First 18 days of development

Question 1

Where does fertilisation occur and when must this happen?

Answer 1

In the ampulla of the fallopian tube

Must occur within 24 hours of ovulation

Question 2

What is an ectopic pregnancy?

Answer 2

A medical emergency -

Fertilised ovum (zygote) embeds in peritoneum, ovary or fallopian tube (or cervix)

Question 3

What is fertilisation?

Answer 3

The combination of nuclei of sperm and ovum ~ 24hours with both sets of haploid chromosomes forming a set of diploid chromosomes.

This combined structure becomes known as a Zygote

Question 4

What is a blastomere?

Answer 4

A zygote begins to split at day 2 by mitosis

• forms 2, then 4, then 8 cells
• these individual cells are know as blastomeres

note: zygote does not grow in size even though number of cells increase

Question 5

What is a morula?

Answer 5

A ball of 16 cells with distinct layers and ability to communicate?

Question 6

What days does the morula form?

Answer 6

In days 3-4

Question 7

What are the three distinct layers of the morula?

Answer 7

Embryoblast

Trophoblast

Zona pelllucida

Question 8

What is the embryoblast?

Answer 8

Inner core of blastomeres which will form embryo

Question 9

What is the trophoblast?

Answer 9

Outer layer of blastomeres which will form supporting structures, eg. placenta

Question 10

What is the Zona pelllucida?

Answer 10

Surrounding layer of glycoprotein

Prevents further sperm entering

Question 11

What days does the blastocyst form?

Answer 11

~ 4 days after fertilisation

Question 12

What is a blastocyst?

Answer 12

Formed from a morula after the following steps:

• has passed into the uterus
• trophoblast cells pull luminal fluid from the uterine cavity into the morula centre
  [] forming blastocoele - blastocyst cavity
• Embryoblast cells are pushed to one end of the cavity
  [] becomes the embryonic pole

Question 13

By what day does the blastocyst enter the uterus?

Answer 13

By day 5

Question 14

What day does implantation occur?

Answer 14

Day 6 (day 20 of menstrual cycle)

Question 15

Where does implantation occur?

Answer 15

In thickened endometrium (decidua)

- contains lots of glands and blood vessels

Question 16

How does the blastocyst implant?

Answer 16

Sticks to endometrial epithelium at embryoblast end

Reactions to implantation:

• decidual reaction
• synctiotrophoblast formation
• embryoblast differentiation

Question 17

What is the decidual reaction?

Answer 17

Epithelium reacts to implantation and allows blastocyst to begin to move into endometrium

Question 18

What is synctiotrophoblast formation?

Answer 18

Trophoblastic cells begin to merge to form a multinuclear synctitium

Question 19

What is embryoblast differentiation?

Answer 19

Embryoblast begins to differentiate into two layers

• the Epiblast
• the Hypoblast

Question 20

What day does the amniotic cavity form?

Answer 20

~ day 8

Question 21

How is the amniotic cavity formed?

Answer 21

Epiblastic cells split after the blastocyst has moved further into the endometrium

• these cells become known as amnioblasts
  = secrete amniotic fluid
• the synctiotrophoblasts continue to grow

Question 22

What day does the primary yolk sac form?

Answer 22

Day 9 - 11

Question 23

What is the primary yolk sac?

Answer 23

The term for the blastocoele after an extracoloemic membrane has formed around it

It provides nutrients to the embryo until the placenta forms

Question 24

Which structure forms between extracoelomic membrane and trophoblastic cells?

Answer 24

Extraembryonic membrane

Question 25

Which days does the chorionic cavity form?

Answer 25

Days 12 - 13

Question 26

How is the chorionic cavity formed?

Answer 26

When the blastocyst becomes completely enveloped in endometrium
- the endothelium closes up over the blastocyst

Question 27

What forms the maternal/foetal blood interface and how?

Answer 27

The synctiotrophoblast forms lacunae which join up, erode into blood vessels and fill with blood

Question 28

What maintains the corpus luteum (maintains endometrium) which is also used to detect pregnancy?

Answer 28

hCG

Question 29

What days does gastrulation take place?

Answer 29

Days 13 - 17

Question 30

What is gastrulation?

Answer 30

Formation of the primitive streak and trilaminar disc, which results in three layers:

• ectoderm
• mesoderm
• endoderm

Question 31

What is the primitive streak?

Answer 31

the earliest trace of the embryo in the fertilised ovum

Question 32

What is the notochord and how is it formed?

Answer 32

Rod down the centre of the mesoderm

Formed from the epiblastic cells which invaginate the primitive pit (surround primitive node) move cephalically and become associated with the hypoblast

Question 33

What is the mesoderm?

Answer 33

Forms from invaginating epiblastic cells

• forms the muscles, connective tissue, CVS, bone and marrow, and reproductive organs

Question 34

What is the ectoderm?

Answer 34

Formed from the epiblast

- froms the epidermis, nervous system and retina

Question 35

What is the endoderm?

Answer 35

Forms the respiratory and GI epithelium, glandular cells of GI organs (e.g. liver, pancreas)