Embryology

Question 0

What happens between fertilisation and compaction?

Answer 0

Cleavage first occurs to give 2 blastomeres surrounded by a zona pellucida. Further mitotic divisions then occur to give a morula.

Question 1

What occurs at day 0?

Answer 1

Fertilisation of the oocyte by sperm in the ampulla of the Fallopian tube to give a zygote.

Question 2

What happens during compaction and at around what day?

Answer 2

Day 4.
The cells differentiate and become pluripotent to give:
- inner cell mass
- trophoblast
- blastocyst cavity

Question 3

What occurs shortly after compaction? (~ day 5)

Answer 3

Hatching of the blastocyst from the zona pellucida = free to grow and interact with uterine surface.

Question 4

How many cells does the embryo have when implantation begins on day 6-7?

Answer 4

107

Question 5

What occurs on day 8?

Answer 5

Differentiation:

• trophoblast develops into cytotrophoblast and syncytiotrophoblast
• inner cell mass develops into epiblast and hypoblast
• formation of amniotic cavity.

Question 6

When and how is implantation completed?

Answer 6

~ day 9-10

Syncytiotrophoblast secretes enzymes which allow penetration of uterine wall.
This is invasive so a fibrin plug develops to block entry site.

Question 7

What does implantation achieve?

Answer 7

Support of the embryo is now haemotrophic and the embryonic tissue is interstitial with the maternal tissue.

Question 8

What are the 2 common implantation defects?

Answer 8

1) . Ectopic pregnancy - where the embryo implants anywhere other than the posterior uterine wall.
2) . Placenta praevia - where they embryo implants in the lower uterine segment. Due to chance of haemorrhage (due to placentas rich blood supply) C sections must be performed.

Question 9

What development occurs on day 9?

Answer 9

Rapid mitotic division of cells in the cytotrophoblast which then migrate to the syncytiotrophoblast and fuse to those cells.

At the abembryonic pole, the hypoblast cells form a membrane that lines the cytotrophoblast = primary yolk sac

Question 10

What forms the extraembryonic reticulum and when?

Answer 10

Around day 11, the yolk sac is pushed away from the cytotrophoblast by extra embryonic reticulum, which will later form the extra embryonic mesoderm.
It then grows to surround the hypoblast and yolk sac.

Question 11

What occurs between the formation of the extraembryonic reticulum at around day 11 and the differentiation of extraembryonic mesoderm around day 14?

Answer 11

The maternal sinusoids invade and become continuous with lacunae = uteroplacental circulation begins.
Secondary yolk sac also pinches off from the primitive.

Question 12

How does extraembryonic mesoderm differentiate and when?

Answer 12

Day 14
Extraembryonic cavities formed by extraembryonic reticulum merge to form chorionic cavity.
Some of the mesoderm differentiates into a column to form the connecting stalk

Question 13

Where and when does the primitive streak appear?

Answer 13

At the start of the 3rd week.

On the dorsal surface of the epiblast.

Question 14

In what direction does the primitive streak appear?

Answer 14

Craniocaudal direction.

Question 15

What is occurring in the primitive streak?

Answer 15

1). Epiblast cells migrate upwards towards the streak, flatten then invaginate between the epiblast and hypoblast. This displaces the hypoblast and creates a third layer = trilaminar disc. The cells continue to spread laterally and cephalad.

Question 16

What does the fate of the epiblast depend on?

Answer 16

Where in the primitive steak they invaginate.

Question 17

How does the primitive streak set the axes of the embryo?

Answer 17

Cillia on the primitive streak waft molecular signals from the primitive node, which ensures correct axial development. This initiates side-specific signalling cascades = right or lefthandedness decided.

Question 18

Where do prenotochordal cells originate?

Answer 18

From the epiblast cells that migrate through the cranial part of the primitive pit.

Question 19

What is the notochord and its purpose?

Answer 19

It is a solid rod of cells that form down the midline of the trilaminar disc.

• defines the axis of the embryo
• drives neurulation

Question 20

What is left behind after the notochord has regressed?

Answer 20

The nucleus pulposus if the intervertebral discs.

Question 21

What occurs during neurulation?

Answer 21

The ectoderm above the notochord begins tho thicken in response to signals the notochord releases. This creates a neural plate, in which the edges curl up until they meet, to form the neural tube.

Question 22

What does mesoderm differentiate and become organised into?

Answer 22

• paraxial mesoderm
• lateral plate
  
  • -> splanchnic mesoderm
  • -> somatic mesoderm
• intermediate mesoderm
• intraembryonic coelom

Question 23

What does the intraembryonic coelom provide?

Answer 23

Space for the body systems to grow.

Question 24

What does splanchnic mesoderm give?

Answer 24

Smooth musculature, vasculature and CT of gut.

Question 25

What does somatic mesoderm give?

Answer 25

The CT of limbs and contributes to the anterior-lateral body wall.

Question 26

How is paraxial mesoderm organised and at what rate?

Answer 26

Organised into 31 pairs of somites at a rate of around 3 pairs per day in the craniocaudal direction.
Appear as a block of regular mesoderm cells surrounding a cavity.

Question 27

What does paraxial mesoderm somites go on to form?

Answer 27

Axial skeleton, dermis and muscles of the limb and ventral body wall.

Question 28

What does organisation of mesoderm into somites give us?

Answer 28

Gives rise to repeating structures (e.g. Ribs, vertebrae, intercostal muscles etc.)
Guides innervation.

Question 29

How is sclerotome produced and what is it?

Answer 29

Sclerotome comes from organised degeneration, where the ventral wall of the somite breaks down. It gives rise to bone/ hard tissue such as the vertebrae and ribs.

Question 30

How is dermamyotome produced and what is combined to form it?

Answer 30

Organisation the dorsal portion if the somites.

Dermatome and myotome.

Question 31

What is dermatome, developmentally and clinically and what does it do?

Answer 31

Developmentally- the part of the somite giving rise to the dermis

Clinically - a strip of skin, supplied by one single spinal nerve

It disperses.

Question 32

What is myotome, developmentally and clinically, and what does it do?

Answer 32

Developmentally - it is the part of the somite from which limb muscles arise

Clinically - A muscle/group of muscles supplied by a single spinal nerve

It proliferates and migrates.

Question 33

What drives cephalocaudal folding?

Answer 33

Somite development

Question 34

What does cephalocaudal folding achieve and what does this do?

Answer 34

It pulls the yolk sac up and the amniotic sac surrounds the embryo. This puts the heart and primordium of the diaphragm and gut in the correct place for development.

Also creates ventral body wall and pulls connecting stalk ventrally.

Question 35

What is situs invertus and what causes it?

Answer 35

Immobile cillia in the primitive streak means that molecular signals to set the axes aren’t correctly ‘wafted’ = a complete mirror image viscera results. There is no associated morbidity.

Question 36

How can embryo splitting occur late?

Answer 36

With duplication of the primitive streak.

Question 37

How can monozygotic twins occur and how can these become conjoined?

Answer 37

1) . Embryo splits after first cleavage = 2 blastomeres
- –> have their own placenta

2) . Inner cell mass of blastocyst splits into 2
- –> share a placenta
- –> if not fully split, conjoined twins may occur

Question 38

How are dizygotic twins formed?

Answer 38

Fertilisation of 2 oocytes at the same time

Question 39

When is an embryo prone to teratogenesis?

Answer 39

During weeks 3-8 with each organ system having its own window in development to where it is particularly prone.

Question 40

What is teratogenesis?

Answer 40

The process in which normal embryological development is disrupted.

Question 41

What are some examples of teratogenic agents?

Answer 41

Alcohol, thalidomide (and other anticonvulsant drugs), warfarin, thalidomide and rubella.