7. Implantation and formation of germ layers

Question 1

Implantation

Answer 1

at 6 days blastocyst adheres to endometrium usually on posterior wall of uterus and nearer fundus than cervix blastocyst gives rise to inner cell mass (5 cells) and trophoblast (55 cells)

Question 2

describe the trophoblast and inner cells mass during implantation

Answer 2

invasive, ingestive and digestive inner cell mass forms embryo

Question 3

what does the inner cell mass do after 6 days

Answer 3

gives rise to primitive ectoderm which surrounds the amniotic cavity

gives rise to primitive endoderm which surrounds the cavity of yolk sac

the blastocyst wall is now 2 layered (cyto and syncytio)

Question 4

day 10

Answer 4

completely embedded in the endoemtrium

epithelial continuity restored

trophoblast and yolk sac produce extraembryonic mesoderm

embryo attaches by trophoblast overlying inner cell mass

Question 5

after day 10

Answer 5

endometrium undergoes DECIDUAL REACTION glycogen/ lipid distended cells, glands swell more. reaction most intense in regions of implantation

extra embryonic mesoderm left lining amnion and yolk sac and trophoblast 2 linings joined by connecting stalk

Question 6

what do the derivatives of trophoblast form

Answer 6

placenta

Question 7

cavities in extraembryonic mesoderm coalesce forms

Answer 7

chorionic cavity= extraembryonic coelom

Question 8

where ectoderm and endoderm lie against each other

Answer 8

flattened bilaminar disc produced- the majority of fetus will be formed

ectoderm of amnion and endoderm of yolk is continuous with disc

Question 9

Implanted Embryo

Answer 9

Question 10

what are the features of the embryo in the second week

Answer 10

2 layers (ecto, endo)

2 cavities (amniotic and yolk sac)

2 trophoblast derivatives (cyto and syncytio)

Question 11

Decidual reaction

Answer 11

oestrogen and progesterone signalling leads to decidualisation- spreads throughout uterus

Bone morpohgenetic protein 2 (BMP2) non active presscursor protein is cleaved by proprotein covertase 5/6 to produce active form

deletion or knock down of these proteins inhibits- implantation failure and infertility

Question 12

Twins: if the zygote is split during days 1-3

Answer 12

Dichorionic/ diamniotic: separate chorions and amnion, own placenta

almost all cases of dizyotic twins

lowest mortality risk

25% monozygotic

Question 13

Twins: cleavage days 4-8

Answer 13

Monochorionic/ diamniotic

same placenta and two amniotic sacs

risk of twin-twin transfusion syndrome

60-70% pregnancies with monozygotic (identical) twins

Question 14

Twins: cleavage day 8-13

Answer 14

Mono/mono

same amnion and placenta

1-2% mz

50-60% survival rate

umbilical cords may tangle and cause miscarriage or cerebral palsy (lack of O2)

Question 15

Twins: cleavage days 13-15

Answer 15

Conjoined twins

highest mortality

shared organs

one twin may be sacrificed

Question 16

Cervical mucus plug

Answer 16

along with decidualisation, oestrogen also stimulates production of mucus from glands at cervix

secreted mucus forms plug/ barrier- mechanical and antibacterial into uterus

Question 17

Abnormal implantation- ectopic pregnancy

Answer 17

implantation into uterine tube (94%)

• spontaneously aborted, or embryo remoed surgically
• uterine epithelium damage (scarring, pelvic inflammatory disease) or zp lost too early- allows premature tubal implantation
• embryo developed through early stages may erode through uterine horn and reattach within peritoneal cavity

Question 18

Hydatidiform Mole- 1% risk of second following first

Answer 18

only conceptus trophoblast layers proliferate- no embryo develops- a hydatidiform mole implants in uterus

trophoblast secrete hCG- pregnancy test normal

diagnosis by ultrasound

can arise from sperm fertilising egg without female pronucleus

Question 19

What happens at the beginning of third week: 1

Answer 19

bilaminar disc becomes pear shaped

Ectodermal cells begin to divide and migrate between ecto and endoderm in region of primitive streak

to form intraembryonic mesoderm

originally believed that difinitive endoderm formed from hypoblast but now

all three germ layers are formed from epiblast (ectoderm)

this pushes primary endoderm of yolk sac aside

Question 20

What happens at the beginning of third week: 2

Answer 20

Endoderm thickens and becomes adherent to overlying ectoderm region of pro or pre chordal plate.

this is the future of mouth region

Question 21

Week 3

Answer 21

Question 22

Week 3: why does the embryo lengthen?

Answer 22

cell production in and migration of the primitive streak

Question 23

week 3: how does the neural plate form?

Answer 23

cells from head/ rostral end of primitve streak forms notochord- this induces ovelrying ectoderm to form neuroectoderm of the neural plate

Question 24

week 3: what happens to do the mesoderm?

Answer 24

continues to be produced from primitive streak

it migrates laterally until it joins up with the extra-embryonic mesoderm

migrated forward but cannot separate prochordal plate endoderm from ectoderm

flows in front of this to form septum transversum

Question 25

week 3: what happens then to the neural plate?

Answer 25

The neural plate forms NEURAL FOLDS (neuroectoderm)

eventually fuses to form neural tube -> brain and spinal cord

loses contact with overlying ectoderm

Question 26

3rd week features

Answer 26

3 germ layers

3 important structures: prochordal plate, notochord, cloacal plate

primitive streak

Question 27

what will the cloacal plate form

Answer 27

anal end of the gut

Question 28

Answer 28

Question 29

Answer 29

Question 30

Answer 30

Question 31

Answer 31

Question 32

Answer 32

Question 33

Answer 33

Question 34

Answer 34

Question 35

Answer 35