Implantation and Germ layers

Question 1

When does implantation occur?

Answer 1

around 6 days, blastocyst adheres to endometrium on posterior wall of the uterus

Question 2

What is the trophoblast’s role in implantation?

Answer 2

decidual reaction - digestion of uterine lining

Question 3

Structure of inner cell mass

Answer 3

Primitive ectoderm surrounds amniotic cavity
primitive endoderm surrounds caity of yolk sac
(blastocyst wall is 2 layered)

Question 4

What is ectopic implantation?

Answer 4

blastocyst implants outside uterus, typically in fallopian tube
can damage uterine epithelium

Question 5

What occurs after implantation?

Answer 5

EEM covers amnion and yolk sac (joined by connecting stalk)

Question 6

What is the bilaminar disc?

Answer 6

where the ecto and endo lie flattened against each other

where majority of fetus is formed

Question 7

Summary of week 2

Answer 7

2 layers - ectoderm and endoderm
2 cavities - amniotic and yolk sac
2 trophoblast derivatives - cyto and syncytio

Question 8

What happens to the bilaminar disc in week 3?

Answer 8

converted into trilaminar disc by gastrulation:

ectoderm cells migrate through the primitive streak to form the intraembryonic mesoderm of the embryo

Question 9

What is derived from the ectoderm?

Answer 9

skin cells, nerve cells, pigment cells

Question 10

What is derived from the intraembryonic mesoderm?

Answer 10

cardiac, skeletal, and smooth muscle

RBCs and tubule cells

Question 11

What is derived from the endoderm?

Answer 11

lung cells, digestive cells and thyroid cells

Question 12

What is the primitive groove?

Answer 12

faint groove along longitudinal midline of the germ disc, which lengthens and deepens over next few days

Question 13

What is the primitive pit and node?

Answer 13

a deeper depression at the cranial end of the embryo, near the centre of the germ disc
mound of ectoderm surrounding the pit

Question 14

What is the primitive streak?

Answer 14

primitive groove + pit + node

establishes the longitudinal axis of the embryo

Question 15

What are the two faint depressions that appear within the ectoderm?

Answer 15

1. cranial end, adjacent to prechordal plate - buccopharyngeal membrane
2. caudal end, behind primitive streak - cloacal membrane

Question 16

Are the CM and BPM bilaminar? Why or why not?

Answer 16

Yes, their ectoderm fuse tightly with the endoderm (excluding intraembryonic mesoderm)

Question 17

What do the cloacal membrane and buccopharyngeal membrane develop to become?

Answer 17

endings of the gut tube

CM - anal end

Question 18

What can the cells that migrate during gastrulation do?

Answer 18

1. rest in the midline in a dense mass, cranial to the primitive pit - prechordal plate
2. rest along the midline in a dense mass, caudal to the primitive pit - notochordal process
3. rest lateral to the midline in a loose sheet-like structure

Question 19

What happens to the notochordal process?

Answer 19

grows in length, completely developed at around day 20

PS regresses from cranial to caudal

Question 20

How is the notochord formed?

Answer 20

notochordal process completely detaches from the endoderm to form a solid cylinder

Question 21

Role of the notochord on day 20?

Answer 21

inducing the neural plate on the surface of the ectoderm

Question 22

Through what process is the neural tube formed?

Answer 22

neuralation

Question 23

What occurs in neuralation?

Answer 23

NP begins to crease along midline = neural folds
these become more concave to form neural tube enclosing neural canal
fusion of neural folds causes separation of neural tube from ectoderm

Question 24

What is the brain and spinal cord’s precursor?

Answer 24

neural tube