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Embryo > The pre-embryonic period > Flashcards

The pre-embryonic period Flashcards

1
Q

How many weeks is a pregnancy? why?

A

40 weeks, because 38 pregnant weeks including the 2 weeks (from last menstrual period date)

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2
Q

Describe what happens in per-embryonic period

A

first 2 weeks
clevage: formation of morula (1st division of zygote - sex cells) cells are totipotent
compaction: formation of blastocyst
implantation begins

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3
Q

Where is the oocyte fertilised?

A

the egg cell is fertilised by sperm in the ampulla of the fallopian tube to become zygote

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4
Q

Where is the ideal site of implantation for the zygote?

A

endometrium

posterior uterine wall

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5
Q

What is the morula?

A

from the 1st mitotic division from the zygote (8-16 cells)

multicellular ball - totipotent

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6
Q

what happens in cleavage of pre-embryonic period?

A

the zygote becomes 2 blastomeres (equal size) 30 hrs after fertilisation

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7
Q

What helps to exclude sperm once the oocyte has been fertilised?

A

zona pellucida - glycoprotein shell which surrounds the plasma membrane containing the 2 blastomeres

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8
Q

What is morula?

A

result of cleavage (1st mitotic division)

all cells totipotent - identical to zygote

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9
Q

Describe what happens in compaction in week 1

A

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

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10
Q

What happens to the cells after compaction?

A

go from totipotent to pluripotent - can be many cell types (multi-lineage potential)

totipotent before compaction - from 1st division after fertilisation

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11
Q

What happens in hatching during week 1?

A

the blastocyst hatches from zona pellucida - no longer constrained - free to enlarge - free to interact with posterior uterine surface (implant)

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12
Q

What happens during implantation in days 6-7?

A

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

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13
Q

What happens in week 2?

A

the week of twos

form 2 distinct cellular layers

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14
Q

What are the 2 distinct layers formed in week 2?

A
  1. outer cell mass ‘trophoblast’: syncytiotrophoblast & cytotrophoblast
  2. inner cell mass ‘embryoblast’: epiblast & hypoblast forming the bilaminar disk
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15
Q

Which part of the embryo attaches to the uterine epithelium?

A

syncytiotrophoblast

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16
Q

What does the epiblast (top) of the embryoblast form?

A

amniotic cavity for the embryo to grow

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17
Q

What does the hypoblast (lower) of the embryoblast form?

A

blastocyst cavity

18
Q

What happens by the end of the 2nd week?

A
  1. conceptus (all structures in zygote) has implanted (onto posterior uterine wall)
  2. embryo has 2 cavities: amniotic cavity & yolk sac (bottom layer)

bilaminar disk

19
Q

what is the embryo suspended by?

A

connecting stalk in a supporting sac (chronic cavity)

20
Q

What happens after implantation? Why?

A

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

21
Q

What happens during implantation? What does this establish?

A

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

22
Q

What is ectopic pregnancy?

A

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

23
Q

What is placenta praevia?

A

implantation in lower uterine segment
can cause haemorrhage
requires C section

24
Q

What is embryonic pole in week 2?

A

rapid development of syncytiotrophoblast

25
Q

What is abembryonic pole?

A

primitive yolk sac forms - yolk sac membrane in contact with cytotrophoblast layer

26
Q

What is primitive yolk sac membrane pushed away from cytotrophoblast by? why?

A

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)

27
Q

How does uteroplacental circulation form?

A

maternal sinusoids blood vessels) invaded by syncytiotrophoblast
lacunae (unfilled space) becomes continous with sinusoids

28
Q

How does secondary yolk sac form?

A

pinch off from primary

both below amniotic cavity

29
Q

How does chorionic cavity form?

A

spaces within extraembryonic mesoderm all merge

30
Q

What is the purpose of the connecting stalk? what is it? what does it become?

A

connecting stalk is a column of mesoderm (early embryonic layer) which suspends the embryo and cavities and becomes the future umbilical cord

31
Q

What happpens during day 14?

A

primitive & secondary yolk sac separate

bleeding around now can be mistaken for menstrual bleeding

32
Q

What is blastocoele?

A

1st cavity (blastocyst) - result of compaction (totipotent to pluripotent)

33
Q

What is the amniotic sac?

A

space where embryo grows

from spaces within the epiblast - cells go onto become embryo

34
Q

What is the primitive yolk sac?

A

by hypoblast lining the blastocoele

35
Q

What is the secondary yolk sac?

A

definitive yolk sac

formed within primitive yolk asc

36
Q

what is the extraembryonic coelom?

A

chorionic cavity - holds connective stalk

from spaces within the extraembryonic reticulum & mesoderm

37
Q

Describe the process from oocyte to conceptus

A

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)

38
Q

What are the main cavities?

A
  1. blastocyst (fluid filled)

2. amniotic cavity (where embryo grows)

39
Q

What does cleavage achieve?

A

formation of morula, fertilised oocyte –> multicellular ball, 2 x blastomere

40
Q

What does compaction achieve?

A

totipotent to pluripotent

41
Q

Define the stages of human development before birth including the weeks

A
  1. weeks 1-2: pre-embryonic period
  2. weeks 3-8: embryonic period
  3. weeks 9-38: fetal

Embryo (4 decks)

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