Tortora & Grabowski 9/e 2000 JWS 29-1 Development and Inheritance From fertilization to birth...

Tortora & Grabowski 9/e 2000 JWS 29-1

Development and Inheritance

• From fertilization to birth– fertilization– implantation– placental development– fetal development– gestation– labor– parturition (birth)


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


From Fertilization to Implantation


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


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


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


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)


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


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 innercell mass will develop into fetal portion of placenta

• Most of inner cell mass will become embryo


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


Implantation

Notice: distinct syncytiotrophoblast and cytotrophoblast layers.


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


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 cellsdivide to fill the spacessurrounding the yolk sac with extraembryonic mesoderm

• spaces develop in that layer to form future ventral body cavity


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


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


Amnion, Yolk sac, Chorion, allantois


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)



Parts of Endometrial Lining• Decidua = all of endometrium lost as placenta

– equals all of the endometrium, except stratum basalis

• Decidua basalis---portion ofendometrium deep to chorion

• Decidua capsularis---part ofendometrial wall that coversimplanted embryo

• Decidua parietalis---part ofendometrial wall not modifiedby embryo until embryo bumps into it as it enlarges

• Decidua capsularis fuses with decidua parietalis


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


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


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


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%


Chorionic Villi Sampling• As early as 8 weeks• Results in few days• Chance of spontaneous

abortion 1-2% • 30 mg of placenta

removed by suctionthrough cervix or with needle through abdomen

• Chromosomal analysis reveals same results as amniocentesis


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


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.


Hormonal Secretion by the Placenta


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


Developmental Changes

• Read Table 29.2 to get a full description of the timing of fetal events during development


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


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%


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


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


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


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


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 blood-containing mucus in the cervical canal

• False labor produces pain at irregular intervals but there is no cervical dilation


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


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


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


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


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• If suckling stops, milk secretion stops


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– hearing a baby’s cry or touching the

genitals


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


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


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


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 offspringof 2 individuals


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


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


Sickle-Cell Inheritance

• 1 normal

• 2 embryos will be sickle-cell trait

• 1 sickle-cell anemia


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)


Polygenic Inheritance• Traits controlled by many genes

– continuous gradations ofsmall differences

– body build, height andskin, hair & eye color

• Skin color controlled by3 genes (Aa, Bb, Cc)– person with genotype of

AABBCC is dark– person aabbcc is light

• Parental generation & F1and F2 generation


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


Human Chromosomes

• 22 pairs of autosomes• 1 pair of sex chromosomes


Sex-Linked Inheritance• Genes found only on X chromosomes

• Red-Green color blindness is lack of either red or greencones, 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 traitwhere 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


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


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


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


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)


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 & chromosome-

damaging 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


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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Tortora & Grabowski 9/e 2000 JWS 29-1 Development and Inheritance From fertilization to birth...

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