes (Aa, Bb, Cc)
– person with genotype of
AABBCC is dark
– person aabbcc is light
• Parental generation & F1
and F2 generation
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Autosomes & Sex Chromosomes
• Each of us has a
pair of sex
chromosomes
• Females XX
• Males have XY
– Y is smaller
– Y is needed to
produce male
development
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Human Chromosomes
• 22 pairs of autosomes
• 1 pair of sex chromosomes
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Sex-Linked Inheritance
• Genes found only on X chromosomes
• Red-Green color blindness
is lack of either red or green
cones, so seen as same color
– XCXC is normal, XCXc is carrier
– XcXc is color blind
– XCY is normal, XcY is color blind
• Hemophilia is sex-linked trait
where blood fails to clot
• Other sex-linked traits
– absence of incisors, night blindness, juvenile glaucoma, and some
types of deafness, diabetes, cataracts, and muscular dystrophy
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X-Chromosome Inactivation
• Females have double dose of X chromosome in
all cells
• One X chromosome is randomly & permanently
inactivated early in development
• Visible as dark-staining Barr body easily seen in
nucleus of neutrophils as “drumstick”
– tightly coiled even in interphase cell
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Environmental Influences
• Phenotype is result of environment effects on
genetic makeup
– more influential on polygenic traits such as height
• Teratogens = cause developmental defects
– Chemicals & Drugs
• fetal alcohol syndrome = slow growth, facial features,
defective heart & CNS
• cocaine = attention problems, hyperirritability, seizures
– Cigarette Smoking
• low birth weight, cleft lip & palate, SIDS
– Irradiation or radioisotopes during first trimester
• mental retardation, microcephaly
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Infertility
• Female
– 10% of reproductive age U.S. population
• ovarian disease or obstruction of uterine tubes
• inadequate or excessive body fat
• Male
– definition is production of adequate quantities of
viable, normal sperm & transport through ducts
• seminiferous ducts sensitive to x-rays, infections, toxins,
malnutrition & high scrotal temperatures
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Alternative Fertilization Techniques
• Fertilization in a laboratory dish -- 16 cell stage placed
into uterus
– in vitro fertilization
• mother given FSH to produce multiple oocytes (surgically removed)
• mixed with solution containing sperm
– intracytoplasmic sperm injection into oocyte
• Embryo transfer
– artificial insemination of oocyte donor
– blastocyst transfer to infertile woman for pregnancy
• Gamete intrafallopian transfer
– FSH & LH stimulate multiple oocytes---aspiration &
fertilization outside the body---reimplantation into uterine tubes
(whole procedure is to skip vagina)
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Down Syndrome (DS)
• Nondisjunction of chromosome 21 causes one
of daughter cells to end up with extra copy
– trisomy 21---2 copies from mom & one from dad
• More common in older mothers
– more exposure to radiation & chromosomedamaging chemicals
• kinetochore microtubules that pull chromosomes apart
sustain damage
• 1 in 800 infants is born with Down syndrome
– mental retardation, distinctive facial structures &
malformation of the heart, ears, hands & feet
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Fragile X Syndrome
• Defective gene on X chromosome
– broken tip of X chromosome
• Causes mental retardation in some of males
with this gene
– learning difficulties, oversized ears, enlarged
testes & double jointedness
– may be involved with autism
• Unaffected males may pass gene onto
daughters whose children may suffer
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