Early embryonic development 1 Flashcards
What is the ideal site of implantation for a zygote?
Posterior uterine wall
How many weeks is the pre embryonic period?
Weeks 1-2
How many weeks is the embryonic period?
Weeks 3-8
How many weeks is the foetal period?
Weeks 9-38
Define zygote
A fertilised ovum/oocyte
Define cleavage
The dividing of cells into two masses known as blastomeres. First cleavage happens ~30 hours after fertilisation (resting period)
Define zona pellucida
The glycoprotein ‘shell’ to prevent polyspermy
Define morula
The result of cleavage of the fertilised oocyte, each cell is Totipotent
Define ovary
The female reproductive organ producing oocytes/eggs for fertilisation
Define fallopian tube
A tube the oocyte travels along to reach the uterus. The egg is fertilised at the ampulla of the fallopian tube.
Define uterus
Female reproductive organ where implantation of the zygote and growth of the embryo occurs
Define blastocyst
Formed from the compaction of cells that make up the morula
Define tropoblast
(Outer cell mass) will later form support structures for the embryo (placenta)
Define embryoblast
(Inner cell mass) will later become the bilaminar disk
Define implantation
The attachment of the Blastocyst to the wall of the uterus at day 6 – 7
Define cytotrophoblast
(Derivative of Trophoblast) Placental membrane around the yolk sac
Define syncytiotrophoblast
(Derivative of Trophoblast) Cells that invade maternal sinusoids (irregular blood vessels) resulting in uteroplacental circulation.
What is compaction?
Blastomeres which make the morula compact to form the blastocyst
What is hatching?
Blastocyst hatches from the zona pellucida
Allows enlargement
Allows implantation
What is implantation?
Interstitial
The uterine epithelium is breached and the conceptus implants within the uterine stroma.
What does the embryoblast differentiate into?
Epiblast
Hypoblast
What does the Trophoblast differentiate into?
Syncytiotrophoblast
Cytotrophoblast
At early stages of development does the embryo or placenta have priority over development?
Placenta
What are the implantation defects?
Ectopic pregnancy
Placenta praevia
What is an ectopic pregnancy?
Implantation at site other than uterine body (most commonly fallopian tube). Can quickly become a life-threatening emergency.
What is placenta praevia?
Implantation in lower uterine segment. Placenta grows across cervical opening. Requires C-section delivery
What is the embryonic pole?
Rapid development of Syncytiotrophoblast
What is the Abembryonic pole?
Primitive yolk sac formed by Heuser’s membrane spreading across the Blastocyst cavity. Yolk sac membrane is in contact with the cytotrophoblast layer
What is the acellular extraembryonic reticulum converted to by cellular migration?
extraembryonic mesoderm
What happens in week 2 (day 11)?
Primitive yolk sac membrane is pushed away from the cytotrophoblast layer by an acellular extraembryonic reticulum.
What happens in week 2 (day 12)?
Maternal sinusoids invaded by syncytiotrophoblast.
Lacunae become continuous with sinusoids.
Uteroplacental circulation begins.
Uterine stroma prepares for support of the embryo.
What happens in week 2 (day 13)?
Formation of secondary yolk sac
Pinches off from the primary yolk sac
A.k.a. Definitive yolk sac
What forms the Chorionic cavity?
Spaces within the extraembryonic mesoderm merge
What happens in week 2 (day 14)?
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.
What can be confused with menstrual bleeding at the end of week 2?
Bleeding around now can be confused with menstrual bleeding.
What has happened by the end of week 2?
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)
