Embryology- pre-embryonic phase (weeks 1 to 2) Flashcards
How does one cell become a multicellular body?
- Growth
- Morphogenesis - development of form and structure
- Differentiation - specialisation for function
Define the stages of human development before birth
- Pre-embryonic
- Embryonic
- Fetal
Which weeks post-fertilisation make up the pre-embryonic stage?
Weeks 1-2
Which weeks post-fertilisation make up the embryonic stage?
Weeks 3-8
Which weeks post-fertilisation make up the fetal stage?
Weeks 9 - birth (38 weeks plus or minus 2 (is normal))
What is the start point which is used to calculate the weeks of the stages of development?
These are calculated from the LMP (last menstrual period). Therefore on average the term of pregnancy is these conception weeks plus 2 (term of pregnancy is 40 weeks) because typically fertilisation occurs 2 weeks into the menstrual cycle.
Which three developmental processes occur in the pre-embryonic stage of development?
- Cleavage - formation of morula by first mitosis
- Compaction - formation of blastocyst
- Implantation begins - blastocyst makes contact with the endometrium of the uterus
Where is the oocyte generally fertilised?
In the ampulla of the fallopian tube
What is the name of a fertilised oocyte?
zygote
What is the ideal site for implantation of the blastocyst?
Posterior uterine wall
Why is there a small window for fertilisation?
An oocyte is only viable for 1 day and sperm for 3 days
What happens during cleavage in the pre-embryonic period?
The first mitotic division.
Results in two blastomeres of equal size surrounded by a glycoprotein shell called the zona pellucida. Blastomeres contained within the zona pellucida are collectively called the morula when they have divided to form 8 blastomeres.
What is the name of the glycoprotein shell that surrounds the blastomeres?
Zona pellucida
What is the morula?
Eight blastomeres and their surrounding zona pellucida.
What is important about the ability of the cells in the morula?
They are totipotent - have the capacity to become any cell type.
At what stage do assisted reproductive techniques transfer the fertilised oocyte into the uterus?
When it has divided to the 4-8 cell stage (morula)
How does pre-implantation genetic diagnosis (PGD) test for heritable conditions?
A cell is safely removed from the morula and tested, prior to transfer of the embryo into the mother
What is the defining occurrence during the pre-embryonic process of compaction?
Formation of the first cavity - blastocyst cavity
What is the name of the total cell mass once compaction has occurred?
blastocyst
What are the two different tissue types that occur during the first differentiation event, compaction?
Inner cell mass “embryoblast”
Outer cell mass “trophoblast”
What does the inner cell mass go on to form?
the embryo
What does the outer cell mass go on to form?
the supporting tissues of the embryo - placenta and membranes
What is the name of the first cavity that is formed in the pre-embryonic period?
Blastocyst cavity
What is the difference in the ability of the cells before and after the compaction event in the pre-embryonic phase of pregnancy?
Before - totipotent: capacity to become any cell type
After - pluripotent: capcity to become 1 of many cell types
After cleavage, hatching occurs. What is meant by the term hatching?
The blastocyst breaks free from the zona pellucida and is no longer constrained, therefore it can enlarge. The cells of the outer cell mass are now able to make contact with the endometrium.
Define conceptus
The product of conception: embryo plus associated membranes
When does implantation begin?
Week 1
Week 2 is called the week of twos. Why?
Both the outer and inner cell mass form two distinct cellular layers:
Outer cell mass: syncytiotrophoblast and cytotrophoblast
Inner cell mass (bi-laminar disk): epiblast and hypoblast
In week two what two cell types make up the bilaminar disk?
Epiblast and hypoblast
In week two what two cell types does the outer cell mass differentiate into?
Synctiotrophoblast
Cytotrophoblast
What two cavities have formed by the end of week 2?
Amniotic cavity and yolk sac
Which of the three processes in pre-embryonic development finishes by the end of week 2?
Implantation
What is the name of supporting sac in which the embryo is suspended at the end of the pre-embryonic phase?
Chorionic cavity
How is the amniotic cavity formed?
By the secretions of cells…
Which of the two cavities formed within the embryo, does the embryo ultimately grow into?
amniotic cavity
Why can there be a bit of bleeding during week 2 of development which can sometimes be mistaken for a light menstruation?
Implantation is an invasive process - the blastocyst breaches the mother’s endometrium using enzymes.
How is the breached endometrium during implantation repaired?
fibrin plug
What is meant by ‘implantation allows support of the embryo to change from histiotrophic to haemotrophic?
It means that support is able to change from simple diffusion to access to the maternal circulation - allowing significant growth to occur.
Name 2 conditions that are linked to implantation defects
Ectopic pregnancy
Placenta praevia
What implantation defect has occurs in ectopic pregnancies?
Implantation at site other than uterine body (most commonly fallopian tube but can be peritoneal or ovarian). Can be caused by cilia problems or blockages in the fallopian tubes.
Why can ectopic pregnancy very quickly become a life-threatening emergency?
Haemorrhage
There is no endometrium and so when the invasive part of development occurs, the conceptus can invade large blood vessels and cause high blood loss.
What implantation defect occurs in placenta praevia?
Implantation in the lower uterine segment.
Why is placenta praevia a problem?
The degree of severity depends on how low in the uterus it has implanted. It can cause haemorrhage and requires a C-section delivery.
Which of the layers syncytiotrophoblast or cytotrophoblast is found on the outer layer of the conceptus?
Synctiotrophoblast layer: synctio- means transport, whereas cyto- means cellular.
Synctiotrophoblast is involved in the formation of lacunae which fill with the maternal blood supply.
What are the name of the pockets that develop in the syncytiotrophoblast?
lacunae
What is the name of the cavity formed when the hypoblast layer spread out from the bilaminar disk and lines the cytotrophoblast layer?
Primitive yolk sac
In week 2 the primitive yolk sac membrane (hypoblast layer) is pushed away from the cytotrophoblast layer by an acellular extraembryonic reticulum (glycoprotein, protein ground substance). What is this reticulum later converted to by cell migration?
Extraembryonic mesoderm
In week 2, the maternal sinusoids are invaded by syncytiotrophoblast and the lacunae become continuous with the sinusoids. The uterine stroma prepares for support of the embryo. What type of circulation can now occur?
Uteroplacental circulation
What is a sinusoid?
A small irregular blood vessel found in certain organs e.g. the liver.
At the end of week 2 the secondary yolk sac is formed. How does this happen?
A small part of the yolk sac is pinched off into the chorionic cavity. The remaining yolk sac is called the secondary or definitive yolk sac.
The connecting stalk is formed from a column of mesoderm. What will it become in the future?
The umbilical cord
What is the name of the first cavity formed as a result of compaction?
Blastocoele
What is the name of the sac formed from spaces within the epiblast?
Amniotic sac
What sac is formed by hypoblast lining the blastocoele?
Primitive yolk sac
What is the name of the sac formed within the primitive yolk sac?
Secondary yolk sac or definitive yolk sac
What cavity is formed from spaces within extraembryonic reticulum and mesoderm?
Chorionic cavity/ extraembryonic coelom
During which weeks are 50% of all zygotes lost?
Weeks 2-3
What percentage of diagnosed pregnancies will miscarry?
15%
What percentage of woman suffer from recurrent miscarriage?
1%
What are the names of the three conceptus stages that it goes through in the pre-embryonic phase?
zygote -> morula -> blastocyst
