Fertilisation and early embryo development

Question 1

Define the fertilisation period

Answer 1

-The time when oocytes are available to be fertilised by sperm, normally few hours after ovulation.
(N.B. the fertile period is the time when mating could result in pregnancy - hence fertile period is longer than fertilisation period)

Question 2

What is the fertile life span of sperm in different species?

Answer 2

Bitch= 5-11 days
Cow= 1-2 days
Mare= 4-5 days
Camelids= 4-5 days 
Some reptiles= 4-5 days

Question 3

Discuss why the timing of ovulation in the dog is different to other species

Answer 3

• Ovulation occurs at prophase 1. Completion of meiosis 2 and formation of second polar bodies occurs after fertilisation
• The fertilisation period in the bitch is shifted so it is longer

Question 4

Describe fertilisation stages (sperm)

Answer 4

• Sperm capacitation: removal of glycoprotein coat and alteration of sperm mitochondria (makes them hypermotile)
• Sperm binding: ZP3 (zona pellucida protein) acts as a receptor for sperm
• Acrosome reaction: loss of acrosome contents, enzymes digest ZP, exposed sperm surface proteins bind to ZP2

Question 5

Describe fertilisation stages (egg)

Answer 5

• Sperm membrane fusion: results in depolarisation of egg membrane (due to influx of Ca2+). Causing cortical reaction which blocks polyspermy. Oocytes is activated allowing completion of meiosis 2.
• Sperm oocyte fusion: membranes between sperm and egg fuse, allowing sperm nuclei to pass into egg cytoplasm. (sperm becomes completely invaginated in the cytoplasm of the egg and the chromatin of the sperm begins to decondense.
• Sperm nucleus decondenses, egg and sperm nucleus fuse (syngamy)

Question 6

When does the embryo become a zygote?

Answer 6

When the two pronuclei fuse together

Question 7

What are cleavage divisions?

Answer 7

When the zygote undergoes a series of mitotic divisions, each cleavage division generates cells which are called blastomeres.

Question 8

Describe cleavage of the zygote

Answer 8

• Cleavage occurs with NO CYTOPLASMS SYNTHESIS

- Hence with each individual blastomere the volume decreases.

Question 9

What is a 16 cell embryo called?

Answer 9

A morula

Question 10

What is a totipotent cell?

Answer 10

Has the ability to form all cells for normal development

N.B totipotency is not demonstrated beyong the morula stage

Question 11

After the morula stage, what can’t happen?

Answer 11

The cell within the embryo cannot give rise to a new individual any longer.

Question 12

How does a blastocyst form?

Answer 12

The outer cells within the morula are more squashed than the outer cells. (Tight junctions form between outer cells and gap junctions form between inner cells)

• The outer cell pump sodium into morula, ionic concentration therefore rises and water then diffuses through the ZP (fluid accumulation)
• Once a distinct cavity (blastoceole) is formed the embryo is called a blastocyst.

Question 13

What do the 2 layers of the blastocyst becomes and what will these layers develop into?

Answer 13

Outer cells= Trophoblast which become chorion and placenta

Inner cells= Inner cell mass which will become the embryo

Question 14

Describe the process of the hatching of the blastocyst

Answer 14

• The blastocyst continues to undergo mitosis and fluid continues to be produced. The fluid causes an increase in pressure.
• Proteolytic enzymes are produced by trophoblast
• ZP weakens
• ZP splits and blastocysts emerges.

Question 15

In which species is progression continuous and in which is it discontinuous?

Answer 15

Continuous: rabbit/ sow
Discontinuous: cow/ ewe

Question 16

What does it mean by the mare undergoing fertilised progression?

Answer 16

Only fertilised eggs will pass into the uterus.

Embryo secretes Prostaglandins E and F which aid migration through oviduct and uterus.

Question 17

What is histiotroph?

Answer 17

Supports embryo until foetal membranes develop.
(Secretions derived from endometrial glands provide nutritional support before placenta is established- secretions called histiotroph and contain carbs, protein, lipids)

Question 18

How is the embryonic disc formed?

Answer 18

Cells of the inner cell mass flatten to form the disc

Question 19

What does the inner cell mass give rise to before grastrulation?

Answer 19

Epiblast (forms amnion) and hypoblast (yolk sac)