Fertilization/Cleavage divisions

Question 1

When does cleavage occur?

Answer 1

cleavage: days 1-5

Question 2

When does implantation occur?

Answer 2

implantation: days 6-9, approx 6 days after ovulation

Question 3

Where is the mammalian egg normally fertilised?

Answer 3

ampulla of the oviduct/fallopian tube

Question 4

What are the two phases at which oogenesis is halted?

Answer 4

primary oocyte halted at prophase 1 until puberty;

secondary oocyte halted at metaphase 2 until fertilised

Question 5

When is spermatogenesis initiated?

Answer 5

only at puberty

Question 6

Average sperm count?

Answer 6

40-100 million spermatozoa (decreasing in Western world)

Question 7

Purpose of the acrosome?

Answer 7

at front of head of spermatozoa - bursts when in contact with receptors in zona pellucida = release of enzymes digesting ZP

Question 8

What prevents polyspermy?

Answer 8

Granules release content when sperm fuse with oocyte plasma membrane, modifying ZP

Question 9

Embryos with maternal Xsomes only?

Answer 9

• gynogenetic embryos

- poorly developed trophoblast

Question 10

Embryos with paternal Xsomes only?

Answer 10

• androgenetic embryos

- over-developed trophoblast, poorly developed embryo

Question 11

What are hydatidiform moles?

Answer 11

• formed when trophoblast grows uncontrollably
• leads to miscarriage, tumour
• complete moles: only paternal Xsomes
• partial moles: 2 sets of paternal Xsomes and 1 set of maternal Xsomes

Question 12

Describe cleavage divisions

Answer 12

• mitosis in fertilised egg as it travels down fallopian tube to uterus
• not accompanied by cellular growth
• large egg splits into smaller cells until they reach normal size
• cleavage divisions take 12-24h, last 5 days in humans

Question 13

Describe compaction

Answer 13

• 16-cell morula
• cells maximise contacts with each other
• outside pop called trophoblast (polar)
• inner pop called inner cell mass (non-polar)
• formation of blastocoel (fluid filled cavity), displacing ICM to 1 pole
• embryo now blastocyst

Question 14

How is regulative development a cause of twins?

Answer 14

• embryo can be divided into individual blastomeres at 2-cell and 4-cell stage, going on to form identical clowns
• reg. dev. responsible for monozygotic/identical twins
• 30% formed 2-cell stage, the rest at blastocyst stage
• proof that blastomeres are totipotent

Question 15

Define tetragametic chimera

Answer 15

• 2 embryos fuse and form normal sized tetragametic chimera (formed by 2 sperm and 2 eggs)
• can happen when fraternal twins fuse to form single embryo

Question 16

What are the functions of the trophoblast?

Answer 16

• implantation
• differentiate chorion
• immunosuppression
• endocrine gland (chorionic gonadotrophin, progesterone…)

Question 17

What is the role of HCG (chorionic gonadotrophin)?

Answer 17

-maintain pregnancy by maintaining steroid production of ovary
-produced by trophoblast during 1st trimester,
maintaining production of progesterone by ovary (required to maintain uterine wall, prevent menstruation)
-in later pregnancy, progesterone produced by trophoblast, no HCG=no ovarian production

Question 18

What is the role of the ZP?

Answer 18

-prevent embryo from implanting into fallopian tube
( + prevent polyspermy through its modification)
-when in uterus, embryo hatches from ZP so it can implant (secretion of emzymes = hole in ZP)

Question 19

What does the ICM give rise to?

Answer 19

• during implantation: epiblast and hypoblast

- epiblast later gives rise to 3 germ layers