Exam 2 - Pregnancy

Question 1

Explain the progression of how many sperm make it throughout the female reproductive tract.

Answer 1

250,000,000 deposited in the vagina
100,000 reach the uterine cavity («1%)
50 or less arrive at the distal end of the fallopian tube («1%)
1 fertilizes the egg

Question 2

When an oocyte arrives in the oviduct/fallopian tube, what stage of mitosis/meiosis is it in? How many chromosomes and chromatids does it have?

Answer 2

has completed meiosis 1 and is arrested in metaphase of meiosis 2 (because it has been ovulated)

23 chromosomes, 46 chromatids

Question 3

What is the viability of the oocyte and sperm after deposition?

Answer 3

oocyte viable for 24 hours after ovulation
sperm viable for 2-3 days (sometimes 5+ days) in the female reproductive tract

Question 4

What is the final stage of sperm development? Where does it happen and what does it allow?

Answer 4

sperm capacitation
final maturation of sperm that occurs in the female reproductive tract, enabling them to fertilize the oocyte

Question 5

Describe the basic steps of how a sperm enters an oocyte (up to membrane fusion)

Answer 5

• sperm passes follicular cells
• sperm binds zona pellucida
• ZP3 binds sperm, causing an acrosomal reaction to release contents
• enzymes from acrosome dissolve zona pellucida
• whip action of tail pushes sperm head to the oocyte membrane
• sperm head/oocyte membranes fuse and contents of sperm enter the oocyte

Question 6

Describe what happens in an oocyte after the sperm and oocyte membranes fuse

Answer 6

• increase Ca++ triggers cortical reaction, granules harden the zona pellucida preventing entry of more sperm cells
• increase Ca++ also induces oocyte completion of meiosis 2 (formation of female pronucleus)
• sperm head forms male pronucleus
• pronuclei fuse

Question 7

At what point does the zona pellucida become hardened?

Answer 7

as soon as the first sperm head has fused membranes with the oocyte membrane (causing a cortical reaction)

Question 8

What occurs in the first week after ovulation (assume the oocyte is going to become fertilized)

Answer 8

ciliary movement draws the oocyte into the tube (stimulated by estrogen)
progesterone secreted by the corpus luteum primes the endometrium while the embryo develops/makes its way through the oviduct
endometrial secretions (glycogen) nourish the arriving blastocyst (stimulated by progesterone)

Question 9

Describe the development of the embryo as it moves through the fallopian tube and into the uterus (by hours and days)

Answer 9

oocyte enters fallopian tube and is fertilized in the ampulla
still in the ampulla:
- zygote (single cell embryo, 2pn)
- two cell stage (39 hours, after first mitotic division)
- four cells stage (42 hours)
- eight cells stage (49 hours)
enters isthmus:
- morula (3-4 days)
enters uterus:
- blastocyst (5-6 days

Question 10

Describe the structure of a blastocyst

Answer 10

Surrounded by the zona pellucida and a single layer of trophoblasts
Inner cell mass that will become the fetus

Question 11

When does implantation begin in relation to ovulation?

Answer 11

approcx. 6 days after

Question 12

Describe the process of implantation, with a general timeline

Answer 12

5 days after fertilization, the embryo hatches out of the zona pellucida and trophoblasts contact the endometrium. they release proteolytic enzymes to “break in.”
Some trophoblasts fuse and penetrate the endometrium to form the syncytiotrophoblast within the decidua
By 12 days after fertilization, a layer of the endometrium completely surrounds the developing fetus.

Question 13

What structures will eventually become the embryo-derived portion of the placenta?

Answer 13

the trophoblast cells (the outer cells, not the inner cell mass)

Question 14

What is the decidua?

Answer 14

highly vascularized endometrium of pregnancy

Question 15

As implantation occurs, what other structures develop inside the blastocyst/embryo?

Answer 15

yolk sac, amniotic cavity, and embryonic disk in between them

Question 16

What is the function of the yolk sac?

Answer 16

functions as an extra-embryonic circulatory system before the internal circulatory system forms

Question 17

What is the structure of the embryonic disk and what structures does it lead to in a fetus?

Answer 17

composed of three embryonic germ layers:
endoderm- lining of respiratory tube, lungs, digestive tube, liver, gall bladder, and pancreas
mesoderm - skeleton, muscles, circulatory system, kidneys
ectoderm - nervous system, outer layer of skin, hair, nails, tooth enamel

Question 18

Between implantation (12 days post fertilization) and 6-7 weeks post fertilization, what embryonic development occurs?

Answer 18

• embryonic disk, amnion, connecting stalk, and yolk sac are derived from the inner cell mass
• the embryonic disc gives rise to the embryo proper
• chorion is derived from the trophoblast (forms an “outer wall”)
• connecting stalk becomes the umbilical cord (connecting the embryo to the chorion)
• chorion eventually fuses with the amnion (because the amnion fills the cavity and touches the chorion)
• yolk sac degenerates

Question 19

What is the distinction between embryo and fetus?

Answer 19

embryo is <8 weeks post fertilization
fetus is >8 weeks post fertilization

Question 20

What are dichorionic/monochorionic and diamnionic/monoamnionic?

Answer 20

dichorionic - two eggs, both fertilized separately, each has their own chorion and placenta
monochorionic - one egg that splits late (so only one placenta) or two eggs that implant close and form a fused placenta
diamniotic - two amniotic sacs
monoamniotic - one amniotic sac (rare and usually dangerous)

Question 21

What are the two types of DC/DA twins?

Answer 21

dizygotic (fraternal) twins - two eggs, implant separately and form two placentas

monozygotic (identical) twins - one egg that splits early (before blastocyst stage), each implant separately and form two placentas

Question 22

What are “MC”/DA twins vs true MC/DA twins?

Answer 22

“MC” : dizygotic (fraternal) twins, two eggs that implant close together and form a fused placenta (rare)

true MC/DA: monozygotic (identical) twins, blastocyst already formed, inner cell mass splits forming two fetuses/two amnion in a single chorion/one placenta

Question 23

What are the four types of identical twin pregnancies?

Answer 23

DC/DA: 25% of identical twins, 1 egg splits before blastocyst and each implants separately with it’s own placenta

MC/DA: 60-70% of identical twins, inner cell mass of a blastocyst splits, forming two fetuses/two amniotic sacs but only one placenta

MC/MA: 1-2% of identical twins, inner cell mass splits later in development, both fetus develop in the same amniotic sac, umbilical cords can twist causing a survival rate of only 50-60%

Conjoined twins: very rare, also technically MC/MA, separate very late so that organ systems are shared by the fetuses, high mortality

Question 24

Describe the blood flow in the placenta

Answer 24

maternal blood flows in maternal arterioles through the uterine decidua, into the intervillus space
fetal blood flows into chorionic villus from the umbilical vein
fetal vessels in the chorionic villus exchange nutrients/waste across the chorionic tissue (a thin layer that covers the villus)
blood returns to mother via mternal venules and fetus via umbilical artery

Question 25

What types of things can diffuse across the chorionic villus/cross the placenta? What things can’t?

Answer 25

small (<500 MW) and hydrophobic molecules
- small: oxygen, carbon dioxide, glucose, amino acids, alcohol, and many drugs
- hydrophobic: thyroid hormone

protein hormones and bacteria cannot because they are too big

Question 26

What are the endocrine functions of the placenta?

Answer 26

protein hormones - hCG and CRH
steroid hormones - progesterone and estrogen (including estriol, the estrogen unique to pregnancy)

Question 27

Describe hCG: site of production, structural similarity, when it is detectable, functions, clinical usage

Answer 27

site of production: syncytiotrophoblast
structural similarity: like LH, glycoprotein family
when it is detectable: 6 days after implantation, peak at 9-12 weeks
functions: signal implantation of the embryo, prevent luteolysis (maintains the corpus luteum), and stimulate luteal production of progesterone
clinical usage: mimic LH surge in infertile women (trigger ovulation), blood or urine tests used to diagnose early pregnancy

Question 28

How are hCG and progesterone related?

Answer 28

hCG is a major regulator of progesterone production, when hCG is produced progesterone levels rise

Question 29

Why does hCG go down after about 12 weeks gestation?

Answer 29

at this point the placental steroidogenesis is sufficient, shifting from the corpus luteum to the placenta, so hCG does not need to maintain CL anymore

Question 30

After about 12 weeks gestation, what are the roles of progesterone and estrogen?

Answer 30

progesterone:
- inhibit uterine contractility
- promote formation of the mucus plug

estrogen:
- stimulate myometrial growth (larger uterus)
- stimulate gap junction and oxytocin receptors in the myometrium (prep for labor)

Question 31

HOW are progesterone and estriol synthesized after 12 weeks gestation? Hint: they are different methods

Answer 31

progesterone: placenta

estriol: feto-placental unit…
- placenta cant convert progesterone to androgens, and the fetus cant convert androgens to estriol, so they work together
1) fetal adrenal cortex converts cholesterol to progesterone to DHEA
2) fetal liver convert DHEA to 16-OH DHEA-S)
3) placenta converts 16-OH-DHEA-S to estriol

Question 32

What is the deal with placental CRH?

Answer 32

• production increases over gestation
• levels at term approach the levels in the hypothalamic portal system during physiological stress
• proposed to play a significant role in the “placental clock” that controls parturition (labor and delivery)
• abnormally high CRH is a risk for premature labor

Question 33

Describe the cascade of events in stage 1 of labor

Answer 33

“uterine awakening”

placenta produces CRH — fetal AP produces ACTH – fetal adrenal cortex produces cortisol and DHEA

cortisol — fetal lungs produce pulmonary surfactant that plays a role in lung maturation/readiness for breathing

DHEA — placenta converts DHEA to estriol/estrogen — increase estrogen causes…
- increase prostaglandin production (cervical softening)
- increase oxytocin receptors in the myometrium (increase uterine responsiveness to low oxytocin)
- increase gap junctions in the myometrial cells (enables uterus to contract as a coordinated unit)

Question 34

What occurs in stage 2 of labor?

Answer 34

neuroendocrine reflex loop (positive feedback) promotes labor:
- oxytocin causes contraction of myometrium
- pushes baby’s head into cervix
- positive feedback to hypothalamus/posterior pit. to secrete more oxytocin

Question 35

What drugs can mimic stage 2 of labor?

Answer 35

pitocin (synthetic oxytocin) and/or prostaglandins

Question 36

What is stage 3 of labor?

Answer 36

delivery of placenta
involution:
- rapid decline in estrogen and progesterone
- uterus shrinks down, remaining endometrial tissue disintegrates
- takes 4-6 weeks
- facilitatesd by oxytocin in lactating mothers