Implantation and the Early Embryo

Question 1

In vivo how many days after fertilisation does implantation occur?

Answer 1

9 days

Question 2

Which cells are the early embryo and which are the early placenta?

Answer 2

Inner cell mass - early embryo

Outer cells - placental membranes

Question 3

What day does the morula reach the uterus?

Answer 3

Around day 3 and 4

Question 4

Why does the blastocyst have to hatch out from the endometrium?

Answer 4

So that it comes into direct contact with the endometrial epithelium

Question 5

What hormone causes the endometrial stroma to accept the blastocyst?

Answer 5

Progesterone

Question 6

What factors cause the conceptus to travel down the ampulla and into the uterus?

Answer 6

Progesterone

Cilia is the most important factor

Question 7

Does the conceptus increase in size during the first few days after fertilisation?

Answer 7

No, whilst in the oviduct in this first few days each cell undergoes a series of cleavage divisions but the size of the conceptus remains the same. This is because each individual blastomere is reduced is size progressively.

Question 8

Do maternal, paternal or embryo derived genes influence the first few days of development. Explain why this is.

Answer 8

Maternal. In the early stages of development there is no mRNA synthesis by the conceptus’s own chromosomes as these are unable to undergo transcription at this point. Therefore, all these early stages are controlled by the products of oogenesis and thus the maternal genome. This total dependance on maternal cytoplasmic inheritence means that any deficiency will result in impaired or failed early development and possible pregnancy loss.

Question 9

On what day does the human conceptus become transcriptionally active?

Answer 9

4 - 8 cell stage (2-3 days)

Question 10

What are the two distinct parts of the blastocyst?

Answer 10

1. An outer ring of trophoblast cells.

2. Inner cell mass (pluripotent cells)

Question 11

What do the trophoblast cells give rise to?

Answer 11

The extra embryonic tissue ie the chorion

Question 12

What are the two functions of the zona pellucida after conception?

Answer 12

1. Prevents the blastomeres of the conceptus from falling apart during early cleavage prior to compaction. (If they do become divided at this time then monozygotic twins arise)
2. Prevents two genetically distinct conceptuses from sticking together.

Question 13

In humans, is implantation invasive or non invasive?

Answer 13

Invasive

Question 14

What cells produce LIF?

Answer 14

The cells of the endometrial glands under the influence of oestrogen

Question 15

What is LIF thought to do with regard to implantation?

Answer 15

Promotes endometrial receptivity to attachment.

Question 16

What is the cervical mucus plug and what hormone causes it to be formed?

Answer 16

Oestrogen
The production of mucus from glands at the cervix to from a plug acting in a mechanical and antibacterial manner to protect the early pregnancy.

Question 17

What 4 things does the inner cell mass give rise to?

Answer 17

1. Embryo
2. Amnion
3. Yolk sac
4. Allantois

Question 18

Describe what happens to the endometrial lining after ovulation in preparation fro implantation?

Answer 18

It turns into a secretory lining in preparation of accepting the embryo. It becomes more vascular and it now known as the decidua

Question 19

What the decidua go on to form if pregnancy occurs?

Answer 19

The maternal part of the placenta

Question 20

What is gastrulation?

Answer 20

The reorganising of a two layer embryo into a three layer embryo. Also gives the embryo a specific head to tail and front to back orientation.

Question 21

What are the three germ layers and what roughly do they develop into?

Answer 21

Ectoderm (Skin cells, neurones, pigment cells)
Endoderm (internal layer: internal lung lining, digestive cells)
Mesoderm (Cardiac and skeletal muscle)

Question 22

When does gastrulation occur? What is the first sign that this is happening?

Answer 22

Around week 3.

The primitive streak (a linear band of cells formed by the migrating epiblast) appears.