Embryology

Question 1

When is the pre-embryonic stage?

Answer 1

First two weeks of development

Question 2

When is the embryonic period?

Answer 2

3rd to 8th weeks of development

Question 3

When is the fetal period?

Answer 3

From the 9th week until birth

Question 4

Oocyte travels from the ____ to the junction with the uterus (_____)

Answer 4

Oocyte travels from the ampulla, along the Fallopian tube, to the junction of the uterus (isthmus)

Question 5

What happens during cleavage?

Answer 5

Cell division, creating two blastomeres initially then continued mitotic division leading to the formation of a morula, ~16 cells

Question 6

How is a blastocyst formed?

Answer 6

Spaces between cells in the morula form one large space by compaction, (blastocoele or blastocyst cavity) creating a hollow sphere of cells

Question 7

What is hatching?

Answer 7

Happens on day 5 of development, the blastocyst hatches from the zona pellucida, allowing it to enlarge and come into contact with the uterine surface for implantation

Question 8

How do the cells change before and after compaction?

Answer 8

Before they are totipotent

After, they are pluripotent

Question 9

What is the outer mass of cells?

Answer 9

The trophoblast, made up of two different types of cells:

• cytiotrophoblast
• syncytiotrophoblast

Trophoblast is a major component of fetal membranes, especially the placenta

Question 10

What is the inner cell mass?

Answer 10

The embryoblast, becomes the embryo itself
Made up of:
-epiblast
-hypoblast

Epiblast and hypoblast make up the bilaminar disk

Question 11

Which cavity is formed from spaces within the epiblast?

Answer 11

The amniotic cavity

Question 12

What happens during implantation?

Answer 12

The uterine epithelial is breached

The conceptus implants within the uterine stroma

Maternal blood flow is established within the placenta, allowing support of the embryo from maternal circulation

The basic structural unit of materno-fetal exchange is established

Question 13

What conditions are linked to implantation defects?

Answer 13

IUGR
Pre-eclampsia
Ectopic pregnancy
Placenta praevia

Question 14

Ectopic pregnancy

Answer 14

Implantation at site other than uterine body (most commonly in the Fallopian tube)

Can be peritoneal or ovarian

Question 15

Placenta praevia

Answer 15

Implantation in the lower uterine segment

Can cause haemorrhage in pregnancy

Requires C-section delivery

Question 16

Which sac is formed from the blastocyst cavity, and how?

Answer 16

Primitive yolk sac

Formed from hypoblast endoderm and extraembryonic mesoderm

Question 17

Formation of extraembryonic mesoderm

Answer 17

Primitive yolk sac membrane is pushed away from cytotrophoblast layer by an acellular extraembryonic reticulum

Reticulum is later converted to extraembryonic mesoderm by cell migration

Question 18

How does uteroplacental circulation begin?

Answer 18

Maternal sinusoids invade synctiotrophoblast

Lacunae become continuous with sinusoids

Uteroplacental circulation begins

Question 19

Secondary yolk sac

Answer 19

secondary yolk sac forms and pinches off from primitive yolk sac

Question 20

Formation of the chronic cavity

Answer 20

Spaces within the extraembryonic mesoderm merge to form the chronic cavity

The embryo and its cavities are suspended by the connecting stalk, a column of mesoderm which is the future umbilical cord

Question 21

Order of cavities, coelom, sacs and spaces

Answer 21

Blastocoele
-1st cavity, as a result of compaction

Amniotic sac
-formed from spaces within epiblast

Primitive yolk sac
-formed by hypoblast lining the blastocoele

Secondary yolk sac
-formed within primitive yolk sac

Extraembryonic coelom

• chorionic cavity
• formed from spaces within extraembryonic reticulum and mesoderm

Question 22

Keys events of early development

Answer 22

Fertilisation and implantation

Gastrulation

Neurulation

Segmentation

Which all lead to

Folding

Question 23

Gastrulation:
When
What
How
Why

Answer 23

When
-3rd week of development, start of embryonic period

What

• bilaminar disk converted to trilaminar disk
• 3 germ layers (ectoderm, mesoderm and endoderm)
• formation of notochord

How

• primitive streak forms in the caudal epiblast
• leads to migration and invagination of epiblast cells into streak
• displacement of hypoblast and creation of 3rd germ layer

Why
-ensure correct placement of precursor tissues to allow subsequent morphogenetic to take place

Question 24

Derivatives of ectoderm, mesoderm and endoderm

Answer 24

Ectoderm

• organs & structures that maintain contact with outside world
• epidermis, CNS, peripheral nervous system

Mesoderm

• supporting tissues
• muscle, cartillage, bone, vascular system including heart and vessels

Endoderm

• internal structures
• epithelial lining of GI tract, respiratory tract, parenchyma of glands

Question 25

Situs Inversus

Answer 25

• complete mirror image
• commonly results from immotile cilia
• no associated morbidity
• problems arise if both normal and mirror-image disposition

Question 26

What drives left-right asymmetry

Answer 26

At primitive node, action of ciliates cells results in left-ward flow of signalling molecules

Ciliated cells beat and push cellular signalling molecules to left side

Differentiating left from right

Lack of action from cilia means down to chance which way signals go which can lead to situs invertus

Initiates side-specific signalling cascade

Question 27

What is the notochord?

Answer 27

Solid rod of cells running in the midline with important signalling role

Directs conversion of overlying ectoderm to neuroectoderm

Notochord signals lead to formation of neural tube

Question 28

What is neurulation

Answer 28

Creation of neural tube

Notochord signals cause overlying ectoderm to thicken, creating a slipper-shaped neural plate

Edges elevate out of the plane of the disk and curl towards each other, forming the neural tube

Question 29

Zones of mesoderm

Answer 29

Paraxial mesoderm

Intermediate mesoderm

Somatic mesoderm

Splanchnic mesoderm

Intraembryonic coelom created between somatic and splanchnic mesoderm

Question 30

Segmentation

Answer 30

Organisation of paraxial mesoderm into segments (somites) gives rise to repeating structures

• vertebrae
• ribs
• intercostal muscles
• spinal cord segments

Spinal cord segments line up with somites, so nerves emerging from spinal cord segments will supply somites and all derivatives

1st pair appear at day 20 in occipital region, more appear in craniocaudal sequence, 3 pairs a day until 42 - 44 pairs are present by end of week 5

Some disappear, leaving 31 pairs of somites

Question 31

Somite derivatives

Answer 31

Dermatome
-dermis

Myotome
-muscles

Sclerotium

• syndetome (tendon)
• arthrotome (vertebral joints)
• endotome (endothelial cells, dorsal aorta)
• vertebrae and ribs

Question 32

What is the buccopharyngeal membrane?

Answer 32

Site where going to create opening to oral cavity, site of future mouth

Question 33

What causes embryonic folding?

Answer 33

growth and development of neural tube pushing down on caudal and cranial end and somites putting pressure laterally.

Question 34

What is cephalocaudal folding

Answer 34

Head fold and tail fold

Question 35

What does embryonic folding entail?

Answer 35

Cephalocaudal folding

Lateral folding

Question 36

Results of folding

Answer 36

Draws together the margins of the disk

• creating a ventral body wall
• pulling amniotic membrane around the disk, embryo becomes suspended within amniotic sac
• pulling connecting stalk centrally

Creates the primordial of the gut, puts heart and primordial of the diaphragm in the right place

Creates a new cavity within the embryo

Question 37

At the end of the 4th week…

Answer 37

The nervous system has started to form

Segments have appeared, assigning specific tasks to specific cells

Embryo has folded, putting everything in the right place