S4 + S6 Embryology

Question 1

How many weeks is the pre-embryonic period? And what happens during this period?

Answer 1

2 weeks long

Cleavage (formation of morula) and implantation begins

Question 2

What is the process when the ovum is travelling from the ovary to the uterus?

Answer 2

1. Oocyte released from ovary
2. Oocyte travels along Fallopian tube
3. Oocyte is fertilised by sperm in the ampulla (of the Fallopian tube)
4. Zygote travels to the uterus

Question 3

What is the name of a fertilised oocyte?

Answer 3

Zygote

Question 4

How long is an oocyte viable for?

Answer 4

1 day

Question 5

How long is a sperm viable for?

Answer 5

Up to 3 days

Question 6

What are the 6 steps that happen in week 1?

Answer 6

1. Cleavage
2. Zona pellucida
3. Morula
4. Compaction
5. Hatching
6. Implantation begins

Question 7

What happens during cleavage?

Answer 7

This is the first mitotic division which begins 30 hours after fertilisation. It results in 2 blastomeres of equal size

Question 8

What is a blastomere?

Answer 8

Each cell of the embryo formed after the first division to produce two cells until the embryo becomes a blastocyst

Question 9

What is the zona pellucida?

Answer 9

A glycoproteins shell that keeps sperm away from the blastocytes.

Question 10

What is the morula?

Answer 10

The stage when all cells are totipotent (capacity to become any cell type). Made up of 16 cells (blastomeres)

Question 11

What is the compaction stage?

Answer 11

The formation of the 1st cavity and the blastocyst.

After compaction the cells are pluripotent (capacity to become many cell types)

Question 12

What is a blastocyst?

Answer 12

A hollow sphere of cells formed from the morula through compaction - fluid is pumped in to create the blastocoele which results in inner and outer cell masses.

Question 13

What is the hatching stage?

Answer 13

The blastocyst hatches from the zona pellucida so it is now free to enlarge and interact with the uterine surface for implantation

Question 14

What happens as implantation begins?

Answer 14

There is now about 100 cells - 8 of these cells make the embryo, the rest (about 92) will contribute to development of feral membranes

Question 15

What two layers is the outer cell mass made out of?

Answer 15

1. Cytotrophoblast (single cell sheet)

2. Syncytiotrophoblast (multi-cell sheet)

Question 16

What two layers make up the initial inner cell mass (bilaminar disk)?

Answer 16

1. Epiblast

2. Hypoblast

Question 17

What happens during week 2?

Answer 17

1. Differentiation
2. Conceptus implanted
3. Cavity formation

Question 18

What are the two cavities in week 2?

Answer 18

1. Amniotic sac

2. Yolk sac

Question 19

What suspends the cavities?

Answer 19

The connecting stalk (in the chorionic cavity)

Question 20

What happens on day 11?

Answer 20

The primitive yolk sac membrane is pushed away from the cytotrophoblast layer by an acellular extraembryonic reticulum

Question 21

What is the acellular extraembryonic reticulum converted in to? And how does this happen?

Answer 21

Extraembryonic mesoderm

By cell migration

Question 22

What happens on day 9?

Answer 22

• at the embryonic pole, there’s rapid development of the synctiotrophoblast
• at the abembryonic pole, the primitive yolk sac is formed (the yolk sac membrane is in contact with the cytotrophoblast layer)

Question 23

What happens on day 12?

Answer 23

• maternal sinusoids invaded by syncytiotrophoblast - lacunae become continuous with sinusoids - this is the start of uteroplacental circulation
• the uterine stroma prepares for support of the embryo

Question 24

What is the uterine stroma?

Answer 24

Layer of connective tissue

Question 25

What does the syncytiotrophoblast contain?

Answer 25

Lacunae

Question 26

What happens on day 13?

Answer 26

Formation of the secondary yolk sac (the definitive yolk sac) - it pinches off from the primitive yolk sac

Question 27

What happens on day 14?

Answer 27

Spaces in the extraembryonic mesoderm merge to form the chorionic cavity

Question 28

What is the role of the outer cell mass?

Answer 28

Designed to produce the tissues supporting the embryo/foetus during development

Question 29

What is the role of the inner cell mass?

Answer 29

Cells inside the blastocyst go on to form the embryo proper

Question 30

What is another name for the inner cell mass?

Answer 30

Embryoblast

Question 31

What are the 5 stages in early development?

Answer 31

1. Fertilisation and implantation 2. Gastrulation 3. Neurulation 4. Segmentation 5. Folding

Question 32

What 4 things does the mesoderm differentiate into?

Answer 32

1. Notochord 2. Somites 3. Intermediate mesoderm 4. Lateral plate

Question 33

What is teratogenesis?

Answer 33

Major congenital malformation

Question 34

When is teratogenesis the biggest risk?

Answer 34

Weeks 3 to 8 (embryonic period) as it’s the period of biggest change

Question 35

When does the primitive streak (and primitive node) form?

Answer 35

In the 3rd week

Question 36

What is the primitive streak?

Answer 36

A thickened region of the epiblast a the caudal end of the bilaminar disc. It is the site from which gastrulation proceeds

Question 37

What happens during migration and invagination?

Answer 37

Epiblast cells migrate to the primitive streak and push through the tight junctions and displace the hypoblast layer, creating 3 layers

Question 38

What is the trilaminar disc made up of?

Answer 38

1. Ectoderm 2. Mesoderm 3. Endoderm All from the epiblast

Question 39

What does the ectoderm form?

Answer 39

* nervous system * epidermis Organs and structures that maintain contact with the outside world

Question 40

What does the mesoderm form?

Answer 40

* muscle * cartilage * bone * vascular system (incl. heart and vessels) Supporting tissues

Question 41

What does the endoderm form?

Answer 41

* epithelial lining of GI tract and respiratory tract * parenchyma of glands Internal structures

Question 42

What is situs inversus? How does it occur?

Answer 42

A congenital condition when the major internal organs are reversed/mirror-image. Occurs due to immotile cilia that develop during the embryonic period. Usually no problems unless the person has both normal and mirror-image disposition.

Question 43

What is responsible for establishing left and right asymmetry?

Answer 43

The primitive node - actions of ciliated cells at the node result in left-ward flow of signaling molecules which leads to a cascade of side-specific signals.

Question 44

What is the notochord and what is its role?

Answer 44

It is a solid rod of cells running in the midline. Has an important signalling role.

Question 45

When does gastrulation occur?

Answer 45

In the 3rd week (start of the embryonic period)

Question 46

What happens in gastrulation?

Answer 46

* bilaminar disc —> trilaminar disc | * formation of the 3 germ layers - ectoderm, mesoderm and endoderm

Question 47

How does gastrulation occur?

Answer 47

Due to the formation of the primitive streak in the caudal epiblast which leads to the migration and invagination of epiblast cells

Question 48

Why does gastrulation occur?

Answer 48

To ensure the correct placement of precursor tissues (left/right) to allow subsequent morphogeneis to occur

Question 49

What does the notochord direct conversion of?

Answer 49

Conversion of the ectoderm to neuroectoderrm

Question 50

What do notochord signals cause?

Answer 50

Causes the ectoderm to thicken and the edges to elevate out of the plane of the disc and curl towards each other - this creates the neural tube

Question 51

What areas does the mesoderm differentiate into?

Answer 51

1. Paraxial mesoderm 2. Intermediate mesoderm 3. Somatic mesoderm 4. Splanchnic mesoderm 5. Intraembryonic coelom

Question 52

What does the intraembryonic coelom (cavity) allow?

Answer 52

The cavity in which organs of the body can grow

Question 53

What does the splanchnic mesoderm differentiate into?

Answer 53

The viscera

Question 54

What does the somatic mesoderm differentiate into?

Answer 54

The body connective tissue

Question 55

What does the paraxial mesoderm form?

Answer 55

It forms somites

Question 56

How many somites form? When do they start forming?

Answer 56

31 (42-44 initially but some are lost) Day 20

Question 57

What do somites undergo to give rise to sclerotome, myotome and dermatome?

Answer 57

Organised degeneration

Question 58

What does the dermatome form?

Answer 58

Dermis

Question 59

What does the myotome form?

Answer 59

Muscles

Question 60

What does the sclerotome form?

Answer 60

Bones

Question 61

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

Answer 61

1. Vertebrae 2. Ribs 3. Intercostal muscles 4. Spinal cord segments

Question 62

What do somites guide?

Answer 62

Innervation

Question 63

What two types of folding does the trilaminar disc undergo?

Answer 63

Cephalocaudal folding and lateral folding

Question 64

Why does folding occur?

Answer 64

* It draws together the margins of the disc to pull the amniotic membrane around the disc * creates a ventral body wall * creates the primordium of the gut * puts primordium of organs in correct places * it creates a new cavity within the embryo

Question 65

What has happened by the end of the fourth week?

Answer 65

* the nervous system has started to form * segments have appeared - assigning specific tasks to specific cells * embryo has folded - puts everything in the right place