Embryology

Question 1

What stage of division in the oocyte during ovulation and what are the defining features?

Answer 1

Metaphase of the second meiotic division and is surrounded by zona pellucida and some granulosa cells

Question 2

How does the ovum move?

Answer 2

1) highly ciliated walls of the fallopian tube
2) during ovulation ovum is swept by fimbrae into oviduct
3) peristaltic waves move ovum to the ampulla

Question 3

How long after ovulation does the oocyte reach the uterus?

Answer 3

Approx. 80 hours

Question 4

What is the sperm journey and survival time, and some of its obstacles?

Answer 4

1) Journey: 15-18cm
2) Survival: 2-5 days
3) Obstacles: pH of vagina, immune response, cervical mucus, physical barriers

Question 5

What is fertilisation and where does it occur?

Answer 5

Fusion of male and female gametes to form a zygote, occurring in the fallopian tube within 24hrs after ovulation

Question 6

How many sperm are deposited into the female tract vs how many reach the cervix?

Answer 6

200-300 million reach the genital tract but only 1% reach the cervix

Question 7

How long do spermatozoa retain their function?

Answer 7

On average 72hrs

Question 8

What is capacitation and how long does it last for?

Answer 8

Process of head/cap removal of sperm (glycoprotein coat and seminal plasma proteins) which lasts up to 7hrs.

Question 9

Why is capacitation important?

Answer 9

Only capacitated sperm can pass through corona cells and undergo the acrosome reaction

Question 10

What are the five stages of fertilisation?

Answer 10

1) Penetration of the corona radiata
2) Penetration of the zona pellucida
3) Cortical and zona reactions
4) Membrane fusion and resumption of second meoitic divisions
5) Formation of pronuclei

Question 11

What happens during the penetration of the corona radiata

Answer 11

1) Of the 300-500 sperm reaching the fertilisation site only one fertilises the egg
2) Capacitated sperm pass freely through the corona ells through flagellar action and enzyme release form acrosome

Question 12

What occurs during zona pellucida penetration and what stage is this in fertilisation

Answer 12

1) Stage 2

2) Sperm perforate acrosome and then ZP glycoprotein shell to reach cytoplasm mediated by ligand ZP3 (a zona protein)

Question 13

What are the cortical and zona reactions of fertilisation

Answer 13

Cortical reaction is the release of cortical vesicles from oocyte plasma membranes when plasma membrane of sperm binds to egg membrane. Hydrolytic enzymes released forming fertilisation membrane (ZP1,ZP2,ZP3 of ZP altered). ZP2 cleaved ZP3 modified.

Question 14

How is polyspermy prevented?

Answer 14

Fertilisation membrane after cortical reaction prevents poly/di/trispermy although if this occurs the fetus nearly always aborts

Question 15

What stage is fusion in fertilisation and what occurs at this stage?

Answer 15

1) Stage 4
2) Head and tail of sperm enter cytoplasm of oocyte. plasma membrane left behind. Secondary oocyte arrested in 2nd meiotic division from puberty finishes division immediately. Daughter (n) produced called second polar body and receives little to no cytoplasm. Other daughter is definite oocyte (2n). Its chromosomes 22+X arrange in vesicular nucleus called female pronucleus.

Question 16

What happens during male and female pronuclei formation and when does this occur?

Answer 16

1) Stage 5 of fertilisation
2) Pronuclei have 23 chromosomes each and indistinguishable. Membranes of each breakdown.
3) Chromosomes arrange for meiotic first cleavage division.
4) Fertilisation complete. Zygote formed.

Question 17

What are the results of fertilisation?

Answer 17

1) Restoration of diploid number of chromosomes
2) Determination of sex of newborn (XX female, XY male)
3) Initiation of cleavage (no fertilisation causes oocyte degeneration 24hrs after fertilisation)

Question 18

What are a few common contraceptive methods?

Answer 18

Pill (25%), Condom (25%), Vasectomy (11%)

Question 19

What happens durning cleavage in fertilisation?

Answer 19

Series of mitotic divisions from zygote to blastomeres.

Zygote forms 2 then 4 then 8 cell embryo, undergo compaction to form morula (16 cell after 4 divisions)

Question 20

What happens in blastocyst formation?

Answer 20

1) 3rd/4th day after fertilisation cavity appears and blastocyst forms
2) Embryoblast (inner cell mass) formed during compaction develops to embryo proper found at one pole of blastocyst
3) Outer cell mass surrounding inner cells and cavity forms throphoblast
NB. Outer cavity called pellucid zone

Question 21

What is the origin of embryonic stem cells?

Answer 21

Pre-implantation embryo - cells can self renew and have pleuripotentcy and can form ecdoderm, endoderm and mesoderm germ layers. IVF forms totipotent cells then blastocyst, then cultured pluripotent stem cells grown.

Question 22

What phase is the uterus in at the time of implantation?

Answer 22

Day 20-23 - secretory phase
Blastocyst implants in endometrium
If no fertilisation menstrual phase begins and endometrial layers are shed with basal layer remaining for regeneration during next cycle

Question 23

What is the process of implantation?

Answer 23

1) Sperm penetrates oocyte in fallopian tube (day 1). 2 cell stage (day 1). 4 cell stage (day 2). 8 cell stage (day 3). 16-32 cell morula stage (day 4). Free bastocyst (day 6) in uterine cavity
2) Cytotrophoblast of blastocyst attaches to epithelium of uterine mucosa. Hypoblast and inner epiblast visible. Syncytiotrophoblast visable on wall.
3) Moves inwards. Hypoblast expands and epiblast forms cavity. Syncytiotrophoblast enlarges.
4) ST spreads outwards. Maternal blood capillaries visable. Hypoblast spreads and amnioitc cavity forms in epiblast.
5) Amnioblasts visable. Fibrin plug forms on uterine wall as ST envelops blastocyst. Trophoblast lacunae form. Hypoblast beings to multiply.
6) Hypoblast grows ventrally and eroded materal capillaries visable.
7) Extraembryonic reticulum forms. Heusers membrane forms over this. Lacunae filled with blood visable.

Question 24

What is the process of IVF and embryo transfer

Answer 24

1) Ovulation induction medication produces follicles
2) Egg retrieval via needle guided ultrasound
3) Insemination and fertilisation in petri dish
4) Embryo transfer via catheter into uterus
5) Luteal phase and given progesterone
6) Embryo freezing (cryopreservation) of those not used

Question 25

What is meant by the concept 3 parent babies?

Answer 25

Mitochondrial disorders due to faulty mitochondria DNA in mother. Donor egg has genes removed and contains healthy DNA, mothers DNA inserted, fathers DNA fertilises egg. No mitochondrial disorders.