Ch 28. Pregnancy & Human Development

what is fertilization?

the process in sexual reproduction that involves the union of a sperm and an egg to form a diploid zygote

what is the fertilization time window?

an oocyte is only viable for 24 hours (at most) after ovulation

how long can sperm survive for in the female reproductive tract?

5 days

how many days in a single month is it possible for fertilization and
pregnancy to occur?

6 day window during ovulation

of the millions of sperm that enter the vagina, only a fraction reach the uterus. why?

1. millions lost immediately from vagina

2. millions more lost by acidic environment of vagina

when sperm is able to make it past the vagina and get to the uterus, what happens to it now?

phagocytes kill off many more
• only a few thousand “survivors”—>reverse peristalsis push sperm
toward the uterine tubes
• sperm have various receptors (chemical, temperature, fluid flow) to
ensure they travel in ”correct” direction

what is the problem of sperm capacitation?

sperm are incapable of fertilizing the oocyte immediately after entering vagina

before fertilization can occur, what must sperm do?

it has to be capacitated

• motility is enhanced

• weakening of sperm cell membrane

what 2 protective structures surround the oocyte?

1. corona radiata

2. zona pellucida

what is the corona radiata?

outer layer, its function is to protect and nourishes the oocyte after it has been ovulated

what is the zona pellucida?

the inner layer, its function is to protect secondary oocyte and it is necessary for fertilization to occur

what happens at the zona pellucida?

acrosomal reaction occurs, where the release of enzymes occurs from the acrosome of the sperm

• sperm bind zona pellucida —> calcium levels in sperm rise

• enzymes from acrosome digest holes in zona pellucida

what happens once the acrosomal reaction has been initiated and the sperm has passed through both layers?

1) Sperm binds to sperm-binding receptors on surface of oocyte
2) Once bound, the two membranes fuse together & sperm
contents enter the oocyte

what is polyspermy?

The entry of more than one sperm cell into the oocyte

why should polyspermy be blocked or prevented?

it must be prevented because extra chromosomes and paternal centrioles cause abnormal, chaotic mitotic divisions that lead to early embryonic death, failing to produce a viable embryo

what happens when oocyte membrane blocks polyspermy after 1 sperm cell has bound?

sperm-binding receptors are shed from oocyte surface
• Sperm unable to bind oocyte surface and fertilize oocyte

what happens in the cortical reaction when blocking polyspermy?

oocyte releases calcium, causes zona pellucida to harden

what happens in meiosis II?

Sperm nucleus travels toward oocyte nucleus
• As it travels, sperm nucleus swells in size to form male pronucleus
• Surge in Ca2+ from cortical reaction causes release of zinc from
oocyte
• This causes completion of meiosis II and it forms polar body & mature ovum

what happens after meiosis II/during fertilization?

female pronucleus forms
• two pronuclei membranes rupture & chromosomes are released
• 2 sets of chromosomes combine to form a diploid zygote—>this is
fertilization

what occurs during twins?

usually one oocyte is ovulated and fertilized to produce a single zygote

what are monozygotic twins

Occurs when a single oocyte is fertilized splits into 2 identical embryos
• Unknown why/how the zygote splits
• Twins usually share a placenta

what are dizygotic twins?

Occurs when 2 oocytes are ovulated and both are fertilized
• Twins have their own placenta

when does development begin?

at fertilization, zygote begins to divide mitotically

what is cleavage?

rapid divisions of the zygote

• First mitotic division produces 2 identical blastomere cells

By 72 hours after fertilization, a morula is formed—>contains
~16 cells

what do morula cells divide to form?

blastocyst

what are the 2 layers of blastocyst?

1. external trophoblast

2. internal embryoblast (inner cell mass)

what do external trophoblast aid in?

embryo implantation, contributes to chorion formation/function, & has immunosuppressive effects

what do internal embryoblast do?

Will eventually form the embryo proper and extraembryonic
membranes

why is endometrium receptive to implanting embryo?

due to high estrogen and progesterone levels

where does trophoblast attach to?

endometrium, this usually occurs high in the uterus

• beneficial bc it ensures proper placenta formation in a vascularized area, reducing risks of miscarriage and complications like placenta previa

once bound, what does trophoblast release?

growth factor and digestive enzymes on epithelium of endometrium

what are the growth factor influences?

1. Endometrium thickens

2. Trophoblast proliferates & releases digestive enzymes to facilitate implantation

what are the digestive enzyme influences

1. Erosion of endometrium around blastocyst
   • Blastocyst buries itself in the blood-rich lining

2. Surrounding endometrial cells proliferate shortly after to enclose
   the blastocyst in lining

what happens when implantation is complete?

embryo releases hormone human chorionic gonadotropin (hCG)
• hCG maintains the corpus luteum for ~12 weeks & suppresses
female immune system

what is the placenta?

a temporary organ originating from embryonic and maternal tissues

what is the function of the placenta?

maintains pregnancy, exchange respiratory gases, provide nutrients to embryo/fetus, dispose of waste, etc

what is the embryonic contribution to placenta formation?

Chorion: membrane that surrounds fetus
• Chorion has chorionic villi
• Form umbilical vein and arteries

what does the functional layer of endometrium develop?

blood-filled lacunae

what does the endometrium that lies underneath embryo become?

decidua basalis

• function=forms placenta with chorionic villi

what does the endometrium found between embryo and lumen of uterus form?

decidua capsularis

• Chorionic villi here degenerate as pregnancy progresses

what is the function of decidua capsularis?

expands the accommodate growing fetus

what are extraembryonic membranes?

Membranes formed during the first few weeks of development that
support & nourish developing embryo/fetus during gestation

what do extramembryonic membranes include?

1. amnion

2. yolk sac

3. allantois

4. chorion

what is amnion?

innermost layer that extends around the embryo

• filled with amniotic fluid
  • functions:
  • 1) Provides buoyancy & protection
  • 2) Maintains consistent temperature
  • 3) Prevents developing parts of embryo from sticking together/fusing
  • 4) Allows movement of embryo/fetus

what is the yolk sac?

sac-like structure
• eventually forms digestive tube
• forms earliest blood cells
• forms precursor gametes

what is allantois?

Helps form the umbilical cord later in fetal development
• Allows for gas exchange, waste disposal, & nutrient exchange

what is chorion?

encloses all other extraembryonic membranes
• Allows exchange of gases, nutrients, & wastes

what is gastrulation?

early developmental process where the blastocyst is reorganized into a 3-layered embryo

what are the 3 primary germ layers?

1) Endoderm
2) Ectoderm
3) Mesoderm

what is the process of gastrulation?

1. Begins with formation of primitive streak

2. Cells migrate & enter the primitive streak
   • These cells form the endoderm—>inferior layer

3. Other cells follow & push between cells of upper and lower
   layers
   • These cells form the mesoderm—>”middle” layer
   • Mesodermal cells immediately beneath primitive streak form the notochord
   • The notochord is the first axial support of the embryo
   • Some cells remain at surface
   • These cells form the ectoderm

what is primitive streak?

a groove that will eventually form the long axis of the embryo

what is organogenesis?

Formation of body organs and organ systems
• Begins with gastrulation
• Cells in 3 primary germ layers will differentiate to form organs &
organ systems
• Early process—>all organ systems are recognizable by week 8

what is the endoderm?

Forms epithelial lining of GI tract
• Respiratory tract forms from outpocketings
• Other glands formed from endothelial outpocketings along the tract
• Opposite ends of the endoderm open to form mouth and anus

what is the ectoderm?

• neurulation

• Induced by chemicals released by notochord
  • Results in formation of a neural tube sitting over the notochord
  • Anterior portion becomes brain, remaining becomes spinal
  cord
  • By week 8—>cerebral hemispheres evident, brain waves
  can be recorded
  • Remaining ectoderm forms epidermis

what is neurulation?

formation of brain and spinal cord

what is the mesoderm?

forms mesodermal blocks called somites that hug the notochord on either side

• Remaining mesoderm forms kidneys & gonads, connective tissues of limbs, heart & blood vessels, dermis of ventral body region, etc.

what are the 3 functional parts of somites?

1. sclerotome

2. dermatome

3. myotome

what is sclerotome?

produce vertebrae and rib at each associated levels

what is dermatome?

forms dermis in dorsal part of body?

what is myotome?

forms skeletal muscle of neck, body trunk, and limbs

are fetal circulation and adult circulation identical?

no they are not

what are the cardiovascular modifications seen only during development?

1. umbilical arteries and vein —> although blood is transported via these structures, gas exchange (O2 and
   CO2), waste elimination, & various other processes occur in placenta

2. vascular shunts

what type of blood do veins carry?

oxygenated blood TO fetus

what type of blood do arteries carry?

oxygen poor blood AWAY from fetus

what are vascular shunts

they redistributes blood to parts of body that need it most, (mostly) bypasses organs not yet used by developing fetus

what are the structures apart of vascular shunts?

1. ductus venosus

2. foramen ovale

3. ductus arteriosus

what does the ductus venosus do?

shunt that bypasses the liver, parents liver does all the work for the fetus

what does foramen ovale do?

shunts blood from right atrium to left atrium

what does ductus arteriosus do?

shunts blood from pulmonary trunk to aorta

• causes blood to bypass lungs

what happens after birth?

1. Umbilical blood vessels constrict & fibrose

2. Umbilical arteries supply bladder & anchors bladder to
   umbilicus

3. Umbilical vein remnant become round ligament of liver

4. Ductus venosus collapses & is converted to ligamentum
   venosum

5. Foramen ovale closes as pulmonary circulation becomes
   functional, forms the fossa ovalis

6. Ductus arteriosus constricts & closes off—>forms the ligamentum arteriosum

what are the anatomical changes due to pregnancy?

1. uterine enlargement—> at beginning uterus is size of pear and at end it is even with xiphoid process, uterus push other organs up and out

2. weight gain—> 28 lbs at beginning of pregnancy and abot 300 extra calories a day to maintain health pregnancy

how does the pelvis widen?

from release of relaxin

what are the metabolic changes during pregnancy?

hormones during pregnancy affect metabolism

what is placental growth hormone?

replaces growth hormone in pregnant women
• stimulates lipolysis and glucose production for growing fetus

what is human placental lactogen (hpL)?

stimulates maturation of breasts for lactation, promotes fetal growth, and is glucose-sparing

what is corticotropin-releasing hormone (CRH)?

rises only toward end of pregnancy
• as CRH rises, parent’s ACTH and cortisol levels rise—>helps during birth
• Cortisol leads to maturation of fetal organs

what is the physiological change in the GI system?

Surge in hCG, estrogen, and progesterone can result in
nausea/vomiting
• “Morning” sickness
• Uterus invading abdomen, pressing on other organsheartburn,
constipation, etc

what is the physiological change in the urinary system?

Increased metabolic rate, higher blood volume, & need to dispose of
extra waste from fetus = more urine produced
• Uterus pushes on bladder—>increases frequency of urination

what is the physiological change in the respiratory system?

Respiratory rate is unaffected
• Increase in tidal volume & decrease in residual volume
• Uterus pushes on diaphragm—>can make breathing difficult by end of pregnancy

what is the physiological change in the cardiovascular system?

• Total blood volume increases by at least 25%, may be as high as 40% by end of 32nd week

• blood pressure decreases

• Uterus pushes on blood vessels—>can block blood flow, forming varicose veins and edema (especially in the legs)

what is the initiation stage of labor?

The fetus determines its own birth date
• Increases in fetal cortisol levels leads to maturation of lung tissue (&
increase in surfactant) and surge in estrogen from the placenta
• Surge in estrogen has 3 main effects

what are the 3 effects of surge in estrogen?

1) Increased synthesis of prostaglandins
2) Increase in oxytocin & prostaglandin receptors in myometrium
3) Addition of more actin, myosin, and other contractile elements

what is released by posterior pituitary gland of parent to stimulate
uterine contractions?

oxytocin

• uterine contractions push head of fetus into cervix

• Stretching of cervix by head stimulates more oxytocin release bythe posterior pituitary

what are the 3 main stages of labor?

1. dilation

2. expulsion

3. placental

what is the dilation stage of labor?

occurs from onset of labor until cervix is fully dilated
• Highly variable duration of time
• Dilation caused by force of baby’s head against cervix
• Full dilation is ~10 cm in diameter
• Contractions begin as weak/short, becomes progressively stronger
• Push of head against cervix will break the amnion

what is the expulsion stage of labor?

occurs from full dilation to delivery
• Contractions occur every 2-3 minutes, last ~1 minute each
• Crowning occurs when the largest portion of infant’s head
distends vulva
• Infant should be in a vertex position

what is the placental stage of labor?

occurs after delivery of infant
• Occurs ~30 minutes after birth
• Parent must also deliver placenta & all of its structures (afterbirth)
• Uterine contractions compress blood vessels and tear placenta from
uterine wall

what are the advantages of the infant being in vertex position?

dilates the cervix for rest of body to pass easier, can suction mucus from oral/nasal passage for breathing to occur sooner

what is the breech position?

butt first

what are extrauterine adjustments?

After birth, the infant must perform all physiological functions
on its own (breathing, excretion, maintain body temperature, etc

what are the major adjustments that MUST occur immediately after birth?

1) Taking the first breath
• Umbilical blood vessels constrict—>prevents removal of CO2 from
infant, helps stimulate first breath
• Lungs must be completely inflated & not collapse
2) Closing of shunts & fetal blood vessels

what happens to your mammary glands when you are not pregnant?

they are not functional, lobules contain glandular alveoli and produce milk during lactation, but not before

what happens in lactation during late pregnancy or after birth?

Rising levels of progesterone, hPL, and estrogens leads to release of prolactin
• Glandular alveoli become active—>produce & release milk
• Prolactin only produced for few weeks after birth—>milk production stimulated by feeding infant after this

what happens when there is a delay in milk production?

colostrum is secreted by mammary glands
• Contains less lactose than milk, very little fat, but contains much more protein, minerals, vitamin A and some antibodies

what are the benefits of breast milk for the infant?

1) Various components of breast milk are more easily metabolized
by the infant
2) Various protective molecules
3) Helps build healthy bacterial flora of infant
4) Serves as a natural laxative
• Cleanses infant bowels of meconium

what are the various protective molecules of breast milk?

IgA, complement, lysozymes, interferon, interleukins, etc.