Flashcards in ToB Embryology 1 - Pre-Embryonic Period Deck (82)
How early in development does the first differentiation take place?
Within the first 2 weeks
What is the first differentiation that occurs?
- Intra-embryonic tissues
- Extra-embryonic tissues
What is meant by 'intra-embryonic tissues'?
Tissues that will become part of the body
What is meant by 'extra-embryonic tissues'?
Tissues which will not become part of the body, but will support the development of the body
Why is there a difference between the length of complete gestation, and the length of pregnancy term?
Complete gestation is 38 weeks, but pregnancy weeks are calculated from the last menstrual period, ie conception weeks + ~2 weeks = 40 weeks
How long is the pre-embryonic period?
What are the 3 main stages from fertilisation to birth?
What is the name of the first division after fertilisation, and what does it produce?
'Cleavage' to produce 2 blastomeres
What is the name of the group of cells which makes contact with the endometrial lining of the uterus?
At what point of the fallopian tube is the oocyte fertilised?
At the ampulla (end nearest the ovary)
What is the name of a fertilised oocyte?
Where is the ideal site for implantation?
Posterior uterine wall, within the endometrial lining of the uterus
How long is an oocyte viable for?
How long is a sperm viable for?
How soon does cleavage occur after fertilisation?
Why are the 2 blastomeres (created via cleavage) the same size as the oocyte?
The cells cannot expand as they are encased by the zona pellucida (glycoprotein shell)
Define zona pellucida:
Thick glycoprotein membrane surrounding the ovum before implantation, which prevents polyspermy
How are other sperm prevented from reaching the oocyte after it has been fertilised?
They are excluded by the zona pellucida
Group of totipotent blastomere cells, formed after cleavage and compaction, distinct from a blastocyst due to lack of cavity.
Why are the cells within the morula described as totipotent?
As the cells are identical at this point, they all have the capacity to become any cell point.
During IVF, what is the name of the group of cells which will be transferred into the uterus?
When can Pre-implantation Genetic Diagnosis occur?
During the morula stage of IVF, before cells are transferred into the mother
What process allows embryos to be screened for serious heritable conditions?
Pre-implantation Genetic Diagnosis (PGD), during IVF
Define the embryonic period of development:
Period 3-8 weeks after fertilisation, during which all the structures and systems are built
Define the fetal period of growth:
Period 9-38 weeks after fertilisation, during which growth is the focus (as all structures and systems have been built). Physiological maturation of the systems occurs.
Define the pre-embryonic period:
Period 0-2 weeks after fertilisation, during which cleavage and the 1st differentiation takes place, and implantation begins.
A fertilised oocyte
The 1st division of the zygote, into two masses known as blastomeres. First cleavage happens ~30 hours after fertilisation
The female reproductive organ producing oocytes/eggs for fertilisation
Define fallopian tube:
Either of a pair of tubes along which oocytes/eggs travel along from the ovaries to the uterus
Female reproductive organ where implantation of the zygote and growth of the embryo occurs
The carrying of an embryo/fetus inside a female reproductive organ.
At what point after fertilisation does a blastocyst form?
During day 4 after fertilisation
How is a morula different to a blastocyst?
-Morula contains identical totipotent cells, whereas the blastocyst contains 2 differentiated types of cells
- Blastocyst contains a cavity, the morula does not
What is the outer layer of the blastocyst?
What is the name of the cell formed after the 1st differentiation event?
What types of cells does the 1st differentiation event produce?
1) Trophoblast cells = outer cell mass
2) Embryoblast cells = inner cell mass
How are trophoblast cells different to embryoblast cells?
Trophoblast cells make up the outer cell mass of the blastocyst,and will become supporting tissue (never part of the body).
Embryoblast cells make up the inner cell mass, and will produce the embryo and fetus.
What event causes the cells of an embryo to change from totipotent to pluripotent?
The capacity of a cell to become any cell type.
The capacity of a cell to become one of many cell types
What is the difference between totipotency, and pluripotency?
Totipotent cells have the capacity to become ANY cell type, whereas pluripotent cells have the capacity to become one of MANY cell types.
With every cell division, why do the blastomeres become smaller?
They are limited by the zona pellucida
What word describes the degradation of the zona pellucida, allowing the blastocyst to enlarge?
At what point after fertilisation does 'hatching' take place?
Day 5 after fertilisation
What is required to occur, before the blastocyst can begin implantation?
Blastocyst must hatch out of the zona pellucida, to allow interaction of the trophoblast cells with the uterine wall.
After hatching, how many cells are present in the conceptus?
How many of the 107 cells present after hatching will make the embryo, and how many will become fetal membranes?`
8 cells will make the embryo
99 will become fetal membanes
At what point after fertilisation does implantation begin?
Day 6-7 after fertilisation
During the early stages of development, which has priority: the making of the embryo, or the making of the placenta?
Formation of the placenta, to be able to support the developing embryo
Why is week 2 known as the 'week of two's'?
The outer and inner cell masses both differentiate into 2 different cell types, (4 different cell types present by end of week 2)
During week 2, what happens to the trophoblast cells?
They differentiate into:
1) Syncytiotrophoblast cells, or
2) Cytotrophoblast cells
What is the role of the syncytiotrophoblast cells?
-Form a syncytium of multi-nucleated cells
-Produce and release lytic enzymes to break down part of the endometrial epithelial cells
-Penetrate basal lamina and invade maternal sinusoids, producing a uteroplacental circulation
-Surround embryo when implantation is complete
-Allow efficient diffusion
Define cytotrophoblast cells:
Non-invasive cells which encase the embryo during implantation, and form the placental membrane
What group of cells within the blastocyst become the bilaminar disk?
Embryoblast cells (inner cell mass)
What 2 groups of cells make up the bilaminar disc?
1) Epiblast cells
2) Hypoblast cells
During week 2, what happens to the embryoblast cells?
They differentiate into:
1) Epiblast cells
2) Hypoblast cells
Describe the embryo at the end of the 2nd week:
- Consists of epiblast and hypoblast cells,
- Two cavities formed: amniotic cavity, and the yolk sac
- In the center of the two cavities, the epiblast and hypoblast cells are joined to form the bilaminar disc
- Is suspended within the chorionic cavity via a connecting stalk
What type of cells surround the amniotic cavity?
In which cavity will the embryo develop, and gestation will occur?
After implantation, what happens to the degraded endometrial lining of the uterus?
The invasion point is filled with a fibrin plug
At what point after fertilisation is implantation complete, and a fibrin plug inserted into the endometrial lining of the uterus?
Day 9-10 after fertilisation
What does the conceptus implant into?
The uterine stroma, through the endometrial lining of the uterus
How does the generation of a uteroplacental circulation change the support of the embryo?
The support of the embryo will change from histiotrophic (tissue fluid simple diffusion) to haemotrophic (relies on nutrients from maternal blood)
What are the 4 main conditions linked to implantation defects?
1) IUGR (Inter Uterine Growth Restriction)
3) Ectopic pregnancy
4) Placenta praevia
Define ectopic pregnancy:
A pregnancy that is not occurring in the uterus
Why is an ectopic pregnancy life-threatening?
If invasive implantation takes place somewhere other than the uterus, can cause haemorrhage elsewhere in the pelvis.
What is placenta praevia?
When the placenta grows across the cervix
What causes placenta praevia?
The placenta will grow wherever implantation takes place, so if implantation occurs in the lower uterine segment there is a high risk of the placenta growing across the cervix.
How will delivery differ between a normal pregnancy, and a pregnancy with placenta praevia?
Pregnancies with placenta praevia usually require Caesarian section delivery, due to very high risk of haemorrhage.
What are the two poles of the conceptus immediately after implantation?
Embryonic pole = first side of embryo to implant, so side where lacunae form
Abembryonic pole = last part of embryo to implant, so side next to fibrin plug
At which pole of the conceptus does the primitive yolk sac form?
Which type of cells form the primitive yolk sac membrane?
At which pole of the conceptus do lacunae first form?
What causes the primitive yolk sac membrane to be separated from the cytotrophoblast layer?
The primitive yolk sac secretes a primitive proteinaceous acellular extraembryonic reticulum
How is the extraembryonic mesoderm formed?
The primitive yolk sac secretes an acellular extraembryonic reticulum, which is later converted to an extraembryonic mesoderm via cell migration
At what point after fertilisation is the uteroplacental circulation developed?
Day 12 (week 2)
What are the function of the lacunae in the syncytiotrophoblast?
The syncytiotrophoblast invades the maternal sinusoids, resulting in the lacunae filling with maternal blood = uteroplacental circulation.
At what point after fertilisation does the definitive/secondary yolk sac form?
Day 13 (week 2)
How is the definitive/secondary yolk sac formed?
Some of the primitive yolk sac (at the abembryonic pole) pinches off
How is the chorionic cavity formed?
Spaces within the extraembryonic mesoderm merge, to form the cavity which encases the embryo