Exam 2 - Pregnancy Flashcards
Explain the progression of how many sperm make it throughout the female reproductive tract.
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
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?
has completed meiosis 1 and is arrested in metaphase of meiosis 2 (because it has been ovulated)
23 chromosomes, 46 chromatids
What is the viability of the oocyte and sperm after deposition?
oocyte viable for 24 hours after ovulation
sperm viable for 2-3 days (sometimes 5+ days) in the female reproductive tract
What is the final stage of sperm development? Where does it happen and what does it allow?
sperm capacitation
final maturation of sperm that occurs in the female reproductive tract, enabling them to fertilize the oocyte
Describe the basic steps of how a sperm enters an oocyte (up to membrane fusion)
- 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
Describe what happens in an oocyte after the sperm and oocyte membranes fuse
- 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
At what point does the zona pellucida become hardened?
as soon as the first sperm head has fused membranes with the oocyte membrane (causing a cortical reaction)
What occurs in the first week after ovulation (assume the oocyte is going to become fertilized)
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)
Describe the development of the embryo as it moves through the fallopian tube and into the uterus (by hours and days)
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
Describe the structure of a blastocyst
Surrounded by the zona pellucida and a single layer of trophoblasts
Inner cell mass that will become the fetus
When does implantation begin in relation to ovulation?
approcx. 6 days after
Describe the process of implantation, with a general timeline
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.
What structures will eventually become the embryo-derived portion of the placenta?
the trophoblast cells (the outer cells, not the inner cell mass)
What is the decidua?
highly vascularized endometrium of pregnancy
As implantation occurs, what other structures develop inside the blastocyst/embryo?
yolk sac, amniotic cavity, and embryonic disk in between them
What is the function of the yolk sac?
functions as an extra-embryonic circulatory system before the internal circulatory system forms
What is the structure of the embryonic disk and what structures does it lead to in a fetus?
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
Between implantation (12 days post fertilization) and 6-7 weeks post fertilization, what embryonic development occurs?
- 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
What is the distinction between embryo and fetus?
embryo is <8 weeks post fertilization
fetus is >8 weeks post fertilization
What are dichorionic/monochorionic and diamnionic/monoamnionic?
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)
What are the two types of DC/DA twins?
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
What are “MC”/DA twins vs true MC/DA twins?
“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
What are the four types of identical twin pregnancies?
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
Describe the blood flow in the placenta
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