01-11-21 - Early Embryonic Development

Question 1

Learning outcomes

Answer 1

• List the sequence of developmental events between fertilization and implantation
• Appreciate the major changes that occur during the formation of the bilaminar disc
• Give examples of abnormal implantation sites and be aware of the implications of ectopic pregnancy
• Understand the major causes of congenital malformations

Question 2

Where is the oocyte released form?

Where does the egg then go?

Where does fertilisation occur?

How soon after fertilization does it occur?

Answer 2

• The oocyte is released form the ovaries
• The egg then travels into the abdominal ostium of the fallopian tubes through contractions of the fimbriae
• The egg then travels through the fallopian tubes
• Fertilisation occurs in the ampulla of the fallopian tubes within 12-24 hours of ovulation

Question 3

How many chromosomes do gametes have?

What is this referred to as?

What does fertilization restore?

How do gametes determine sex?

What is cleavage and how soon does it occur after fertilization?

What kind of differentiative capability do zygotes have?

When does this cease to be the case?

Answer 3

• Gametes (sperm and ovum/oocyte) have 23 chromosomes, and are referred to as haploid cells
• When fertilization occurs, the original number of 46 chromosomes is restored, creating a diploid cell
• Sex is determined by the sperm – Oocytes have X chromosomes, whereas the sperm can have either X or Y
• Cleavage is the first few cellular divisions of a zygote (fertilized egg), and is initiated within 36 hours of fertilization
• Initially, cells are totipotent (can give rise to any cell type or a complete embryo)
• This changes at the blastocyst stage

Question 4

What do ova have surrounding them when released from the ovaries?

What barrier does it have?

What is meiosis?

When do oocytes begin meiosis?

When is it completed?

Answer 4

• Ova have a cloud of cells called the corona radiata surrounding them when released form the ovaries.
• They also have a barrier called the zona pullucida, which is an important barrier for the penetration of sperm. Like a jelly coat
• Meiosis is a type of cell division that reduces the number of chromosomes in the parent cell by half, and produces four gametes
• Oocytes begin meiosis in the foetal ovary, but only complete it when fertilized in the adult reproductive tract

Question 5

What are the 3 parts of the sperm?

What do they each contain?

Answer 5

• Head
• Acrosome (lysosome-related organelle) that contains proteolytic enzyme (like hyaluronidase and acrosin) that allow the sperm to penetrate the ovum (singular egg)
• The head has a nucleus that contains genetic material which combines with DNA from the female
• Midpiece
• Contains mitochondria around filamentous core
• Provides energy for the tail (engine room of sperm)
• Tail
• Specialised flagellum that propels the sperm forward

Question 6

When is fertilization complete?

Describe the steps 7 of fertilization?

What 4 things does the fusion of the sperm and oocyte membrane trigger?

Answer 6

• Fertilization is complete when the sperm penetrates the outer surface of the egg, forming a zygote.

1) Sperm is released into the vagina via ejaculation
2) Sperm capacitation occurs, which refers to physiological changes that occur in the sperm that allow it to penetrate the egg e.g alkalinization of sperm cytoplasm, particularly the flagellum
3) The oocyte release chemicals to lure sperm via chemotaxis. This causes the sperm to use their flagellum to swim towards the egg
4) When they reach the egg, sperm will use proteolytic enzymes, like hyaluronidase, to penetrate the corona radiate (cloud of cells around egg)
5) Sperm interact with ZP3 receptors on the egg, which triggers the sperm to release its proteolytic enzymes via exocytosis
6) This allows the sperm to digest the zona pullucida and go towards the oocyte membrane
7) The sperm membrane and the oocyte membrane fuse

• The fusion of the sperm and oocyte membrane triggers:

1) The completion of meiosis in the oocyte
2) Cortical granules to be released to degrade ZP3 receptors and screen to prevent second penetration
3) An increase in membrane negative charge to screen against a second penetration
4) The release of the nucleus, midpiece and tail of the sperm into the cytoplasm of the oocyte (not known if sperm mitochondria survive)

Question 7

What happens to the sperm nucleus when it enters the oocytes cytoplasm?

Answer 7

• When the sperm nucleus enters the oocyte cytoplasm, it decondenses and forms a typical nucleus
• A male and female pronucleus form
• The pronuclei then fuse together to produce a cell with a diploid number of chromosomes

Question 8

What occurs in the maternal cytoplasm after fusion of the pronuclei?

What is present in the cytoplasm?

Where do offspring inherit their mitochondria from?

Why is this important?

How many genes does the mitochondrial genome have?

How many are protein coding?

If we know the mother has a mitochondrial genetic defect, how can this be avoided in the offspring?

Answer 8

• There is not a lot of transcription going on at this point
• There is a lot of Mrna present that will code for proteins
• There are also mitochondria present that will give rise to the mitochondria of the embryo?
• Mitochondria in offspring is only inherited from the mother
• The mitochondrial genome contains 37 genes, and 13 protein coding genes
• This is important as the mitochondria have their own genome, and if there is a mutation present, there could be a maternal mitochondrion gene that could cause genetic disorders in the offspring
• If the mother is known to have a mitochondrial genetic defect, the nuclear material of the fertilised egg can be taken out and placed into a healthy donor egg
• This is known as a three-person embryo-cytoplasm donation of healthy mitochondria

Question 9

How soon after fertilization does initiation of cleavage (division) occur?

How does the first mitosis occur?

How is the process of mitosis different in early cell cycles from how it normally occurs?

How much gene expression is present at this stage?

When does compaction begin?

What occurs in this process?

Answer 9

• Initiation of cleavage occurs within 36 hours of fertilization

1) Maternal and paternal chromosomes line up on the spindle equation
2) The sister chromatids separate

• There is no cell growth in early cell cycles, only back-to-back mitosis and synthesis
• The cell size and maternal cytoplasm are halved, but the volume stays.
• There is little expression of parental genes at this stage

Compaction begins at 8 cells
• The cluster of rounded cells becomes compacted into a homogenous ball
• Central cells become linked together, and communicate, through gap junctions
• The post-axis of the embryo is established but not visible at this point

Question 10

What is a Morula?

Where is it transported to?

How many cells Is it by this time?

What occurs to the zona pellucida at this point?

What do the inner and outer cells of the Morula divide to form?

Answer 10

• A morula is a solid ball of 16+ cells with inner and outer layers
• The Morula is transported alone the uterine tube (fallopian tube) and arrives at the uterus approximately 3-4 days after fertilization, and is 30-40 cells by this point
• The zona pellucida now disappears
• The outer cells of the morula divide to become the trophectoderm (trophoblasts)
• The inner cells divide to from an inner cell mass/embryoblast

Question 11

What is formed at 4.5 days after fertilization?

What does this consist of?

What is the differentiative capability of the cells at this point?

What cells of this structure is now removed for genetic testing and why is this ok?

What could this be used to detect?

What cells were previously used?

Why was this bad?

Answer 11

• After 4.5 days, a blastocyst formed, which is a trophectoderm consisting of trophoblasts around an embryoblast (inner cell mass)
• This blastocyst also contains an inner cavity called a blastocoel
• The cells at this point are still totipotent, and could generate any cell type or a whole new individual
• At 5-6 days, the trophectoderm can be sampled
• The trophectoderm can be used as it will not be part of the embryoid cell, so sampling it will not cause damage to the embryo
• This allows single gene disorders or chromosomes abnormalities to be detected, and preimplantation genetic diagnosis to be made
• Previously the inner mass was sampled, which is bad as the embryo will form from these cells

Question 12

What is the timeline of the blastocyst formation like?

Answer 12

Question 13

How long after fertilization does binding to the uterine wall take place?

How does this occur?

What 3 proteins are involved in the initial penetration of the uterine cell wall?

Answer 13

• Blastocyst binding to the uterine wall takes place 5.5-6 days after fertilization
• This is downregulation of anti-adhesion molecules (MUC-1) in the epithelium of the uterine wall
• This allows for selectins from the embryo to bind to glyco-components on the epithelial cells of the uterine wall (similar mechanism to WBC adhesion to blood vessel walls)
• Integrins (transmembrane proteins), laminin and fibronectin (both linking proteins) are involved in the initial penetration of the uterine cell wall

Question 14

What occurs 6-7 days after fertilization?

What do trophoblasts become?

How are these cells made?

What do these newly made cells then form?

What do these cells then do?

What occurs to prevent immune reactions during this process?

Why does this happen?

Answer 14

• 6-7 days after fertilization, blastocysts implantation into the uterine wall begins
• The trophoblasts that make up the trophectoderm divide into the invasive Syncytiotrophoblast (SCT) and cytotrophoblast
• Syncytiotrophoblast are multi-nucleate cells formed by nuclei dividing, but cells not dividing
• Syncytiotrophoblasts form the syncytium in the periphery of the blastocyst
• Syncytiotrophoblasts invade the epithelium of the uterine wall using metalloproteases, which are any enzymes whose catalytic reaction involves metal
• There is immunosuppression of host/graft and graft/host reactions
• This is because the blastocyst contains foreign material from the father, so immunosuppression is required to avoid immune reactions

Question 15

What occurs 8 days after fertilization?

What occurs in the embryo?

How many cavities are now present in the embryo?

Answer 15

• After 8 days from fertilization, Syncytiotrophoblasts continue to invade the uterine wall
• Two layers form in the embryo – epiblast (dorsal/posterior) and hypoblast (vertebral/anterior)
• The amniotic cavity begins to form as a space within the epiblast
• 2 cavities now present in the embryo – The amniotic cavity, and the blastocoel cavity in the blastocyst

Question 16

What happens to the 2 cavities in the embryo 9 days after fertilization?

What happens in the Syncytiotrophoblasts?

What will soon begin to form?

Answer 16

• Hypoblasts cells from the embryo migrate to coat the blastocyst cavity to form the primary yolk sac
• The amniotic cavity has now formed as a space within the epiblast of the embryo
• In the Syncytiotrophoblasts, spaces begin to develop
• There will be proliferation of the mesoderm between the layers of cells lining the primary yolk sac and the cytotrophoblast (at the white arrow on the diagram)

Question 17

What has the blastocyst done 11-12 days after fertilization?

What has formed by now?

What forms in this structure?

What occurs in the Syncytiotrophoblasts?

What does this provide for the embryo?

What is the embryonic disc composed of at this point?

What will these layers form?

Answer 17

• 11-12 days after fertilization, the blastocyst has completely burrowed into the endometrium of the uterine wall
• The mesoderm has been generated as a layer between the primary yolk sac and the cytotrophoblast
• The chorionic cavity in the mesoderm is formed through the fusion of many smaller spaces
• Large maternal capillaries bleed into the gaps in the Syncytiotrophoblast cells, which provides primitive placental circulation
• The Syncytiotrophoblasts then erode through the walls of these large maternal capillaries
• The embryonic disc consists of two layers (bi-layered) – the epiblast (dorsal) and hypoblast (ventral)
• The epiblast will form the ectoderm (hasn’t happened yet)
• The hypoblast will form the endoderm (hasn’t happened yet)

Question 18

After 2 weeks from fertilization occurring (When woman will expect her next period):
• What 2 layers in the trophoblast have formed?
• What 2 layers in the inner cell mass (embryoblast) has formed?
• What 2 cavities have formed?
• What happens to the primary yolk sac?

Answer 18

• Elements of the primary yolk sac get left behind to form the secondary yolk sac

Question 19

What normal occurs in foetal development around 20-22 weeks into a pregnancy?

How is this affected in preeclampsia?

What does this cause?

What symptoms does preeclampsia present with?

Answer 19

• After 20-22 weeks of pregnancy, the cytotrophoblast erodes spiral arteries in the uterus
• In preeclampsia, this invasion is defective
• This results in the spiral arteries keeping their muscular walls and acting as high resistance vessels
• This results in high blood pressure (hypertension) and proteinuria (increased levels of protein in the urine) which effects the kidneys

Question 20

What are the chances of getting from conception to a live birth?

Answer 20

• 1/3 chance of getting from conception to a live birth

Question 21

What is an ectopic pregnancy?

Where is the most common site of ectopic pregnancy?

What can this cause?

When are ectopic more likely?

What is a differential diagnosis?

What is ectopic pregnancy a differential diagnosis for?

Answer 21

• An ectopic pregnancy is when a fertilised egg implants itself outside of the womb
• The ampulla is the most common sit of ectopic pregnancy
• This can cause tube rupture and blood loss
• Ectopics are more likely in inflammatory disease of the pelvis
• Differential diagnosis looks at the possible disorders that could be causing your symptoms
• Ectopic pregnancy is an important differential diagnosis for appendicitis, which is why doctors ask patients when they had their last period

Question 22

What are congenital malformations?

When are birth defects most likely to occur?

What are the main causes of Congenital malformation?

Answer 22

• Congenital malformations are structural or functional anomalies that occur during intrauterine life (between conception and birth)
• Birth defects are most likely to occur within the first 3 months of pregnancy, when the organs are forming

Question 23

What are the 3 main environmental causes of malformations?

What is another name for these?

Answer 23

•	These can be referred to as teratogens
•	The 3 main teratogens are:
1) Drug chemicals e.g nicotine/alcohol
2) Infectious agents e.g rubella
3) Ionizing radiation

Question 24

What causes foetal alcohol spectrum disorder (FASD)?

What are 3 effects FASD has on a child?

Answer 24

• FASD is caused by someone who drank too much during pregnancy
• FASD causes:
1) Growth retardation
2) CNS damage
3) Effects on facial development – neural crest cells responsible for development of facial features fail to migrate e.g small eye openings, cleft lip, thin upper lip

Question 25

How are axes in the embryo determined?

Answer 25

• Dorsal (top) and ventral (bottom) determined in the oocyte by unclear mechanisms
• Left and right – determined by cilia movement in the node
• Rostral caudal -