Implantation and Formation of Germ Layers

Question 1

When does implantation take place?

Answer 1

6 days after fertilisation

Question 2

Where does implantation take place?

Answer 2

Adheres to endometrium (wall of uterus lined by epithelium).
On the posterior wall of the uterus- near the fundus rather than the cervix.

Question 3

What is the endometrium?

Answer 3

Wall of uterus- has epithelial covering
Endometrium: mixture of connective tissue vessel, muscle and various glands
- Secretes a lot of nutrient which move via diffusion into the uterine cavity

Question 4

Explain process of implantation and features of blastocyst at this stage

Answer 4

1) - Blastocyst: Inner cell mass (5 cells) and outer cell mass (55 cells)
- zona pellucidum disappears
- Inner cell mass starts differentiation to primitive ectoderm (epiblast) and endoderm (hypoblast)
- outer cell mass becomes trophoblast
2)Embryo implanted: epithelium has ben breeched and blastocyst penetrated into uterus
Outer structures- blastocyst wall 2 layered:
- Trophoblast differentiated into cytotrophoblast (single cells) and huge mass of cellular material with many nuclei (green)- syncytiotrophoblast
Inner structures- blastocyst also 2 layered:
- Yellow cells (primitive endoderm/ epiblast) line the cavity- yolk sack
- Blue cells (primitive ectoderm/ hypoblast) form their own cavity- amnion
These cavities allow for expansion of structures within them- easier pushing fluid out of way rather than cells
3) By day 10, completely embedded and epithelial continuity restored. Trophoblast produces extraembryonic mesoderm- pushing embryo from trophoblast. Within mesoderm cavities form.
5) - Cavity expanded (called chorionic cavity/ extraembryonic choelom)- around embryo
- Extraembryonic mesoderm split into that which covers trophoblast and that which covers embryo
- Embryo connected to trophoblast via connecting stalk (goes on to form umbilical chord)
- Trophoblast still in 2 forms
- Uterus undergoes decidual reaction- reacting to the fact the implantation has taken place
- Glycogen/lipids cells distend and glands swell and embryo still gets nutrients via diffusion
- Completely implanted into uterine wall (decidua)
○ Actually gone through wall and surrounded by uterine tissue
Epithelium then repairs itself
6) Ectoderm and endoderm touching to form the bilaminar disc

Question 5

What is ectopic implantation? And consequence

Answer 5

• Ectopic implantation- implant in the fallopian tube rather than uterus- have to be removed as not enough space- can damage and possibly cause this side of uterus to become infertile
  Also egg can go out of fimb

Question 6

What happens if egg escapes to peritoneal cavity?

Answer 6

Also egg can go out of fimbriae and float away in peritoneal cavity-most shrivel and die as run out of nutrients or can implant in the liver and grow very large- surgery to remove

Question 7

If egg implanted wrong- why is mother not aware of this?

Answer 7

Detected by scan despite normal hCG levels indicating normal pregnancy on test

Question 8

Describe what happens in week 2

Answer 8

• 2 layers (ecto and endoderm)
• 2 cavities (amniotic and yolk sack
  2 trophoblast derivatives (cyto and syncytio trophoblast

Question 9

Why is the decidual reaction important and what is it caused by?

Answer 9

• By oestrogen and progesterone production
• Important for implantation of embryo as glycogen/lipids cells distend and glands swell and embryo still gets nutrients via diffusion
• If no reaction- embryo lost within 2 days

Question 10

What is the cervical mucus plug and what causes this to form?

Answer 10

Oestrogen stimulates production of mucus from glands at the cervix
This forms a barrier/ plug that acts in a mechanical and antibacterial manner

Question 11

Outline different types of twins

Answer 11

• dichorionic-diamniotic twins (cleavage at morula 1-3) - own chorion and amnium- can be dizygotic (mostly) or monozygotic (identical 25%)- lowest mortality risk 9%
• monochorionic/ diamniotic twins (cleavage at blastocyst 4-8)- same placenta but 2 amniotic sacs- equal blood supply
• monochorionic/monoamniotic (implanted blastocyst 8-13)- risk of transfusion of blood from 1 to other- cerebral palsy or miscarriage due to lack O2
• Conjoined twins (cleavage 13-15after formed embryonic disc) mortality highest as share same organs

Question 12

Why does abnormal implantation occur?

Answer 12

• If epithelium damaged (scarring, pelvic inflammatory disease) or if zona pellucida lost too early, allowing premature tubal implantation.
• Or sperm going into egg with no pronucleus
• Can get into peritoneum
• Only trophoblast layers proliferate forming mole (partial mole, complete mole, persistent gestational trophoblastic tumor)
• If not spontaneously aborted, surgical intervention required
• following 1st molar pregnancy 1% risk of 2nd

Question 13

What happens in week 3

Answer 13

WEEK 3- 3 germ layers and 3 important structures (primitive streak, notochord and neural tube)

Question 14

Explain formation of notochord, neural tube and trilaminar disc

Answer 14

1) Indentation in the ectoderm called the primitive streak. This forms a new layer of cells- intraembryonic mesoderm (pushes yolk sack and amnion apart)
2) Continues to occur going from primitive plate to prechordal plate- but endoderm thickens in prechordal plate and adheres to endoderm so no separation of these layers- continues going AROUND this plate
3) From the primitive streak we see the primitive node/ knot/ pit forming. From this the notochord forms all the way up to the prechordal plate (mouth). The embryo also grows going from round–> slipper shape
4) Cloacal plate also forms below- anus
4) Notochord gives off signals to the cells above it- ectoderm- signalling that it will become neuronal tissue. Signals are seen as neural plate surrounding notochord.

Question 15

Explain formation of notochord, neural tube and trilaminar disc

Answer 15

1) Indentation in the ectoderm called the primitive streak. In this region ectodermal cells divide and forms a new layer of cells- intraembryonic mesoderm (pushes yolk sack and amnion apart)- becomes continuous with extraembryonic mesoderm.
2) Continues to occur going from primitive plate to prechordal plate- but endoderm thickens in prechordal plate region and adheres to endoderm so no separation of these layers- continues going AROUND this plate (so to either side we still see intraembryonic mesoderm)
3) From the primitive streak (indentation in ectoderm) we see the primitive node/ knot/ pit forming. From this the notochord forms all the way up to the prechordal plate (mouth). Also gives rise to 3 new layers in embryo (definitive ectoderm and endoderm and intracellular mesoderm) The embryo also grows going from round–> slipper shape
4) Cloacal plate also forms below- anus
4) Notochord gives off signals to the cells above it- ectoderm- signalling that it will become neuronal tissue. Signals cause neural plate to form (thickening on ectoderm above notochord).
5) Above the prechordal plate we have the septum transversum- this will form the diaphragm.
6) Then neurulation takes place- where neural plate forms neutral tube
Neural plate lengthens and its lateral edges elevate forming 2 neural folds
Neural groove is the depressed region of neural plate.
This will eventually become pinched off to make the neural tube above notochord. This forms the brain and spinal chord. Ectoderm continuous again over top. Happens in middle of embryo first then migrates to head and tail region.
7) Allantois- extension of yolk sack into extraembryonic mesoderm - important for disposal of waste- not too important in humans as we have the placenta

Question 16

Physiologically what is the cloacal plate and pre/Prochordal plate?

Answer 16

Thickening of endoderm (therefore no intraembryonic mesoderm can form in these areas)

Question 17

Physiologically what is the primitive streak? Therefore what is the primitive node and what is the primitive pit?

Answer 17

Indentations of the ectoderm
Primitive node is the end of the primitive streak
Primitive pit= depressed area in the primitive node

Question 18

Physiologically what is the neural plate?

Answer 18

Appearance of the notochord inducing the overlying ectoderm to thicken

Question 19

Why is the primitive streak so important?

Answer 19

Gives rise to 3 new layers in the embryo- whole embryo formed form the epiblast/ ectoderm!
Primitive streak (invagination) causes some cells to displace primitive endoderm (hypoblast)--> definitive endoderm
Others cells lie between ectoderm, and new definitive endoderm- forming mesoderm
Cells remaining in the primitive ectoderm form definitive ectoderm