Life Before Birth

Question 1

What are the developmental stages of life before birth?

Answer 1

• Week 1; Pre-implantation stage
• Week 2-8; Embryonic Stage (organogenesis)
• Week 9-38; Fetal Stage (growth and development)

Question 2

What are some examples of important early embryological events?

Answer 2

• Cleavage (week 1)
• Morula Formation (week 1)
• Blastocyst formation (week 1)

Question 3

What is cleavage?

Answer 3

This is the mitotic divisions of fertilised oocyte.
The overall size remains the same:
-allows passage down narrowest part of the uterine
tube
-Surrounded by tough glycoprotein- ZONA PELLUCIDA to prevent immature implantation.

Question 4

What is morula formation?

Answer 4

Around day 4 after fertilisation, cells maximise contact with each other, forming a cluster of cells held together by tight junctions, this enters the uterus.

Question 5

What is blastocyst formation?

Answer 5

This is the 1st sign of cellular differentiation.
This consists of:
-Inner cell mass; goes onto form the embryo and extraembryonic tissues.
-Trophoblasts; contributes to the placenta
As embryo enters the uterine cavity, fluid enters via the ZONA PELLUCIDA into the spaces of the inner cell mass. This forms a fluid filled blastocyst cavity.

Question 6

What is the major event in week 2 of development?

Answer 6

Implantation (interaction between embryo and endometrial layer of the uterus)

Question 7

What is the process of hatching in week 2?

Answer 7

Blastocyst starts to run out of nutrients, needs to implant.
ICM cells undergo proliferation and the fluid builds up in the cavity, eventually resulting in the blastocyst hatching from the ZONA PELLUCIDA to facilitate implantation.

Question 8

What is the process of implantation?

Answer 8

Blastocyst contacts endometrium of uterus, decidulisation occurs in the stromal cells of the uterus, this triggers production of several molecules and promotes trophoblasts to become invasive.
Implanting trophoblast cells differentiate.

Question 9

What are the stromal cells important for?

Answer 9

These cells form maternal component of placenta.

Question 10

What do implanting trophoblast cells differentiate into?

Answer 10

Cells closest to inside of the embryo become single layer of cells called cytotrophoblast, the outer layer ic called the syncytiotrophoblast.
The inner cell mass forms 2 layers known together as the bilaminar disc (epiblast and hypoblast).

Question 11

What happens at the end of week 2?

Answer 11

Implanting cells start talking to maternal side of placenta, a connection is established and diffusion of O2, waste and nutrients via blodd supply is enabled.

Question 12

What is detected in preganancy kits?

Answer 12

At end of week 2 syncytiotrphoblasts produce hormone hCG which is detected.

Question 13

What is ectopic implantation?

Answer 13

When implantation occurs at abnormal sites, this often leads to miscarriage as a fully functioning placenta is not formed.
Fertility is not usually affected if treated quick but sometimes uterine tubes are removed.
If miscarriage doesnt occur the embryo must be removed by drugs or surgery as otherwise invasive trophoblasts may cause haemorhagge.

Question 14

How are extraembryonic membranes formed?

Answer 14

They are derived from the blastocyst structures that don’t contribute to the embryo.

Question 15

What are some examples of extraembryonic membranes?

Answer 15

Amnion
Yolk Sac
Chorion
Allantois

Question 16

Describe the amnion?

Answer 16

• continous with epiblast of bilaminar disc, associated with ectoderm
• Lines amniotic cavity which is filled with fluid and protects developing embryo
• present until birth

Question 17

Describe the chorion?

Answer 17

• double layered membrane formed by trophoblasts and extra embryonic membranes
• lines chorionic cavity, this cavity dissapears due to expansion of amniotic cavity
• forms fetal component of placenta

Question 18

Describe the yolk sac?

Answer 18

• continous with hypoblast of bilaminar disc
• associated with endoderm
• important in nutrient transfer in weeks 2 and 3, disappears by week 20
• important in blood cell and gut formation

Question 19

Describe the allantois?

Answer 19

• forms as outgrowth of yolk-sac

- contributes to umbilical arteries and connects t fetal bladder

Question 20

What is the major event that occurs in week 3 of development?

Answer 20

Gastrulation

Question 21

What is gastrulation?

Answer 21

A process of cell division and migration resulting in formation of 3 germ layers.

Question 22

What are the 3 germ layers and from what are they formed?

Answer 22

-Ectoderm
-Endoderm
-Mesoderm
Formed from the bilaminar epiblast

Question 23

What are some of the derivatives of the germ layers?

Answer 23

• Ectoderm; include epidermis, cornea, hair, teeth, epithelial lining (outside things)
• Endoderm; GI tract, resp. tract, liver, pancreas
• Mesoderm; muscle, bones

Question 24

What other 3 structures are formed during gastrulation?

Answer 24

• Primitive streak
• Notochord
• Neural tube

Question 25

What is the purpose of the primitive streak?

Answer 25

Encourages migration of epiblasts downwards, creating mesoderm.
Some cells push further and displace the hypoblast to the side to form the endoderm.
The cells left at the top become the ectoderm.
This process is called invagination.