Module 2 Section 5

Question 1

what is fertilization

Answer 1

• in humans, fertilization is the joining of female egg and a male sperm
• this most commonly occurs in the ampulla of the fallopian tube

Question 2

what is the first stage of fertilization - ovum transport to the oviduct

Answer 2

• at the end of the ovarian follicular phase, an ovum is released into the abdominal cavity
• at the end of the oviducts are finger-like projections called fimbriae
• the fimbriae guide the ovum into the oviduct and then peristaltic contractions move the ovum to the ampulla
• fertilization normally happens in the ampulla of the oviduct

Question 3

what is the second stage of fertilization - sperm transport to the oviduct

Answer 3

• after being deposited in the vagina, the first barrier sperm face is the cervical canal
• under the influence of progesterone, cervical mucus is very thick and prevents sperm from passing through the canal
• the high levels of estrogen at ovulation cause the mucus to become thin enough to allow sperm passage
• the passage of sperm into the uterus is limited to only a few days per month and coincides with ovulation

Question 4

what happens once sperm is in the uterus

Answer 4

• myometrial contractions rapidly disperse the sperm and help propel them to the oviducts
• once in the oviducts, peristaltic contractions help to move the sperm to the ampulla
  -sperm start to arrive in the ampulla around 30 min following ejaculation

Question 5

what is the third stage of fertilization

Answer 5

• once both the ovum and sperm are in the ampulla, fertilization can occur
• for this to happen, the sperm must be able to penetrate both the corona radiata (the layer of follicular cells surrounding the zona pellucida) and then the zona pellucida itself

Question 6

how does sperm penetrate the corona radiata

Answer 6

• by means of membrane-bound enzymes on its head
• to penetrate to zona pellucida, it must first bind to it
• the plasma membrane of the sperm contains a protein called fertilin, which binds to ZP3, a glycoprotein on the outer layer of the zona pellucida
• this binding causes the acrosome membrane to be disrupted and releases the acrosomal enzymes
• these enzymes digest the zona pellucida

Question 7

what happens to the first sperm that reaches the ovum

Answer 7

• it fuses with the ovum’s membrane
• the sperm head enters the ovum and leaves the tail behind
• this also triggers the release of intracellular Ca2+, which has several actions
• within an hour of fertilization, the sperm and ovum nuclei have fused and the ovum is now called a zygote

Question 8

what does Ca2+ do in the ovum

Answer 8

1. causes the inactivation of ZP3 so no further sperm can bind to the zona pellucida
2. hardens the zona pellucida so no further sperm can penetrate it
3. it triggers the second meiotic division in the ovum

Question 9

explain the fourth step of fertilization - implantation

Answer 9

• the zygote remains in the ampulla for a few days where it undergoes several mitotic divisions and becomes the morula, a solid ball of cells
• by now, the corpus luteum is secreting large amounts of progesterone, which helps to relax the oviduct and allow the morula to move to the uterus
• once in the uterus, the developing embryo floats freely for several days being nourished by the endometrial secretions
• within a week of ovulation the endometrium is suitable for implantation

Question 10

what happens once the endometrium is suitable for implantation

Answer 10

• the morula has transformed into a blastocyst, a hollow structure containing a cluster of cells from which the fetus arises
• when the free-floating blastocyst adheres to the endometrial lining, its cells begin to penetrate the endometrium
• when implantation is finished, the entire blastocyst is completely buried in the endometrium

Question 11

what causes identical and fraternal twins

Answer 11

• identical twins occur when a single egg is fertilized with a sperm cell, forms one zygote, and then spontaneously divides into two separate embryos
• fraternal twins occur after two eggs are released from the ovary. both eggs are fertilized by two different sperm cells, which are implanted in the uterine wall at the same time

Question 12

explain ectopic pregnancy

Answer 12

• when the fertilized egg implants itself outside of the main uterine cavity, an ectopic pregnancy occurs
• most often occurs in the fallopian tubes which is known as tubal pregnancy
• it can also occur in the abdominal cavity, the ovaries, or the cervix (neck of the uterus)

Question 13

what is the placenta

Answer 13

• the endometrial glycogen stores are sufficient to provide nutrients to the embryo for only a few weeks
• to sustain the embryo for the duration of the pregnancy, the placenta, a specialized organ for exchange between the maternal and fetal blood, must develop

Question 14

explain the placenta at 4 weeks

Answer 14

• 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

Question 15

explain the placenta at 8 weeks

Answer 15

even though not fully developed, the placenta is well established and operational by five weeks after implantation

Question 16

explain the placenta after 12 weeks

Answer 16

• development of maternal blood supply to the placenta is complete by the end of the first trimester of pregnancy (approx 12-13 weeks)
• at this point, the placenta has developed all the necessary structures to support the embryo for the remainder of the pregnancy

Question 17

explain placental hormones

Answer 17

• the placenta is also very active in secreting hormones and becomes the major endocrine organ during pregnancy
• it is a temporary endocrine organ and it secretes hormones without any external control
• its hormonal secretions vary with the different stages of pregnancy

Question 18

explain the placenta after 40 weeks

Answer 18

• once fully developed, the placenta continues to grow throughout pregnancy
• it functions as the digestive system, respiratory system, and kidneys of the fetus
• exchange of nutrients and wastes between the maternal and fetal blood occurs across the thin barrier that is the placenta

Question 19

explain the placental hormone - human chorionic gonadotropin (hCG)

Answer 19

• the first hormone to be secreted by the developing placenta
• it is a peptide hormone functionally and structurally similar enough to LH such that it can bind to LH receptors
• hCG stimulates and maintains the corpus luteum
• the corpus luteum gets larger, becomes the corpus luteum of pregnancy, and is the primary source of estrogen and progesterone until the placenta takes over at around 10 weeks of gestation
• once placenta secretes sufficient estrogen and progesterone, hCG secretions decrease to a low and steady leevl
• the corpus luteum of pregnancy remains until after birth

Question 20

what is the role of hCG in male fetuses

Answer 20

it stimulates the precursor Leydig cells to secrete testosterone, which is necessary for the masculinization of the reproductive tract

Question 21

what can hCG determine in early pregnancy

Answer 21

• it can be detected in maternal urine long before an embryo can be detected by examination, it is used in pregnancy tests
• it also coincides with the timing and duration of morning sickness

Question 22

explain the placental hormone - estrogen

Answer 22

• the placenta itself does not contain the enzymes necessary to convert cholesterol into estrogen
• the fetal adrenal cortex produced and secretes dehydroepiandrosterone (DHEA) into the fetal blood
• the placenta then takes this DHEA and converts it into estrogen, which it then secretes into the maternal blood
• the main estrogen it produces is estriol

Question 23

explain the role of estrogen during pregnancy

Answer 23

• it takes time for the fetal adrenal glands to begin secreting sufficient DHEA, which is why the corpus luteum of pregnancy is the primary source of estrogen for the first trimester
• the role of estrogen during pregnancy is mainly to increase 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

Question 24

explain the placental hormone - progesterone

Answer 24

• almost immediately after implantation, the placenta can begin secreting progesterone
• the placentas 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 pregnancy, maternal circulating levels of progesterone increase throughout
• progesterone maintains the cervical mucus plug, stimulates milk gland development, and suppresses uterine contractions

Question 25

explain parturition

Answer 25

- child birth, or 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 -during early gestation, estrogen levels are relatively low and progressively increase - towards the end of the normal gestational period, there is an increase in estrogen production that helps to prepare both the cervix and uterus for delivery - estrogen has several roles

Question 26

what are the roles of estrogen to prepare the cervix and uterus for delivery

Answer 26

1. normally uterine myometrium smooth muscle cells are not connected by gap junctions. 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 increases the synthesis of oxytocin receptors in he myometrium

Question 27

explain the positive feedback cycle of parturition

Answer 27

- oxytocin is a hormone released by the posterior pituitary and is a powerful stimulator of uterine smooth muscle contraction - levels are relatively constant throughout pregnancy so the secretion of oxytocin is not thought to initiate parturition - it is possible that when the estrogen-stimulated increase in oxytocin receptor levels get to a critical level, the uterus can then respond to these "normal" levels of oxytocin - as high levels of estrogen increase 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 down toward the cervix

Question 28

what are the 2 effects of the pressure of a contraction

Answer 28

1. helps to force open the cervix 2. a neuroendocrine reflex causes the release of oxytocin. this release strengthens the uterine contractions, which 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

Question 29

why is positive feedback necessary for parturition and what would happen if positive feedback did not occur

Answer 29

- 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 increases the amount of oxytocin released - without this increasing level of oxytocin, there would not be enough cervical pressure, and birth would not occur

Question 30

explain the 4 hormones that control the preparation of lactation

Answer 30

1. estrogen: high level of estrogen promotes the development of milk collecting ducts 2. progesterone: stimulates the formation of alveoli 3. prolactin & hCG: both prolactin and hCG stimulate the synthesis of enzymes necessary for milk production

Question 30

what is lactation

Answer 30

- during gestation the mammary glands, or breasts, are prepared for milk production - a breast prepared for lactation has a network of ducts that branch out from the nipple and get progressively smaller until they terminate in lobules - these lobules are epithelial-lines milk producing glands called alveoli - milk is synthesized in the epithelial cells and secreted into the alveoli, where it can then flow through the milk collecting duct to the nipple

Question 31

when and how are the breasts capable of secreting milk

Answer 31

- by the middle of gestation - doesn't occur because of high levels of estrogen and progesterone that suppress prolactin secretion - after parturition estrogen and progesterone levels decrease, removing this inhibition

Question 32

explain the stimulus of lactation

Answer 32

- the stimulation of lactation is under the control of two hormones, prolactin and oxytocin - release of both of these hormones is stimulates by a neuroendocrine reflex triggered by suckling

Question 33

explain the diagram

Answer 33

- suckling: milk production and ejection happen in response to the suckling reflex - hypothalamus: the infants suckling of the nipple activates afferent nerve endings to the hypothalamus - posterior pituitary: hypothalamus then directly releases oxytocin from the posterior pituitary - oxytocin: oxytocin from posterior pituitary is responsible for milk ejection - contraception of myoepithelial cells surrounding alveoli: infant cannot "suck" milk out of alveoli, it must be forcibly ejected - PIH/PRH: hypothalamus also releases PRH and decreases PIH into the hypothalamic-hypophyseal portal system, which results in prolactin release from the anterior pituitary - prolactin: released from anterior pituitary under the control of the hypothalamic hormones PIH and PRH. PIH is known to be dopamine and PRH is suspected to be oxytocin - secretion by alveolar epithelial cells: prolactin triggers the alveolar epithelial cells to replace the ejected milk