Fertilisation And Cleavage

Question 1

Outline the stages of embryonic development and their lengths

Answer 1

• Cleavage days 1-5
• Implantation days 6-9
• Gastrulation weeks 3-4
• Body plan week 4
• Organogenesis weeks 4-8
• Fetal development weeks 8-40

Question 2

How long do cleavage divisions take?

Answer 2

First is 24 hours

Others are 12-24

Question 3

Where are primordial germ cells formed?

Answer 3

Caudal mesoderm of late gastrulae

Then migrate to developing gonads and multiply by mitosis

Question 4

How are oocytes formed?

Answer 4

• Meiosis begins in fetal ovary
• Suspended at Prophase I (primary oocytes) until a few are activated during each menstrual cycle
• Secondary oocytes are held at metaphase II until fertilised
• Single oocyte released and wafted into ampulla of nearest oviduct

Question 5

Describe an oocyte

Answer 5

• surrounded by zona pellucida and a layer of somatic granulosa cells
—> glycoprotein membrane
• beneath the membrane are cortical granules- membrane bound vesicles

Question 6

Where does the first oocyte polar body form?

Answer 6

Between the plasma membrane and zona pellucida

Question 7

How are spermatozoa formed?

Answer 7

1. Spermatogonia-> primary spermatocytes by mitosis
2. -> 2 secondary spermatocytes by meiosis I
3. -> 4 spermatids by meiosis II
4. -> 4 sperm by spermatogenesis
5. Move to epididymis for storage

Question 8

What is parthenogenesis?

Answer 8

Eggs developing without fertilisation

Question 9

What are androgenetic and gynogenetic embryos?

Answer 9

Abnormal trophoblast (outer layer of blastocyst)

Question 10

What is a hydatidiform mole?

Answer 10

Tumour
• Complete hydatidiform mole= diploid, only male chromosomes
• Partial hydatidiform mole= Tripoli’s, two male one female

Question 11

How common are hydatidiform moles?

Answer 11

1/500

Usually miscarry or lead to choriocarcinoma tumour

Question 12

Why are both male and female chromosomes required for development in mammals?

Answer 12

• Genomic imprinting- DNA methylated during gametogenesis so adjacent gene is inactive during early development
• Male and female must combine for a complete set of active genes

Question 13

Outline normal fertilisation

Answer 13

1. In ampulla of oviduct, contact between sperm and zona pellucida
2. Acrosome reaction- enzymes released
3. Sperm fuses with plasma membrane- increase in cytosolic calcium, spreads throughout egg
4. Cortical granules fuse with plasma and release contents
5. Enzymes modify zona pellucida
6. Prevents polyspermy

Question 14

Describe the cleavage of a fertilised egg

Answer 14

• Egg completes meiosis- haploid female pro nucleus + second polar body
• No growth with first divisions
• Compaction- before 32-cell stage adhesion molecules are added to plasma membrane

Question 15

Describe the early development of a fertilised egg

Answer 15

• Trophoblast forms, blastomeres polarised
• Cell division generates ICM
• Trophoblast cells pump sodium into central region- water- blastocoel
• Embryo is now blastocyst

Question 16

What will the Trophoblast and ICM form?

Answer 16

Trophoblast- chorionic tissues of placenta

ICM- everything else

Question 17

What potency are 4 stage blastomeres and the ICM

Answer 17

4 stage- totipotent

ICM- pluripotent

Question 18

What percentage of monozygotic twins have a shared placenta?

Answer 18

70%

Question 19

What causes conjoined twins?

Answer 19

Shared placenta and amnion

Question 20

What is tetragametic chimerism?

Answer 20

Organisms of 2 genetically distinct cell populations

- each embryo contributes to chimera

Question 21

Outline implantation

Answer 21

• Blastocyst burrows into uterine wall
• Embryo hatches from zona pellucida and Trophoblast attaches to endometrium
• Syncytiotrophoblast forms- in contact with endometrium
• Trophoblast cells lining blastocoel- cyrotrophoblast
• syncytiotrophoblast penetrates uterine wall with enzymes- 9-10 days

Question 22

What happens once the embryo has implanted in the uterine wall?

Answer 22

• Lacunae cavities form- blood
• Hormone chorionic gonadotropin (HCG) produced
• HCG maintains progesterone production by corpus luteum
• Progesterone maintains uterine endometrium

Question 23

How does the blastocyst develop during implantation?

Answer 23

• ICM-> hypoblast and epiblast
• Hypoblast forms Heuser’s membrane
• Epiblast forms embryonic germ layers, amniotic membrane, extraembryonic mesoderm
• Extraembryonic mesoderm forms blood vessels of umbilical cord and placenta

Question 24

Describe the embryo at 14 days

Answer 24

• Suspended by connecting stalk of extraembryonic mesoderm

* Syncytiotrophoblast, cytotrophoblast and extraembryonic mesoderm extend into lacunae to form stem villi- increase SA