Flashcards in The pre-embryonic period Deck (41):
How many weeks is a pregnancy? why?
40 weeks, because 38 pregnant weeks including the 2 weeks (from last menstrual period date)
Describe what happens in per-embryonic period
first 2 weeks
clevage: formation of morula (1st division of zygote - sex cells) cells are totipotent
compaction: formation of blastocyst
Where is the oocyte fertilised?
the egg cell is fertilised by sperm in the ampulla of the fallopian tube to become zygote
Where is the ideal site of implantation for the zygote?
posterior uterine wall
What is the morula?
from the 1st mitotic division from the zygote (8-16 cells)
multicellular ball - totipotent
what happens in cleavage of pre-embryonic period?
the zygote becomes 2 blastomeres (equal size) 30 hrs after fertilisation
What helps to exclude sperm once the oocyte has been fertilised?
zona pellucida - glycoprotein shell which surrounds the plasma membrane containing the 2 blastomeres
What is morula?
result of cleavage (1st mitotic division)
all cells totipotent - identical to zygote
Describe what happens in compaction in week 1
formation of 1st cavity: blastocyst cavity
is a fluid filled space - secretes fluid
has embryoblast which will go onto become embryo
trophoblast: outer cell mass which supports the embryo during pregnancy
and zona pellucida - restricting overall size
What happens to the cells after compaction?
go from totipotent to pluripotent - can be many cell types (multi-lineage potential)
totipotent before compaction - from 1st division after fertilisation
What happens in hatching during week 1?
the blastocyst hatches from zona pellucida - no longer constrained - free to enlarge - free to interact with posterior uterine surface (implant)
What happens during implantation in days 6-7?
there is now 107 cells in conceptus (product of conception)
99 cells develop into foetal membrane
8 cells go onto become the embryo
the trophoblast (outer cell mass) attaches to the uterine epithelium
What happens in week 2?
the week of twos
form 2 distinct cellular layers
What are the 2 distinct layers formed in week 2?
1. outer cell mass 'trophoblast': syncytiotrophoblast & cytotrophoblast
2. inner cell mass 'embryoblast': epiblast & hypoblast forming the bilaminar disk
Which part of the embryo attaches to the uterine epithelium?
What does the epiblast (top) of the embryoblast form?
amniotic cavity for the embryo to grow
What does the hypoblast (lower) of the embryoblast form?
What happens by the end of the 2nd week?
1. conceptus (all structures in zygote) has implanted (onto posterior uterine wall)
2. embryo has 2 cavities: amniotic cavity & yolk sac (bottom layer)
what is the embryo suspended by?
connecting stalk in a supporting sac (chronic cavity)
What happens after implantation? Why?
repair where the syncytiotrophoblast part of conceptus broke through the endometrium lining (uterine epithelium) to hatch
because this is a controlled invasive process, so need to repair lining by fibrin plug
What happens during implantation? What does this establish?
the blastocyst lies within endometrium
the uterine epithelium is breached and the conceptus implants within uterine stroma
to establish MATERNAL blood flow within placenta - embryo change from histiotrophic (diffusion) to haemotrophic (rely on maternal circulation): maternal-foetal exchange
What is ectopic pregnancy?
implantation at other site NOT uterine body
normally fallopian tube
can quickly be life threatening (haemorrhage, invasive implantation large vessel in pelvis - femoral)
peritoneal / ovarian
What is placenta praevia?
implantation in lower uterine segment
can cause haemorrhage
requires C section
What is embryonic pole in week 2?
rapid development of syncytiotrophoblast
What is abembryonic pole?
primitive yolk sac forms - yolk sac membrane in contact with cytotrophoblast layer
What is primitive yolk sac membrane pushed away from cytotrophoblast by? why?
pushed by acellular extraembryonic reticulum (AER)
AER later converted to extraembryonic mesoderm by cell migration (1 of 3 early embryonic layers: ecto & endo are the others)
How does uteroplacental circulation form?
maternal sinusoids blood vessels) invaded by syncytiotrophoblast
lacunae (unfilled space) becomes continous with sinusoids
How does secondary yolk sac form?
pinch off from primary
(both below amniotic cavity)
How does chorionic cavity form?
spaces within extraembryonic mesoderm all merge
What is the purpose of the connecting stalk? what is it? what does it become?
connecting stalk is a column of mesoderm (early embryonic layer) which suspends the embryo and cavities and becomes the future umbilical cord
What happpens during day 14?
primitive & secondary yolk sac separate
bleeding around now can be mistaken for menstrual bleeding
What is blastocoele?
1st cavity (blastocyst) - result of compaction (totipotent to pluripotent)
What is the amniotic sac?
space where embryo grows
from spaces within the epiblast - cells go onto become embryo
What is the primitive yolk sac?
by hypoblast lining the blastocoele
What is the secondary yolk sac?
definitive yolk sac
formed within primitive yolk asc
what is the extraembryonic coelom?
chorionic cavity - holds connective stalk
from spaces within the extraembryonic reticulum & mesoderm
Describe the process from oocyte to conceptus
oocyte --> fertilised by sperm in ampulla of fallopian tube --> zygote --> morula (totipotent) --> COMPACTION --> blastocyst (pluripotent) --> trophoblast (syncytiotrophoblast & cytotrophoblast - placenta membranes) & embryoblast (epiblast (embryo) & hypoblast (support) - bilaminar disc)
What are the main cavities?
1. blastocyst (fluid filled)
2. amniotic cavity (where embryo grows)
What does cleavage achieve?
formation of morula, fertilised oocyte --> multicellular ball, 2 x blastomere
What does compaction achieve?
totipotent to pluripotent