Module 2 Section 5 (Fertilization, Pregnancy, Parturition & Lactation) Flashcards
Describe the processes by which male and female gametes unite.
It occurs through 4 stages:
1) Ovum transport to the oviduct
- Ovum released -> abdominal cavity
- At end of oviducts = fimbriae (finger-like projections) which guide the ovum into the oviducts. The perostaltic contractions move ovum -> ampulla
- Fertilization normally happens in the ampulla of the oviduct.
2) Sperm transport to the oviduct
- Sperm have barriers when they are in the vagina:
• Cervical canal: due to progesterone, the cervical musus is very thick which prevents sperm from -> canal. But w/ high levels of estrogen at ovulation, the mucous become thin enough for sperm -> canal (only for a few days).
- When in the uterus, the myometrial contractions rapidly disperse the sperm -> oviducts
- When in oviducts, peristaltic contractions move sperm -> ampulla
• Sperm arrives in ampulla about 30mins after ejaculation
3) Fertilization
- Fertilization can occur once the ovum and sperm are both in the ampulla
- Sperm needs to penetrate the corona radiata (layer of follicular cells surrounding the zona pullucida) and the zona pullucida itself.
• Sperm penetrates corona radiata w/ its membrane-bound head
• Sperm penetrates zona pellucida by fist binding to it. Plasma membrane of sperm has a protein called fertilin, which binds to ZP3 (a glycoprotein on the outer layer of the pellucida). This binding -> causes acrosome membrane to be disrupted -> releasing acrosomal enzymes which digest the zona pellucida
- The first sperm that reaches the ovum -> fuses w/ the ovum’s membrane
- Sperm’s head -> enters ovum and leaves the tail behind which -> triggers the release of inctracellular Ca, has several actions:
• Causes the inactivation of ZP3 so no further sperm can bind to the zona pellucida
• Hardens the zona pellucida so no futher sperm can penetrate it
• Triggers the second meitotic division in the ovum
- Implantation
- Zygote stays in ampulla for days where it undergoes several mitotic divisions and becomes the morula (solid ball of cells)
- Corpus letuem secretes large amounts of progesterone (helps relax the oviduct and allows morula -> move to uterus
- In uterus, the developing embryo floats freely for days while being noursished by the endometrial secretions
- Within a week of ovulation, endometrium is suitable for implantation
- By now, the morula transformed -> blastocyst (hollow structure containing a cluster of cells from which the fetus arises)
- When free-floating blastocyst adheres -> endometrial lining, its cells penetrate -> endometrium
- When implantation is done, the entire blastocyst is completely buries in the endometrium
Compare and contrast the roles of the corpus luteum and the placenta in secreting hormones to
support the fetus.
The placenta is very active in secreting hormones and becomes the major endocrine organ during pregnancy. It secretes hormones w/o any external control. The secretions vary with the different stages of pregnancy
1) Human Chorionic Gonadotropin (hCG):
- 1st hormone secreted by placenta.
- It’s a peptide hormone similar enough to LH that is can bind to LH receptors. B/c of this, hCG stimulates and maintains the corpus leutum.
• The corpus luteum gets larger, becomes the corpus luteum of pregnancy (remains until after birth), and is the primary source of estrogen and progesterone until the placenta takes over at around 10 weeks of gestation.
- Once the placenta secretes enough
estrogen and progesterone, hCG secretions decr to a low and steady level.
- hCG also plays a role in the development of male fetuses. It stimulates the precursor Leydig cells to secrete testosterone (necessary for the masculinization of the reproductive tract).
- It also has a role in morning sickness (reason is unknown).
- It’s also used as the marker for pregnancy tests since it’s detected in the urine early on.
2) Estrogen:
- Placenta can’t convert cholesterol -> estrogen. However, the fetal adrenal cortex produces and secretes DHEA into the fetal blood. The placenta takes it and converts -> estrogen, which then secretes into the maternal blood.
- The main estrogen it produces = estriol (circulating estriol levels can be used to assess fetal viability)
- The role of estrogen during pregnancy = incr the size of the myometrium as the uterus expands and to make sure the muscle mass is sufficient for childbirth.
- Estriol also promotes the development of the ducts within the mammary glands
* * Review pathway on slide 9**
3) Progesterone
- Almost immediately after implantation, the placenta can begin secreting progesterone. However, the placenta’s ability to secrete this hormone is proportional to the placental mass.
- It takes about 10 weeks for the placenta to produce enough progesterone to support the endometrium and replace the role of the corpus luteum.
- Since the placenta continues to grow throughout the entire pregnancy, maternal circulating levels of progesterone also incr throughout pregnancy.
- During pregnancy progesterone maintains the cervical mucus plug, stimulates milk gland
development, and suppresses uterine contractions.
** Review pathway on slide 9**
Describe the process of parturition with an emphasis on the positive feedback loop.
Child birth (parturition) requires both the dilation of the cervical canal through which the fetus will pass and the contractions of the uterine myometrium to forcibly expel the fetus. - Oxytocin is a hormone released by the posterior pituitary and is a powerful stimulator of uterine smooth muscle contraction. Levels = relatively constant throughout pregnancy so the secretion of oxytocin is not thought to initiate parturition. - As high levels of estrogen incr the uterine responsiveness to oxytocin to the point that uterine contractions begin, it initiates a positive feedback cycle. With each contraction, the fetus is pushed downwards towards the cervix: 1) It helps to force open the cervix. 2) A neuroendocrine reflex causes the release of oxytocin, which strengthens the uterine contractions and puts even more pressure on the cervix so even more oxytocin is released. This cycle continues until birth occurs and cervical pressure is removed, returning oxytocin levels to normal.
** Check positive feedback cycle on slide 11 **
Describe the process of lactation with an emphasis on its hormonal control.
1) Estrogen:
The high level of estrogen promotes -> development of milk collecting ducts.
2) Progesterone:
- Stimulates the formation of alveoli.
3) Prolactin and hCG:
- Both prolactin and hCG stimulate the synthesis of enzymes necessary for milk production.
By the middle of gestation, the breasts are capable of secreting milk, but this does not occur b/c of the high levels of estrogen and progesterone that suppress prolactin secretion. However, after parturition estrogen and progesterone levels decr, removing this inhibition.
What is fertilization? Where does is commonly occur?
Female egg + male sperm
It most commonly occurs in the ampulla of the Fallopian tube.
Using your knowledge of the steps of fertilization, choose if the following statements are true of
false.
1) Sperm start to arrive in the ampulla around 30 minutes following ejaculation
2) The zygote remains in the ampulla for a few hours.
3) The release of intracellular Ca2+ in the ovum causes the zona pellucida to harden.
4) Fertilization normally happens in endometrium of the uterus.
5) After implantation, the corpus luteum is secreting large amounts of estrogen which
helps to relax the oviduct and allow the morula to move to the uterus
1) Sperm start to arrive in the ampulla around 30 minutes following ejaculation
- True
2) The zygote remains in the ampulla for a few hours.
- False
3) The release of intracellular Ca2+ in the ovum causes the zona pellucida to harden.
- True
4) Fertilization normally happens in endometrium of the uterus.
- False
5) After implantation, the corpus luteum is secreting large amounts of estrogen which helps to relax the oviduct and allow the morula to move to the uterus
- False
Twins are 2 offspring produced by the same pregnancy and can either be monozygotic (i.e. identical) or dizygotic (i.e. fraternal). From what you have learned about fertilization and implantation, what do you think occurs in
the case of identical twins? What about fraternal twins?
Identical twins occur when a single egg is fertilized with a sperm cell, forms 1 zygote, and then spontaneously divides into 2 separate embryos.
Fraternal twins occur after 2 eggs are released from the ovary. Both eggs are fertilized by 2 different sperm cells, which are implanted in the uterine wall at the same time.
What’s an ectopic pregnancy?
In a normal pregnancy the fertilized egg implants itself to the uterine lining.
However, when the fertilized egg implants itself outside of the main uterine cavity, an ectopic pregnancy occurs.
It usually occurs in the fallopian tubes (tubal pregnancy). It can
also occur in the abdominal cavity, the ovaries, or the cervix (neck of the uterus).
What are the risk factors of ectopic pregnancy?
- History of pelvic inflammatory disease
- STIs
- Previous ectopic pregnancy
- Previous tubal surgery (for infertility)
What are the treatments of an ectopic pregnancy?
Meds or surgery to prevent complications
What is the purpose of the placenta?
The placenta is a specialized organ used for the exchange b/w the maternal and fetal blood.
Discuss the development of the placenta in the uterus.
4 Weeks:
- By day 12 the embryo is completely embedded in the endometrial tissue at the implantation site.
- This tissue contributes to the development of the placenta.
8 Weeks:
- Even though not fully developed, the placenta is well established and operational by 5 weeks after implantation.
12 Weeks:
- Development of maternal blood supply -> placenta is complete by the end of the 1st trimester of pregnancy (approximately 12-13 weeks).
- At this point, the placenta has developed all the necessary structures to support the embryo for the remainder of the pregnancy.
40 Weeks:
- Once fully developed, the placenta continues to grow throughout pregnancy.
- It functions as the
digestive system, the respiratory system, and the kidneys of the fetus.
- Exchange of nutrients and wastes b/w the maternal and fetal blood occurs across it.
What is the role of estrogen during child birth?
During early gestation, estrogen levels are relatively low and progressively incr.
- Towards the end
of the normal gestational period, there is an incr in estrogen production that helps to prepare both the cervix and uterus for delivery.
What are the roles of estrogen?
Estrogen has several roles:
1) Normally uterine myometrium smooth muscle cells are not connected by gap junctions. However,
the late surge in estrogen causes them to synthesize connections and form gap junctions so that the myometrium can contract synchronously.
2) The rise in estrogen also incr the synthesis of oxytocin receptors in the myometrium.
** Review diagram on slide 10 **
From what you have learned about positive feedback loops, describe in your own words why positive feedback is necessary for parturition, and what would happen if positive feedback did
not occur.
During parturition, positive feedback loops are necessary as they increase the release of oxytocin, which strengthens uterine contractions. These contractions put pressure on the cervix, necessary for the opening of the birthing canal, which in turn incr the amount of oxytocin released.
W/o this increasing level of oxytocin, there would not be enough cervical pressure, and birth would not occur.
