L3) Embryology 1 ✔ Flashcards
define the pre-embryonic period of human development
first two weeks of development
processes involved include: cleavage, compaction and implantation begins
how does fertilisation occur?
oocyte released from ovary
travels along fallopian tube
fertilised by sperm in ampulla
what is a fertilised oocyte called?
zygote
what is the ideal site for implantation of the zygote?
posterior uterine wall
briefly describe what occurs in week 1 - cleavage
30 hrs after fertilisation
cleavage results in two blastomeres of equal size
zona pellucida sustained
what is the zone pellucida?
protective glycoprotein shell
briefly describe what occurs in week 1 - post cleavage
after zygote undergoes 4 cleavage divisions
embryo now defined as morula (16 cells)
briefly describe what occurs in week 1 - compaction
morula cells shift to form the first cavity - blastocyst cavity
inner cell mass - embryoblast
outer cell mass - trophoblast
zona pellucida sustained
now defined as blastocyst
what is the function of the zona pellucida being sustained?
remains constant in size
hence any successive division of zygote results in smaller cell size
briefly describe what occurs in week 1 - hatching?
blastocyst hatches from zona pellucida
no longer constrained - can enlarge
now defined as a conceptus
can now interact with uterine wall
briefly describe what occurs in week 1 - implantation begins?
conceptus now = 100 cells
8/100 = form embryo
92/100 = begin development of metal membranes
broadly describe what occurs in week 2?
differentiation of outer cell mass and inner cell mass
what does the outer cell mass give rise to in week 2?
syncytiotrophoblast
cytotrophoblast
what does the inner cell mass give rise to in week 2?
epiblast
hypoblast
aka the bilaminar disc
what is the potency of cells before compaction?
totipotent
capacity to become any cell type
what is the potency of cells after compaction?
pluripotent
capacity to become one of many cell types
briefly describe the development of a conceptus by the end of week 2
the conceptus has implanted
the embryo has two cavities (amniotic cavity and yolk sac)
and is suspended from the endometrium by a connecting stalk
within a supporting sac (chorionic cavity)
broadly describe the process of implantation
invasive process
interstitial
establishes maternal blood flow within placenta
establishes the basic structural unit of matron-fetal exchange
why is implantation interstitial?
uterine epithelium is breached
conceptus implants within the uterine stroma
why is it important to establish a maternal blood flow within the placenta?
allow support of the embryo from maternal circulation
what is the basic structural unit of matron-fetal exchange?
the chorionic villus
what are some assisted reproductive techniques?
in vitro - oocytes fertilised in vitro (lab) and allowed to divide to 4 or 8 cell stage
morula then transferred to uterus
allows for PGD
what is PGD?
Pre-implantation Genetic Diagnosis
removal of cell from morula
to test fro serious heritable conditions prior to transfer of embryo into mother
what are some conditions linked to implantation defects?
placenta praevia
ectopic pregnancy
what is placenta praaevia?
implantation in the lower uterine segment
can cause haemorrhage in pregnancy
placenta blocks the cervical canal
requires C-section
what is ectopic pregnancy?
implantation at site other than uterine body (mostly Fallopian tube)
can be peritoneal or ovarian
can quickly become life-threatening emergency
briefly describe what occurs in week 2 - day 9?
at the embryonic pole - the rapid development of syncytiotrophoblast
at the abembryonic pole - primitive yolk sac formed - this is in contact with the cytotrophoblast layer
briefly describe what occurs in week 2 - day 11?
primitive yolk sac membrane pushed away from cytotrophoblast layer by acellular extraembryonic reticulum
reticulum later converted to extraembryonic mesoderm by cell migration
briefly describe what occurs in week 2 - day 12?
maternal sinusoids invaded by syncytiotrophoblast
lacunae becomes continuous with sinusoids
uteroplacental circulation begins
uterine stroma prepares for support of the embryo
what are maternal sinusoids?
uterine capillaries that develop after implantation allow the exchange of gas and nutrients with the mother
briefly describe what occurs in week 2 - day 13?
formation of secondary (definitive) yolk sac
pinches off the primitive yolk sac
briefly describe what occurs week 2 - day 14?
spaces within extraembryonic mesoderm merge to form chorionic cavity
the embryo and its cavities are suspended by a connecting stalk
bleeding around now may be confused with menstrual bleeding
what is the connecting stalk?
a column of mesoderm
future umbilical cord
what is the balstocoele?
first cavity
formed as result of compaction
what is the amniotic sac?
formed from spaces within epiblast
what is the primitive yolk sac?
the exocoelomic cavity
formed from hypoblast lining blastocoele
what is the secondary yolk sac
the difinitive yolk sac
formed within primitive YS
what is the extraembryonic coelom?
the chorionic cavity/sac
formed from spaces within extraembryonic reticulum and mesoderm
epidemiology of early pregnancy loss
app. 50% of all zygotes lost in the first 2-3 weeks
15% diagnosed pregnancies will miscarry
1% women suffer from recurrent miscarriages
why is a Caesarean section indicated in placenta praevia?
as the placenta blocks the cervical canal
vaginal delivery can cause severe bleeding
why is ectopic pregnancy a life-threatening emergency?
can cause your fallopian tube to burst open. without treatment, the ruptured tube can lead to life-threatening bleeding
