Fertilization and early stages of the embryo development(week-1 and week 2)

Question 1

Where does fertilization typically occur in the female reproductive tract?

Answer 1

Fertilization usually occurs in the ampulla of the uterine tube.

Question 2

How is the ovulated oocyte transported into the uterine tube?

Answer 2

The fimbriae of the uterine tube sweep the ovulated oocyte into the tube.

Ciliary action and muscular contractions within the uterine tube move the oocyte towards the ampulla.

Question 3

What are the key mechanisms for sperm transport in the female reproductive tract?

Answer 3

Muscular contractions of the uterus and uterine tubes.

Chemoattraction by signals released from the oocyte or surrounding cells.

Thermotaxis, where sperm are guided by temperature gradients

Question 4

What is the acrosome reaction, and when does it occur?

Answer 4

The acrosome reaction is triggered when the sperm binds to glycoproteins (e.g., ZP3) on the zona pellucida.

It involves the release of hydrolytic enzymes, such as hyaluronidase and acrosin, which digest the zona pellucida, allowing the sperm to penetrate and reach the oocyte.

Question 5

How is polyspermy prevented after fertilization?

Answer 5

Fast block to polyspermy: 2-3 seconds; Rapid depolarization of the oocyte membrane prevents additional sperm binding. Lasts a few minutes

Cortical reaction (permanent block): Ca++ waves/oscillations (egg
activation)
Fusion of cortical granules with plasma membrane: release of content in perivitelline space
Swelling of the space
Zona reaction: hydrolyzation of sperm receptor proteins (ZP)
Changes in the plasma membrane of the egg (sperm receptor proteins), release of Juno

Question 6

What are the steps of sperm-oocyte interaction during fertilization?

Answer 6

Sperm binding: Sperm binds to specific receptors on the zona pellucida (e.g., ZP3 glycoproteins).

Acrosome reaction: Ca+ influx, increases intracellular pH -> fusion of the sperm plasma membrane with the anterior region of the acrosome membrane

Membrane fusion: The sperm and oocyte plasma membranes fuse with the anterior region of the acrosome membrane delivering the sperm nucleus.

Penetration of the sperm through the ZP: digestion and swimming, Entering the perivitelline space

Oocyte activation: Calcium oscillations within the oocyte trigger completion of the second meiotic division, forming the female pronucleus.

Question 7

What happens to the sperm nucleus after fusion with the oocyte?

Answer 7

The sperm nucleus decondenses, forming the male pronucleus.

The male and female pronuclei migrate toward each other and fuse to form the zygote nucleus (diploid), marking the beginning of a new organism.

Question 8

What is the role of the zona pellucida during fertilization?

Answer 8

Acts as a barrier to polyspermy by undergoing structural modifications (cortical reaction).

Provides a binding site (ZP3 glycoprotein) for sperm during fertilization.

Maintains the integrity of the oocyte before and after sperm entry.

Question 9

How is the zygote transported to the uterus after fertilization?

Answer 9

Ciliary beating in the uterine tube epithelium propels the zygote.

Smooth muscle contractions of the uterine tube aid in movement.

The transport takes 3-4 days, allowing early embryonic development (up to the morula stage) before entering the uterus.

Question 10

What are the barriers sperm must overcome to reach the oocyte?

Answer 10

Cervical mucus: Becomes thinner during ovulation but still filters less motile or abnormal sperm.

Uterine environment: Contractions may expel weak sperm.

Zona pellucida: Requires capacitated sperm to bind and penetrate.

Cumulus oophorus: Surrounds the oocyte and must be digested by sperm enzymes.

Question 11

Why does fertilization typically occur in the ampulla of the uterine tube?

Answer 11

The ampulla provides a slow transport environment, ensuring the oocyte is present when sperm arrive.

It is rich in nutrients and chemical signals that promote sperm capacitation and survival.

The ampulla’s environment supports the early stages of zygote formation.

Question 12

What are the two phases of sperm transport in the female reproductive tract?

Answer 12

Rapid phase: Sperm reach the uterine tube within minutes due to uterine contractions.

Sustained phase: Sperm are gradually released from crypts in the uterine tube, ensuring a steady supply for potential fertilization.

Question 13

What is the importance of the cortical reaction in fertilization?

Answer 13

Prevents polyspermy by hardening the zona pellucida and inactivating sperm-binding receptors.

Establishes a block to further sperm penetration.

Ensures the oocyte focuses on pronucleus formation and zygote development.

Question 14

What is the role of prostaglandins in sperm transport?

Answer 14

Prostaglandins in seminal fluid stimulate contractions of the uterus and uterine tube, aiding in sperm transport toward the oocyte.

Question 15

What changes occur in the uterine tube during ovulation to facilitate fertilization?

Answer 15

Increased blood flow to the uterine tube.
Enhanced ciliary activity to move the oocyte toward the ampulla.
Secretion of nutrients to support sperm capacitation and the oocyte.

Question 16

How do sperm recognize and bind to the zona pellucida?

Answer 16

Sperm recognize and bind to the zona pellucida through interactions between sperm membrane proteins and glycoproteins on the zona pellucida, particularly ZP3, which acts as a receptor for sperm.

Question 17

What triggers the acrosome reaction?

Answer 17

Binding of sperm to ZP3 glycoproteins in the zona pellucida triggers the acrosome reaction, releasing enzymes to penetrate the zona pellucida.

Question 18

What is the role of calcium oscillations in the oocyte after fertilization?

Answer 18

Calcium oscillations in the oocyte trigger:
Activation of enzymes for the cortical reaction.
Resumption and completion of meiosis II.
Formation of the female pronucleus.

Question 19

What is the role of hyaluronidase during fertilization?

Answer 19

Hyaluronidase, released during the acrosome reaction, helps sperm digest the hyaluronic acid matrix of the cumulus oophorus to reach the zona pellucida.

Question 20

What occurs during syngamy?

Answer 20

Syngamy is the process where the male and female pronuclei fuse, forming the zygote nucleus and restoring the diploid chromosome number.

Question 21

In addition to the acrosomal reaction what helps sperm penetrate the corona radiata?

Answer 21

Swimming movements

Question 22

What are the three main phases of fertilization?

Answer 22

Penetration of the corona radiata.
Penetration of the zona pellucida.
Fusion of the sperm and oocyte membranes.

Question 23

How does the sperm fuse with the oocyte membrane?

Answer 23

After penetrating the zona pellucida, the sperm binds to receptors on the oocyte membrane (e.g., Izumo1 on the sperm and Juno on the oocyte) and fuses with the plasma membrane.

Question 24

What is the significance of calcium oscillations in the oocyte?

Answer 24

Activate the oocyte for further development.
Trigger the cortical reaction to prevent polyspermy.
Stimulate the completion of Meiosis II.

Question 25

How does the sperm contribute to the zygote?

Answer 25

Haploid DNA from its nucleus to the zygote.
Centrioles, which organize the zygote’s first mitotic spindle.

Question 26

What are the stages of early zygote development after fertilization?

Answer 26

Zygote formation (diploid).
Cleavage: Rapid mitotic divisions.
Morula: A solid ball of cells.
Blastocyst: A hollow structure that implants in the uterus.

Question 27

What happens during the process of cleavage in early embryonic development?

Answer 27

Cleavage is a series of rapid mitotic divisions of the zygote, resulting in a progressively smaller cell population called blastomeres. These divisions do not involve growth, leading to a decrease in cell size. Cleavage continues until a morula is formed, a solid ball of around 16 cells, before the embryo begins to develop a fluid-filled cavity.

Question 28

What is the role of the zona pellucida during the early stages of cleavage?

Answer 28

The zona pellucida is a protective glycoprotein layer surrounding the early embryo. During cleavage, it prevents the blastomeres from prematurely adhering to the uterine wall. As cleavage progresses and the morula forms, the zona pellucida also helps the embryo to maintain its shape and structure.

Question 29

Describe the formation of the inner cell mass and its role in early development.

Answer 29

The inner cell mass (ICM) is formed by the blastomeres that are located in the interior of the blastocyst. These cells will give rise to the embryo proper and the extraembryonic tissues. The ICM is pluripotent, meaning it can differentiate into any of the three germ layers—ectoderm, mesoderm, or endoderm.

Question 30

What happens during the transition from a morula to a blastocyst?

Answer 30

As the morula continues to divide, a blastocyst cavity (blastocoel) forms within the mass of cells. The outer layer of the morula becomes the trophoblast, which later contributes to the formation of the placenta, while the inner cell mass forms the embryo. This process marks the differentiation between trophoblast and inner cell mass.

Question 31

How do cells of the trophoblast contribute to implantation?

Answer 31

The trophoblast cells begin to express adhesion molecules that enable them to adhere to the uterine wall. The trophoblast differentiates into two layers: the cytotrophoblast (inner layer) and the syncytiotrophoblast (outer layer). The syncytiotrophoblast secretes enzymes that break down the maternal endometrial tissue, allowing the blastocyst to embed itself into the uterine lining.

Question 32

What is the function of syncytiotrophoblast cells during implantation?

Answer 32

Syncytiotrophoblast cells invade the maternal endometrium and secrete enzymes that break down the surrounding tissues. This action facilitates the embedding of the blastocyst in the uterine wall. These cells also begin to produce human chorionic gonadotropin (hCG), a hormone that supports pregnancy by maintaining the corpus luteum.

Question 33

What is the significance of Oct-4 in early embryonic development?

Answer 33

Oct-4 is a transcription factor essential for maintaining the pluripotency of cells in the inner cell mass (ICM). It is expressed in all blastomeres up to the morula stage and helps regulate genes that control the undifferentiated state of the ICM. Loss of Oct-4 expression leads to developmental arrest.

Question 34

What is the role of Nanog in the inner cell mass?

Answer 34

Nanog is a key transcription factor expressed in the late morula and early blastocyst stages. It works alongside Oct-4 to maintain the pluripotency of the inner cell mass, ensuring that these cells do not differentiate prematurely. In the absence of Nanog, cells of the ICM will differentiate into primitive endoderm (hypoblast).

Question 35

How does the embryo “hatch” from the zona pellucida during implantation?

Answer 35

As the blastocyst continues to grow, it produces enzymes that degrade the zona pellucida, allowing the embryo to “hatch” from the protective layer. This is a crucial step for implantation, as it allows the trophoblast to make contact with the maternal uterine lining and initiate the process of embedding into the endometrium.

Question 36

What is the fate of cells in the trophoblast layer during implantation?

Answer 36

Cells of the trophoblast layer undergo differentiation into two main components:

Cytotrophoblast: The inner, mitotically active layer that contributes to the formation of the syncytiotrophoblast.

Syncytiotrophoblast: The outer, multinucleated layer that invades the uterine wall, facilitating implantation and hormone secretion (e.g., hCG).

Question 37

What is polarity in early embryonic development, and how is it established?

Answer 37

Polarity refers to the distinct distribution of cellular components, leading to the differentiation of the embryo into distinct regions. In the early stages, polarity is established by asymmetric division of cells in the morula, with inner cells becoming the inner cell mass and outer cells contributing to the trophoblast. This polarization influences subsequent development of the embryo and its extraembryonic structures.

Question 38

What role do integrins play in implantation?

Answer 38

Integrins are adhesion receptors expressed on both the trophoblast and endometrial epithelial cells. They mediate the attachment of the blastocyst to the endometrial lining, facilitating successful implantation.

Question 39

What is compaction and when does it occur?

Answer 39

Compaction occurs between the 8-cell and 16-cell stages of cleavage. It involves the tight adhesion of blastomeres due to the action of E-cadherin, which leads to the loss of individual cell identity and the formation of a more cohesive embryo.

Question 40

What is the decidual reaction?

Answer 40

The decidual reaction is the transformation of the maternal endometrial cells into decidual cells in response to implantation.
Fibroblasts in contact with the syncytiotrophoblast become bigger,
rounded and epitheliod-like

filled with fluids, lipids and glycogen

form a massive cellular matrix surrounding the embryo and later occupying the whole endometrium

Formation of an immunologically privileged site

Question 41

What is the importance of human chorionic gonadotropin (hCG) during implantation?

Answer 41

hCG is produced by the syncytiotrophoblast after implantation. It maintains the corpus luteum, which in turn secretes progesterone to support the pregnancy and prevent menstruation.

Question 42

What factors contribute to embryo failure and spontaneous abortion?

Answer 42

Common causes of embryo failure include chromosomal abnormalities (50%), defects in implantation, and immunological rejection of the embryo by the mother. Many embryos with chromosomal abnormalities do not progress past the cleavage stages, resulting in early miscarriage.

Question 43

What is the structure of the morula, and how does it differ from the blastocyst?

Answer 43

The morula is a solid ball of cells formed by the cleavage divisions of the zygote.
As cleavage continues, the blastocyst forms, characterized by a fluid-filled blastocoel inside the embryo. The blastocyst also has a trophoblast layer and an inner cell mass (ICM), whereas the morula has no cavity and is a cluster of cells without differentiation.

Question 44

Describe the formation of the primary villi during implantation.

Answer 44

Primary villi are formed by the syncytiotrophoblast and cytotrophoblast.
These villi are finger-like projections that invade the maternal endometrium and increase the surface area for exchange between maternal blood and the developing embryo. They eventually become the chorionic villi involved in nutrient exchange.

Question 45

What is the function of the decidua in pregnancy?

Answer 45

The decidua is the modified maternal endometrium that provides nutrients to the implanting embryo.
It also acts as a barrier to excessive trophoblastic invasion, preventing damage to maternal tissues.

Question 46

How does the embryo acquire nutrients during early implantation before placenta formation?

Answer 46

During the early stages of implantation, the embryo receives nutrients from the endometrial glands through the decidua.
The trophoblast invades the uterine lining to form the maternal-fetal interface, where nutrients and gases are exchanged.

Question 47

How does the embryo form the primitive streak, and what is its significance?

Answer 47

The primitive streak forms in the epiblast of the embryo during the gastrulation phase.
It defines the anteroposterior axis of the embryo and is crucial for the formation of the three germ layers: ectoderm, mesoderm, and endoderm.

Question 48

What is the process of gastrulation, and how does it contribute to embryonic development?

Answer 48

Gastrulation is the process where the three primary germ layers—ectoderm, mesoderm, and endoderm—are formed.
This process defines the body plan and establishes the foundation for all organ systems.

Question 49

What are transcription factors?

Answer 49

Proteins that help turn specific genes “on” or “off”

Activators boost a gene’s transcription
Repressors decrease transcription

Groups of transcription factor binding sites called enhancers and silencers can turn a gene on/off in specific parts of the body

Allow cells to perform logic operactions and combine different sources of information to “decide” whether to express a gene

Question 50

What is the difference between developmental potency and
developmental fate?

Answer 50

Developmental potency refers to a cell’s potential to differentiate into various types of cells or tissues, ranging from totipotent (all cell types) to unipotent (one cell type). Developmental fate is the specific cell type or tissue that a cell will become based on its position and environmental signals during development.

Question 51

What is mosaicism in the context of embryonic development, and why can it be clinically relevant?

Answer 51

Mosaicism occurs when a mitotic error at the cleavage stage leads to a mosaic cleavage-stage embryo. In this case, the mosaic cell line persists and is present throughout both the trophectoderm and inner cell mass as the embryo forms a blastocyst. Mosaicism can be clinically relevant because it may affect the developmental potential and lead to genetic disorders or differentiation problems in the embryo.

Question 52

What is produced within 36 to 48 hours after fertilization, and why is it important?

Answer 52

Within 36 to 48 hours post-fertilization, the embryo produces early pregnancy factor (EPF), which acts as an immunosuppressant to prevent the mother’s immune system from rejecting the embryo.

Question 53

What happens to the corona radiata within 48 hours of fertilization?

Answer 53

It is lost

Question 54

How long does the oocyte remain in the ampullary portion of the uterine tube before reaching the uterus?

Answer 54

About 3 days

Question 55

How long does it take for the embryo to cross the isthmus, and what role does progesterone play in this process?

Answer 55

The embryo takes about 8 hours to cross the isthmus, with progesterone playing a key role by relaxing the utero-tubal junction, allowing smoother passage through the uterine tube.

Question 56

What occurs during etching of the blastocyst and early implantation?

Answer 56

During etching, microvilli from the trophoblast extend to the zona pellucida and release proteases that begin to digest the zona pellucida, allowing the blastocyst to protrude and prepare for implantation.

Question 57

What happens after 6-7 days post-fertilization during the hatching of the blastocyst?

Answer 57

After 6-7 days, the zona pellucida is fully digested and shed, releasing the blastocyst into the uterine environment, a process known as hatching.

Question 58

What is the “implantation window,” and how long does it last?

Answer 58

The implantation window is a period of time during which the endometrium is reception-ready for the blastocyst. This phase lasts 4 days and begins 6 days after the LH peak.

Question 59

What happens during the implantation window, and can the blastocyst still be eliminated?

Answer 59

During the implantation window, the endometrium becomes receptive to the blastocyst. However, the blastocyst can still be eliminated if it is flushed out before successful implantation.

Question 60

What changes occur in the endometrium to prepare for apposition and adhesion?

Answer 60

The glycocalyx of the endometrial epithelium becomes thinner.

Microvilli disappear, preparing a flattened surface.

Pinopodes appear on the endometrial surface, which aid in the blastocyst’s attachment.

Question 61

What role do adhesion molecules play during apposition and adhesion?

Answer 61

Both trophoblastic cells and endometrial cells express adhesion molecules that facilitate the attachment of the blastocyst to the endometrium during the process of apposition and adhesion.

Question 62

How does signaling contribute to implantation?

Answer 62

Signaling regulates immunotolerance, allowing the maternal immune system to tolerate the embryo while facilitating its implantation. This ensures that the immune response does not reject the developing blastocyst.

Question 63

What are the stages of implantation?

Answer 63

apposition, adhesion, and invasion

Question 64

How does the syncytiotrophoblast surround the embryo during implantation?

Answer 64

The syncytiotrophoblast surrounds the developing embryo as it invades the maternal tissue, forming a protective barrier and facilitating the exchange of nutrients and waste products between the embryo and the maternal circulation.

Question 65

What is spotting during invasion?

Answer 65

Some bleeding may take place during invasion (possible confusion with a menstrual period)

Question 66

What can high or low levels of human chorionic gonadotropin mean?

Answer 66

High levels of hCG can be the sign twinning or of a problem (e.g.choriocarcinoma,
Hydatiform mole)

Low levels of hCG can also be the sign of a problem (fetal death, incomplete
miscarriage …)

Question 67

What do decidual cells in the uterine mucosa secrete, and how do these factors influence implantation?

Answer 67

Decidual cells secrete interleukins, growth factors, and other molecules that can either support implantation (proinvasive) or inhibit implantation (antiinvasive), regulating the balance between embryo invasion and maternal tissue protection.

Question 68

What is the most common location for ectopic pregnancy (ETP), and what complications can arise?

Answer 68

95-98% of ectopic pregnancies occur in the fallopian tube, leading to potential uterine tube rupture and hemorrhage, which can be life-threatening.

Question 69

What symptoms are commonly associated with ectopic pregnancy (ETP)?

Answer 69

Internal bleeding
Abnormal uterine bleeding
Pelvic pain
Abdominal pain
Low back pain
Massive bleeding in the first trimester.

Question 70

How might an ectopic pregnancy be mistaken for other conditions?

Answer 70

The differential diagnosis for ectopic pregnancy includes conditions like appendicitis (if the implantation is in the right tube) or an aborting intrauterine pregnancy.

Question 71

How does ectopic pregnancy (ETP) affect human chorionic gonadotropin (hCG) levels?

Answer 71

Ectopic pregnancies produce β-hCG at a slower rate, which can lead to a false negative result on pregnancy tests in the early stages.

Question 72

How is transvaginal ultrasonography helpful in diagnosing ectopic pregnancy?

Answer 72

Transvaginal ultrasonography is a valuable tool for detecting early tubal pregnancies, allowing for

better visualization of the implantation site and

aiding in early diagnosis.

Question 73

What happens to the inner cell mass during the second week of development?

Answer 73

During the second week, the inner cell mass rearranges into an epithelium, forming the embryonic shield, which marks the beginning of the development of the bilaminar embryo.

Question 74

What two layers are formed in the bilaminar embryonic disk during the second week of development?

Answer 74

Epiblast (cells expressing Nanog)
Hypoblast or primitive endoderm (cells expressing Gata 6)

Question 75

What axis is established during the formation of the bilaminar disk in the second week?

Answer 75

The primitive dorso-ventral axis is established, defining the upper (dorsal) and lower (ventral) body orientation of the embryo.

Question 76

How is the amniotic cavity formed during the second week of development?

Answer 76

Cavitation occurs in the epiblast region, forming the amniotic cavity by day 8. This cavity is crucial for the development of the amniotic membrane and supports the embryo’s development.

Question 77

What is the amniotic membrane, and how is it formed?

Answer 77

The amniotic membrane is formed from the epiblast cells surrounding the amniotic cavity. It will eventually enclose the embryo in the amniotic fluid.

Question 78

What is the primary yolk sac, and how is it formed?

Answer 78

The primary yolk sac is formed by the parietal endoderm (from the hypoblast) and lines the cytotrophoblast. This structure provides early nourishment to the embryo before the placenta becomes functional.

Question 79

What is the role of Heuser’s membrane during the second week of development?

Answer 79

Heuser’s membrane is a layer of parietal endoderm that forms the lining of the primary yolk sac, which provides the early nutritional support for the developing embryo.

Question 80

What is the formation of extraembryonic mesoderm, and what is its function?

Answer 80

Extraembryonic mesoderm forms between the cytotrophoblast and the yolk sac and amniotic membrane. It supports the epithelium of the amnion, yolk sac, chorionic villi, and developing blood vessels.

Question 81

How does the embryo receive nutrients during the first 2 weeks?

Answer 81

1st week
The embryo eliminate wastes and receives nutrients by diffusion
2nd week
Development of the uteroplacental circulation

Question 82

What is spontaneous abortion (SA), and when does it typically occur?

Answer 82

Spontaneous abortion (SA), also known as a miscarriage, refers to the natural loss of a pregnancy before the 20th week of gestation. It most commonly occurs during the 3rd week after fertilization.

Question 83

What is the estimated percentage of recognized pregnancies that end in spontaneous abortion (SA)?

Answer 83

Approximately 25%-30% of recognized pregnancies end in spontaneous abortion, usually during the first 12 weeks of pregnancy.

Question 84

Why is the frequency of early spontaneous abortion (SA) difficult to assess?

Answer 84

The frequency of early spontaneous abortion is difficult to assess because a woman may not be aware that she is pregnant, and reported rates for early SA range from 50%-70%.

Question 85

How is spontaneous abortion (SA) often mistaken for a different condition?

Answer 85

Spontaneous abortion (SA) occurring several days after the first missed period is often mistaken for delayed menstruation due to similar symptoms.

Question 86

What percentage of spontaneous abortions (SA) result from chromosomal abnormalities?

Answer 86

More than 50% of known spontaneous abortions are caused by chromosomal abnormalities in the fetus.

Question 87

What are some other causes of spontaneous abortion (SA) apart from chromosomal abnormalities?

Answer 87

Failure of blastocyst implantation due to a poorly developed endometrium or lack of immunotolerance.
Fetal structural abnormalities, such as neural tube defects, cleft lip, and cleft palate.

Question 88

What is the association between spontaneous abortion (SA) and fetal abnormalities?

Answer 88

After the 10th gestational week, about 25-40% of spontaneous abortions are related to fetal causes (e.g., chromosomal defects, neural tube defects), and 5-10% are related to maternal causes.

Question 89

How do placental causes contribute to spontaneous abortion (SA)?

Answer 89

Placental causes account for 25-35% of spontaneous abortions, often due to placental insufficiency, which interferes with fetal nutrient and oxygen supply.