(8) EMRYOLOGY SESSION 1 - First two weeks Flashcards
Define pre-embryonic
A fertilized ovum up to 14 days old, before it becomes implanted in the uterus
Embryonic
- The fertilized egg of a vertebrate animal following cleavage before definite form arises. 3-8 weeks.
Fetal
In humans the fetal period extends from the first day of the ninth week of intrauterine life until birth. 9-38 weeks.
Discuss the changes taking place as the fertilised human ovum (zygote) travels down the Fallopian (uterine) tube to the uterus
- 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
Zygote
A fertilised ovum/oocyte
Cleavage
The dividing of cells into two masses known as blastomeres. First cleavage happens ~30 hours after fertilisation (resting period)
Zona Pellucida
The glycoprotein ‘shell’ to prevent polyspermy
Morula
The result of cleavage of the fertilised oocyte, each cell is Totipotent
Ovary
The female reproductive organ producing oocytes/eggs for fertilisation
Fallopian tube
A tube the oocyte travels along to reach the uterus. The egg is fertilised at the ampulla of the fallopian tube.
Uterus
Female reproductive organ where implantation of the zygote and growth of the embryo occurs
Blastocyst
Formed from the compaction of cells that make up the morula
Trophoblast
(Outer cell mass) will later form support structures for the embryo (placenta)
Embryoblast
(Inner cell mass) will later become the bilaminar disk
Implantation
The attachment of the Blastocyst to the wall of the uterus at day 6 – 7
Cytotrophoblast
(Derivative of Trophoblast) Placental membrane around the yolk sac
Syncytiotrophoblast
(Derivative of Trophoblast) Cells that invade maternal sinusoids (irregular blood vessels) resulting in uteroplacental circulation.
What happens in week 1, within 30 hours of oocyte fertilisation?
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.
What happens at week 1, day 4?
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)
What changes occur in cells before and after compaction?
Before compaction, all cells totipotent. Afterwards, simply have multilineage potential.
Outline an assisted reproduction technique `
Oocytes are fertilised in vitro and allowed to divide to the 4-8 cell stage.
Morula is then transferred into the uterus.
What is PGD?
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.
What happens at day 5?
Hatching
- Blastocyst hatches from the zona pellucida
- No longer constrained and free to enlarge
- Can now interact with the uterine surface to implant
What happens at days 6-7?
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
Why is second week called the week of twos?
Differentiation
Embryoblast –> Epiblast, Hypoblast
Trophoblast –> Syncytiotrophoblast, Cytotrophoblast
The bilaminar disk and amniotic cavity are formed.
Describe the process of implantation
Implantation is interstitial (invasive, surrounded and embedded). The uterine epithelium is breached and the conceptus implants within the uterine stroma.
What problems can occur as a results of an implantation defect? (Implantation at inappropriate site)
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.
Describe Week 2, Day 9 – 10
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.
Week 2, Day 11
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
Week 2, Day 12
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.
Week 2, Day 13
Formation of secondary yolk sac
Pinches off from the primary yolk sac
A.k.a. Definitive yolk sac
Week 2, Day 14
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.
What has happened at the the end of the second week?
- 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)
Give some early pregnancy loss statistics
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)
