Early development of embryo & implantation

Question 1

Where does fertilisation normally occur in female genital tract?

Answer 1

Ampulla of fallopian tube

Question 2

What type of cell division occurs in early conceptus (before balstocyst stage)?

Answer 2

Cleavage divisions

Question 3

What are cleavage divisions?

Answer 3

Parent cell is cleaved into 2 equal halves to form daughter cells. There is an increase in cell number but not volume.

Question 4

What is the function of the zona pellucida after fertilisation?

Answer 4

1. Holds dividing blastomeres together
2. Prevents premature implantation
3. Prevents multiple genetically distinct conceptuses from fusing together

Question 5

What type of genetic information mediates the first few divisions of the conceptus?

Answer 5

The first 1 or 2 divisions are regulated by maternal proteins inherited from the oocyte as the conceptus only begins synthesising its own proteins at the 4-8 cell stage.

Question 6

What is compaction and when does it occur?

Answer 6

• At the 8-16 cell stage, compaction begins.
• This is when the cells change in shape so that their outer surfaces become convex and their inner surfaces become concave, maximising the contact between adjacent cells.

Question 7

How are cells polarised during proess of compaction?

Answer 7

• Cells change from well-rounded, symmetrical blastomeres to polarised epithelioid cells.
• The daughter cells have the opportunity to inherit different membrane proteins from parent cell (apical or basal), which ultimately decides their fate in the blastocyst.
• Basal derived cells differentiate into inner cell mass while apical derived cells differentiate into the trophoblast.

Question 8

What is the blastocoel?

Answer 8

Cavity that forms in the centre of the morula (16-cell conceptus), separating it into one-cell thick trophoblast and inner cell mass. Formation of this cavity results in entry into blastocyst stage of development.

Question 9

What events mediate the formation of the blastocoel?

Answer 9

Absorption of uterine fluid from the uterine cavity as a result of:

1. Formation of tight junctions between epithelioid cells of morula.
2. Efflux of Na⁺ from morula cells into the ECF surrounding the cells.

These 2 events create an osmotic gradient between uterine cavity and ECF of morula, causing movement of water into ECF to create blastocoel.

Question 10

What are the fates of the different components of the blastocyst?

Answer 10

Trophoblast → Placenta

ICM → Embryo and mesoderm of placenta

Question 11

What genes are important in differentiation of the trophoblast and ICM?

Answer 11

1. Cdx2
2. Oct4

Question 12

What is the molecular basis of differentiation in the blastocyst?

Answer 12

• During differentiation of the trophoblast and the ICM, Oc4 expression is restricted to the ICM while Cdx2 is restricted to the trophoblast.
• This differentiation is thought to be regulated by kinase called Hippo.

Question 13

How is Hippo expression regulated?

Answer 13

1. In non-polarised cells on the inside of the morula, there is great degree of contact, allowing Hippo to be expressed, that then phosphorylates and inhibits Yap, thus inhibiting activation of Cdx2 expression.
2. In polarised cells on the outside of the morula, there is very little contact, inhibiting Hippo expression, thus Yap is active and promotes expression of Cdx2.

Question 14

What mediates transport of conceptus down the fallopian tube after fertilisation?

Answer 14

• Pulsation of the cilia and contraction of smooth muscle in the fallopian tube (although contraction not necessary).
• Mediated by high progesterone:oestrogen ratio in luteal phase.

Question 15

When does the conceptus enter the uterus?

Answer 15

~4 days after fertilisation

Question 16

What is the process of hatching and what is it mediated by?

Answer 16

Breaking down of zona pellucida by proteases secreted from the blastocyst, allowing the blastocyst to grow freely.

Question 17

What is the typical site of implantation?

Answer 17

Fundus of uterus

Question 18

What is placenta previa?

Answer 18

Implantation occurs in the lower uterine segment, resulting in obstruction of the cervix later in pregnancy.

Question 19

What re ectopic pregnancies?

Answer 19

Implantation of the conceptus outside the uterus

Question 20

What are the stages of placentation?

Answer 20

1. Repulsion (non-receptive)
2. Apposition (receptive)
3. Adhesion (receptive)
4. Invasion (non-receptive)

Question 21

What are the characteristics of the uterus that prevent implantation during the repulsion stage?

Answer 21

1. High expression of progesterone receptors
2. Thick mucin glycoprotein coat
3. Negative charge on surface of endometrium
4. Long microvilli
5. Expression of Muc1 repels blastocyst from attaching to uterine walls.

Question 22

What are the characteristics of the uterus that promote implantation during the receptive phase?

Answer 22

1. Decreased expression of progesterone receptors
2. Thinning of mucin glycoprotein coat.
3. Loss of surface negative charge
4. Appearance of swellings on apical surface called pinopodes
5. Shortening of surface microvilli
6. Down-regulation of Muc1 allows blastocyst to attach to endometrium.

Question 23

What are the roles of pinopodes during receptive phase?

Answer 23

The pinopodes absorb uterine fluid, decreasing the volume of the uterine cavity and bring opposing endometrium into close contact (occlusion).

Question 24

What are the roles of hormones during transition of non-receptive to receptive stage?

Answer 24

1. Non-receptive phase:
   - Expression of progesterone receptors high in endometrium.
   - This mediates production of uterine secretion by glands but inhibits secretion.
   - Also mediates thickening of endometrium.
   - Receptive phase:
   - Expression of progesterone receptors down-regulated.
   - This causes dis-inhibition of endometrial secretion (and promotion by oestrogens as its levels rise in luteal phase).

Question 25

What is the molecular basis of blastocyst adhesion to the endometrium?

Answer 25

1. Secretion of LIF from cells of endometrial glands
2. LIF binds to LIF receptors on the blastocyst, which may cause positive feedback loop that increases LIF secretion in local area of endometrium where blastocyst is located
3. LIF causes upregulation of HB-EGF expression on endometrial wall, which binds to HSPG on blastocyst to mediate adhesion
4. This induces α2 integrin expression on endometrium and α6/β5 expression on trophoblast, which may mediate adhesion.

Question 26

What factors mediate process of invasion of endometrium by conceptus?

Answer 26

Matrix metalloprotease (MMP) secretion from conceptus

Question 27

Do cells of the endometrium undergo apoptosis following invasion?

Answer 27

No

Question 28

What is the decidual reaction?

Answer 28

After implantation, endometrial cells transform from spindly cells to round decidual cells as they accumulate lipids and glucose.

Question 29

What is decidualisation?

Answer 29

Transformation of the whole uterine endometrium into decidua as all cells undergo decidual reaction.

Question 30

What is the sequence of events in decidualisation?

Answer 30

1. Primary decidual reaction: Initially, only the endometrial cells in region of implantation undergo decidual reaction.
2. Secondary decidual reaction: Eventually, the whole of the endometrium undergoes the reaction.

Question 31

What are the parts of the decidua?

Answer 31

1. Basalis: Beneath conceptus
2. Capsularis: Overlying implanted conceptus
3. Parietalis: Arount the rest of uterus

Question 32

How is pregnancy maintained after implantation?

Answer 32

1. Directly after implantation, hCG is secreted from trophoblast that acts to maintain corpus luteum, which secretes progesterone and inhibits menstruation.
2. After placentation occurs (~10 weeks), the placenta continues to produce progesterone, inhibiting menstruation and sustaining pregnancy.

Question 33

What are possible factors mediating pregancy loss?

Answer 33

1. Chromosomal abnormalities
2. Fertilisation errors due to aged sperm/oocyte
3. Uterine factors (e.g. endometrial defects)
4. Physical factors

Question 34

What is the vanishing twin phonomenon?

Answer 34

• 2 gestational sacs present but only 1 conceptus present (other twin is lost)
• May be responsible for discrepency between number of twin pregancies compared to twin births