Fertilisation and embryo development

Question 1

When in the cell cycle does ovulation and fertilisation occur?

Answer 1

Time of ovulation is ‘earlier’ in canine species, ovulation occurs at metaphase of meiosis I. Completion of meiosis II and formation of second polar body occurs after fertilisation.

Question 2

where does fertilisation normally occur?

Answer 2

Fertilisation normally occurs in the first third of the oviduct.

Question 3

What sperm events must happen before fertilisation?

Answer 3

Sperm capacitation- Removal of glycoprotein coat and alteration of sperm mitochondria

Sperm binding - ZP3 acts as a receptor for sperm

Acrosome reaction - Loss of acrosome contents, enzymes digest zona pellucida, exposed sperm surface proteins bind to ZP2

Sperm membrane fusion - Causes depolarisation of egg membrane and the cortical reaction

Question 4

What happens to the nuclei when fertilization occurs?

Answer 4

When fertilisation occurs there is sperm nucleus decondensation and the egg and sperm nucleus fuse (syngamy).
The two pronuclei approach each other (when both are visible the cell is called an ootid)

Question 5

Once the two pronuclei fuse what is it called?

Answer 5

Following fusion of the pronuclei the single cell embryo is called a zygote

Question 6

Once the single cell zygote is formed what is the next step?

Answer 6

The zygote undergoes a series of mitotic divisions called cleavage divisions
The first cleavage division generates a two-cell embryo (The cells of which are called blastomeres)
Cleavage occurs with no cytoplasm synthesis

Question 7

What happens to the volume of each blastomere as the zygote divides?

Answer 7

Cleavage occurs with no cytoplasm synthesis

Therefore individual blastomere volume decreases

Question 8

How are identical twins formed?

Answer 8

Identical twins are derived from blastomeres of a two-cell embryo that divide independently to form two separate embryos (different to twins that originate from two separate ovulations)

Question 9

When the embryo has over 16 cells it is called a ……….

Answer 9

Morula

Question 10

The blastomeres have the ability to form all cells necessary for normal development so cell number can be decreased or increased without adverse effect to the embryo, what does this make the cells?

Answer 10

The cells at this stage are Totipotent

Question 11

The outer cells within the morula are more squashed than the inner cells, what does this cause?

Answer 11

Outer cells form cell-cell adhesions called ‘tight junctions’
Inner cells have looser lines of communication called ‘gap junctions’

Question 12

How does fluid accumulate inside the zona pellucida?

Answer 12

The outer cells pump sodium into the morula, ionic concentration therefore rises and water then diffuses through the zona pellucida

Question 13

When does the morula become a blastocyst?

Answer 13

When a distinct cavity is formed the embryo is called a blastocyst

Question 14

The two cell types (inner and outer) of the morula become what cells in the blastocyst?

Answer 14

Outer cells -> Trophoblast -> chorion and placenta

Inner cells -> Inner Cell Mass (ICM) -> Embryo

Question 15

The blastocyst continues to undergo mitosis, fluid continues to be produced (increase of pressure) and enzymes are produced by the trophoblast, why does this happen?

Answer 15

So the zona pellucida splits and the blastocyst hatches

Question 16

Does the fertilised egg move along the oviduct at a constant speed?

Answer 16

Slow continuous progression – rabbit, sow

Discontinuous progression – cow, ewe, human
Prolonged stay in the uterine tube
Anti-peristaltic contractions maintain egg for 2-3 days
Stronger, peristaltic contractions then transport egg onwards
Does not matter whether egg is fertilized or not

Fertilised progression – mare
Only fertilised eggs pass into uterus
Embryo secretes prostaglandins E and F that are hypothesized to promote embryo migration through oviduct as well as in the uterus

Question 17

How do eggs move along the oviduct in hamsters and rats?

Answer 17

Rats and hamster are different where fertilised eggs secret motility regulatory factors so move more rapidly and synchronously

Question 18

How is the free floating blastocyst supported and nourished before the placenta is formed?

Answer 18

Secretions derived from endometrial glands provide nutritional support before the placenta is established
The secretions contain carbohydrate, proteins and lipids and are described as uterine milk or histiotroph.

Question 19

What three layers are produced from the embryonic epiblast?

Answer 19

An outer ectoderm and inner endoderm is formed
Later a third layer (mesoderm) is produced
These 3 layers form the primary germ layers

Question 20

Before gastrulation what does the inner cell mass give rise to?

Answer 20

ICM gives rise to Epiblast and Hypoblast

Question 21

What does the Epiblast give rise to?

Answer 21

Epiblast forms amnionic ectoderm (amnion)

Question 22

The hypoblast forms a primitive endoderm, what does this develop into?

Answer 22

The yolk sac that provides nutrition

Question 23

How is the chorioallantoic membrane formed?

Answer 23

A third fluid filled sac forms from an out-pouching of the hind gut (the allantois)
The allantois makes contact with the chorion and eventually fuses to form the chorioallantoic membrane
The amniotic folds form the amnion and the yolk sac regresses

Question 24

Where does gastrulation arise from? What happens during gastrulation?

Answer 24

Gastrulation begins at the posterior end of the embryo, where the node arises
Cells from the epiblast migrate through the primitive streak and differentiate into ectoderm, mesoderm and endoderm