Fertilisation and Early Embryo Development

Question 1

What is the stimulant for ovulation?

Answer 1

A spike of plasma LH

Question 2

What are the two types of ovulation?

Answer 2

Spontaneous & induced ovulation

Question 3

What are the four key evens of ovulation?

Answer 3

Follicular wall bursts, oocyte & granulosa cells released from GC anchor point, oocyte carried with follicular fluid into the peritoneal cavity, captured by fimbria of the infundibulum

Question 4

What is the difference between the fertilisation period and the fertile period?

Answer 4

Fertilisation period is the time when oocytes are available to be fertilised by sperm, fertile period is the time when mating could result in pregnancy

Question 5

When does ovulation occur in canine species?

Answer 5

Ovulation occurs at metaphase of meiosis I, completion of meiosis II and formation of second polar body occurs after fertilisation – this means, unlike other species, the fertilisation period is not immediately after ovulation (actually 2 days to 5 days after)

Question 6

What three events must sperm go through in order for fertilisation to occur?

Answer 6

Sperm capacitation, sperm binding, acrosome reaction

Question 7

What are the two binding sites for sperm?

Answer 7

Primary zona binding region & acrosome reaction promoting ligand

Question 8

What is the function of the sperm and egg membrane fusion?

Answer 8

Allows sperm nuclei passage into egg cytoplasm

Question 9

What two events are stimulated by sperm membrane fusion?

Answer 9

Causes depolarisation of egg membrane & the cortical reaction (cortical granules are released into the perivitelline space – changes ZP3 so that sperm can no longer bind)

Question 10

What happens during the second meiotic division?

Answer 10

Oocyte activation & extrusion of second polar body

Question 11

What is an ootid?

Answer 11

When the two pronuclei are both visible in the cell

Question 12

What do the two pronuclei fuse to form?

Answer 12

A zygote

Question 13

What are cleavage divisions?

Answer 13

A series of mitotic divisions undergone by the zygote

Question 14

What are the cells of the two-cell embryo called?

Answer 14

Blastomeres

Question 15

What is the 16 cell embryo formed from subsequent divisions of the blastomere called?

Answer 15

Morula

Question 16

When does totipotency cease?

Answer 16

Beyond the 16-cell stage

Question 17

What do the outer and inner cells within the morula form?

Answer 17

Outer cells form cell-cell adhesions called tight junctions -> trophoblast – chorion and placenta
Inner cells have looser lines of communication called gap junctions -> inner cell mass – embryonic disc

Question 18

How does a blastocyst form?

Answer 18

Outer cells pump sodium into the morula, ionic concentration therefore rises and water then diffuses through the zona pellucida – fluid accumulation

Question 19

How does hatching of the blastocyst occur?

Answer 19

Proteolytic enzymes are produced by trophoblast, zona pellucida weakens, zona pellucida splits and the blastocyst emerges (hatching)

Question 20

What is uterine milk/histiotroph?

Answer 20

Secretions derived from the endometrial glands containing carbohydrate, proteins and lipids that provide nutritional support to the blastocyst before the placenta is established

Question 21

What are the three types of embryo progression?

Answer 21

Slow, continuous progression – rabbit, sow
Discontinuous progression – cow, ewe, human – prolonged stay in the uterine tube
Fertilised progression – mare – only fertilised eggs pass into uterus

Question 22

Which three layers make up the primary germ layers?

Answer 22

Outer ectoderm and inner endoderm, later a third layer (mesoderm) is produced

Question 23

What does the ICM give rise to?

Answer 23

Epiblast – forms anionic ectoderm & Hypoblast – form primitive endoderm (yolk sac)

Question 24

What happens to the extraembryonic mesoderm?

Answer 24

Continues to grow and forms a sac that surrounds the yolk, and folds dorsally to form the amniotic folds

Question 25

How does the allantois form?

Answer 25

From an out-pouching of the hindgut

Question 26

What is the chorion?

Answer 26

Membranes between the developing fetus and the mother – formed by extraembryonic mesoderm and the two layers of trophoblast

Question 27

Where does gastrulation begin?

Answer 27

At the posterior end of the embryo

Question 28

What happens during gastrulation?

Answer 28

Cells from the epiblast migrate through the primitive streak and differentiate into ectoderm, mesoderm and endoderm