Embryology - 1

Question 1

How long can sperm and oocyte survive?

Answer 1

Sperm - 5 days
Oocyte - 24 hours after ovulation

Question 2

Describe the journey of the egg during fertilisation

Answer 2

Egg leaves ovary and enters fallopian tube
Sperm enters egg and unites with nucleus
Fertilised egg divides - cell numbers increase and blastocyst is formed
Cells attach to uterus

Question 3

Describe the process of fertilisation

Answer 3

Sperm makes contact with egg
Acrosome reacts with zona pellucida - release hydrolytic enzymes
Acrosome reacts with perivitelline space
Plasma membranes of sperm and egg fuse
Sperm nucleus enters egg

Question 4

What happens when sperm nucleus enters egg?

Answer 4

Cortical granules fuse with egg plasma membrane which render the vitelline layer impermeable to sperm

Question 5

Describe early embryo development

Answer 5

Day 1 - formation of pronuclei
2 - first cell division
3 - cell division continues
4 - formation of morula (ball of cells)
5 - blastocyst forms

Question 6

What does the cells in the blastocyst organise into?

Answer 6

2 layers - trophoblast (outer) where placenta develops and inner cell mass where embryo develops into foetus

Question 7

Which layer of the blastocyst invades the maternal endometrium for implantation?

Answer 7

Trophoblast

Question 8

What are the 4 steps of implantation?

Answer 8

Hatching
Apposition
Adhesion
Invasion

Question 9

Describe the hatching stage of implantation

Answer 9

Trophectoderm cells produce protease to dissolve the zona in preparation for implantation

Question 10

Describe the apposition stage of implantation

Answer 10

First connection between blastocyst and endometrium, apposes to microvilli like structure called pinopodes expressed on receptive endometrium

Question 11

Describe adhesion stage of implantation

Answer 11

Trophoblast of the blastocyst adheres to the epithelial layer of maternal endometrium, embryonic tissue starts to actively secrete hCG

Question 12

Describe the invasion stage of implantation

Answer 12

Trophoblast proliferation, differentiation, crossing of epithelial membrane and invasion of endometrial stroma to form placenta
Uterine spiral arteries remodelled by invasive trophoblast

Question 13

What happens if there is inability to hatch?

Answer 13

Can result in infertility and premature hatching can result in abnormal implantation in the uterine tube - ectopic can result

Question 14

What is the stages of apposition and adherence mediated by?

Answer 14

Growth factors and cytokines
Hormones
Free oxygen radicals

Question 15

What is decidualisation?

Answer 15

Pre-requisite for trophoblast invasion
Involves transformation of stromal cells of maternal endometrium every month under progesterone

Question 16

What are the changes which happen during decidualisation?

Answer 16

Stromal cell differentiation converted to decidual
Angiogenesis - new blood vessel formation
Increased macrophages, lymphocytes and decidual leukocytes for maternal immune tolerance

Question 17

What do the changes of decidualisation transform the endometrium into?

Answer 17

Vascular receptive tissue for blastocyst invasion

Question 18

What are the stages of placental formation?

Answer 18

Blastocyst comes into contact with endometrium
Trophoblast differentiation and villi formation
Placenta is fully derived from embryo

Question 19

What forms primary chorionic villi in placentation?

Answer 19

Invading trophoblast

Question 20

What makes primary chorionic villi become secondary?

Answer 20

With infiltration of extra embryonic mesoderm

Question 21

When do secondary chorionic villi become tertiary?

Answer 21

When capillaries form

Question 22

What does invasion of cytotrophoblast do in placentation?

Answer 22

Remodels spiral arteries of endometrium to increase blood flow

Question 23

What is the mature placenta formed of?

Answer 23

Chorionic villi
Intervillous space
Placental septae
Cotyledons

Question 24

What are the functions of the placenta?

Answer 24

Acts as an immunological barrier
Gas exchange
Nutrient exchange
Waste excretion
Endocrine functions