6. Embryology Flashcards
how does one cell become a multicellular body?
- growth
- morphogenesis
- differentiation
What is morphogenesis?
development of form and structure
What is differentiation?
specialisation for function
During weeks 1 -2 what is the stage called?
Pre embryonic
During weeeks 3-8 what is the stage called?
Embryonic
During weeks 9-38 what is the stage called?
Fetal
When are pregnancies dated from?
• Pregnancies are dated from the first day after the last menstrual period and fertilisation is assumed to be at 2 weeks after that date
What happens during fertilisation?
- Oocyte is released from the ovary
- Travels along the Fallopian (uterine) tube
- Is fertilised by sperm in the ampulla
- Fertilised oocyte is called the zygote
What is the ideal site for implantation ?
Ideal site for implantation is the posterior uterine wall
What 4 things happen during week one of the pre-embryonic period?
- Cleavage
- Compaction
- Hatching
- Implantation begins
What happens during cleavage?
- Cleavage - splitting - first mitosis division
• results in 2 blastomeres of equal size
• Mitosis continues until a morula is formed
What is the zone pellucida?
The glycoprotein shell that surrounded the ovum and ensures only one sperm gets in
What stage of differentiation are the cells of the morula?
Totipotent
What happens during compaction?
formation of the first cavity - blastocyst cavity — fluid filled space
The whole structure is now called the blastocyst
What happens during hatching?
- blastocyst hatches from zona pellucida
- no longer constrained - now free to enlarge
- can now interact with uterine surface to implant
What two things are occurring at the same time during the pre embryonic stage?
- Making an embryo
2. Making a placenta to support the embryo
At the very early stages formation of what has priority?
Placenta
What does Totipotent mean?
The capacity to become ANY cell type
What does pluripotent mean?
The capacity to become one of MANY cell types - not placenta
Before compaction are the cells Totipotent or pluripotent?
Totipotent
After compaction, are the cells Totipotent or pluripotent?
Pluripotent
What happens during assisted reproductive techniques?
- Oocytes are fertilised in vitro and allowed to divide to the 4- or 8- cell stage
- The morula is then transferred into the uterus
What structures are present in the blastocyst?
- Embryoblast (inner cell mass): which forms the embryo
- Trophoblast (outer cell mass): forms supporting tissue for embryo
- Blastocoele (blastocyst cavity): first cavity formed
What happens during week 2 of the pre embryonic period?
- Differentiation
- Implantation complete
- rapid development of syncytiotrophoblast at embryonic pole
- Primitive yolk sac formation
- primitive yolk sac membrane is pushed away from cytotrophoblast layer
- Maternal sinusoids invaded by syncytiotrophoblast
- Formation of secondary yolk sac
- Spaces within the extraembryonic mesoderm merge to form the chorionic cavity
What happens during differentiation?
two distinct cellular layers emerge from:
> outer cell mass
• syncytiotrophoblast
• cytotrophoblast
> inner cell mass becomes the bilaminar disk
• epiblast
• hypoblast
What happens during completion of implantation?
• implantation is interstitial
>the uterine epithelium is breached and the conceptus implants within the uterine stroma
• establishes maternal blood flow within the placenta
> Allows support of the embryo from maternal circulation
•establishes the basic structural unit of materno-fetal exchange
>The chorionic villus
What happens during week 2 day 9?
• Embryonic pole >rapid development of syncytiotrophoblast • Abembryonic pole >Primitive yolk sac formed
• Yolk sac membrane in contact
with cytotrophoblast layer
What happens during week 2 day 11?
• primitive yolk sac membrane is pushed away from cytotrophoblast layer by an acellular extraembryonic reticulum • reticulum later converted to extraembryonic mesoderm by cell migration
What happens during 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 during week 2, day 13?
- Formation of secondary yolk sac
- a.k.a. definitive yolk sac
- pinches off from primitive yolk sac
What happens during week 2 say 14?
• Spaces within the extraembryonic mesoderm merge to form the chorionic cavity • The embryo and its cavities are suspended by the connecting stalk > column of mesoderm > future umbilical cord • Bleeding around now can be confused with menstrual bleeding
What is a blastocoele?
first cavity, formed as a result of compaction
What is the amniotic sac?
formed from spaces within the epiblast
What is the primitive yolk sac?
- a.k.a exocoelomic cavity
* formed by hypoblast lining blastocoele
What is the secondary yolk sac?
- a.k.a definitive yolk sac
* formed within primitive YS
What is the Extraembryonic coelom?
- a.k.a chorionic cavity
* formed from spaces within extraembryonic reticulum & mesoderm
What are two implantation defects?
Ectopic pregnancy
Placenta praevia
What is ectopic pregnancy?
> implantation at site other than uterine body
(Most commonly Fallopian tube)
Can be peritoneal or ovarian
can very quickly become life threatening emergency
What is placenta praevia?
> implantation in the lower uterine segment
can cause haemorrhage in pregnancy
requires c section delivery
Which period is the period of greatest change?
Embryonic period
At which period is there the greatest risk of major congenital malformation (i.e. teratogenesis) due to
environmental exposure or drug
therapy?
Embryonic period
In general, what happens during the early embryonic period?
Foundations laid for the development of the body systems
What are Key events and processes in early development?
- fertilisation & implantation
- gastrulation
- neurulation
- segmentation
- folding
At the end of the second week the epiblast is a?
Uniform disc
In the third week, what is the new feature that appears on the dorsal surface of the epiblast?
primitive streak
What does the primitive streak indicate?
That gastrulation is going to start
What is the head end (away from edge of the disk )of the primitive streak called?
Primitive node
- Within the primitive streak lies a primitive node at the cranial end, and within the primitive node lies the primitive pit.
When does gastrulation occur?
in the third week of development, marking
the start of the embryonic period
Why does gastrulation occur?
Prior to gastrulation the embryonic disk
is bilaterally symmetrical
Describe what happens during gastrulation
• primitive streak forms in the caudal epiblast,
leading to migration & invagination of epiblast cells.
• The hypoblast is responsible for establishing a primitive yolk space - so it is displaced and degenerates and then the 3rd layer is created
• the bilaminar disk is converted to a trilaminar disk
What does the ectoderm become?
organs & structures that maintain contact with outside world
e.g. nervous system, epidermis
What does the mesoderm become?
supporting tissues
e.g. muscle, cartilage, bone, vascular system (including heart & vessels)
What does the endoderm become?
internal structures
e.g. epithelial lining of GI tract, respiratory tract; parenchyma of glands
What is situs inversus?
complete mirror-image viscera
What is the viscera?
the internal organs in the main cavities of the body, especially those in the abdomen, e.g. the intestines.
What causes situs inversus?
commonly results from immotile cilia
What are the associated morbidities of situs inversus?
no associated morbidity (usually)
When do problems arise in individuals with situs inversus ?
problems arise if there is both normal and
mirror-image disposition
What is the shape of the embryonic disk prior to gastrulation/
Prior to gastrulation the embryonic disk
is bilaterally symmetrical
How do we go from a bilaterally symmetrical embryonic disk to having left and right differences?
Action of ciliates cells at the primitive node results in left wards flow of signalling molecules for left side and right wards flow of signalling molecules for right side
What is the notochord?
Solid rod of cells running in the midline(middle of mesoderm) with important signalling role.
How is the neuroectoderm created?
Notochord Releases signalling molecules which diffuse and act on cells in the ectoderm.
Notochord directs conversion of overlying ectoderm to
NEURECTODERM
How is the neural tube created?
• Notochord signals cause overlying ectoderm to thicken
• Slipper-shaped neural plate
• Edges elevate out of the plane of the disk and curl towards each other, creating
the neural tube
