Section 5

Question 1

What are the stages of fertilization in humans?

Answer 1

1. Ovum Transport to the Oviduct: The ovum is released into the abdominal cavity and guided by fimbriae into the oviduct, where peristaltic contractions move it to the ampulla.
2. Sperm Transport to the Oviduct: After being deposited in the vagina, sperm face the cervical canal. Cervical mucus, influenced by progesterone and estrogen, allows sperm passage. Myometrial contractions in the uterus propel sperm to the oviducts.
3. Fertilization: Sperm penetrates the corona radiata and zona pellucida using enzymes. The binding of fertilin to ZP3 triggers acrosomal enzyme release. The first sperm fuses with the ovum, initiating the second meiotic division and forming a zygote.
4. Implantation: The zygote becomes a morula in the ampulla, moves to the uterus, and transforms into a blastocyst. Blastocyst adheres to the endometrial lining, penetrating it for implantation.

Question 2

Describe the fertilization process in sperm penetration.

Answer 2

1. Penetration of Corona Radiata: The sperm must penetrate the corona radiata, the layer of follicular cells surrounding the zona pellucida. Membrane-bound enzymes on the sperm’s head aid in this penetration.
2. Binding to Zona Pellucida: The sperm binds to the zona pellucida by the protein fertilin, which interacts with ZP3, a glycoprotein on the zona pellucida’s outer layer. This binding causes disruption of the acrosome membrane, releasing acrosomal enzymes.
3. Digestion of Zona Pellucida: Acrosomal enzymes digest the zona pellucida, allowing the sperm to reach the ovum.
4. Fusion with Ovum: The first sperm to reach the ovum fuses with the ovum’s membrane. The sperm head enters the ovum, leaving the tail behind.
5. Intracellular Ca2+ Release: Fusion triggers the release of intracellular Ca2+, leading to several actions:
   Inactivation of ZP3, preventing further sperm binding.
   - Hardening of the zona pellucida to prevent additional sperm penetration.
   - Triggering the second meiotic division in the ovum.
6. Formation of Zygote: Within an hour of fertilization, the sperm and ovum nuclei fuse, forming a zygote.

Question 3

Describe the process of implantation in human reproduction.

Answer 3

1. Morula Formation: After fertilization, the zygote remains in the ampulla, undergoing several mitotic divisions, and forms a solid ball of cells known as the morula.
2. Corpus Luteum Secretion: The corpus luteum, formed from the remnants of the ovarian follicle, starts secreting large amounts of progesterone.
3. Morula Movement to Uterus: Progesterone helps relax the oviduct, allowing the morula to move from the ampulla to the uterus.
4. Free-Floating Stage: In the uterus, the developing embryo, now a morula, floats freely for several days, nourished by endometrial secretions.
5. Formation of Blastocyst: The morula transforms into a blastocyst, a hollow structure with a cluster of cells that will give rise to the fetus.
6. Implantation Preparation: Within a week of ovulation, the endometrium becomes suitable for implantation.
7. Adhesion and Penetration: The blastocyst adheres to the endometrial lining, and its cells begin to penetrate the endometrium.
8. Complete Implantation: Once implantation is finished, the entire blastocyst is completely buried in the endometrium.

Question 4

T or F: Sperm start to arrive in the ampulla around 30 minutes following ejaculation

Answer 4

T

Question 5

T or F: The zygote remains in the ampulla for a few hours.

Answer 5

F

Question 6

T or F: The release of intracellular Ca2+ in the ovum causes the zona pellucida to harden.

Answer 6

T

Question 7

T or F: Fertilization normally happens in endometrium of the uterus.

Answer 7

F

Question 8

T or F: 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

Answer 8

F

Question 9

Twins are two 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?

Answer 9

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.

Comparison of zygote development in monozygotic and dizygotic twins.

Question 10

Define ectopic pregnancy and identify common locations for ectopic implantation.

Answer 10

Ectopic Pregnancy Definition: In an ectopic pregnancy, the fertilized egg implants itself outside the main uterine cavity.

Common Locations:
- Fallopian Tubes (Tubal Pregnancy): The most frequent site for ectopic pregnancies.
- Abdominal Cavity: Implantation can occur within the abdominal cavity.
- Ovaries: Ectopic pregnancies may happen in the ovaries.
- Cervix (Neck of the Uterus): Rarely, implantation can occur in the cervix.

Question 11

What is the placenta, and how does it support the embryo during pregnancy?

Answer 11

Placenta Definition: The placenta is a specialized organ that develops in the uterus to facilitate exchange between maternal and fetal blood during pregnancy.

Support for Embryo:
Nutrient Supply: The placenta provides essential nutrients to the embryo, sustaining it throughout pregnancy.

Development Timeline:
- 4 Weeks: The embryo is embedded in endometrial tissue, contributing to placental development.

• 8 Weeks: The placenta is well established and operational, though not fully developed.
• 12 Weeks: Maternal blood supply to the placenta is complete, and it has developed necessary structures to support the embryo.
• 40 Weeks: Fully developed placenta serves as the digestive, respiratory, and renal system for the fetus, facilitating nutrient and waste exchange between maternal and fetal blood.

Question 12

What role does the placenta play in the endocrine system during pregnancy?

Answer 12

The placenta becomes the major endocrine organ, secreting hormones without external control. Its hormonal secretions vary throughout pregnancy, supporting fetal development and regulating maternal physiological changes.

Question 13

Why is the placenta considered a temporary endocrine organ?

Answer 13

The placenta is temporary because it forms during pregnancy and is expelled after childbirth. Its endocrine functions are crucial for maintaining pregnancy and facilitating fetal development.

Question 14

What are some hormones secreted by the placenta, and how do they contribute to pregnancy?

Answer 14

Placental hormones include human chorionic gonadotropin (hCG), human placental lactogen (hPL), and estrogen. They regulate pregnancy by supporting corpus luteum function, ensuring a nutrient-rich environment for the fetus, and adapting maternal physiology.

Question 15

What is the function of Human Chorionic Gonadotropin (hCG) during pregnancy?

Answer 15

hCG is the first hormone secreted by the developing placenta. It stimulates and maintains the corpus luteum, leading to increased production of estrogen and progesterone, crucial for the early stages of pregnancy. It also has a role in the development of male fetuses, stimulating testosterone secretion.

Question 16

How does hCG support the corpus luteum during pregnancy?

Answer 16

hCG binds to LH receptors, mimicking the action of LH. This stimulation enhances the corpus luteum, transforming it into the corpus luteum of pregnancy, which produces estrogen and progesterone until the placenta takes over around 10 weeks of gestation.

Question 17

What role does hCG play in the development of male fetuses?

Answer 17

hCG stimulates precursor Leydig cells to secrete testosterone, contributing to the masculinization of the reproductive tract in male fetuses.

Question 18

How is hCG utilized in pregnancy tests?

Answer 18

hCG is a marker for pregnancy tests because it can be detected in maternal urine before an embryo is visible through examination. Its presence confirms pregnancy.

Question 19

How does the placenta obtain the precursors for estrogen synthesis during pregnancy?

Answer 19

The fetal adrenal cortex produces dehydroepiandrosterone (DHEA), which is secreted into the fetal blood. The placenta takes up DHEA and converts it into estrogen, mainly estriol, before releasing it into the maternal blood.

Question 20

What is the significance of estriol in assessing fetal viability during pregnancy?

Answer 20

Estriol is the main estrogen produced by the placenta, differing from the ovaries’ primary estrogen, estradiol. Monitoring circulating estriol levels can be used as an indicator of fetal viability during pregnancy.

Question 21

Why is the corpus luteum of pregnancy the primary source of estrogen during the first trimester?

Answer 21

The fetal adrenal glands take time to produce sufficient dehydroepiandrosterone (DHEA). Therefore, the corpus luteum of pregnancy becomes the primary source of estrogen, especially during the initial stages of pregnancy.

Question 22

What are the primary roles of estrogen during pregnancy?

Answer 22

Estrogen plays a crucial role in increasing the size of the myometrium to accommodate uterine expansion during pregnancy. Additionally, it promotes the development of ducts within the mammary glands.

Question 23

When does the placenta begin secreting progesterone, and how long does it take for the placenta to produce enough to replace the corpus luteum’s role?

Answer 23

The placenta starts secreting progesterone almost immediately after implantation. It takes about 10 weeks for the placenta to produce sufficient progesterone to support the endometrium and replace the role of the corpus luteum.

Question 24

What are the functions of progesterone during pregnancy?

Answer 24

Progesterone during pregnancy maintains the cervical mucus plug, stimulates the development of milk glands, and suppresses uterine contractions.

Question 25

What are the factors affecting uterine contractions during parturition?

Answer 25

During parturition, uterine contractions are influenced by the increase in estrogen levels towards the end of gestation. Estrogen plays a crucial role by causing the synthesis of connections between uterine myometrium smooth muscle cells, forming gap junctions for synchronous contractions. Additionally, estrogen increases the synthesis of oxytocin receptors in the myometrium.

Question 26

Describe the positive feedback cycle of parturition.

Answer 26

The positive feedback cycle of parturition is initiated by estrogen-stimulated increases in oxytocin receptor levels in the uterine myometrium. When these receptor levels reach a critical point, the uterus becomes responsive to normal levels of oxytocin. This leads to uterine contractions, pushing the fetus downwards. The pressure on the cervix helps force it open and triggers a neuroendocrine reflex, releasing more oxytocin. This reinforces uterine contractions, creating a cycle that continues until birth, at which point cervical pressure is relieved, and oxytocin levels return to normal.

Question 27

When does the uterus become responsive to normal levels of oxytocin during parturition?

Answer 27

The uterus becomes responsive to normal levels of oxytocin when the estrogen-stimulated increase in oxytocin receptor levels reaches a critical point.

Question 28

What are the effects of pressure on the cervix during parturition?

Answer 28

Pressure on the cervix during parturition helps force it open and triggers a neuroendocrine reflex, causing the release of oxytocin.

Question 29

How does the positive feedback cycle of parturition continue until birth?

Answer 29

With each contraction, the fetus is pushed downwards, putting pressure on the cervix. This pressure releases oxytocin, strengthening contractions, and creating a cycle that continues until birth.

Question 30

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.

Answer 30

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 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 31

What is the structure of a breast prepared for lactation?

Answer 31

A breast prepared for lactation has a network of ducts branching out from the nipple, getting smaller until terminating in lobules, which are epithelial-lined milk-producing glands called alveoli.

Question 32

What hormones control the preparation of mammary glands for milk production?

Answer 32

The preparation of mammary glands for milk production is controlled by estrogen, progesterone, prolactin, and hCG.

Question 33

What is the role of estrogen in lactation?

Answer 33

High levels of estrogen promote the development of milk collecting ducts in the mammary glands.

Question 34

How does progesterone contribute to lactation?

Answer 34

Progesterone stimulates the formation of alveoli, which are epithelial-lined milk-producing glands within the mammary glands.

Question 35

What is the role of prolactin and hCG in milk production?

Answer 35

Both prolactin and hCG stimulate the synthesis of enzymes necessary for milk production in the mammary glands.

Question 36

When are breasts capable of secreting milk during gestation?

Answer 36

By the middle of gestation, breasts are capable of secreting milk, but this is inhibited by high levels of estrogen and progesterone.

Question 37

What hormones control the stimulation of lactation?

Answer 37

The stimulation of lactation is under the control of two hormones: prolactin and oxytocin.

Question 38

What triggers the release of prolactin and oxytocin during lactation?

Answer 38

The release of prolactin and oxytocin is triggered by the neuroendocrine reflex induced by suckling.

Question 39

What role does oxytocin play in lactation?

Answer 39

Oxytocin from the posterior pituitary is responsible for milk ejection by facilitating the contraction of myoepithelial cells surrounding alveoli.

Question 40

How does the suckling reflex contribute to milk production?

Answer 40

Milk production and ejection occur in response to the suckling reflex, preventing inappropriate milk ejection. This reflex can be conditioned to stimuli like the sound of a crying baby.

Question 41

What role does the hypothalamus play in lactation stimulation?

Answer 41

The infant’s suckling activates afferent nerve endings to the hypothalamus, leading to the release of oxytocin from the posterior pituitary.

Question 42

What hormone is responsible for the release of prolactin from the anterior pituitary?

Answer 42

Prolactin is released from the anterior pituitary under the control of hypothalamic hormones, including PIH (prolactin-inhibiting hormone) and PRH (prolactin-releasing hormone).