Life Before Birth

Question 1

What are the 3 developmental stages of life before birth? what are their timings and what happens briefly?

Answer 1

Pre-implantation stage: week 1 (fertilised eggs implanted in uterus)
Embryonic stage: week 2-8 (organogen)
Fetal stage: week 9-38 (growth and development)

Question 2

Describe cleavage?

Answer 2

When zygote undergoes a process of mitotic division and the cells in the embryo are cleaved and multiply in number but size of zygote remains the same.

Question 3

Why is it necessary that the zygote does not increase in size?

Answer 3

So that zygote can pass through the isthmus (narrowest part of the Fallopian tube)

Question 4

What helps protect the zygote from premature implantation?

Answer 4

Zygote is surrounded by the ZONA PELLUCIDA - which is a tough, transparent coat made up of glycoproteins. This helps ti along the Fallopian tube.

Question 5

What is a morula?

Answer 5

Cluster of around 16 cells/blastomeres

Question 6

Describe morula formation and when doe sit occur?

Answer 6

Occurs in day 4 after fertilisation

As cells continue to multiply, they run out of space and cluster together. They maximise contact with one another and form tight junctions to communicate with one another.

Question 7

Describe blastocyst formation and when does it form

Answer 7

Day 5
Cells in the embryo either differentiate to form:
Inner cell mass (which forms the embryo and extra embryonic cells)
OR
Trophoblast cells (which forms the placenta)

Question 8

Describe the structure of a blastocyst

Answer 8

Trophoblast cells line the outside
Clump of inner cell mass in one side
Blastocyst Cavity is filled with fluid (enters via the ZONA PELLUCIDA)

Question 9

What triggers the hatching process and how does it occur?

Answer 9

Cells start to run out of nutrients and need to implant on the endometrium.
Inner cell mass begins to proliferate and fluids builds up in the blastocyst cavity. the pressure eventually causes the blastocyst to hatch out of the zone pellucid to allow implantation

Question 10

When does the hatching process occur?

Answer 10

Days 5-7

Question 11

What week and days does implantation occur in?

Answer 11

Week 2

Days 7-12 after fertilisation

Question 12

What is implantation?

Answer 12

Interaction between mother’s endometrium and implanting embryo

Question 13

Which cells implant into the endometrium first?

Answer 13

trophoblast cells

Question 14

What do trophoblast cells differentiate into? which ones are inner and outer?

Answer 14

Syncytiotrophoblast cells (outer)
Cytotrophoblast cells (inner)

Question 15

What does the bilaminar embryonic disc comprise of?

Answer 15

Epiblast

Hypoblast

Question 16

Give examples of abnormal implantation

Answer 16

Ectopic pregnancy: embryo implants itself in the Fallopian tube
Ectopic sites: embryo escapes the Fallopian tube and sticks onto external surface of uterus

Question 17

Describe the 2 cavities around the bilaminar disc

Answer 17

Amniotic sac: extends from the epiblast, cavity filled with amniotic fluid which surround and protects developing embryo

Yolk sac: extends from the hypoblast, providing nutrients to the embryo in week 2-3. Disappears at around week 20

Question 18

List extra embryonic membraneS?

Answer 18

Amnion
Chorion
Yolk sac

Question 19

Describe chorion, what is it made from?

Answer 19

Made from trophoblast and mesoderm cells. It contributes to the formation of the fetal component of the placenta. Only seen in early pregnancy as it disappears when the amnion gets bigger.

Question 20

Define gastrulation and what week does it occur in?

Answer 20

Gastrulation is the process of cell division and migration, resulting in formation of three germ layers (trilaminar embryo)

Question 21

What are 3 important structures in gastrulation?

Answer 21

Primitive streak
Notochord
Neural tube

Question 22

What is the primitive streak?

Answer 22

Thickening of cells in the caudal end of the embryo

Question 23

Describe the process of invagination

Answer 23

Cells in the epiblast move towards the primitive streak and push down where the PS is. The cells then push outwards and form a layer underneath called the Mesoderm.

Question 24

Describe what happens after imagination and what the 3 germ layers are

Answer 24

After invagination, cells replace the hypoblast to form the endoderm (bottom layer).
Cell that lie between the endoderm and the epiblast = mesoderm
Remaining cells in the epiblast form the ectoderm (top layer)

Question 25

What are the derivatives of ectoderm?

Answer 25

Epidermis of skin and its derivatives (sweat glands, hair follicles)
Epithelial lining of mouth and anus
Cornea and lens of eye
Epithelium of pineal and pituitary gland
Nervous system
Sensory receptors in epidermis 
Adrenal medulla
Tooth enamel 
Nervous system

Question 26

What are the derivatives of mesoderm?

Answer 26

Skeletal system
Muscular system
Muscular layer of stomach and intestine
Excretory system
Reproductive system 
Adrenal Cortex
Circulatory and lymphatic system
Notochord 
Dermis of skin
Lining of body cavity

Question 27

What are the derivatives of endoderm?

Answer 27

Epithelial lining of digestive system
Epithelial lining of respiratory system 
Thymus
Pancreas 
Liver 
Lining of urethra, urinary bladder and reproductive system
Thyroid and parathyroid glands