Lecture 4: Fertilization and Pregnancy & Readings

Question 1

Describe the viability period of an oocyte after ovulation.

Answer 1

An oocyte is viable for 12 to 24 hours after it is ejected from the ovary.

Question 2

Summarize the conditions necessary for successful fertilization.

Answer 2

Successful fertilization requires intercourse to occur no more than two days before ovulation and no later than 24 hours after ovulation.

Question 3

Explain how long sperm can survive in the female reproductive tract.

Answer 3

Sperm may survive in the female reproductive tract for up to a week.

Question 4

Define sperm capacitation and its importance in fertility.

Answer 4

Sperm capacitation occurs following ejaculation once sperm are in the female tract, and it is crucial for enabling sperm to fertilize the oocyte.

Question 5

Describe the journey of sperm after ejaculation.

Answer 5

After ejaculation, sperm must navigate through the cervical canal to reach the uterine cavity, where many are destroyed by phagocytes.

Question 6

Explain the process of fertilization.

Answer 6

Fertilization occurs when a sperm’s chromosomes combine with those of an egg to form a fertilized egg, or zygote.

Question 7

Summarize the main features of early embryonic development up to implantation.

Answer 7

Early embryonic development includes the formation of a zygote, cleavage, and the development of a blastocyst that eventually implants in the uterine wall.

Question 8

Describe the role of uterine contractions in sperm movement.

Answer 8

Uterine contractions help propel sperm throughout the uterine cavity, aiding their journey toward the oocyte.

Question 9

Indicate the distance an oocyte travels after ovulation before fertilization can occur.

Answer 9

The oocyte travels approximately one-third of the way down the uterine tube after ovulation.

Question 10

Explain the significance of timing in relation to ovulation for conception.

Answer 10

The chance of pregnancy drops to almost zero the day after ovulation, making timing crucial for conception.

Question 11

Describe the process sperm must undergo to penetrate an oocyte.

Answer 11

Sperm must first undergo capacitation, a delicate biochemical process that enhances their motility and makes their membranes fragile, allowing the release of hydrolytic enzymes from their acrosomes.

Question 12

Explain the significance of capacitation in sperm function.

Answer 12

Capacitation is crucial because it prepares sperm to penetrate the oocyte by enhancing motility and allowing the release of enzymes necessary for breaching the oocyte’s protective layers.

Question 13

How do sperm navigate through the female reproductive tract?

Answer 13

Sperm have receptors that detect fluid flow, temperature, and certain chemicals, which guide them upstream through the uterine tubes.

Question 14

Define polyspermy and its implications in human reproduction.

Answer 14

Polyspermy is the entry of multiple sperm into an egg. In humans, it is prevented to ensure that only one sperm penetrates the oocyte, maintaining monospermy.

Question 15

What are the fast and slow blocks to polyspermy?

Answer 15

The fast block to polyspermy involves a rapid change in the egg’s membrane potential, preventing additional sperm from entering. The slow block involves the release of enzymes that modify the zona pellucida, making it impenetrable to other sperm.

Question 16

Describe the role of the acrosomal reaction in fertilization.

Answer 16

The acrosomal reaction is central to fertilization as it allows sperm to release enzymes that help breach the zona pellucida and corona radiata surrounding the oocyte.

Question 17

Explain why sperm must wait for capacitation even if they reach the oocyte quickly.

Answer 17

Sperm must wait for capacitation to occur because they are not yet capable of penetrating the oocyte until their membranes are fragile and their motility is enhanced.

Question 18

What happens to sperm membranes as they move through the female reproductive tract?

Answer 18

As sperm move through the cervical mucus, uterus, and uterine tubes, protective membrane proteins and cholesterol are removed, making their membranes more fragile.

Question 19

Why is it important for acrosomal enzymes to be released only after sperm bind to the zona pellucida?

Answer 19

Releasing acrosomal enzymes prematurely in the male reproductive tract could lead to autolysis of the male reproductive organs, which is prevented by the timing of the acrosomal reaction.

Question 20

What factors in seminal fluid inhibit capacitation?

Answer 20

Seminal fluid contains capacitation inhibiting factors that prevent sperm from undergoing capacitation until they are in the female reproductive tract.

Question 21

Describe the consequences of polyspermy in embryos.

Answer 21

Polyspermy leads to embryos containing too much genetic material, which results in their inability to successfully complete mitosis and ultimately causes their death.

Question 22

Explain the mechanisms that ensure monospermy during fertilization.

Answer 22

Two mechanisms that help ensure monospermy are the oocyte membrane block and the cortical reaction.

Question 23

How does the oocyte membrane block function after sperm binding?

Answer 23

When a sperm binds to the oocyte membrane’s sperm-binding receptors, it triggers the oocyte membrane block, causing the oocyte to shed remaining sperm-binding receptors, preventing additional sperm from binding.

Question 24

What triggers the cortical reaction in the oocyte?

Answer 24

The cortical reaction is triggered by waves of Ca2+ released from the oocyte’s endoplasmic reticulum into its cytoplasm after the sperm and oocyte plasma membranes have fused.

Question 25

Define the zona reaction and its significance in fertilization.

Answer 25

The zona reaction involves changes in the zona pellucida, where enzymes destroy sperm-binding receptors and harden the zona pellucida, effectively clearing sperm out and preventing further fertilization.

Question 26

How does the sperm's centrosome contribute to fertilization?

Answer 26

The sperm’s centrosome assembles microtubules that propel the sperm’s nucleus toward the oocyte nucleus, causing the sperm's nucleus to swell and form the male pronucleus.

Question 27

What role do calcium surges play in the activation of the secondary oocyte?

Answer 27

Calcium surges activate the secondary oocyte from its semidormant state and also trigger the cortical reaction.

Question 28

Describe the phenomenon of zinc sparks during fertilization.

Answer 28

Zinc sparks are bursts of zinc ions that are released from the egg following fluctuations in calcium levels, occurring during the activation of the secondary oocyte.

Question 29

Describe the role of zinc efflux in oocyte maturation.

Answer 29

The efflux of zinc allows the oocyte to complete meiosis II, resulting in the formation of the ovum nucleus and the second polar body.

Question 30

Explain the process that occurs after the ovum nucleus swells.

Answer 30

After the ovum nucleus swells, it becomes the female pronucleus, and both pronuclei replicate their DNA as they move closer together.

Question 31

How do the pronuclei interact during fertilization?

Answer 31

As a mitotic spindle develops between the pronuclei, their membranes rupture, releasing chromosomes into the vicinity of the spindle.

Question 32

Define the moment of fertilization in human reproduction.

Answer 32

The true moment of fertilization occurs when the maternal and paternal chromosomes combine to produce the diploid zygote, or fertilized egg.

Question 33

What is necessary for the formation of a zygote in humans?

Answer 33

For a zygote to form in humans, the chromosomes in the male and female pronuclei must actually combine.

Question 34

Explain the significance of the zygote in human development.

Answer 34

The zygote is the first cell of a new individual and is ready to undergo the first mitotic division of the conceptus.

Question 35

Define zygote.

Answer 35

A zygote is the initial cell formed when a sperm cell fertilizes an egg cell, marking the beginning of embryonic development.

Question 36

Summarize the developmental steps that occur while the embryo is traversing the oviduct.

Answer 36

During its journey through the oviduct, the zygote undergoes cleavage divisions, producing a series of identical cells called blastomeres, which eventually form a morula, and then a blastocyst as it reaches the uterus.

Question 37

Explain the goal of the first phase of embryonic development.

Answer 37

The goal of the first phase of development is to produce small cells with a high surface-to-volume ratio, enhancing nutrient and oxygen uptake while facilitating waste disposal, and providing numerous cells as building blocks for the embryo.

Question 38

Describe the process that occurs 36 hours after fertilization.

Answer 38

About 36 hours after fertilization, the zygote undergoes its first cleavage division, resulting in two identical cells known as blastomeres.

Question 39

How does the morula form during embryonic development?

Answer 39

The morula forms as the zygote continues to divide, producing a berry-shaped cluster of 16 or more cells by approximately 72 hours after fertilization.

Question 40

What happens to the embryo by day 4 or 5 after fertilization?

Answer 40

By day 4 or 5 after fertilization, the embryo consists of about 100 cells, floats freely in the uterus, and begins to accumulate fluid within an internal cavity.

Question 41

Explain the significance of the zona pellucida during early embryonic development.

Answer 41

The zona pellucida is a protective layer surrounding the embryo that helps maintain its structure during early development and eventually breaks down to allow the blastocyst to hatch.

Question 42

Describe the transition from morula to blastocyst.

Answer 42

The transition from morula to blastocyst occurs as the morula develops an internal cavity and begins to accumulate fluid, leading to the formation of the blastocyst.

Question 43

Describe the structure of a blastocyst.

Answer 43

The blastocyst is a fluid-filled hollow sphere composed of a single layer of large, flattened cells called trophoblast cells and a small cluster of 20 to 30 rounded cells known as the embryoblast or inner cell mass.

Question 44

Explain the process of cleavage in early embryonic development.

Answer 44

Cleavage is a period of rapid mitotic divisions of the zygote that occurs without intervening growth, leading to the formation of the blastocyst.

Question 45

Define the role of trophoblast cells during embryonic development.

Answer 45

Trophoblast cells are crucial for implanting the embryo in the uterine wall, contribute to the formation of the chorion (the embryo's portion of the placenta), and protect the conceptus from the pregnant person's immune cells.

Question 46

How does the embryoblast contribute to embryonic development?

Answer 46

The embryoblast develops into a two-layered embryonic disc, which forms the embryo proper and the extraembryonic membranes.

Question 47

What is the significance of implantation in the context of the blastocyst?

Answer 47

Implantation is a critical event where the blastocyst attaches to the uterine wall, allowing for further development and nourishment of the embryo.

Question 48

Identify the components of the blastocyst and their functions.

Answer 48

The blastocyst consists of trophoblast cells, which aid in implantation and placenta formation, and the embryoblast, which develops into the embryo and extraembryonic membranes.

Question 49

Discuss the protective function of trophoblast cells.

Answer 49

Trophoblast cells protect the conceptus from the immune response of the pregnant person, ensuring the embryo's safety during early development.