BL S9 Embryology

Question 1

What is embryology?

Answer 1

The study of the progression from a single cell to a baby in 9 months.
Including the molecular, cellular and structural factors involved.

Question 2

What is ovulation?

What happens to the endometrium in prep for ovulation?

What is the role of fimbrae?

What moves the egg along the Fallopian tube?

Where does fertilisation occur specifically?

If the egg isn’t fertilised what happens to it?

What occurs 2 week after ovulation?

Answer 2

The discharge of an egg from the ovary.

Thickens

Move egg to neighbouring Fallopian tube.

Contractions in tube wall and ciliates epithelium of tube.

Ampulla

Absorbed by the body

Uterus lining shed through vagina - menstruation.

Question 3

Describe the various selective pressures on sperm in its journey through to fertilise the egg.

5 points.

Answer 3

1. Some flow out of vagina
2. May become trapped in cervix mucous and folds.
3. Uterus - resident immune cells - destroy sperm
4. Half go down wrong Fallopian tube
5. Sperm act against motion of cilia.

Question 4

In the uterus what moves the sperm towards the egg?

Answer 4

Uterine contractions.

Question 5

Where does capacitation occur and what 2 things does it involve?

Answer 5

Uterus

Destabilisation of the acrosomal sperm head allowing it penetrate the outer later of the egg.

Changed to tail that allow for increased mobility - hyperactive.

Question 6

What is the outermost layer of the egg?

Answer 6

Corona radiata

Question 7

Once sperm reach zona pellucida what do they do?

What is the perivitelline space and what is its significance?

Answer 7

Bind to specialised sperm receptors which causes digestive enzymes to be released.

Fluid filled matrix between zona pellucida and egg cell membrane.

First sperm to make contact will fertilise the egg.

Question 8

What measures are in place to stop polyspermy?

Answer 8

Egg cell membrane changes preventing other sperm from attaching.
Zona pellucida hardens trapping sperm and they can no longer attach to it.
Egg releases chemical push away sperm

Question 9

How long must sperm be introduced into the female reproductive tract prior to ovulation?

How long are sperm viable?

How long is secondary oocyte viable?

Answer 9

5 days

5days

12-24 hours

Question 10

What are sperm not immunologically rejected by the body?

Answer 10

Contact between sperm and blood must be made to trigger immune response.

Sertoli cells form extensive tight junctions between them to avoid contact between blood and sperm cells.

Question 11

Define

Embryonic Age

Gestational age

Germinal Stage

Embryonic period

Foetal period

Answer 11

Time since fertilisation

Time since last menstruation - embryonic age + 2 weeks.

Fertilisation to end of second week.

Start of 3rd to end of 8th week.

Start of ninth to birth at 28 weeks.

Question 12

Overall what occurs in week 1?

Answer 12

Oocyte fertilised by sperm in ampullary region and implant in the superior posterior uterine wall.

Question 13

Overall what occur in week 2?

Answer 13

Trophoblast differentiates into

• Syncitiotrophoblast
• Cytotrophoblast

Embryoblast differentiates into bilaminar disc:

• Epiblast
• Hypoblast

Question 14

Overall what occurs in week 3?

Answer 14

Primitive steak forms
Gastrulation occurs
Neuralation

Question 15

Overall what occurs in week four?

Answer 15

Somitogenesis - organisation of the paraxial mesoderm

Folding

Question 16

Describe briefly the following week 1 processes.

Cleavage

Compaction

Answer 16

Cleavage - First mitotic division forms 2 blastomeres - continues - morula forms - 16 blastomeres

Compaction - differentiation to 2 cell types
- embryo blast
- trophoblast
Both pluripotent and blastocyst cavity forms also.

Question 17

Describe the following week 1 processes:

Hatching

Implantation

Answer 17

Polyspermy threat declined - blastocyst hatches out of zona pellucida to allow for growth and implantation

Conceptus (8 embryonic cells + 99 trophoblast cells) penetrates through uterine mucosa simple columnar epithelium and implant in the uterine stroma

Question 18

After implantation has occurred, what closes the gap that was opened by the conceptus?

How is access of the maternal vasculature allowed for?

What forms the primitive yolk sac?

What pushes away the primitive yolk sac from the cytotrophoblast?

How is the secondary yolk sac formed?

What forms the chorionic cavity?

The secondary yolk sac forms the connecting stalk and what else?

What is the role of the connecting stalk?

Answer 18

Fibrin clot

Lacunae of syncitiotrophoblast becomes continuous with maternal sinusoids.
Uteroplacentalk circulation begins.

Hypoblast and exocoelomic membrane form the primitive yolk sac.

Extraembryonic mesoderm

Primary yolk sac pinches off.

Spaces within the extraembryonic mesoderm.

Definitive yolk sac

Joins bilaminar disc to the cytotrophoblast

Question 19

Describe the location of the primitive streak and primitive node.

Answer 19

Primitive streak forms on the dorsal surface of the epiblast.

Primitive node forms at cephalic end.

Question 20

Describe the process of gastrulation.

What is structurally distinct about one of the layers formed?

Answer 20

Epiblast cells migrate to the primitive streak and invaginate.

Hypoblast displaced and 3 new layers are formed.

• Ectoderm
• Mesoderm
• Endoderm

Mesoderm has gaps for mouth and anus.

Question 21

How is the notochord formed?

What is the role of the notochord.

Answer 21

Action of ciliated cells at the primitive node during gastrulation cause former epiblast cells to migrate to cranial end of primitive steak and form the notochord.

Drives neuralation.

Question 22

In week 4 segmentation occurs, describe this process.

What differentiates first?

What is derived from each tome etc.

Answer 22

Mesoderm - arranged into discrete zones and areas.

Paraxial mesoderm segments into 31 pairs of somites.

Somites differentiate to form

• Myotomes - skeletal muscle derived from this
• Dermatomes - area of skin innervated by 1 spinal nerve
• Sclerotomes - forms bones.

Sclerotomes differentiate first leaving dermomyotomes.

• myotome proliferates
• dermatome disperses

Question 23

What does the organisation of paraxial mesoderm into somites give rise to?

Answer 23

Repeating structures:

• Ribs
• Vertebrae
• Intercostal muscle
• Spinal cord segments

Question 24

What does folding in week 4 achieve?

Answer 24

Creates dorsal, ventral distinction 
Creates a ventral body wall
Embryo suspended in amniotic sac
Creates the interembryonic coelom 
Pull connecting stalk ventrally

Question 25

What does cephalocaudal folding achieve?

What does lateral folding achieve?

Answer 25

Heart moves away from cranial end towards the future chest.

Creates primordium of the gut - intraembryonic coelom

Question 26

Describe the process of neuralation.

Answer 26

Notochord - diffusion limited signalling molecules
Signal to above ectoderm cells to differentiate into neuroectoderm cells.
Causing thickening of ectoderm in this region
- neural plate forms
Edges of neural plate rise and curl - neural tube forms.

Question 27

What can be said about the structure of the neural tube?

Answer 27

Wider at cephalic end compared to caudal end.

Question 28

What is situs inversus and what is its cause?

Answer 28

Complete reverse mirror image viscera

Occurs as result of immobile cilia in primitive node.

Question 29

What it derived from the ectoderm?

Answer 29

Organs and structures that maintain contact with the exterior.
Epidermis
Nervous system

Question 30

What derives from the mesoderm?

Answer 30

Muscle
Cartilage
Bone
Vasculature

Question 31

What derives from the endoderm?

Answer 31

Epithelium of GI and Respiratory tract
Panachyma of gland
Internal structures

Question 32

What is teratogenesis?

Answer 32

The process by which congenital malformations are produced in a foetus.