(8) EMRYOLOGY SESSION 1 - First two weeks

Question 1

Define pre-embryonic

Answer 1

A fertilized ovum up to 14 days old, before it becomes implanted in the uterus

Question 2

Embryonic

Answer 2

• The fertilized egg of a vertebrate animal following cleavage before definite form arises. 3-8 weeks.

Question 3

Fetal

Answer 3

In humans the fetal period extends from the first day of the ninth week of intrauterine life until birth. 9-38 weeks.

Question 4

Discuss the changes taking place as the fertilised human ovum (zygote) travels down the Fallopian (uterine) tube to the uterus

Answer 4

• Oocyte is released from the ovary
• Travels along the Fallopian (uterine) tube
• Is fertilized by sperm in the ampulla
• Fertilised oocyte is called the zygote
• Ideal site for implantation is the posterior uterine wall

Question 5

Zygote

Answer 5

A fertilised ovum/oocyte

Question 6

Cleavage

Answer 6

The dividing of cells into two masses known as blastomeres. First cleavage happens ~30 hours after fertilisation (resting period)

Question 7

Zona Pellucida

Answer 7

The glycoprotein ‘shell’ to prevent polyspermy

Question 8

Morula

Answer 8

The result of cleavage of the fertilised oocyte, each cell is Totipotent

Question 9

Ovary

Answer 9

The female reproductive organ producing oocytes/eggs for fertilisation

Question 10

Fallopian tube

Answer 10

A tube the oocyte travels along to reach the uterus. The egg is fertilised at the ampulla of the fallopian tube.

Question 11

Uterus

Answer 11

Female reproductive organ where implantation of the zygote and growth of the embryo occurs

Question 12

Blastocyst

Answer 12

Formed from the compaction of cells that make up the morula

Question 13

Trophoblast

Answer 13

(Outer cell mass) will later form support structures for the embryo (placenta)

Question 14

Embryoblast

Answer 14

(Inner cell mass) will later become the bilaminar disk

Question 15

Implantation

Answer 15

The attachment of the Blastocyst to the wall of the uterus at day 6 – 7

Question 16

Cytotrophoblast

Answer 16

(Derivative of Trophoblast) Placental membrane around the yolk sac

Question 17

Syncytiotrophoblast

Answer 17

(Derivative of Trophoblast) Cells that invade maternal sinusoids (irregular blood vessels) resulting in uteroplacental circulation.

Question 18

What happens in week 1, within 30 hours of oocyte fertilisation?

Answer 18

Cleavage begins 30 hours after fertilisation
Results in two blastomeres of equal size
This forms a morula, made up of totipotent cells.

Zona pellucida forms, which prevents polyspermy.

Question 19

What happens at week 1, day 4?

Answer 19

Compaction

At eight cell stage, blastomeres making up morula compact to form the Blastocyst

Formation of first blastocyst cavity

Embryoblast (Inner cell mass)
Trophoblast (Outer cell mass)

Question 20

What changes occur in cells before and after compaction?

Answer 20

Before compaction, all cells totipotent. Afterwards, simply have multilineage potential.

Question 21

Outline an assisted reproduction technique `

Answer 21

Oocytes are fertilised in vitro and allowed to divide to the 4-8 cell stage.
Morula is then transferred into the uterus.

Question 22

What is PGD?

Answer 22

Pre-implantation genetic diagnosis

a single cell can be safely removed from the morula and tested for serious heritable condition prior to transfer of the embryo to the mother.

Question 23

What happens at day 5?

Answer 23

Hatching

• Blastocyst hatches from the zona pellucida
• No longer constrained and free to enlarge
• Can now interact with the uterine surface to implant

Question 24

What happens at days 6-7?

Answer 24

Implantation Begins (Day 6–7)

• The conceptus now has 107 cells.
• 8 will make the embryo
• 99 will begin the development of the foetal membrane

Question 25

Why is second week called the week of twos?

Answer 25

Differentiation
Embryoblast –> Epiblast, Hypoblast
Trophoblast –> Syncytiotrophoblast, Cytotrophoblast

The bilaminar disk and amniotic cavity are formed.

Question 26

Describe the process of implantation

Answer 26

Implantation is interstitial (invasive, surrounded and embedded). The uterine epithelium is breached and the conceptus implants within the uterine stroma.

Question 27

What problems can occur as a results of an implantation defect? (Implantation at inappropriate site)

Answer 27

Ectopic Pregnancy – Implantation at site other than uterine body (most commonly fallopian tube). Can quickly become a life-threatening emergency.

Placenta Praevia – Implantation in lower uterine segment. Placenta grows across cervical opening. Requires C-section delivery.

Question 28

Describe Week 2, Day 9 – 10

Answer 28

Embryonic pole (top bit)
Rapid development of Syncytiotrophoblast
Abembryonic pole (bottom bit)
Primitive yolk sac formed by Heuser’s membrane spreading across the Blastocyst cavity. Yolk sac membrane is in contact with the cytotrophoblast layer.

Question 29

Week 2, Day 11

Answer 29

Primitive yolk sac membrane is pushed away from the cytotrophoblast layer by an acellular extraembryonic reticulum.
Reticulum is later converted to extraembryonic mesoderm by cell migration

Question 30

Week 2, Day 12

Answer 30

Maternal sinusoids invaded by syncytiotrophoblast.
Lacunae (blood lakes) in trophoplast become continuous with sinusoids.
Uteroplacental circulation begins.
Uterine stroma prepares for support of the embryo.

Question 31

Week 2, Day 13

Answer 31

Formation of secondary yolk sac
Pinches off from the primary yolk sac
A.k.a. Definitive yolk sac

Question 32

Week 2, Day 14

Answer 32

Spaces within the extraembryonic mesoderm merge to form the chorionic cavity
The Connecting Stalk suspends the embryo and its cavities in the chorionic cavity.
The connecting stalk is a column of mesoderm, and is the future umbilical cord.
Bleeding around now can be confused with menstrual bleeding.

Question 33

What has happened at the the end of the second week?

Answer 33

• The conceptus has implanted
• The embryo and its two cavities (Amniotic cavity and Yolk sac) are
• Suspended by a connecting stalk in a
• Supporting Sac (Chorionic Cavity)

Question 34

Give some early pregnancy loss statistics

Answer 34

Åpproximately 50% of all zygotes are lost in the first 2-3 weeks
15% of all diagnosed pregnancies will miscarry
1% women suffer from recurrent miscarriage (miscarriage in 3 consecutive pregnancies)