Fertilisation and early embryo development

Question 1

What is the fertilisastion period?

Answer 1

The time when oocytes are available to be fertilised by sperm

Question 2

how long is the fertile life span of sperm in the bitch?

Answer 2

5-11 days (prolonged)

Question 3

How long is the fertile lifespan of sperm in the cow?

Answer 3

1-2 days

Question 4

How long is the fertile life span of sperm in the mare?

Answer 4

4-5 days

Question 5

What is different about the time of ovulation in dogs?

Answer 5

Ovulation occurs at metaphase of meiosis I (immature oocytes)
Completion of meiosis II and formation of second polar body occurs after fertilisation

Question 6

What is embryonic diapause?

Answer 6

Temporary arrest of embryo development characterised by delayed implantation in the uterus to allow birth at time for optimal survival chance for offspring

Question 7

What causes embryonic diapause?

Answer 7

Descreased daylight => increased melatonin => increase in prolactin => luteal suppressive effect => decreased progesterone => embryonic diapause

Question 8

What ends embryonic diapause in carnivores?

Answer 8

increased daylight decrease melatonin => increased prolactin => luteotrophic effects => increased progesterone => embryo reactivated

Question 9

Where does fertilisation usually occur?

Answer 9

first 1/3 of oviduct

Question 10

What is an ootid?

Answer 10

When both pronuclei from the sperm and egg are visible

Question 11

What is syngamy?

Answer 11

Egg and sperm nuclei fuse

Question 12

What occurs to the sperm in fertilisation?

Answer 12

Sperm capacitation - removal of glycoprotein coat and alteration of sperm mitochondria => hypermotile
Sperm binding - ZP3 acts as a receptor
Acrosome reaction - loss of acrosomal contents, enzymes digest zona pellucida, exposed sperm surface proteins bind to ZP2

Question 13

What is an embryo called a zygote?

Answer 13

When the pronuclei of then sperm and egg are fused

Question 14

Describe the process of the first cleavage division

Answer 14

Zygote becomes a 2-cell embryo via mitotic division
DNA replicates but cytoplasm does not so the individual cells (blastomeres) have a lower volume

Question 15

How do identical twins develop?

Answer 15

In the 2-cell embryo, each blastomere has the potential to develop into separate offspring by dividing independently to form 2 separate embryos

Question 16

How does the morula form?

Answer 16

The 2-celled embryo undergoes more cleavage divisions within the zona pellucida eventually resulting in a 16 celled embryo = morula

Question 17

What is totipotency and where is it found?

Answer 17

Blastomeres have the ability to form all cells necessary for normal development
Not seen beyond the 16-cell stage (morula)

Question 18

Label the early cell divisions of the embry

Answer 18

a - zygote
b - 2 cell embryo

Question 19

Describe the formation of the blastocyst

Answer 19

The outer cells within the morula are more squashed than the inner cells
- outer cells form tight junctions
- inner cells have looser communication lines cause gap junctions
- outer cells pump sodium into morula from extracellular space
- ionic conc rises and water diffuses into zona pellucida (fluid accumulation)
- fluid forms a distinct cavity => blastocyst

Question 20

Describe the cell types within a blastocyst and what they give rise to

Answer 20

Outer cells = trophoblast => chorion and placenta
Inner cells = inner cell mass => embryo

Question 21

Describe the process of hatching of the blastocyst

Answer 21

Blastocyst continues to undergo mitosis and fluid production => increased pressure
Proteolytic enzymes produced by trophoblast
Zona pellucida weakens and splits => hatching of blastocyst

Question 22

Describe discontinuous progression of the egg in cows and ewes

Answer 22

Question 23

Describe fertilised progression of the egg in mares

Answer 23

Question 24

What happens to the blastocyst after hatching?

Answer 24

becomes free floating embryo
secretions derived from endometrial glands (histiotroph) provide nutritional support before placenta is established

Question 25

What tissues arise from the inner cell mass of the blastocyst?

Answer 25

ICM gives rise to epiblast and hypoblast:
- epiblast gives rise to amnionic ectoderm (amnion) and the primary germ layers
- hypoblast gives rise to extraembyonic or primitive endoderm (yolk sac)

Question 26

Describe the formation of the yolk sac, amnion and allantois

Answer 26

The primitive endoderm forms a lining which results in a cavity (yolk sac) when complete.
The extra embryonic mesoderm grows around the yolk sac and also fold dorsally to form the amniotic folds which forms the amnion.
A 3rd fluid filled sac forms from an outpouching of the hind gut (allantois)
The yolk sac regresses

Question 27

How does the chorioallantoic membrane form?

Answer 27

The allantois makes contact with the chorion

Question 28

What is the chorion?

Answer 28

membranes between the developing fetus and mother
Formed by extraembryonic mesoderm and layers of trophoblast

Question 29

Describe the process of gastrulation

Answer 29

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