Development

Human DevelopmentHuman Development

Chapter 17Chapter 17

FertilizationFertilization• Fertilization is the union of the sperm and egg to form Fertilization is the union of the sperm and egg to form zygotezygote• Egg is surrounded by an outer matrix called the Egg is surrounded by an outer matrix called the zona of zona of

pellucidapellucida• Outside this matrix has a few layers of follicular cells Outside this matrix has a few layers of follicular cells

collectively called the collectively called the corona radiatacorona radiata• Steps of fertilization:Steps of fertilization:

1.1. Several sperm penetrate the corona radiata Several sperm penetrate the corona radiata 2.2. Acrosomal enzymes digests a portion of the zona Acrosomal enzymes digests a portion of the zona

pellucidapellucida3.3. The sperm binds to and fuses with the egg’s plasma The sperm binds to and fuses with the egg’s plasma

membranemembrane4.4. Sperm nucleus enters the eggSperm nucleus enters the egg5.5. Sperm and egg nucleus will fuseSperm and egg nucleus will fuse

What prevents more than one sperm from entering the egg?What prevents more than one sperm from entering the egg?

• The egg’s plasma membrane changes to prevent other sperm The egg’s plasma membrane changes to prevent other sperm from bindingfrom binding

• Vesicles within the egg release enzymes that cause the zona Vesicles within the egg release enzymes that cause the zona pellucida to become impenetrable and sperm cannot bindpellucida to become impenetrable and sperm cannot bind

Embryonic DevelopmentEmbryonic Development• CleavageCleavage – cells undergo division (2, 4, 8 etc) without the – cells undergo division (2, 4, 8 etc) without the

embryo increasing in sizeembryo increasing in size

• GrowthGrowth – cells undergo division as well as increase in size – cells undergo division as well as increase in size

• MorphogenesisMorphogenesis – the embryo begins to take shape as cells – the embryo begins to take shape as cells migratemigrate

• DifferentiationDifferentiation – when cells take on specific structure and – when cells take on specific structure and function (the nervous system is the first visible system)function (the nervous system is the first visible system)

Extra-embryonic MembranesExtra-embryonic Membranes• ChorionChorion – fetal half of the – fetal half of the

placenta, the organ that placenta, the organ that provides the embryo with provides the embryo with nourishment and gets rid of nourishment and gets rid of wasteswastes

• AllantoisAllantois – gives rise to the – gives rise to the bladder and the blood vessels bladder and the blood vessels of the umbilical cord that of the umbilical cord that carry blood to and from the carry blood to and from the fetusfetus

• Yolk sac Yolk sac – contains many – contains many blood vessels and where blood vessels and where blood cells first form (little blood cells first form (little yolk in humans)yolk in humans)

• AmnionAmnion – contains amniotic – contains amniotic fluid that cushions and fluid that cushions and protects the embryoprotects the embryo

Stages of DevelopmentStages of Development• Pre-embryonic development Pre-embryonic development - 1st week of development after - 1st week of development after

fertilizationfertilization

• Embryonic development Embryonic development – 2nd week after fertilization until – 2nd week after fertilization until the end of the 2nd monththe end of the 2nd month

• Fetal development Fetal development – the 3rd through the 9th months of – the 3rd through the 9th months of developmentdevelopment

• Development after birth Development after birth – stages of life including infancy, – stages of life including infancy, childhood, adolescence and adulthoodchildhood, adolescence and adulthood

Pre-embryonic Development: Week 1Pre-embryonic Development: Week 1• CleavageCleavage – cell division that increases the number of cells – cell division that increases the number of cells

• MorulaMorula – compact ball of embryonic cells – compact ball of embryonic cells

• Early blastocyst Early blastocyst – inner cell mass that becomes the embryo – inner cell mass that becomes the embryo covered by a layer of cells that becomes the chorioncovered by a layer of cells that becomes the chorion

• ImplantationImplantation – embryo embeds into the uterus around day 6 – embryo embeds into the uterus around day 6

Pre-embryonic Development: Week 1Pre-embryonic Development: Week 1

Embryonic Development: Week 2Embryonic Development: Week 2• Pregnancy begins after implantationPregnancy begins after implantation• Human chorionic gonadotropin (HCG) is secreted maintaining Human chorionic gonadotropin (HCG) is secreted maintaining

the corpus luteum and the endometrium the corpus luteum and the endometrium • HCG is the basis for a pregnancy testHCG is the basis for a pregnancy test• The inner cell mass detaches itself and becomes the The inner cell mass detaches itself and becomes the

embryonic disk that will go through embryonic disk that will go through gastrulationgastrulation to become 3 to become 3 primary germ layers (primary germ layers (endodermendoderm, , mesodermmesoderm and and ectodermectoderm))

Embryonic Development: Week 3Embryonic Development: Week 3• Nervous system begins to developNervous system begins to develop• The posterior neural tube will become the spinal cord and The posterior neural tube will become the spinal cord and

brainbrain• Development of the heart beginsDevelopment of the heart begins

Embryonic Development: Weeks 4 and 5Embryonic Development: Weeks 4 and 5• 4th week:4th week:– Embryo is slightly larger Embryo is slightly larger

than the height of the than the height of the print in your bookprint in your book

– Chorionic villi formChorionic villi form– Umbilical cord formsUmbilical cord forms– Limb buds form (later Limb buds form (later

develop into legs and develop into legs and arms)arms)

– Hands and feet are Hands and feet are apparentapparent

• 5th week:5th week:– Head enlargesHead enlarges– Eyes, ears and nose Eyes, ears and nose

become prominentbecome prominent

Week 5Week 5

Embryonic Development: Weeks 6 to 8Embryonic Development: Weeks 6 to 8• Embryo begins to look like a human beingEmbryo begins to look like a human being• Reflex actions occurReflex actions occur• All organ systems have been establishedAll organ systems have been established• 38mm by the end of this period and weight ~ the same as an 38mm by the end of this period and weight ~ the same as an

aspirin tabletaspirin tablet

Fetal DevelopmentFetal Development• During fetal development organs continue to develop and During fetal development organs continue to develop and

growgrow– Fetal development begins in the eighth week of pregnancyFetal development begins in the eighth week of pregnancy

• Birth defects include structural and functional defects that Birth defects include structural and functional defects that appear in the newbornappear in the newborn– About 31 % of all successful fertilizations end in a About 31 % of all successful fertilizations end in a

miscarriagemiscarriage– About 10-12 % of all newborns have birth defects About 10-12 % of all newborns have birth defects – Birth defects can arise from chemical, biological, and Birth defects can arise from chemical, biological, and

physical agents known as teratogensphysical agents known as teratogens– The effect of teratogenic agents is related to the time of The effect of teratogenic agents is related to the time of

exposure, the nature of the agent, and the doseexposure, the nature of the agent, and the dose

Fetal Development: 3 to 4 MonthsFetal Development: 3 to 4 Months• Hair developsHair develops• The head slows in growth so that the body size can catch upThe head slows in growth so that the body size can catch up• Cartilage begins to be replaced by boneCartilage begins to be replaced by bone• Able to distinguish female from male (month 3)Able to distinguish female from male (month 3)• Can hear heartbeat with the stethoscope (month 4)Can hear heartbeat with the stethoscope (month 4)• By the end of this period the fetus is ~6 inches and ~6 ouncesBy the end of this period the fetus is ~6 inches and ~6 ounces

Fetal Development: 5 to 7 MonthsFetal Development: 5 to 7 Months• Fetal movement can be felt by the Fetal movement can be felt by the

mothermother• Fetus is in fetal positionFetus is in fetal position• Eyelids are fully openEyelids are fully open• Fetus size has increased to ~12 inches Fetus size has increased to ~12 inches

and ~3 poundsand ~3 pounds

Fetal Development: 8 to 9 MonthsFetal Development: 8 to 9 Months• Weight gain is about a pound per weekWeight gain is about a pound per week

• Fetus rotates so that the head is pointed towards the cervixFetus rotates so that the head is pointed towards the cervix

• At the end of fetal development the fetus weighs ~7.5 pounds At the end of fetal development the fetus weighs ~7.5 pounds and ~20.5 inchesand ~20.5 inches

Fetal CirculationFetal Circulation

Note: Fetal and maternal blood do not mix because exchange of Note: Fetal and maternal blood do not mix because exchange of materials between the two occurs at the chorionic villi.materials between the two occurs at the chorionic villi.

Development of Sex OrgansDevelopment of Sex Organs• Sex of an individual is determined at conception (XX is Sex of an individual is determined at conception (XX is

female and XY is male)female and XY is male)

• If the If the SRYSRY (the sex determining region on the Y (the sex determining region on the Y chromosome) gene is present at ~ week 6 then the embryo chromosome) gene is present at ~ week 6 then the embryo develops into a maledevelops into a male

• Anti-Mullerian hormone Anti-Mullerian hormone secreted by the testes prevents the secreted by the testes prevents the development of female sex organsdevelopment of female sex organs

• At 14 weeks primitive testes and ovaries with gametes are At 14 weeks primitive testes and ovaries with gametes are already developingalready developing

• The development of the external organs is dependent on The development of the external organs is dependent on the presence or absence of the presence or absence of dihydrotestosterone (DHT) dihydrotestosterone (DHT) produced by the testesproduced by the testes

Abnormal Sex Organ DevelopmentAbnormal Sex Organ Development• XY female syndrome XY female syndrome - an individual develops into a male - an individual develops into a male

because piece of the Y chromosome containing the SRY gene because piece of the Y chromosome containing the SRY gene is missingis missing

• XX male syndrome XX male syndrome – an individual develops into a male – an individual develops into a male because the same small piece of the Y containing the SRY because the same small piece of the Y containing the SRY gene is present on an X chromosomegene is present on an X chromosome

Ambiguous Sex DeterminationAmbiguous Sex Determination• Results from the absence of testosterone, Anti-Mullerian Results from the absence of testosterone, Anti-Mullerian

hormone and/or DHThormone and/or DHT

• Androgen insensitivity syndromeAndrogen insensitivity syndrome: all hormones are made but : all hormones are made but testosterone receptors on cells are ineffective thus the testosterone receptors on cells are ineffective thus the individual has testes that do not descend and outwardly individual has testes that do not descend and outwardly appear to be femaleappear to be female

• Male pseudo-hermaphroditismMale pseudo-hermaphroditism: an individual appears female : an individual appears female until puberty when anti-Mullerian hormone is produced but until puberty when anti-Mullerian hormone is produced but the testes never produce testosterone or DTHthe testes never produce testosterone or DTH

BirthBirth• True labor is characterized by uterine contraction that occur every True labor is characterized by uterine contraction that occur every

15-20 minutes and last for at least 40 seconds15-20 minutes and last for at least 40 seconds• 3 stages:3 stages:• 1st: 1st: • effacement occurs in which the cervical canal slowly disappears and effacement occurs in which the cervical canal slowly disappears and

the babies head acts as a wedge to cause cervical dilationthe babies head acts as a wedge to cause cervical dilation• 2nd: 2nd: • uterine contractions every 1-2 minutes lasting for ~1 minute eachuterine contractions every 1-2 minutes lasting for ~1 minute each• an incision is made to the opening to help the baby as its head an incision is made to the opening to help the baby as its head

reaches the exterior;reaches the exterior;• once the baby is born the umbilical cord is cut and tiedonce the baby is born the umbilical cord is cut and tied• 3rd: 3rd: • The afterbirth is delivered usually about 15 minutes after the birth The afterbirth is delivered usually about 15 minutes after the birth

of the babyof the baby

BirthBirth• Childbirth occurs in three stages Childbirth occurs in three stages

1.1. The dilation stage - cervix dilatesThe dilation stage - cervix dilates2.2. The expulsion stage - delivery of the childThe expulsion stage - delivery of the child

• Uterine muscle contractions are stimulated by a change Uterine muscle contractions are stimulated by a change in hormonal levels. in hormonal levels.

• It’s believed that fetal oxytocin stimulates the release It’s believed that fetal oxytocin stimulates the release of prostaglandins by the placentaof prostaglandins by the placenta

• Oxytocin and prostaglandins stimulate contractions in Oxytocin and prostaglandins stimulate contractions in the sensitized uterine musculaturethe sensitized uterine musculature

3.3. The placental stage - placenta is expelledThe placental stage - placenta is expelled